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Savings have been described as the ability of healthy humans to relearn a previously acquired motor skill faster than the first time, which in the context of motor adaptation suggests that the learning rate in the brain could be adjusted when a perturbation is recognized. Alternatively, it has been argued that apparent savings were the consequence of a distinct process that instead of reflecting a change in the learning rate, revealed an explicit re-aiming strategy. Based on recent evidence that feedback adaptation may be central to both planning and control, we hypothesized that this component could genuinely accelerate relearning in human adaptation to force fields during reaching. Consistent with our hypothesis, we observed that upon re-exposure to a previously learned force field, the very first movement performed by healthy volunteers in the relearning context was better adapted to the external disturbance, and this occurred without any anticipation or cognitive strategy because the relearning session was started unexpectedly. We conclude that feedback adaptation is a medium by which the nervous system can genuinely accelerate learning across movements.Significance StatementSavings describe the ability of healthy humans to faster relearn a previously acquired motor skill. It is debated whether savings result from implicit changes in the learning rate or an explicit strategy. Given recent evidence that feedback control plays a key role in movement adaptation, we hypothesized that this component also contributed to savings in the context of force field adaptation. We confirm that relearning was faster and demonstrate that the very first relearning movement was better adapted to the external disturbance, i.e., the online corrections in the first re-exposure trial carried imprints of feedback adaptation from the previous sessions in the absence of any explicit strategy. We conclude that this non-explicit feedback component can genuinely accelerate motor learning.

Most human movements require coordinated activation of multiple muscles. Although many studies reported associations between arm, leg, and trunk muscles during functional tasks, their neural interaction mechanisms still remain unclear. Therefore, the aim of our study was to investigate arm-trunk or arm-leg neural interactions in the corticospinal tract during different arm muscle contractions. Specifically, we examined corticospinal excitability of the erector spinae (ES; trunk extensor), rectus abdominis (RA; trunk flexor), and tibialis anterior (TA; leg) muscles while participants exerted: (1) wrist flexion and (2) wrist extension isometric contraction at various contraction intensity levels ranging from rest to 50% of maximal voluntary contraction (MVC) effort. Corticospinal excitability was assessed using motor evoked potentials (MEPs) elicited through motor cortex transcranial magnetic stimulation (TMS). Results showed that ES MEPs were facilitated even at low contractions (>5% MVC) during wrist flexion and extension, while stronger contractions (>25% MVC) were required to facilitate RA MEPs. The extent of facilitation of ES MEPs depended on contraction intensity of wrist extension, but not flexion. Moreover, TA MEPs were facilitated at low contractions (>5% MVC) during wrist flexion and extension, but contraction intensity dependence was only shown during stronger wrist extension contractions (>25% MVC). In conclusion, trunk extensor corticospinal excitability seems to depend on the task and the intensity of arm contraction, while this is not true for trunk flexor and leg muscles. Our study therefore demonstrated task- and intensity-dependent neural interactions of arm-trunk connections, which may underlie anatomic and/or functional substrates of these muscle pairs.

Knee joint trauma can cause an osteochondral defect (OD), a risk factor for osteoarthritis and cause of debilitating pain in patients. Rodent OD models are less translatable due to their smaller joint size and open growth plate. This study proposes sheep as a translationally relevant model to understand the neuronal basis of OD pain. Unilateral 6 mm deep OD was induced in adult female sheep. 2-6 weeks post-operation, lumbar dorsal root ganglia (DRG) neurons were collected from the contralateral and OD side of operated sheep. Functional assessment of neuronal excitability and activity of the pain-related ion channels TRPV1 and P2X3 was carried out using electrophysiology and Ca2+-imaging. Immunohistochemistry was utilized to verify expression of pain-related proteins. We observed that an increased proportion of OD DRG neurons (sheep, N = 3; Ctrl neurons, n =15, OD neurons, n = 16) showed spontaneous electrical excitability (Ctrl: 20.33 ± 4.5%; OD: 50 ± 10%; p = 0.009, unpaired t-test) and an increased proportion fired a greater number of spikes above baseline in response to application of a TRPV1 agonist (capsaicin) application (Ctrl: 40%; OD: 75%; p = 0.04, chi-sq test). Capsaicin also produced Ca2+ influx in an increased proportion of OD DRG neurons isolated (Ctrl: 25%; OD: 44%; p = 0.001, chi-sq test). Neither protein expression, nor functionality of the P2X3 ion channel were altered in OD neurons. Overall, we provide evidence of increased excitability of DRG neurons (an important neural correlate of pain) and TRPV1 function in an OD sheep model. Our data show that functional assessment of sheep DRG neurons can provide important insights into the neural basis of OD pain and thus potentially prevent its progression into arthritic pain. Significance statement Pain is the primary symptom of osteoarthritis (OA) and often the main reason for patients seeking medical care. Understanding pain mechanisms in OA can boost the development of disease specific pain relief. While small animals such as mouse and rat have been widely used in OA pain studies, the genetic and anatomical differences between rodents and humans can hinder clinical translation. Here, we studied pain in an early osteochondral defect (OD) model in sheep, a commonly used large animal model in OA research. We found increased excitability and TRPV1 function in dorsal root ganglia (DRG) neurons innervating the site of OD. This study thus demonstrates the utility of using a large animal, such as sheep, for studying mechanisms of joint pain.

In the mature mouse retina, Otx2 is expressed in both Retinal Pigmented Epithelium (RPE) and photoreceptor (PR) cells, but Otx2 knockout in the RPE alone results in PR degeneration. To study the cell-autonomous function of OTX2 in PRs, we performed PR-specific Otx2 knockout (cKO) in adults. As expected, the protein disappears completely from PR nuclei but is still observed in PR inner and outer segments while its level concomitantly decreases in the RPE, suggesting a transfer of OTX2 from RPE to PRs in response to Otx2 ablation in PRs. The ability of OTX2 to transfer from RPE to PRs was verified by viral expression of tagged-OTX2 in the RPE. Transferred OTX2 distributed across the PR cytoplasm, suggesting functions distinct from nuclear transcription regulation functions. PR-specific Otx2 cKO did not alter the structure of the retina but impaired the translocation of photoreceptor arrestin-1 upon illumination changes, making mice photophobic. RNA-seq analyses following Otx2 knockout revealed down-regulation of genes involved in the cytoskeleton that might account for the arrestin-1 translocation defect, and of genes involved in extracellular matrix and signalling factors that may participate in the enhanced transfer of OTX2. Interestingly, several RPE-specific OTX2 target genes involved in melanogenesis were downregulated, lending weight to a decrease of OTX2 levels in the RPE following PR-specific Otx2 cKO. Our study reveals a new role of endogenous OTX2 in PR light adaptation and demonstrates the existence of OTX2 transfer from RPE to PR cells, which is increased upon PR-specific Otx2 ablation and might participate in PR neuroprotection.Significance statementOTX2 homeoprotein is expressed in retinal pigmented epithelium (RPE), photoreceptors (PRs) and bipolar cells. The function of endogenous PR OTX2, which, in contrast with RPE OTX2, is dispensable for PR maintenance, is unknown. We performed PR-specific Otx2 knockout and found that removal of endogenous PR OTX2 leads to impaired arrestin-1 translocation associated with photophobia, specific modifications of PR and RPE gene expression, and to increased transfer of OTX2 protein from the RPE to the PR cytoplasm. Thus, several PR activities, including light adaptation, rely on endogenous nuclear OTX2, while PR neuroprotection seems to require RPE OTX2, highlighting the importance of non-cell-autonomous OTX2 in the adult retina.

The local circuitry within olfactory bulb glomeruli filters, transforms, and facilitates information transfer from olfactory sensory neurons to bulb output neurons. Two key elements of this circuit are glutamatergic tufted cells (TCs) and GABAergic periglomerular (PG) cells, both of which actively shape mitral cell activity and bulb output. A subtype of TCs, the external tufted cells (eTCs), can synaptically excite PG cells, but there are unresolved questions about other aspects of the glomerular connections, including the extent of connectivity between eTCs and the precise nature of reciprocal interactions between TCs and PG cells. We combined patch-clamp recordings in OB slices and optophysiological tools to investigate local functional connections within glomeruli of mice and rats. When TCs that express cholecystokinin (CCK) were optically suppressed, excitatory post-synaptic currents (EPSCs) in “uniglomerular” PG cells that extend dendrites to one glomerulus were decreased, consistent with TC activation being required for most excitation of these PG cells. However, TC suppression had no effect on EPSCs in eTCs, arguing that TCs make few, if any, direct glutamatergic synaptic connections onto eTCs. The absence of synaptic connections between eTCs was also supported by recordings in eTC pairs. Lastly, we show using similar optical suppression methods that GAD65-expressing PG cells, mainly uniglomerular cells, provide strong inhibition onto eTCs. Our results imply that the local network of CCK-expressing TCs form potent reciprocal chemical synaptic connections with GAD65-expressing uniglomerular PG cells but not eTCs. This configuration favors local inhibition over recurrent excitation within a glomerulus, limiting its output. Significance Statement The brain circuits involved with the initial processing of sensory information are comprised of networks of excitatory and inhibitory neurons. The strength of local connections can impact the weighting of excitation versus inhibition and determine whether sensory signals are passed on to higher-order brain structures. We investigated the connectivity between excitatory and inhibitory neurons in discrete glomerular structures of the rodent olfactory bulb. We found that excitatory neurons make potent chemical synaptic connections with a class of inhibitory cells with dendrites confined to one glomerulus. However, excitatory cells do not form chemical synaptic connections with each other. This circuit configuration suggests that sensory input is generally biased toward driving more local inhibition than recurrent excitation, thus suppressing bulbar output.

Congenital sensorineural hearing loss (SNHL) affects thousands of infants each year and results in significant delays in speech and language development. Previous studies have shown that early exposure to a simple augmented acoustic environment (AAE) can limit the effects of progressive SNHL on hearing sensitivity. However, SNHL is also accompanied by hearing loss that is not assessed on standard audiological examinations, such as reduced temporal processing acuity. To assess whether sound therapy may improve these deficits, a mouse model of congenital SNHL was exposed to simple or temporally complex AAE. The DBA/2J mouse strain develops rapid, base to apex, progressive SNHL beginning at birth and is functionally deaf by six months of age. Hearing sensitivity and auditory brainstem function was measured using otoacoustic emissions, auditory brainstem response (ABR) and extracellular recording from the inferior colliculus (IC) in mice following exposure to 30 d of continuous AAE. Peripheral function and sound sensitivity in auditory midbrain neurons improved following exposure to both types of AAE. However, exposure to a novel, temporally complex AAE more strongly improved a measure of temporal processing acuity, neural gap-in-noise detection in the auditory midbrain. These experiments suggest that targeted sound therapy may be harnessed to improve hearing outcomes for children suffering from congenital SNHL.

In humans, finely tuned gamma synchronization (60-90 Hz) rapidly appears at movement onset in a motor control network involving primary motor cortex, the basals ganglia and motor thalamus. Yet the functional consequences of brief movement-related synchronization are still unclear. Distinct synchronization phenomena have also been linked to different forms of motor inhibition, including relaxing antagonist muscles, rapid movement interruption and stabilizing network dynamics for sustained contractions. Here I will introduce detailed hypotheses about how intra- and inter-site synchronization could interact with firing rate changes in different parts of the network to enable flexible action control. The here proposed cause-and-effect relationships shine a spotlight on potential key mechanisms of cortico-basal ganglia-thalamo-cortical communication. Confirming or revising these hypotheses will be critical in understanding the neuronal basis of flexible movement initiation, invigoration and inhibition. Ultimately, the study of more complex cognitive phenomena will also become more tractable once we understand the neuronal mechanisms underlying behavioural readouts.Significance statementIn spite of tremendous progress in describing how neuronal activity unfolds before and during movements, the mechanisms that trigger the switch from movement preparation to execution, regulate movement vigour and enable movement inhibition remain unknown. Brief synchronization of neural activity within and between cortical sites and the basal ganglia may be a key factor in controlling these mechanisms. Here I review the evidence and describe in detail how synchronization may shape firing rates in distinct sites of the cortico-basal ganglia-thalamo-cortical network to enable flexible action control.

Motor behaviors are central to many functions and dysfunctions of the brain, and understanding their neural basis has consequently been a major focus in neuroscience. However, most studies of motor behaviors have been restricted to artificial, repetitive paradigms, far removed from natural movements performed “in the wild.” Here, we leveraged recent advances in machine learning and computer vision to analyze intracranial recordings from 12 human subjects during thousands of spontaneous, unstructured arm reach movements, observed over several days for each subject. These naturalistic movements elicited cortical spectral power patterns consistent with findings from controlled paradigms, but with considerable neural variability across subjects and events. We modeled interevent variability using 10 behavioral and environmental features; the most important features explaining this variability were reach angle and day of recording. Our work is among the first studies connecting behavioral and neural variability across cortex in humans during unstructured movements and contributes to our understanding of long-term naturalistic behavior. Significance Statement Understanding the neural basis of human movement has long been a key focus in neuroscience. However, researchers often study constrained, monotonous tasks that differ greatly from the rich and diverse natural movements we actually make. Here, we use data-intensive computational approaches to reveal patterns in neural activity underlying naturalistic human arm movements. While such movements match previous experimental findings on average, there is substantial neural variability from one movement to the next. We partially explain this variability by aspects of the movement observed, though much variability remains unaccounted for. Our study sheds light on how the brain generates natural arm movements and emphasizes the critical need to study brain activity in more unstructured settings that mimic daily life.

Visual Abstract, Loss of olfactory sensory neurons (OSNs) after injury to the olfactory epithelium (OE) triggers the generation of OSNs that are incorporated into olfactory circuits to restore olfactory sensory perception. This study addresses how insulin receptor-mediated signaling affects the functional recovery of OSNs after OE injury. Insulin levels were reduced in mice by ablating the pancreatic β cells via streptozotocin (STZ) injections. These STZ-induced diabetic and control mice were then intraperitoneally injected with the olfactotoxic drug methimazole to selectively ablate OSNs. The OE of diabetic and control mice regenerated similarly until day 14 after injury. Thereafter, the OE of diabetic mice contained fewer mature and more apoptotic OSNs than control mice. Functionally, diabetic mice showed reduced electro-olfactogram (EOG) responses and their olfactory bulbs (OBs) had fewer c-Fos-active cells following odor stimulation, as well as performed worse in an odor-guided task compared with control mice. Insulin administered intranasally during days 8–13 after injury was sufficient to rescue recovery of OSNs in diabetic mice compared with control levels, while insulin administration between days 1 and 6 did not. During this critical time window on days 8–13 after injury, insulin receptors are highly expressed and intranasal application of an insulin receptor antagonist inhibits regeneration. Furthermore, an insulin-enriched environment could facilitate regeneration even in non-diabetic mice. These results indicate that insulin facilitates the regeneration of OSNs after injury and suggest a critical stage during recovery (8–13 d after injury) during which the maturation of newly generated OSNs is highly dependent on and promoted by insulin.

Visual Abstract, Neuropeptides are implicated in control of lateralized processes in the brain. A unilateral brain injury (UBI) causes the contralesional sensorimotor deficits. To examine whether opioid neuropeptides mediate UBI induced asymmetric processes we compared effects of opioid antagonists on the contralesional and ipsilesional hindlimb responses to the left-sided and right-sided injury in rats. UBI induced hindlimb postural asymmetry (HL-PA) with the contralesional hindlimb flexion, and activated contralesional withdrawal reflex of extensor digitorum longus (EDL) evoked by electrical stimulation and recorded with EMG technique. No effects on the interossei (Int) and peroneaus longus (PL) were evident. The general opioid antagonist naloxone blocked postural effects, did not change EDL asymmetry while uncovered cryptic asymmetry in the PL and Int reflexes induced by UBI. Thus, the spinal opioid system may either mediate or counteract the injury effects. Strikingly, effects of selective opioid antagonists were the injury side-specific. The μ-antagonist β-funaltrexamine (FNA) and κ-antagonist nor-binaltorphimine (BNI) reduced postural asymmetry after the right but not left UBI. In contrast, the δ-antagonist naltrindole (NTI) inhibited HL-PA after the left but not right-side brain injury. The opioid gene expression and opioid peptides were lateralized in the lumbar spinal cord, and coordination between expression of the opioid and neuroplasticity-related genes was impaired by UBI that together may underlie the side-specific effects of the antagonists. We suggest that mirror-symmetric neural circuits that mediate effects of left and right brain injury on the contralesional hindlimbs are differentially controlled by the lateralized opioid system.

eNeuro / Society for Neuroscience 8(5), ENEURO.0179-21.2021 (2021). doi:10.1523/ENEURO.0179-21.2021, Published by Society for Neuroscience, Washington, DC

GABAergic somatodendritic inhibition in the preBötzinger complex (preBötC), a medullary site for the generation of inspiratory rhythm, is involved in respiratory rhythmogenesis and patterning. Nevertheless, whether GABA acts distally on presynaptic terminals, evoking presynaptic inhibition is unknown. Here, we begin to address this problem by measuring presynaptic Ca2+ transients in preBötC neurons, under rhythmic and non-rhythmic conditions, with two variants of genetically encoded Ca2+ indicators (GECIs). Organotypic slice cultures from newborn mice, containing the preBötC, were drop-transduced with jGCaMP7s, or injected with jGCaMP7f-labeling commissural preBötC neurons. Then, Ca2+ imaging combined with whole-cell patch-clamp or field stimulation was obtained from inspiratory preBötC neurons. We found that rhythmically active neurons expressed synchronized Ca2+ transients in soma, proximal and distal dendritic regions, and punctate synapse-like structures. Expansion microscopy revealed morphologic characteristics of bona fide synaptic boutons of the en passant and terminal type. Under non-rhythmic conditions, we found that bath application of the GABAA receptor agonist muscimol, and local microiontophoresis of GABA, reduced action potential (AP)-evoked and field stimulus-evoked Ca2+ transients in presynaptic terminals in inspiratory neurons and commissural neurons projecting to the contralateral preBötC. In addition, under rhythmic conditions, network rhythmic activity was suppressed by muscimol, while the GABAA receptor antagonist bicuculline completely re-activated spontaneous activity. These observations demonstrate that the preBötC includes neurons that show GABAergic inhibition of presynaptic Ca2+ transients, and presynaptic inhibition may play a role in the network activity that underlies breathing.

Mitral/tufted (MT) cells of the olfactory bulb (OB) show diverse temporal responses to odorant stimulation that are thought to encode odor information. Much of this diversity is thought to arise from inhibitory OB circuits, but the dynamics of excitatory input to MT cells, which is driven in a feedforward manner by sensory afferents, may also be important. To examine the contribution of excitatory input dynamics to generating temporal diversity in MT cells, we imaged glutamate signaling onto mitral and tufted cell dendrites in anesthetized and awake mice. We found surprising diversity in the temporal dynamics of these signals. Inhalation-linked glutamate transients were variable in onset latency and duration, and in awake mice the degree of coupling to inhalation varied substantially with odorant identity and concentration. Successive inhalations of odorant produced nonlinear changes in glutamate signaling that included facilitating, adapting and suppressive responses and which varied with odorant identity and concentration. Dual-color imaging of glutamate and calcium signals from MT cells in the same glomerulus revealed highly correlated pre- and postsynaptic signals across these different response types. Suppressive calcium responses in MT cells were nearly always accompanied by suppression in the glutamate signal, providing little evidence for MT cell suppression by lateral or feedforward inhibition. These results indicate a high degree of diversity in the dynamics of excitatory input to MT cells, and suggest that these dynamics may account for much of the diversity in MT cell responses that underlies OB odor representations. Significance statement Temporal patterns of excitation and inhibition among olfactory bulb output neurons (mitral and tufted, or MT, cells) play important roles in coding olfactory information. Using the glutamate sensor iGluSnFR expressed on MT cells, we found surprising diversity in the dynamics of their excitatory input: odorant-evoked glutamate signals varied within a respiratory cycle and across repeated samples of odorant, and varied with both odorant identity and concentration. Simultaneous imaging of presynaptic glutamate and postsynaptic calcium from MT cells revealed high correspondence in the dynamics of glutamatergic input and MT cell output. These results suggest that the dynamics of excitatory input alone can account for much of the diversity of MT cell responses that underlie odor representations in vivo.

International audience; Background noise strongly penalizes auditory perception of speech in humans or vocalizations in animals. Despite this, auditory neurons are still able to detect communications sounds against considerable levels of background noise. We collected neuronal recordings in cochlear nucleus (CN), inferior colliculus (IC), auditory thalamus, and primary and secondary auditory cortex in response to vocalizations presented either against a stationary or a chorus noise in anesthetized guinea pigs at three signal-to-noise ratios (SNRs; -10, 0, and 10 dB). We provide evidence that, at each level of the auditory system, five behaviors in noise exist within a continuum, from neurons with high-fidelity representations of the signal, mostly found in IC and thalamus, to neurons with high-fidelity representations of the noise, mostly found in CN for the stationary noise and in similar proportions in each structure for the chorus noise. The two cortical areas displayed fewer robust responses than the IC and thalamus. Furthermore, between 21% and 72% of the neurons (depending on the structure) switch categories from one background noise to another, even if the initial assignment of these neurons to a category was confirmed by a severe bootstrap procedure. Importantly, supervised learning pointed out that assigning a recording to one of the five categories can be predicted up to a maximum of 70% based on both the response to signal alone and noise alone.Significance Statement In daily situations, humans and animals are faced with various background noises in which they have to detect behaviorally salient signals. Noise resistance is often viewed as an emergent property of cortical networks, but only a few studies have characterized the relative contribution of cortical and subcortical neurons. Our results demonstrate that the neuronal resistance to noise is distributed along the auditory system with a more important fraction of robust neurons in subcortical structures compared with auditory cortex, and is relatively well predictable based on the responses to the signal alone and the noise alone. Our results also suggest that noise-invariant representations of communication sounds coexist with accurate noise representations, which are detected as early as the cochlear nucleus (CN).

Visual Abstract, The dorsal cochlear nucleus (DCN) is a region of particular interest for auditory and tinnitus research. However, lack of useful genetic markers for in vivo manipulations hinders elucidation of the DCN contribution to tinnitus pathophysiology. This work assesses whether adeno-associated viral vectors (AAV) containing the calcium/calmodulin-dependent protein kinase 2α (CaMKIIα) promoter and a mouse line of nicotinic acetylcholine receptor α2 subunit (Chrna2)-Cre can target specific DCN populations. We found that CaMKIIα cannot be used to target excitatory fusiform DCN neurons as labeled cells showed diverse morphology indicating they belong to different classes of DCN neurons. Light stimulation after driving Channelrhodopsin2 (ChR2) by the CaMKIIα promoter generated spikes in some units but firing rate decreased when light stimulation coincided with sound. Expression and activation of CaMKIIα-eArchaerhodopsin3.0 in the DCN produced inhibition in some units but sound-driven spikes were delayed by concomitant light stimulation. We explored the existence of Cre+ cells in the DCN of Chrna2-Cre mice by hydrogel embedding technique (CLARITY). There were almost no Cre+ cell bodies in the DCN; however, we identified profuse projections arising from the ventral cochlear nucleus (VCN). Anterograde labeling in the VCN revealed projections to the ipsilateral superior olive and contralateral medial nucleus of the trapezoid body (MNTB; bushy cells), and a second bundle terminating in the DCN, suggesting the latter to be excitatory Chrna2+ T-stellate cells. Exciting Chrna2+ cells increased DCN firing. This work shows that cortical molecular tools may be useful for manipulating the DCN especially for tinnitus studies.

Many receptive fields in the early visual system show standard (center-surround) structure and can be analyzed using simple drifting patterns and a difference-of-Gaussians (DoG) model, which treats the receptive field as a linear filter of the visual image. But many other receptive fields show nonlinear properties such as selectivity for direction of movement. Such receptive fields are typically studied using discrete stimuli (moving or flashed bars and edges) and are modelled according to the features of the visual image to which they are most sensitive. Here, we harness recent advances in tomographic image analysis to characterize rapidly and simultaneously both the linear and nonlinear components of visual receptive fields. Spiking and intracellular voltage potential responses to briefly flashed bars are analyzed using non-negative matrix factorization (NNMF) and iterative reconstruction tomography (IRT). The method yields high-resolution receptive field maps of individual neurons and neuron ensembles in primate (marmoset, both sexes) lateral geniculate and rodent (mouse, male) retina. We show that the first two IRT components correspond to DoG-equivalent center and surround of standard [magnocellular (M) and parvocellular (P)] receptive fields in primate geniculate. The first two IRT components also reveal the spatiotemporal receptive field structure of nonstandard (on/off-rectifying) receptive fields. In rodent retina we combine NNMF-IRT with patch-clamp recording and dye injection to directly map spatial receptive fields to the underlying anatomy of retinal output neurons. We conclude that NNMF-IRT provides a rapid and flexible framework for study of receptive fields in the early visual system. Significance Statement We present new means to characterize rapidly the linear and nonlinear properties of receptive fields in early stages of visual processing. We analyze light-evoked response properties using new tomographic methods developed for medical imaging. The tomographic method is rapid, can be used to characterize many cells simultaneously, and reveals detailed structure of receptive field organization in monkey and mouse visual system.

The detection of temporal variations in amplitude of light intensity, or temporal contrast sensitivity (TCS), depends on the kinetics of rod photoresponse recovery. Uncharacteristically fast rod recovery kinetics are facets of both human patients and transgenic animal models with a P23H rhodopsin mutation, a prevalent cause of retinitis pigmentosa. Here, we show that mice with this mutation (RhoP23H/+) exhibit an age- and illumination-dependent enhancement in TCS compared to controls. At retinal illumination levels producing ≥1000 R*/rod/s or more, P30 RhoP23H/+ mice exhibit a 1.2 to 2-fold increase in retinal and optomotor TCS relative to controls in response to flicker frequencies of 3, 6, and 12 Hz despite significant photoreceptor degeneration and loss of flash ERG b-wave amplitude. Surprisingly, the TCS of RhoP23H/+ mice further increases as degeneration advances. Enhanced TCS is also observed in a second model (rhodopsin heterozygous mice, Rho+/-) with fast rod recovery kinetics and no apparent retinal degeneration. In both mouse models, enhanced TCS is explained quantitatively by a comprehensive model that includes photoresponse recovery kinetics, density and collecting area of degenerating rods. Measurement of TCS may be a non-invasive early diagnostic tool indicative of rod dysfunction in some forms of retinal degenerative disease. Significance Statement Retinal degeneration in humans causes loss of retinal cells, loss of retinal function, and eventual blindness. Understanding the retinal and visual changes that occur early in retinal degenerative disease is critical for improving therapeutic strategies and treatment outcomes. We show here an enhanced ability to detect flickering lights that develops during early retinal degeneration in a mouse model of a human disease. This surprising gain-of-function was caused by a pathological acceleration of the temporal properties of rod photoresponses. In humans, advanced rod dysfunction is currently diagnosed using full-field ERG and perimetry. Measurement of retinal or visual sensitivity to flickering lights such as used here could prove useful as a rapid test for early rod dysfunction in retinal degeneration.

Responding to a stimulus requires transforming an internal sensory representation into an internal motor representation. Where and how this sensory-motor transformation occurs is a matter of vigorous debate. Here, we trained male and female mice in a whisker detection go/no-go task in which they learned to respond (lick) following a transient whisker deflection. Using single unit recordings, we quantified sensory-related, motor-related, and choice-related activities in whisker primary somatosensory cortex (S1), whisker region of primary motor cortex (wMC), and anterior lateral motor cortex (ALM), three regions that have been proposed to be critical for the sensory-motor transformation in whisker detection. We observed strong sensory encoding in S1 and wMC, with enhanced encoding in wMC, and a lack of sensory encoding in ALM. We observed strong motor encoding in all three regions, yet largest in wMC and ALM. We observed the earliest choice probability in wMC, despite earliest sensory responses in S1. Based on the criteria of having both strong sensory and motor representations and early choice probability, we identify whisker motor cortex as the cortical region most directly related to the sensory-motor transformation. Our data support a model of sensory encoding originating in S1, sensory amplification and sensory-motor transformation occurring within wMC, and motor signals emerging in ALM after the sensory-motor transformation.

Our eyes are constantly in motion and the various kinds of eye movements are closely linked to many aspects of human cognitive processing. Measuring all possible eye movements unobtrusively is not achievable with current methods. Video-based eye-trackers only measure rotational but not translational motion of the eye, require a calibration process relying on the participant’s self-report of accurate fixation, and do not work if vision of the eyeball is blocked. Scleral search coils attach physical weight on the eyeball and also do not measure translation. Here, we describe a novel and fully automated method to use real-time magnetic resonance imaging (MRI) for eye tracking. We achieved a temporal resolution sufficient to measure eye rotations and translations as short as those that occur within a blink and behind a closed eyelid. To demonstrate this method, we measured the full extent of the blink-related eye movement for two individuals, suggesting that the eye approaches a holding position during lid closure and can move by as much as 35° in rotation and 2 mm in translation. We also investigated the coordination of gaze shifts with blinks. We found that the gaze shift is tightly coupled in time to the translational blink movement and that blinks can induce significant temporal shifts of the gaze trajectory between left and right eye. Our MR-based Eye Tracking (MREyeTrack) method allows measurement of eye movements in terms of both translation and rotation and enables new opportunities for studying ocular motility and its disorders.

Excitatory amino acid transporters (EAATs) remove glutamate from the synaptic cleft. In the retina, EAAT1 and EAAT2 are considered the major glutamate transporters. However, it has not yet been possible to determine how EAAT5 shapes the retinal light responses because of the lack of a selective EAAT5 blocker or EAAT5 knock-out animal model. In this study, EAAT5 was found to be expressed in a punctate manner close to release sites of glutamatergic synapses in the mouse retina. Light responses from retinae of wild-type and of a newly generated model with a targeted deletion of EAAT5 (EAAT5-/-) were recorded in vitro using multi-electrode arrays. Flicker resolution was considerably lower in EAAT5-/- retinae than in wild-type retinae. The close proximity to the glutamate release site makes EAAT5 an ideal tool to improve temporal information processing in the retina by controlling information transfer at glutamatergic synapses.Significance statementNeurons communicate with other neurons at synaptic connections by release of neurotransmitters acting at postsynaptic receptors. Neurotransmitters are removed from the synaptic cleft by transporters. Using the mouse retina as a model for the central nervous system, the role of EAAT5 that functions as glutamate transporter and as glutamate-gated ion channel was investigated in retinal information processing. EAAT5 was found highly localized to the glutamate release site at retinal synapses, suggesting a role in shaping of synaptic responses. In a mouse model devoid of EAAT5, temporal resolution of the retina was severely compromised. The results demonstrate that glutamate transporters like EAAT5 can exert a tremendous effect on information processing in neuronal networks.

Motor neurons (MNs) innervating the digit muscles of the intrinsic hand (IH) and intrinsic foot (IF) control fine motor movements. The ability to reproducibly label specifically IH and IF MNs in mice would be a beneficial tool for studies focused on fine motor control. To this end, we find that a CRE knock-in mouse line of Atoh1, a developmentally expressed basic helix-loop-helix (bHLH) transcription factor, reliably expresses CRE-dependent reporter genes in ∼60% of the IH and IF MNs. We determine that CRE-dependent expression in IH and IF MNs is ectopic because an Atoh1 mouse line driving FLPo recombinase does not label these MNs although other Atoh1-lineage neurons in the intermediate spinal cord are reliably identified. Furthermore, the CRE-dependent reporter expression is enriched in the IH and IF MN pools with much sparser labeling of other limb-innervating MN pools such as the tibialis anterior (TA), gastrocnemius (GS), quadricep (Q), and adductor (Ad). Lastly, we find that ectopic reporter expression begins postnatally and labels a mixture of α and γ-MNs. Altogether, the Atoh1 CRE knock-in mouse strain might be a useful tool to explore the function and connectivity of MNs involved in fine motor control when combined with other genetic or viral strategies that can restrict labeling specifically to the IH and IF MNs. Accordingly, we provide an example of sparse labeling of IH and IF MNs using an intersectional genetic approach.

With the world population shifting its proportions towards higher ages, it is imperative to increase healthspan rather than lifespan, both for the sakes of community sustainability as well as individual quality of life. One of the main means to obtain healthy ageing, is a concept called active ageing, as activity has shown to slow down the natural ageing process. Active ageing is threatened by a decline in balance performance and an increase of fall-related concerns, as activity curtailment is a major consequence. Fall-related concerns are prevalent in a large portion of the older population. Current understanding of its mechanisms is largely hypothesised, based on few observational studies in which single factors – mostly psychological – are researched. Postural control is the system that helps us maintain balance and control while moving or being still. Yet, the interactions between fall-related concerns and postural control are not very well described. Sensory input and processing have been identified as possibly an important part, but more in-depth investigations are needed. To investigate the prevalence and factors that may play a part in fall-related concerns, 153 older adults (70 years and over) were visited in their home. During these visits questionnaires and clinical tests were administered (among them FES-I, PGCM, SPPB, MMSE). To investigate the relationship between fall-related concerns and postural control more thoroughly, 45 of the original sample were also tested in the movement laboratory. Several modalities of sensory function were recorded, as well as lower limb strength and quiet stance centre of pressure. The home visits showed that 70% of people 70 years and older, living in the community experience at least one of the constructs of fall-related concerns and that associated factors are of both psychological and physical nature (fear, morale, and physical performance). Large and significant differences between men and women were discovered not only in prevalence, but also for the associated factors. Hierarchical multivariate modelling of the laboratory data showed that an increase in sway size and velocity during open eyes quiet stance can explain a rise in fall-related concerns and that that relationship in turn can be explained by declines in specific parts of the postural control system (lower limb strength, reaction time, vision, and proprioception of the ankle, knee, and neck). The more in-depth methods of frequency domain and multivariate discriminant analysis revealed that older participants could be grouped in ‘declined and concerned’ and ‘not declined and not concerned.’ The first showing power in lower frequencies of the postural sway signal than the latter and showing an inability to adjust control strategy when closing their eyes. Finally, the changing and adapting postural control of older people was successfully modelled by applying dimension reduction, neural networks, and an internal feedback of 150ms. The model can predict postural corrections up to 1 second ahead of time. This gave more insight in key features of successful adaptation to an ageing system and provides a virtual environment for future model and theory testing. In conclusion, I have gained more understanding of the complexity and interaction of the different systems involved in the concern-and-balance relationship and have contributed to the field by showing methods that will improve the quality of analyses when testing, evaluating, and researching these phenomena. BAHRT

The processing of visual motion is carried out by dedicated pathways in the primate brain. These pathways originate with populations of direction-selective neurons in the primary visual cortex, which project to dorsal structures like the middle temporal (MT) and medial superior temporal (MST) areas. Anatomical and imaging studies have suggested that area V3A might also be specialized for motion processing, but there have been very few studies of single-neuron direction selectivity in this area. We have therefore performed electrophysiological recordings from V3A neurons in two macaque monkeys (one male and one female) and measured responses to a large battery of motion stimuli that includes translation motion, as well as more complex optic flow patterns. For comparison, we simultaneously recorded the responses of MT neurons to the same stimuli. Surprisingly, we find that overall levels of direction selectivity are similar in V3A and MT and moreover that the population of V3A neurons exhibits somewhat greater selectivity for optic flow patterns. These results suggest that V3A should be considered as part of the motion processing machinery of the visual cortex, in both human and non-human primates.Significance statementAlthough area V3A is frequently the target of anatomy and imaging studies, little is known about its functional role in processing visual stimuli. Its contribution to motion processing has been particularly unclear, with different studies yielding different conclusions. We report a detailed study of direction selectivity in V3A. Our results show that single V3A neurons are, on average, as capable of representing motion direction as are neurons in well-known structures like MT. Moreover, we identify a possible specialization for V3A neurons in representing complex optic flow, which has previously been thought to emerge in higher-order brain regions. Thus it appears that V3A is well-suited to a functional role in motion processing.

Generalization of sensorimotor adaptation across limbs, known as interlimb transfer, is a well-demonstrated phenomenon in humans, yet the underlying neural mechanisms remain unclear. Theoretical models suggest that interlimb transfer is mediated by interhemispheric transfer of information via the corpus callosum. We thus hypothesized that lesions of the corpus callosum, especially to its midbody connecting motor, supplementary motor and premotor areas of the two cerebral hemispheres, would impair interlimb transfer of sensorimotor adaptation. To test this hypothesis, we recruited three patients: two rare stroke patients with recent, extensive callosal lesions including the midbody and one patient with complete agenesis. A prismatic adaptation paradigm involving unconstrained arm reaching movements was designed to assess interlimb transfer from the prism-exposed dominant arm to the unexposed non-dominant arm for each participant. Baseline results showed that spatial performance of each patient did not significantly differ from controls, for both limbs. Further, each patient adapted to the prismatic perturbation, with no significant difference in error reduction compared to controls. Crucially, interlimb transfer was found in each patient. The absolute magnitude of each patient’s transfer did not significantly differ from controls. These findings show that sensorimotor adaptation can transfer across limbs despite extensive lesions or complete absence of the corpus callosum. Therefore, callosal pathways connecting homologous motor, premotor and supplementary motor areas are not necessary for interlimb transfer of prismatic reach adaptation. Such interlimb transfer could be mediated by transcallosal splenium pathways connecting parietal, temporal and visual areas, ipsilateral cortico-spinal pathways or subcortical structures such as the cerebellum. Significance Statement Theoretical models suggest that interlimb transfer of sensorimotor adaptation is mediated by interhemispheric interactions via the corpus callosum, specifically between motor cortices. We thus hypothesized that interlimb transfer of prism adaptation in a reaching task would be impaired in patients with callosal abnormalities, especially those affecting midbody pathways connecting the motor cortices. Contrarily, we found interlimb transfer in each patient, to a level comparable to that of controls. Our findings show that callosal pathways connecting motor, premotor and supplementary motor areas are not necessary for the interlimb transfer of prismatic reach adaptation. Alternatively, this transfer could be mediated by ipsilateral cortico-spinal pathways, subcortical structures such as the cerebellum or callosal splenium pathways connecting parietal, temporal and visual areas.

Although moving sound-sources abound in natural auditory scenes, it is not clear how the human brain processes auditory motion. Previous studies have indicated that, although ocular localization responses to stationary sounds are quite accurate, ocular smooth pursuit of moving sounds is very poor. We here demonstrate that human subjects faithfully track a sound’s unpredictable movements in the horizontal plane with smooth-pursuit responses of the head. Our analysis revealed that the stimulus–response relation was well described by an under-damped passive, second-order low-pass filter in series with an idiosyncratic, fixed, pure delay. The model contained only two free parameters: the system’s damping coefficient, and its central (resonance) frequency. We found that the latter remained constant at about 0.6 Hz throughout the experiment for all subjects. Interestingly, the damping coefficient systematically increased with trial number, suggesting the presence of an adaptive mechanism in the auditory pursuit system. This mechanism functions even for unpredictable sound-motion trajectories endowed with fixed, but covert, frequency characteristics in open-loop tracking conditions. We conjecture that the auditory pursuit system optimizes a trade-off between response speed and effort. Taken together, our data support the existence of a pursuit system for auditory head-tracking, which would suggest the presence of a neural representation of a spatial auditory fovea. Significance statement Inspired by the visual ocular smooth-pursuit system, several studies have used eye movements to track moving sounds, but obtained poor pursuit performance, which led to the idea that the auditory system lacks sensitivity to sound velocity. We here demonstrate accurate head-pursuit of sounds, moving along unpredictable trajectories in the horizontal plane. Interestingly, the auditory pursuit responses adapted to the covert movement spectrum of the stimulus ensemble, from which we infer that the system may optimize a trade-off between movement speed and effort. Our results support the existence of an auditory pursuit system, and we discuss its implications for the neural mechanisms that represent and track moving sounds.

Visual Abstract, γ Band plays a key role in the encoding of visual features in the primary visual cortex (V1). In rodents V1 two ranges within the γ band are sensitive to contrast: a broad γ band (BB) increasing with contrast, and a narrow γ band (NB), peaking at ∼60 Hz, decreasing with contrast. The functional roles of the two bands and the neural circuits originating them are not completely clear yet. Here, we show, combining experimental and simulated data, that in mice V1 (1) BB carries information about high contrast and NB about low contrast; (2) BB modulation depends on excitatory-inhibitory interplay in the cortex, while NB modulation is because of entrainment to the thalamic drive. In awake mice presented with alternating gratings, NB power progressively decreased from low to intermediate levels of contrast where it reached a plateau. Conversely, BB power was constant across low levels of contrast, but it progressively increased from intermediate to high levels of contrast. Furthermore, BB response was stronger immediately after contrast reversal, while the opposite held for NB. These complementary modulations were reproduced by a recurrent excitatory-inhibitory leaky integrate-and-fire network provided that the thalamic inputs were composed of a sustained and a periodic component having complementary sensitivity ranges. These results show that in rodents the thalamic-driven NB plays a specific key role in encoding visual contrast. Moreover, we propose a simple and effective network model of response to visual stimuli in rodents that might help in investigating network dysfunctions of pathologic visual information processing.

The G-protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors (GPCRs) in the olfactory system function to sense the surrounding environment and respond to various odorants. The genes coding for olfactory receptors in Caenorhabditis elegans are larger in number in comparison to those in mammals, suggesting complexity in the receptor-odorant relationships. Recent studies have shown that the same odorant in different concentrations could act on multiple receptors in different neurons to induce attractive or repulsive responses. The ASH neurons are known to be responsible for responding to high concentrations of volatile odorants. Here, we characterize a new GPCR, SRX-97. We found that the srx-97 promoter drives expression specifically in the head ASH and tail PHB chemosensory neurons of C. elegans. Moreover, the SRX-97 protein localizes to the ciliary ends of the ASH neurons. Analysis of clustered regularly interspaced short palindromic repeats (CRISPR)-based deletion mutants of the srx-97 locus suggests that this gene is involved in recognition of high concentrations of benzaldehyde. This was further confirmed through rescue and neuronal ablation experiments. Our work brings novel insights into concentration-dependent receptor function in the olfactory system, and provides details of an additional molecule that helps the animal navigate its surroundings.

Visual Abstract, There has been a long-standing debate regarding the role of peripheral afferents in mediating rapid-onset anorexia among other responses elicited by peripheral inflammatory insults. Thus, the current study assessed the sufficiency of peripheral afferents expressing toll-like receptor 4 (TLR4) to the initiation of the anorexia caused by peripheral bacterial lipopolysaccharide (LPS). We generated a Tlr4 null (Tlr4LoxTB) mouse in which Tlr4 expression is globally disrupted by a loxP-flanked transcription blocking (TB) cassette. This novel mouse model allowed us to restore the endogenous TLR4 expression in specific cell types. Using Zp3-Cre and Nav1.8-Cre mice, we produced mice that express TLR4 in all cells (Tlr4LoxTB X Zp3-Cre) and in peripheral afferents (Tlr4LoxTB X Nav1.8-Cre), respectively. We validated the Tlr4LoxTB mice, which were phenotypically identical to previously reported global TLR4 knock-out mice. Contrary to our expectations, the administration of LPS did not cause rapid-onset anorexia in mice with Nav1.8-restricted TLR4. The later result prompted us to identify Tlr4-expressing vagal afferents using in situ hybridization (ISH). In vivo, we found that Tlr4 mRNA was primarily enriched in vagal Nav1.8 afferents located in the jugular ganglion that co-expressed calcitonin gene-related peptide (CGRP). In vitro, the application of LPS to cultured Nav1.8-restricted TLR4 afferents was sufficient to stimulate the release of CGRP. In summary, we demonstrated using a new mouse model that vagally-expressed TLR4 is selectively involved in stimulating the release of CGRP but not in causing anorexia.

Visual Abstract, Extinction learning, the ability to update previously learned information by integrating novel contradictory information, is of high clinical relevance for therapeutic approaches to the modulation of maladaptive memories. Insect models have been instrumental in uncovering fundamental processes of memory formation and memory update. Recent experimental results in Drosophila melanogaster suggest that, after the behavioral extinction of a memory, two parallel but opposing memory traces coexist, residing at different sites within the mushroom body (MB). Here, we propose a minimalistic circuit model of the Drosophila MB that supports classical appetitive and aversive conditioning and memory extinction. The model is tailored to the existing anatomic data and involves two circuit motives of central functional importance. It employs plastic synaptic connections between Kenyon cells (KCs) and MB output neurons (MBONs) in separate and mutually inhibiting appetitive and aversive learning pathways. Recurrent modulation of plasticity through projections from MBONs to reinforcement-mediating dopaminergic neurons (DAN) implements a simple reward prediction mechanism. A distinct set of four MBONs encodes odor valence and predicts behavioral model output. Subjecting our model to learning and extinction protocols reproduced experimental results from recent behavioral and imaging studies. Simulating the experimental blocking of synaptic output of individual neurons or neuron groups in the model circuit confirmed experimental results and allowed formulation of testable predictions. In the temporal domain, our model achieves rapid learning with a step-like increase in the encoded odor value after a single pairing of the conditioned stimulus (CS) with a reward or punishment, facilitating single-trial learning.

In the dorsal horn of the spinal cord, peripheral nerve injury induces structural and neurochemical alterations through which aberrant synaptic signals contribute to the formation of neuropathic pain. However, the role of injured primary afferent terminals in such plastic changes remain unclear. In this study, we investigated the effect of nerve injury on the morphology of cell adhesion molecule L1-CAM (total L1-CAM [tL1-CAM])-positive primary afferent terminals and on the synaptic contact pattern in the dorsal horn. In the confocal images, the tL1-CAM-positive terminals showed morphological changes leading to the formation of hypertrophic varicosities in the c-fiber terminal. These hypertrophic varicosities in the dorsal horn were co-labeled with phosphorylated (Ser1181) L1-CAM (pL1-CAM) and shown to store neurotransmitter peptides, but not when co-labeled with the pre-synaptic marker, synaptophysin. Quantitative analyses based on three-dimensional reconstructed confocal images revealed that peripheral nerve injury reduced dendritic synaptic contacts but promoted aberrant axo-axonic contacts on the tL1-CAM-positive hypertrophic varicosities. These tL1-CAM-positive varicosities co-expressed the injury-induced alpha2delta-1 (α2δ-1) subunit of the calcium channel in the dorsal horn. Administration of the anti-allodynic drug, pregabalin, inhibited accumulation of α2δ-1 and pL1-CAM associated with a reduction in hypertrophic changes of tL1-CAM-positive varicosities, and normalized injury-induced alterations in synaptic contacts in the dorsal horn. Our findings highlight the formation of aberrant spinal circuits that mediate the convergence of local neuronal signals onto injured c-fibers, suggesting that these hypertrophic varicosities may be important contributors to the pathological mechanisms underlying neuropathic pain. Significance Statement We describe, for the first time, morphological changes in L1-CAM-positive injured c-fiber terminals that lead to the formation of hypertrophic varicosities, in which we found phosphorylation of L1-CAM and increased expression of the calcium channel subunit α2δ-1. Moreover, we found alterations in synaptic contacts and discovered that peripheral nerve injury increased axo-axonic contacts onto L1-CAM-positive varicosities while decreasing axo-dendritic contacts. These plastic changes indicate the convergence of neuronal signals onto the pain pathway and may represent a pathological mechanism underlying neuropathic pain. Administration of the anti-allodynic drug pregabalin reversed the injury-induced synaptic alternations. These data highlight the unique role of injured c-fibers in peripheral nerve injury and their role as a possible therapeutic target for neuropathic pain.

Nitrous oxide (N2O) is a hypnotic gas with antidepressant and psychedelic properties at subanesthetic concentrations. Despite long-standing clinical use, there is insufficient understanding of its effect on neural dynamics and cortical processing, which is important for mechanistic understanding of its therapeutic effects. We administered subanesthetic (70%), inhaled N2O and studied the dynamic changes of spiking rate, spectral content, and somatosensory information representation in primary motor cortex (M1) in two male rhesus macaques implanted with Utah microelectrode arrays in the hand area of M1. The average sorted multiunit spiking rate in M1 increased from 8.1 ± 0.99 to 10.6 ± 1.3 Hz in Monkey W (p < 0.001) and from 5.6 ± 0.87 to 7.0 ± 1.1 Hz in Monkey N (p = 0.003). Power spectral densities increased in beta- and gamma-band power. To evaluate somatosensory content in M1 as a surrogate of information transfer, fingers were lightly brushed and classified using a naive Bayes classifier. In both monkeys, the proportion of correctly classified fingers dropped from 0.50 ± 0.06 before N2O inhalation to 0.34 ± 0.03 during N2O inhalation (p = 0.018), although some fingers continued to be correctly classified (p = 0.005). The decrease in correct classifications corresponded to decreased modulation depth for the population (p = 0.005) and fewer modulated units (p = 0.046). However, the increased single-unit firing rate was not correlated with its modulation depth (R2< 0.001,p = 0.93). These data suggest that N2O degrades information transfer, although no clear relationship was found between neuronal tuning and N2O-induced changes in firing rate.

Neuronal activity plays critical roles in the development of sensory circuits in the mammalian brain. Experimental procedures are now available to alter the function of specific taste transduction pathways and have been especially useful in studying how stimulus-specific taste activity influences the development of central gustatory circuits. We previously used a mouse knock-out (KO) model in which the transduction channel necessary for sodium taste is removed from taste bud cells throughout life. In these KO mice, the terminal fields that carry taste information from taste buds into the nucleus of the solitary tract (NST) fail to mature, suggesting that sodium-elicited taste activity is important for the proper development of central gustatory circuits. Here, we tested the hypothesis that the development and maintenance of the dendritic architecture of NST relay cells, the primary postsynaptic partner of gustatory nerve terminal fields, are similarly dependent on sodium-elicited taste activity. The dendritic fields of NST relay cells, from adult male and female mice in which the α-subunit of the epithelial sodium channel (αENaC) was conditionally deleted in taste bud cells throughout life, were up to 2.4× larger and more complex than that of age-matched control mice. Interestingly, these differences in dendritic architecture did not appear until after the age when terminal fields begin “pruning,” after postnatal day (P)20. Overall, our results suggest that ENaC-mediated sodium taste activity is necessary for the maintenance of dendritic fields of relay cells in the gustatory NST.

The human auditory system is exceptional at comprehending an individual speaker even in complex acoustic environments. Because the inner ear, or cochlea, possesses an active mechanism that can be controlled by subsequent neural processing centers through descending nerve fibers, it may already contribute to speech processing. The cochlear activity can be assessed by recording otoacoustic emissions (OAEs), but employing these emissions to assess speech processing in the cochlea is obstructed by the complexity of natural speech. Here we develop a novel methodology to measure otoacoustic emissions that are related to the time-varying harmonic structure of speech (speech-DPOAEs). We then employ the method to investigate the effect of selective attention on the speech-DPOAEs. We provide tentative evidence that the speech-DPOAEs are larger when the corresponding speech signal is attended than when it is ignored. Our development of speech-DPOAEs opens up a path to further investigations of the contribution of the cochlea to the processing of complex real-world signals. Significance Statement Real-world environments, such as a loud pub or restaurant, are often noisy. The detection of sound occurs in the inner ear, which also possesses an active mechanism to mechanically amplify sound vibrations. Because the active mechanism can be regulated by the nervous system, it may already play a part in analyzing complex acoustic scenes. However, investigations of these questions have been hindered by a lack of experimental tools to assess the inner ear9s activity in relation to speech processing. Here we develop a method to record otoacoustic emissions that relate to the harmonic structure of speech, a key feature of many speech parts. We use the novel tool to provide tentative evidence that the inner ear contributes to selective attention.

Horizontal cells (HCs) form reciprocal synapses with rod and cone photoreceptors, an arrangement that underlies lateral inhibition in the retina. HCs send negative and positive feedback signals to photoreceptors, but how HCs initiate these signals remains unclear. Unfortunately, because HCs have no unique neurotransmitter receptors, there are no pharmacological treatments for perturbing membrane potential specifically in HCs. Here we use transgenic zebrafish whose HCs express alien receptors, enabling cell-type-specific control by cognate alien agonists. To depolarize HCs, we used the Phe-Met-Arg-Phe-amide (FMRFamide)-gated Na+channel (FaNaC) activated by the invertebrate neuropeptide FMRFamide. To hyperpolarize HCs we used a pharmacologically selective actuator module (PSAM)-glycine receptor (GlyR), an engineered Cl–selective channel activated by a synthetic agonist. Expression of FaNaC or PSAM-GlyR was restricted to HCs with the cell-type selective promoter for connexin-55.5. We assessed HC-feedback control of photoreceptor synapses in three ways. First, we measured presynaptic exocytosis from photoreceptor terminals using the fluorescent dye FM1-43. Second, we measured the electroretinogram (ERG) b-wave, a signal generated by postsynaptic responses. Third, we used Ca2+imaging in retinal ganglion cells (RGCs) expressing the Ca2+indicator GCaMP6. Addition of FMRFamide significantly decreased FM1-43 destaining in darkness, whereas the addition of PSAM-GlyR significantly increased it. However, both agonists decreased the light-elicited ERG b-wave and eliminated surround inhibition of the Ca2+response of RGCs. Taken together, our findings show that chemogenetic tools can selectively manipulate negative feedback from HCs, providing a platform for understanding its mechanism and helping to elucidate its functional roles in visual information processing at a succession of downstream stages.

Visual Abstract, The mechanotransduction (MT) complex in auditory hair cells converts the mechanical stimulation of sound waves into neural signals. Recently, the MT complex has been suggested to contain at least four distinct integral membrane proteins: protocadherin 15 (PCDH15), transmembrane channel-like protein 1 (TMC1), lipoma HMGIC fusion partner-like 5 (LHFPL5), and transmembrane inner ear protein (TMIE). However, the composition, function, and regulation of the MT-complex proteins remain incompletely investigated. Here, we report previously undescribed splicing isoforms of TMC1, LHFPL5, and TMIE. We identified four alternative splicing events for the genes encoding these three proteins by analyzing RNA-seq libraries of auditory hair cells from adult mice [over postnatal day (P)28], and we then verified the alternative splicing events by using RT-PCR and Sanger sequencing. Moreover, we examined the tissue-specific distribution, developmental expression patterns, and tonotopic gradient of the splicing isoforms by performing semiquantitative and quantitative real-time PCR (qRT-PCR), and we found that the alternative splicing of TMC1 and LHFPL5 is cochlear-specific and occurs in both neonatal and adult mouse cochleae. Our findings not only reveal the potential complexity of the MT-complex composition, but also provide critical insights for guiding future research on the function, regulation, and trafficking of TMC1, LHFPL5, and TMIE and on the clinical diagnosis of hearing loss related to aberrant splicing of these three key genes in hearing.

Visual Abstract, While it is well established that there are robust circadian rhythms of arginine vasopressin (AVP) in the cerebrospinal fluid (CSF), the route whereby the peptide reaches the CSF is not clear. A, AVP neurons constitute the largest fraction of the SCN neuronal population. Here, we show that processes of AVP-expressing SCN neurons cross the epithelium of the 3rd ventricular wall to reach the CSF (black arrows). Additionally, we report rostro-caudal differences in AVP neuron size and demonstrate that the localization of cells expressing the clock protein PER2 extend beyond the AVP population, thereby indicating that the size of this nucleus is somewhat larger than previously understood. B, Following lateral ventricle (LV) injection of cholera toxin β subunit (CTβ ; magenta) the retrograde tracer is seen in AVP neurons of the SCN, supporting the anatomical evidence that AVP neuronal processes directly contact the CSF. Arginine vasopressin (AVP) expressing neurons form the major population in the brain’s circadian clock located in the hypothalamic suprachiasmatic nucleus (SCN). They participate in inter-neuronal coupling and provide an output signal for synchronizing daily rhythms. AVP is present at high concentrations in the cerebrospinal fluid (CSF) and fluctuates on a circadian timescale. While it is assumed that rhythms in CSF AVP are of SCN origin, a route of communication between these compartments has not been delineated. Using immunochemistry (ICC) and cell filling techniques, we determine the morphology and location of AVP neurons in mouse and delineate their axonal and dendritic processes. Cholera toxin β subunit (CTβ) tracer injected into the lateral ventricle tests whether AVP neurons communicate with CSF. Most importantly, the results indicate that AVP neurons lie in close proximity to the third ventricle, and their processes cross the ventricular wall into the CSF. We also report that contrary to widely held assumptions, AVP neurons do not fully delineate the SCN borders as PER2 expression extends beyond the AVP region. Also, AVP neurons form a rostral prong originating in the SCN medial-most and ventral-most aspect. AVP is lacking in the mid-dorsal shell but does occur at the base of the SCN just above the optic tract. Finally, neurons of the rostral SCN are smaller than those lying caudally. These findings extend our understanding of AVP signaling potential, demonstrate the heterogeneity of AVP neurons, and highlight limits in using this peptide to delineate the mouse SCN.

The posterior dorsomedial striatum (pDMS) is mainly composed of medium spiny neurons (MSNs) expressing either dopamine D1 receptors (D1Rs) or D2Rs. Activation of these two MSN types produces opposing effects on addictive behaviors. However, it remains unclear whether pDMS D1-MSNs or D2-MSNs receive afferent inputs from different brain regions or whether the extrastriatal afferents express distinct dopamine receptors. To assess whether these afferents also contained D1Rs or D2Rs, we generated double transgenic mice, in which D1R-expressing and D2R-expressing neurons were fluorescently labeled. We used rabies virus-mediated retrograde tracing in these mice to perform whole-brain mapping of direct inputs to D1-MSNs or D2-MSNs in the pDMS. We found that D1-MSNs preferentially received inputs from the secondary motor, secondary visual, and cingulate cortices, whereas D2-MSNs received inputs from the primary motor and primary sensory cortices, and the thalamus. We also discovered that the bed nucleus of the stria terminalis (BNST) and the central nucleus of the amygdala (CeA) contained abundant D2R-expressing, but few D1R-expressing, neurons in a triple transgenic mouse model. Remarkably, although limited D1R or D2R expression was observed in extrastriatal neurons that projected to D1-MSNs or D2-MSNs, we found that cortical structures preferentially contained D1R-expressing neurons that projected to D1-MSNs or D2-MSNs, while the thalamus, substantia nigra pars compacta (SNc), and BNST had more D2R-expressing cells that projected to D2-MSNs. Taken together, these findings provide a foundation for future understanding of the pDMS circuit and its role in action selection and reward-based behaviors.

Visual Abstract, The neural code relates the activity of the nervous system to the activity of the muscles to the generation of behavior. To decipher it, it would be ideal to comprehensively measure the activity of the entire nervous system and musculature in a behaving animal. As a step in this direction, we used the cnidarian Hydra vulgaris to explore how physiological and environmental conditions alter simple contractile behavior and its accompanying neural and muscle activity. We used whole-body calcium imaging of neurons and muscle cells and studied the effect of temperature, media osmolarity, nutritional state, and body size on contractile behavior. In mounted Hydra preparations, changes in temperature, nutrition state, or body size did not have a major effect on neural or muscle activity, or on contractile behavior. But changes in media osmolarity systematically altered contractile behavior and foot detachments, increasing their frequency in hypo-osmolar media solutions and decreasing it in hyperosmolar media. Similar effects were seen in ectodermal, but not in endodermal muscle. Osmolarity also bidirectionally changed the activity of contraction burst (CB) neurons, but did not affect the network of rhythmic potential (RP) neurons in the ectoderm. These findings show osmolarity-dependent changes in the activity of CB neurons and ectodermal muscle, consistent with the hypothesis that CB neurons respond to media hypo-osmolarity, activating ectodermal muscle to generate CBs. This dedicated reflex could serve as an excretory system to prevent osmotic injury. This work demonstrates the feasibility of studying an entire neuronal and muscle activity in a behaving animal.

OTX2 is a homeoprotein transcription factor expressed in photoreceptors and bipolar cells in the retina. OTX2, like many other homeoproteins, transfers between cells and exerts non-cell autonomous effects such as promoting survival of retinal ganglion cells that do not express the protein. Here we used a genetic approach to target extracellular OTX2 in the retina by conditional expression of a secreted single chain anti-OTX2 antibody. Compared to control mice, the expression of this antibody by Parvalbumin-expressing neurons in the retina is followed by a reduction in visual acuity in one-month-old mice with no alteration of the retinal structure or cell type number or aspect. A- and b-waves measured by electroretinogram were also indistinguishable from control mice, suggesting no functional deficit of photoreceptors and bipolar cells. Mice expressing the OTX2-neutralizing antibody did show a significant doubling in the flicker amplitude, consistent with a change in inner retinal function. Our results show that interferingin vivowith OTX2 non-cell autonomous activity in the postnatal retina leads to an alteration in inner retinal cell functions and causes a deficit in visual acuity.Significance statementOTX2 is a homeoprotein transcription factor expressed in retinal photoreceptors and bipolar cells. Although theOtx2locus is silent in the inner retina, the protein is detected in cells of the ganglion cell layer consistent with the ability of this class of proteins to transfer between cells. We expressed a secreted single chain antibody (scFv) against OTX2 in the retina to neutralize extracellular OTX2. Antibody expression leads to reduced visual acuity with no change in retinal structure, or photoreceptor or bipolar physiology; however, activity in the inner retina was altered. Thus, interfering with OTX2 non-cell autonomous activity in postnatal retina alters inner retinal function and causes vision loss, highlighting the physiological value of homeoprotein direct non-cell autonomous signaling.

When performing a long chain of actions in rapid sequence, future movements need to be planned concurrently with ongoing action. However, how far ahead we plan, and whether this ability improves with practice, is currently unknown. Here we designed an experiment in which healthy volunteers produced sequences of 14 finger presses quickly and accurately on a keyboard in response to numerical stimuli. On every trial, participants were only shown a fixed number of stimuli ahead of the current keypress. The size of this viewing window varied between 1 (next digit revealed with the pressing of the current key) and 14 (full view of the sequence). Participants practiced the task for five days and their performance was continuously assessed on random sequences. Our results indicate that participants used the available visual information to plan multiple actions into the future, but that the planning horizon was limited: receiving information about more than 3 movements ahead did not result in faster sequence production. Over the course of practice, we found larger performance improvements for larger viewing windows and an expansion of the planning horizon. These findings suggest that the ability to plan future responses during ongoing movement constitutes an important aspect of skillful movement. Based on the results, we propose a framework to investigate the neuronal processes underlying simultaneous planning and execution.Significance StatementAlthough skill learning has typically focused on the training of specific movement sequences, practice improves performance even for random sequences. Here we hypothesize that a fundamental aspect of skilled sequential behavior is the ability to plan multiple actions into the future, both before and during execution. By controlling the amount of visual information available for motor planning, we show that people plan at least three movements beyond current action and that this planning horizon expands with practice. Our findings suggest that coordinating ongoing movement and planning of future actions is an essential component of skilled sequential behavior and offer testable predictions for the neural implementation of online motor planning.

Environmental enrichment is beneficial to sensory functions. Thus, elucidating the neural mechanism underlying improvement of sensory stimulus discrimination is important for developing therapeutic strategies. We aim to advance the understanding of such neural mechanism. We found that tactile enrichment improved tactile stimulus feature discrimination. The neural correlate of such improvement was revealed by analyzing single-cell information coding in both the primary somatosensory cortex and the premotor cortex of awake behaving animals. Our results show that environmental enrichment enhances the decision-information coding capacity of cells that are tuned to adjacent whiskers, and of premotor cortical cells. Significance statement This study advances the understanding of the neural mechanisms underlying the improvement of tactile discrimination induced by tactile environmental enrichment. We demonstrate that enrichment improves the information-coding capacity of adjacent-whisker tuned cells in the barrel cortex and premotor cortex, in awake animals performing a single whisker discrimination task. This understanding contributes to the development of therapeutic strategies for sensory function improvement utilizing environmental enrichment, which is a promising non-invasive therapy for many neurodegenerative diseases and traumatic brain injury and stroke recovery.

Retinofugal synapses serve as models for understanding how sensory signals from the periphery are relayed to the brain. Past studies have focused primarily on understanding the postsynaptic glutamatergic receptor subtypes involved in signal transmission, but the mechanisms underlying glutamate release at presynaptic retinal terminals remains largely unknown. Here we explored how different calcium (Ca2+) channel subtypes regulate glutamatergic excitatory synaptic transmission in two principal retinorecipient targets, the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) of the mouse. We used an in vitro slice preparation to record the synaptic responses of dLGN and SC neurons evoked by the electrical stimulation of optic tract (OT) fibers before and during the application of selective Ca2+ channel blockers. We found that synaptic responses to paired or repetitive OT stimulation were highly sensitive to extracellular levels of Ca2+ and to selective antagonists of voltage gated Ca2+ channels, indicating that these channels regulate the presynaptic release of glutamate at retinal synapses in both dLGN and SC. Bath application of selective Ca2+ channel blockers revealed that P/Q-type Ca2+ channels primarily operate to regulate glutamate release at retinal synapses in dLGN, while N-type Ca2+ channels dominate release in the SC.

A region within songbird cortex, dorsal intermediate arcopallium (AId), is functionally analogous to motor cortex in mammals and has been implicated in song learning during development. Non-vocal factors such as visual and social cues are known to mediate song learning and performance, yet previous chronic-recording studies of regions important for song behavior have focused exclusively on neural activity in relation to song production. Thus, we have little understanding of the range of non-vocal information that single neurons may encode. We made chronic recordings in AId of freely behaving juvenile zebra finches and evaluated neural activity during diverse motor behaviors throughout entire recording sessions, including song production as well as hopping, pecking, preening, fluff-ups, beak interactions, scratching, and stretching. These movements are part of natural behavioral repertoires and are important components of both song learning and courtship behavior. A large population of AId neurons showed significant modulation of activity during singing. In addition, single neurons demonstrated heterogeneous response patterns during multiple movements (including excitation during one movement type and suppression during another), and some neurons showed differential activity depending on the context in which movements occurred. Moreover, we found evidence of neurons that did not respond during discrete movements but were nonetheless modulated during active behavioral states compared with quiescence. Our results suggest that AId neurons process both vocal and non-vocal information, highlighting the importance of considering the variety of multimodal factors that can contribute to vocal motor learning during development.

Visual Abstract, Loss of vestibular function causes severe acute symptoms of dizziness and disorientation, yet the brain can adapt and regain near to normal locomotor and orientation function through sensory substitution. Animal studies quantifying functional recovery have yet been limited to reflexive eye movements. Here, we studied the interplay between vestibular and proprioceptive graviception in macaque monkeys trained in an earth-vertical visual orientation (subjective visual vertical; SVV) task and measured the time course of sensory substitution for gravity perception following complete bilateral vestibular loss (BVL). Graviceptive gain, defined as the ratio of perceived versus actual tilt angle, decreased to 20% immediately following labyrinthectomy, and recovered to nearly prelesion levels with a time constant of approximately three weeks of postsurgery testing. We conclude that proprioception accounts for up to 20% of gravity sensing in normal animals, and is re-weighted to substitute completely perceptual graviception after vestibular loss. We show that these results can be accounted for by an optimal sensory fusion model.

Eye-centered (egocentric) and landmark-centered (allocentric) visual signals influence spatial cognition, navigation and goal-directed action, but the neural mechanisms that integrate these signals for motor control are poorly understood. A likely candidate for ego/allocentric integration in the gaze control system is the supplementary eye fields (SEF), a mediofrontal structure with high-level ‘executive’ functions, spatially tuned visual/motor response fields, and reciprocal projections with the frontal eye fields (FEF). To test this hypothesis, we trained two head-unrestrained monkeys (Macaca mulatta) to saccade toward a remembered visual target in the presence of a visual landmark that shifted during the delay, causing gaze end points to shift partially in the same direction. 256 SEF neurons were recorded, including 68 with spatially tuned response fields. Model fits to the latter established that, like the FEF and superior colliculus, spatially tuned SEF responses primarily showed an egocentric (eye-centered) target-to-gaze position transformation. However, the landmark shift influenced this default egocentric transformation: during the delay, motor neurons (with no visual response) showed a transient but unintegrated shift (i.e., not correlated with the target-to-gaze transformation), whereas during the saccade-related burst visuomotor neurons showed an integrated shift (i.e., correlated with the target-to-gaze transformation). This differed from our simultaneous FEF recordings (Bharmauria et al., 2020), which showed a transient shift in visuomotor neurons, followed by an integrated response in all motor responses. Based on these findings and past literature, we propose that prefrontal cortex incorporates landmark-centered information into a distributed, eye-centered target-to-gaze transformation through a reciprocal prefrontal circuit. Significance Statement It is thought that the brain integrates egocentric (self-centered) and allocentric (landmark-centered) visual signals to generate accurate goal-directed movements, but the neural mechanism is not known. Here, by shifting a visual landmark while recording frontal cortex activity in awake behaving monkeys, we show that the supplementary eye fields (SEF) incorporates landmark-centered information (in memory and motor activity) when it transforms target location into future gaze position commands. We propose a circuit model in which the SEF provides control signals to implement an integrated gaze command in the frontal eye fields (Bharmauria et al., 2020). Taken together, these experiments explain normal ego/allocentric integration and might suggest rehabilitation strategies for neurological patients who have lost one of these visual mechanisms.

When the eyes rotate during translational self-motion, the focus of expansion (FOE) in optic flow no longer indicates heading, yet heading judgements are largely unbiased. Much emphasis has been placed on the role of extraretinal signals in compensating for the visual consequences of eye rotation. However, recent studies also support a purely visual mechanism of rotation compensation in heading-selective neurons. Computational theories support a visual compensatory strategy but require different visual depth cues. We examined the rotation tolerance of heading tuning in macaque area MSTd using two different virtual environments, a frontoparallel (2D) wall and a 3D cloud of random dots. Both environments contained rotational optic flow cues (i.e., dynamic perspective), but only the 3D cloud stimulus contained local motion parallax cues, which are required by some models. The 3D cloud environment did not enhance the rotation tolerance of heading tuning for individual MSTd neurons, nor the accuracy of heading estimates decoded from population activity, suggesting a key role for dynamic perspective cues. We also added vestibular translation signals to optic flow, to test whether rotation tolerance is enhanced by non-visual cues to heading. We found no benefit of vestibular signals overall, but a modest effect for some neurons with significant vestibular heading tuning. We also find that neurons with more rotation tolerant heading tuning typically are less selective to pure visual rotation cues. Together, our findings help to clarify the types of information that are used to construct heading representations that are tolerant to eye rotations.

Sensory cortical areas receive glutamatergic callosal projections that link information processing between brain hemispheres. In primary auditory cortex (A1), ipsilateral principal cells from a particular tonotopic region project to neurons in matching frequency space of the contralateral cortex. However, the role of interhemispheric projections in shaping cortical responses to sound and frequency tuning in awake animals is unclear. Here, we use translaminar single-unit recordings and optogenetic approaches to probe how callosal inputs modulate spontaneous and tone-evoked activity in A1 of awake mice. Brief activation of callosal inputs drove either short-latency increases or decreases in firing of individual neurons. Across all cortical layers, the majority of responsive regular spiking (RS) cells received short-latency inhibition, whereas fast spiking (FS) cells were almost exclusively excited. Consistent with the callosal-evoked increases in FS cell activityin vivo, brain slice recordings confirmed that parvalbumin (PV)-expressing cells received stronger callosal input than pyramidal cells or other interneuron subtypes. Acutein vivosilencing of the contralateral cortex generally increased spontaneous firing across cortical layers and linearly transformed responses to pure tones via both divisive and additive operations. The net effect was a decrease in signal-to-noise ratio for evoked responses and a broadening of frequency tuning curves. Together, these results suggest that callosal input regulates both the salience and tuning sharpness of tone responses in A1 via PV cell-mediated feedforward inhibition.

The brains of male and female mice are shaped by genetics and hormones during development. The enzyme aromatase helps establish sex differences in social behaviors and in the neural circuits that produce these behaviors. The medial amygdala of mice contains a large population of aromatase neurons and is a critical hub in the social behavior network. Moreover, the neural representation of social stimuli in the medial amygdala displays clear sex differences that track developmental changes in social behaviors. Here, we identify a potential anatomic basis for those sex differences. We found that sensory input from the accessory olfactory bulb (AOB) to aromatase neurons is derived nearly exclusively from the anterior AOB, which selectively responds to chemosensory cues from conspecific animals. Through the coordinated use of mouse transgenics and viral-based circuit-tracing strategies, we demonstrate a clear sex difference in the volume of synapses connecting the accessory olfactory bulb to aromatase-expressing neurons in the medial amygdala of male versus female mice. This difference in anatomy likely mediates, at least in part, sex differences in medial amygdala-mediated social behaviors.

Visual Abstract, The self is a distinct entity from the rest of the world, and actions and sensory feedback are our channels of interaction with the external world. This study examined how the sense of control influences people’s perception of sensorimotor input under the framework of categorical perception. Twenty human participants (18 males, two females) took part in both experiments. Experiment 1 showed that the sensitivity (d′) of detecting a 20% change in control from no change was higher when the changes occurred at the control-category boundary than within each category. Experiment 2 showed that the control categories greatly affected early attention allocation, even when the judgment of control was unnecessary to the task. Taken together, these results showed that our perceptual and cognitive systems are highly sensitive to small changes in control that build up to a determinant change in the control category within a relatively narrow boundary zone between categories, compared with a continuous, gradual physical change in control.