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The brain is able to determine angular self-motion from visual, vestibular, and kinesthetic information. There is compelling evidence that both humans and non-human primates integrate visual and inertial (i.e., vestibular and kinesthetic) information in a statistically optimal fashion when discriminating heading direction. In the present study, we investigated whether the brain also integrates information about angular self-motion in a similar manner. Eight participants performed a 2IFC task in which they discriminated yaw-rotations (2-s sinusoidal acceleration) on peak velocity. Just-noticeable differences (JNDs) were determined as a measure of precision in unimodal inertial-only and visual-only trials, as well as in bimodal visual-inertial trials. The visual stimulus was a moving stripe pattern, synchronized with the inertial motion. Peak velocity of comparison stimuli was varied relative to the standard stimulus. Individual analyses showed that data of three participants showed an increase in bimodal precision, consistent with the optimal integration model; while data from the other participants did not conform to maximum-likelihood integration schemes. We suggest that either the sensory cues were not perceived as congruent, that integration might be achieved with fixed weights, or that estimates of visual precision obtained from non-moving observers do not accurately reflect visual precision during self-motion. [ABSTRACT FROM AUTHOR]

The mechanism underlying cybersickness during virtual reality (VR) exposure is still poorly understood, although research has highlighted a causal role for visual-vestibular sensory conflict. Recently established methods for reducing cybersickness include galvanic vestibular stimulation (GVS) to mimic absent vestibular cues in VR, or vibration of the vestibular organs to add noise to the sensory modality. Here, we examined if applying noise to the vestibular system using noisy-current GVS affects sickness severity in VR. Participants were exposed to one of the two VR games that were classified as either moderately or intensely nauseogenic. The VR content lasted for 50 min and was broken down into three blocks: 30 min of gameplay during exposure to either noisy GVS (± 1750 μA) or sham stimulation (0 μA), and 10 min of gameplay before and after this block. We characterized the effects of noisy GVS in terms of post-minus-pre-exposure cybersickness scores. In the intense VR condition, we found a main effect of noisy vestibular stimulation on a verbal cybersickness scale, but not for questionnaire measures of cybersickness. Participants reported lower cybersickness scores during and directly after exposure to GVS. However, this difference was quickly extinguished (~ 3-6 min) after further VR exposure, indicating that sensory adaptation did not persist after stimulation was terminated. In contrast, there were no differences between the sham and GVS group for the moderate VR content. The results show the potential for reducing cybersickness with non-invasive sensory stimulation. We address possible mechanisms for the observed effects, including noise-induced sensory re-weighting.

Knowing when the head moves is crucial information for the central nervous system to maintain a veridical representation of the self in the world for perception and action. Previous studies have shown that active head movement onset has to precede a sound by approximately 80 ms to be perceived as simultaneous, suggesting that the perceived timing of head movement is slow. However, this research was conducted with closed eyes. Given that visual information is available for most natural head movements, could perceptual delays in head movement onset be related to removing vision? Here, we examined whether visual information affects the perceived timing of active head movement onset. Participants performed a series of temporal order judgment tasks between their active head movement and an auditory tone presented at various stimulus onset asynchronies. Visual information was either absent (eyes closed) or present while either maintaining fixation on an earth or head-fixed target in the dark or in the light. Results show that head movement onset has to precede a sound by ~76 ms with eyes closed confirming previous work. The results also suggest that head movement onset must still precede a sound when fixating targets in the dark with a trend for the head having to move with less lead time with visual information and with the vestibulo-ocular reflex active or suppressed (~70 to 48 ms). Together, these results suggest that the perception of head movement onset is persistently delayed and is not fully resolved even with full field visual input.

Perceptual binding of multisensory events occurs within a limited time span known as the temporal binding window. Failure to correctly identify whether multisensory events occur simultaneously, what their temporal order is, or whether they should be causally bound can lead to inaccurate representations of the physical world, poor decision-making, and dangerous behavior. It has been shown that the ability to discriminate simultaneity, temporal order, and causal relationships among stimuli can become increasingly difficult as we age. In the present study, we assessed the relationship between these three attributes of temporally processing multisensory information in both younger and older adults. Performance on three tasks (temporal order judgment: TOJ, simultaneity judgment: SJ, and stream/bounce illusion) was compared using a large sample within-subjects design consisting of younger and older adults to determine aging effects as well as relationships between the three tasks. Older adults had more difficulty (larger temporal binding window) discriminating temporal order and perceived collision than younger adults. Simultaneity judgments in younger and older adults were indistinguishable. Positive correlations between TOJ and SJ as well as SJ and stream/bounce tasks were found in younger adults, which identify common (SJ) and distinct (TOJ, stream/bounce) neural mechanisms that sub-serve temporal processing of audiovisual information that is lost in older adults. We conclude that older adults have an extended temporal binding window for TOJ and stream/bounce tasks, but the temporal binding window in SJ is preserved, suggesting that age-related changes in multisensory integration are task specific and not a general trait of aging.

Perceiving vertical self-motion is crucial for maintaining balance as well as for controlling an aircraft. Whereas heave absolute thresholds have been exhaustively studied, little work has been done in investigating how vertical sensitivity depends on motion intensity (i.e., differential thresholds). Here we measure human sensitivity for 1-Hz sinusoidal accelerations for 10 participants in darkness. Absolute and differential thresholds are measured for upward and downward translations independently at 5 different peak amplitudes ranging from 0 to 2 m/s(2). Overall vertical differential thresholds are higher than horizontal differential thresholds found in the literature. Psychometric functions are fit in linear and logarithmic space, with goodness of fit being similar in both cases. Differential thresholds are higher for upward as compared to downward motion and increase with stimulus intensity following a trend best described by two power laws. The power laws' exponents of 0.60 and 0.42 for upward and downward motion, respectively, deviate from Weber's Law in that thresholds increase less than expected at high stimulus intensity. We speculate that increased sensitivity at high accelerations and greater sensitivity to downward than upward self-motion may reflect adaptations to avoid falling.

In this paper, we show that differences in reaction times (RT) to self-motion depend not only on the duration of the profile, but also on the actual time course of the acceleration. We previously proposed models that described direction discrimination thresholds for rotational and translational motions based on the dynamics of the vestibular sensory organs (otoliths and semi-circular canals). As these models have the potential to describe RT for different motion profiles (e.g., trapezoidal versus triangular acceleration profiles or varying profile durations), we validated these models by measuring RTs in human observers for a direction discrimination task using both translational and rotational motions varying in amplitude, duration and acceleration profile shape in a within-subjects design. In agreement with previous studies, amplitude and duration were found to affect RT, and importantly, we found an influence of the profile shape on RT. The models are able to fit the measured RTs with an accuracy of around 5 ms, and the best-fitting parameters are similar to those found from identifying the models based on threshold measurements. This confirms the validity of the modeling approach and links perceptual thresholds to RT. By establishing a link between vestibular thresholds for self-motion and RT, we show for the first time that RTs to purely inertial motion stimuli can be used as an alternative to threshold measurements for identifying self-motion perception models. This is advantageous, since RT tasks are less challenging for participants and make assessment of vestibular function less fatiguing. Further, our results provide strong evidence that the perceived timing of self-motion stimulation is largely influenced by the response dynamics of the vestibular sensory organs.

The perception of simultaneity between auditory and vestibular information is crucially important for maintaining a coherent representation of the acoustic environment whenever the head moves. It has been recently reported, however, that despite having similar transduction latencies, vestibular stimuli are perceived significantly later than auditory stimuli when simultaneously generated. This suggests that perceptual latency of a head movement is longer than a co-occurring sound. However, these studies paired a vestibular stimulation of long duration (~1 s) and of a continuously changing temporal envelope with a brief (10-50 ms) sound pulse. In the present study, the stimuli were matched for temporal envelope duration and shape. Participants judged the temporal order of the two stimuli, the onset of an active head movement and the onset of brief (50 ms) or long (1,400 ms) sounds with a square- or raised-cosine-shaped envelope. Consistent with previous reports, head movement onset had to precede the onset of a brief sound by about 73 ms in order for the stimuli to be perceived as simultaneous. Head movements paired with long square sounds (~100 ms) were not significantly different than brief sounds. Surprisingly, head movements paired with long raised-cosine sound (~115 ms) had to be presented even earlier than brief stimuli. This additional lead time could not be accounted for by differences in the comparison stimulus characteristics (temporal envelope duration and shape). Rather, differences between sound conditions were found to be attributable to variability in the time for head movement to reach peak velocity: the head moved faster when paired with a brief sound. The persistent lead time required for vestibular stimulation provides further evidence that the perceptual latency of vestibular stimulation is greater than the other senses.

Understanding the dynamics of vestibular perception is important, for example, for improving the realism of motion simulation and virtual reality environments or for diagnosing patients suffering from vestibular problems. Previous research has found a dependence of direction discrimination thresholds for rotational motions on the period length (inverse frequency) of a transient (single cycle) sinusoidal acceleration stimulus. However, self-motion is seldom purely sinusoidal, and up to now, no models have been proposed that take into account non-sinusoidal stimuli for rotational motions. In this work, the influence of both the period length and the specific time course of an inertial stimulus is investigated. Thresholds for three acceleration profile shapes (triangular, sinusoidal, and trapezoidal) were measured for three period lengths (0.3, 1.4, and 6.7 s) in ten participants. A two-alternative forced-choice discrimination task was used where participants had to judge if a yaw rotation around an earth-vertical axis was leftward or rightward. The peak velocity of the stimulus was varied, and the threshold was defined as the stimulus yielding 75 % correct answers. In accordance with previous research, thresholds decreased with shortening period length (from ~2 deg/s for 6.7 s to ~0.8 deg/s for 0.3 s). The peak velocity was the determining factor for discrimination: Different profiles with the same period length have similar velocity thresholds. These measurements were used to fit a novel model based on a description of the firing rate of semi-circular canal neurons. In accordance with previous research, the estimates of the model parameters suggest that velocity storage does not influence perceptual thresholds.

The brain can know about an active head movement even in advance of its execution by means of an efference copy signal. In fact, sensory correlates of active movements appear to be suppressed. Passive disturbances of the head, however, can be detected only by sensory feedback. Might the perceived timing of an active head movement be speeded relative to the perception of a passive movement due to the efferent copy (anticipation hypothesis) or delayed because of sensory suppression (suppression hypothesis)? We compared the perceived timing of active and passive head movement using other sensory events as temporal reference points. Participants made unspeeded temporal order and synchronicity judgments comparing the perceived onset of active and passive head movement with the onset of tactile, auditory and visual stimuli. The comparison stimuli had to be delayed by about 45 ms to appear coincident with passive head movement or by about 80 ms to appear aligned with an active head movement. The slow perceptual reaction to vestibular activation is compatible with our earlier study using galvanic stimulation (Barnett-Cowan and Harris 2009). The unexpected additional delay in processing the timing of an active head movement is compatible with the suppression hypothesis and is discussed in relation to suppression of vestibular signals during self-generated head movement.

A conflict between an egocentric and an external reference frame can be highlighted by examining the marked deficit observed with tactile temporal order judgments (TOJ) when the hands are crossed. The anecdotally-reported large individual differences in the magnitude of this crossed-hands deficit were explored here by testing a large group of participants (48; 24 female). Given that females have been shown to be more visually dependent than males in the potentially related rod-and-frame test (RFT), we hypothesized that females would show a larger influence of the external reference frame (i.e., a larger crossed-hands deficit). As predicted, female participants produced larger tactile TOJ deficits compared to our male participants. We also administered the RFT in these participants with hands crossed and uncrossed. Crossing the hands increased the effect of the frame in the RFT, more so for females than males, further highlighting the potential difference in the way that each sex accommodates reference frame conflicts. Finally, examining the relation between the two tasks revealed a significant correlation, with larger frame effects associated with larger crossed-hands TOJ deficits, but this only held for males. We speculate that sex-specific differences in multisensory processing and spatial ability may explain why females are less able to disambiguate a crossed-hands posture than are males.

Different senses have different processing times. Here we measured the perceived timing of galvanic vestibular stimulation (GVS) relative to tactile, visual and auditory stimuli. Simple reaction times for perceived head movement (438 +/- 49 ms) were significantly longer than to touches (245 +/- 14 ms), lights (220 +/- 13 ms), or sounds (197 +/- 13 ms). Temporal order and simultaneity judgments both indicated that GVS had to occur about 160 ms before other stimuli to be perceived as simultaneous with them. This lead was significantly less than the relative timing predicted by reaction time differences compatible with an incomplete tendency to compensate for differences in processing times.