Search

By focusing on arthroscopic rotator cuff repair in a worker’s compensation patient population that are without personal injuries, previous ipsi-lateral shoulder injury, any concomitant pathologies or being federal employees, the rate of return-to-work (RTW) can be better isolated as an outcome of the surgery. RTW rate for a consecutive 50 patient cohort was 98% with 58% achieving full duty. A single patient with a frozen shoulder was unable to return to work at any level. These RTW rates contrast with several previous studies that reported significantly lower RTW rates for workers with compensation than workers without compensation. All but three patients improved their UCLA scores after the repair.

Octopus cells in the ventral cochlear nucleus (VCN) have been difficult to study because of the very features that distinguish them from other VCN neurons. We performed in vivo recordings in cats on well-isolated units, some of which were intracellularly labeled and histologically reconstructed. We found that responses to low-frequency tones with frequencies

Work-related aspects of carpal tunnel syndrome (CTS) have engendered considerable research and debate resulting from its frequent occurrence in the population along with the cost of addressing it in the work place. Some studies have gone so far as to deny that CTS has any relation to work exposure, rather it largely results from predisposition of individuals. As such, the condition is non compensable leaving workers without any benefits due to their condition. This is despite the fact that high force and repetition of hand movements have been identified as CTS causes for years. As computer use has become endemic in recent years, several studies have concluded that the resulting exposures are insufficient to attribute any occupational causation to it. Rather, it is argued that genetic or worker pre-existing conditions are responsible for CTS development. This review analyses the existing literature to assess occupational carpal tunnel causation arising out of exposure to highly manual and repetitive jobs as well as intense exposure to keyboarding. We believe the literature supports a strong causal connection to carpal tunnel causation when exposed to highly repetitive and forceful jobs and a causal connection to keyboarding when the exposure dose is sufficient.

The von Bekesy Medal is presented to individuals, irrespective of nationality, age, or society affiliation, who have made outstanding contributions to the area of psychological or physiological acoustics as evidenced by publication of research results in professional journals or by other accomplishments in the fields.

Much of what is known about how the cochlear nuclei participate in mammalian hearing comes from studies of non-primate mammalian species. To determine to what extent the cochlear nuclei of primates resemble those of other mammalian orders, we have recorded responses to sound in three primate species: marmosets, cynomolgus macaques, and squirrel monkeys. These recordings show that the same types of temporal firing patterns are found in primates that have been described in other mammals. Responses to tones of neurons in the ventral cochlear nucleus have similar tuning, latencies, post-stimulus time and interspike interval histograms as those recorded in non-primate cochlear nucleus neurons. In the dorsal cochlear nucleus, too, responses were similar. From these results it is evident that insights gained from non-primate studies can be applied to the peripheral auditory system of primates.

The mammalian cochlear nucleus (CN) consists of a diverse set of neurons both physiologically and morphologically that are involved in processing different aspects of the sound signal. One class of CN neurons that is located near the entrance of the auditory nerve (AN) to the CN has an oval soma with an eccentric nucleus and a short-bushy dendritic tree and is called a globular/bushy cell (GBC). They contact the principal cells of the medial nucleus of the trapezoid body (MNTB) with the very large calyx of Held that is one of the most secure synapses in the brain. Because MNTB cells provide an inhibitory input to the lateral superior olive (LSO), a structure purported to play a role in lateralizing high frequency sounds, GBC physiology is of great interest. Results were obtained with intracellular recording and subsequent labeling with neurobiotin of 32 GBCs along with a number of cells characterized extracellularly as likely GBCs in the cochlear nucleus (CN) of cat. Their poststimulus discharge response pattern to repeated tones varies from a primarylike pattern, i.e. similar to the AN, to a primarylike pattern with a 0.5–2 ms notch after the initial spike, to an onset pattern with a low-sustained rate. They can represent low frequency tones and amplitude modulated signals exceptionally well with a temporal code.

Basilar membrane (BM) vibration was measured using a displacement measuring interferometer for single-tone and two-tone suppression (2TS) paradigms in the 6-9 kHz region of sensitive chinchilla cochleae that had gains near or better than 60 dB. Based on prior studies of basilar membrane vibration, three significant differences remain between BM and auditory nerve (AN) 2TS responses: (1) suppression thresholds in the tail of tuning curves were much higher in BM than the auditory nerve (AN); (2) rates of suppression were significantly higher in AN than BM; and (3) the amplitude of vibration with low-frequency suppressors was always greater than the single-tone displacement rendering it impossible to explain 2TS rate suppression in the AN. The first two differences are eliminated by the results of the present study while the third remains. Suppression amplitudes greater than 40 dB and rates of suppression larger than 2.5 dB/dB were found for low-frequency suppressors. A correlation between both the gain and nonlinearity of the cochlea and 2TS properties indicates that when sensitive cochleae are studied. The third difference between BM and AN behavior could be strictly a function of the high-pass filter characteristic of the inner hair cells.

The vibration of the basilar membrane in the 6-9 kHz region in the chinchilla cochlea has been studied using a displacement sensitive interferometer. Displacements of 0.7-1.4 nm at 0 dB sound pressure level have been obtained. At the characteristic frequency (CF), rate-of-growth (ROG) functions computed as the slope of input-output (IO) functions can be as low as 0.1 dB/dB. IO functions for frequenciesCF have ROGs near 0 dB/dB and can have notches characterized by both negative slopes and expansive ROGs, i.e.,1 dB/dB. For frequencies0.6*CF, ROGs1.2 dB/dB were found. Cochlear gain is shown to be greater than 60 dB in sensitive preparations with a single cochlea having nearly 80 dB gain. The compressive nature of the cochlea remains at all levels though it is masked at frequenciesCF when the amplitude of a compression wave exceeds that of the traveling wave. The compression wave produces the plateau region of the mechanical response at high intensities and has a nearly constant phase versus frequency function implying a high velocity. The summation of the traveling and compression waves explains the occurrence of the notches in both the IO and iso-intensity functions. Vibration of the osseous spiral limbus may alter the drive to inner hair cells.

Evidence for work-related exposure as a cause of shoulder tendinopathy is updated. Previous studies indicated that forceful movements combined with hands above head increases the likelihood of shoulder disorders. Hands above the shoulder and arm flexion or abduction at greater than 60° combined with forceful or repetitious motions results in significantly increased occurrence of shoulder tendinopathy. Shoulder disease is multifactorial with genetics, age, and bodyweight correlated with increased occurrence though they are minor factors for young workers. Normal aging of the rotator cuff may underlie much of shoulder disease for individuals older than 60 thereby presenting a complication for the determination of work-related rotator cuff tears in this group. In contrast, there is sufficient evidence to support work-related musculoskeletal disorders of the shoulder in younger individuals who have not undergone the aging processes. Numerous reviews along with individual cross sectional and longitudinal studies conclude that shoulder tendinopathy can result from work-related exposures based on the odds and relative risks of occurrence in a working population compared to a group that has not been similarly exposed to comparable manual effort.

After carpal tunnel syndrome, cubital tunnel syndrome (CuTS) is the 2 nd most frequent upper extremity musculoskeletal disorder resulting from ulnar nerve entrapment at the elbow. Insurance companies often deny that this condition results from exposure at work labeling it idiopathic despite numerous studies that support a causal relation between high repetitions and force the occurrence and CuTS. Odds Ratios for work-related CuTS are significantly higher than would be expected if there were no work factors associated with the condition. Reviews of the literature by several noteworthy scientific organizations support workrelatedness of CuTS. While additional study of the work-relatedness of CuTS is frequently suggested, significant support for this relation already exists.

Comparison of a novel triple-row (NTR) rotator cuff repair (RCR) was made to single row (SR) and double row (DR) repairs as measured by gap formation, Ultimate Failure Load (UFL), and mode of failure. Porcine humeri were divided into three groups of forty and the infraspinatus tendons were completely released and then repaired using generic (PEEK) anchors. Biomechanical testing of RCRs consisted of 3500 cycles of a load varied from 10 to 180 Newtons (N). Gap size at all cycle counts was significantly larger for SR and DR than NTR RCRs. 84% of NTR repairs completed the 3500 cycle test versus 66% of DR and 61% of SR repairs. The tendon pulled from the sutures 60%, 49% and 33% of the time for SR, DR and NTR RCRs respectively. UFLs were 464 N for the NTR repairs, 394 N for DR and 414 N for SR repairs. In summary, NTR repairs resulted in smaller gap formation, higher UFLs, and fewer instances of tendons tearing at the sutures than SR or DR repairs. Results of NTR repairs demonstrate that the tendon footprint is securely restored with small gap formation for the range of forces that a shoulder will be subjected to in normal use.

Behavioral or neural measures of speech encoding are often taken from animals with auditory systems that differ substantially from those of humans. Absolute distance between spectral peaks of speech sounds along the basilar membrane is typically much greater in humans than in smaller animals. To address this problem, a synthesizer was developed for creating speech scaled for nonhuman cochleae in which spectra are warped to account for differences in cochlear physiology. Absolute cochlear distance between spectral peaks (formants) is held constant across species while formant band- widths are also scaled to span equal numbers of hair cells as for humans. This was accomplished via a flexible reimplementation of the KLATT80 speech synthesizer in MATLAB. © 2001 Acoustical Scociety of America

Multicomponent stimuli consisting of two to seven tones were used to study suppression of basilar-membrane vibration at the 3-4-mm region of the chinchilla cochlea with a characteristic frequency between 6.5 and 8.5 kHz. Three-component stimuli were amplitude-modulated sinusoids (AM) with modulation depth varied between 0.25 and 2 and modulation frequency varied between 100 and 2000 Hz. For five-component stimuli of equal amplitude, frequency separation between adjacent components was the same as that used for AM stimuli. An additional manipulation was to position either the first, third, or fifth component at the characteristic frequency (CF). This allowed the study of the basilar-membrane response to off-CF stimuli. CF suppression was as high as 35 dB for two-tone combinations, while for equal-amplitude stimulus components CF suppression never exceeded 20 dB. This latter case occurred for both two-tone stimuli where the suppressor was below CF and for multitone stimuli with the third component=CF. Suppression was least for the AM stimuli, including when the three AM components were equal. Maximum suppression was both level- and frequency dependent, and occurred for component frequency separations of 500 to 600 Hz. Suppression decreased for multicomponent stimuli with component frequency spacing greater than 600 Hz. Mutual suppression occurred whenever stimulus components were within the compressive region of the basilar membrane.

The response of chinchilla basilar membrane in the basal region of the cochlea to multicomponent (1, 3, 5, 6, or 7) stimuli was studied using a laser interferometer. Three-component stimuli were amplitude-modulated signals with modulation depths that varied from 25% to 200% and the modulation frequency varied from 100 to 2000 Hz while the carrier frequency was set to the characteristic frequency of the region under study (approximately 6.3 to 9 kHz). Results indicate that, for certain modulation frequencies and depths, there is enhancement of the response. Responses to five equal-amplitude sine wave stimuli indicated the occurrence of nonlinear phenomena such as spectral edge enhancement, present when the frequency spacing was less than 200 Hz, and mutual suppression. For five-component stimuli, the first, third, or fifth component was placed at the characteristic frequency and the component frequency separation was varied over a 2-kHz range. Responses to seven component stimuli were similar to those of five-component stimuli. Six-component stimuli were generated by leaving out the center component of the seven-component stimuli. In the latter case, the center component was restored in the basilar-membrane response as a result of distortion-product generation in the nonlinear cochlea.

Basilar membrane (BM) responses to two types of broadband stimuli-clicks and Schroeder-phase complexes--were recorded at several sites at the base of the chinchilla cochlea. Recording sites (characteristic frequency, CF, in the range of 5.5-18 kHz) span the 1-4-mm basal region of the basilar membrane. BM responses to clicks consisted of undamped oscillations with instantaneous frequency that increased over time until it reached a value around CF. The time constant of this glide is CF dependent. Throughout the entire region under study, BM vibration exceeded umbo motion by up to 60 dB. Nonlinear properties of BM responses to clicks resemble those found in the more studied 8-10-kHz region. Amplitude spectra of Schroeder-phase complex stimuli, which consist of a series of sinusoidal components summed in negative (-SCHR) and positive Schroeder phase (+SCHR), are flat. The envelope of BM responses to +SCHR stimuli contains valleys, or dips, that are wider than those found in responses to the -SCHR stimuli. Hence, BM responses to the former stimuli are "peakier" than responses to the latter. Differences in response waveforms are less obvious in linear cochleae. Suppression of a near-CF tone by -SCHR stimuli was larger than that evoked by +SCHR stimuli.

Measurements from the 1-4-mm basal region of the chinchilla cochlea indicate the basilar membrane in the hook region (12-18 kHz) vibrates essentially as it does more apically, in the 5-9-kHz region. That is, a compressive nonlinearity in the region of the characteristic frequency, amplitude-dependent phase changes, and a gain relative to stapes motion that can attain nearly 10,000 at low levels. The displacement at threshold for auditory-nerve fibers in this region (20 dB SPL) was approximately 2 nm. Measurements were made at several locations in individual animals in the longitudinal and radial directions. The results indicate that there is little variability in the phase of motion radially and no indication of higher-order modes of vibration. The data from the longitudinal studies indicate that there is a shift in the location of the maximum with increasing stimulus levels toward the base. The cochlear amplifier extends over a 2-3-mm region around the location of the characteristic frequency.

Amplitude modulated (AM) signals have often been used as precisely defined partial analogs of speech sounds. This study considers the response to an AM complex with 200% sinusoidal modulation, that is, the amplitudes of the three AM components are equal. By varying the carrier frequency across the entire frequency range of unit response, it is shown that units in the cochlear nucleus of cat are relatively insensitive to variation in the carrier frequency, which is to say that population response to an AM signal at a fixed locus will be widespread. These stimuli and procedures result in the presentation of both harmonic and inharmonic complexes, and thus permit assessment of neural responses for the information needed to make spectral or time-domain pitch matches. It is shown that the reciprocals of the modes (favored intervals) in the interspike interval histogram reflect the first effect of pitch shift, which is defined psychophysically as a proportional shift in pitch to the change in carrier frequency. In particular, interspike intervals of units with a widespread spectral response provide a basis to explain phase and dominant component pitch behavior that early narrow-band pitch theories found problematical. The amplitude of phase locking to individual AM components varies systematically though there are some unexplained variations across the frequency-intensity plane that could be due to combination tones. The unit response to a quasifrequency modulated (QFM) stimulus shows that if pitch is based on interspike intervals, it would remain the smae as pitch for an AM signal. The magnitude of the synchrony response to QFM stimuli is less than to AM stimuli for the majority of cochlear nucleus units; however, there are exceptions.

A software routine to reconstruct individual spike trains from multi-neuron, single-channel extracellular recordings was designed. Using a neural network algorithm that automatically clusters and sorts the spikes, the only user input needed is the threshold level for spike detection and the number of unit types present in the recording. Adaptive features are included in the algorithm to allow for tracking of spike trains during periods of amplitude variation and also to identify noise spikes. The routine will operate on-line during extracellular studies of the cochlear nucleus in cats.

1. Amplitude modulation (AM) is a pervasive property of acoustic communication systems. In the present study we investigate neural temporal mechanisms in the auditory nerve and cochlear nuclei of the pentobarbital sodium-anesthesized cat associated with the neural coding of 100% AM tones, both in quiet and in the presence of wideband, quasi-flat-spectrum noise. The AM carrier frequency was set to the neuron's characteristic frequency (CF) and the sound pressure level (SPL) of acoustic stimuli was varied over a wide dynamic range of intensities (< or = 40 dB). The temporal AM-encoding capability of auditory neurons was measured by computing the synchronization coefficient (SC) of the neural response to the signal's modulation and carrier frequency. The temporal modulation transfer function (tMTF) of a neuron was then computed by measuring the SC of the response to signals of variable fmod (50-2550 Hz). 2. Neurons in the cochlear nuclei synchronize on average more highly to the modulation frequency than fibers of comparable CF, threshold, and spontaneous rate in the auditory nerve. The disparity in performance is greatest at high SPLs and low signal-to-noise ratios. However, there is a significant degree of diversity in AM-encoding capability among neurons in both the cochlear nuclei and auditory nerve. Among auditory nerve fibers (ANFs), low- and medium-spontaneous-rate (SR) units (SR < 18 spike/s) phase-lock with greater precision than comparable high-SR units at any given frequency, particularly at moderate to high SPLs, consistent with previous studies. 3. The phase-locking capabilities of neurons in the cochlear nucleus are considerably more variable than in the auditory nerve. Moreover, the variability itself depends on two distinct measures of phase-locking performance. Most ANFs are capable of phase-locking to frequencies as high as 3–4 kHz. In the cochlear nucleus many unit types do not phase-lock to modulation frequencies > 1 kHz. As a result, phase-locking performance is measured on the basis of two parameters, maximum synchronization, irrespective of stimulus frequency, and the upper frequency limit for significant phase-locking. 4. Cochlear nucleus neurons may be divided into three distinct groups on the basis of maximum synchronization capability. In group 1 are the primary-like (PL) units of the anteroventral division, whose phase-locking capabilities are comparable with those of high-SR ANFs.(ABSTRACT TRUNCATED AT 400 WORDS)

Two-tone suppression and two-tone distortion were investigated at the level of the basilar membrane in the hook region of cat and guinea pig cochleae using a displacement-sensitive laser interferometric measurement system. The system allowed measurements to be performed at physiological stimulus levels in the cochlear region tuned to 30-35 kHz in cat and 29 kHz in guinea pig. The amplitude of vibration of the basilar membrane due to a probe tone at the characteristic frequency (CF) was attenuated during the presentation of a simultaneous suppressor tone either above or below CF. The amount of suppression depended on the intensities of both probe and suppressor, and the relationship of the suppressor frequency to the CF. Suppressors at frequencies more than an octave below the CF attenuated the responses to the CF probe at a rate of up to 1 dB/dB, with little variation based on suppressor frequency. As the suppressor frequency was increased above CF the rate of suppression decreased rapidly. The lowest suppressor intensity at which attenuation of the probe response was observed did not vary in direct proportion to the probe intensity. This suppression threshold often varied only a few dB SPL when the probe was varied over a 20 dB SPL range. In a few instances the rate of attenuation was as much as a factor of two greater at the lowest probe intensities than at higher intensities. It is noteworthy that suppression was found when the frequency of the suppressor was either above or below CF in the same preparation. Low frequency suppressor tones suppress basilar membrane motion at the CF when the basilar membrane undergoes displacement toward either scala. The maximum suppression occurs around 100 microseconds after the peak excursions caused by the low frequency biasing tone. Two-tone distortion products were often observed even at stimulus levels below those causing two-tone suppression at the site studied. The cubic difference tone (CDT) was the most prominent of the distortion products. The level of the CDT component varied nonmonotonically with the level of either of the primary tones. Responses at the difference frequency between the two primaries were usually below the noise floor of the recording system. The existence of both two-tone distortion and two-tone suppression was dependent on the presence of a cochlear nonlinearity.

A heterodyne laser interferometer was used to observe the movements of small (approximately 20 microns) stainless-steel beads placed on the basilar membrane in the hook region of cat and guinea-pig cochleae. In several preparations, the displacement patterns observed exhibited sharp nonlinear tuning; in one cat this tuning was comparable to that commonly observed in single auditory-nerve fibers. The most sensitive frequencies of the preparations ranged from 31-40 kHz in the cat, and 28-32 kHz in the guinea-pig. The sharp tuning and nonlinearity of the basilar membrane responses was not apparent in surgically or acoustically traumatized preparations. The response nonlinearities were susceptible to temporary threshold shifts and disappeared within a few minutes post-mortem. Stimulus-related shifts in the baseline position of the basilar membrane were not apparent at low stimulus levels. Such shifts were occasionally observed at higher stimulus levels (e.g., > 90 dB SPL), but never approached the fundamental (oscillatory) component of basilar membrane vibration in magnitude. These findings are discussed in relation to previous observations by other workers.

Middle-ear to basilar membrane (BM) velocity transfer functions are reported for seven locations in the hook region of a single cat cochlea. These transfer functions were recorded at high sound pressure levels in a linearized, or passive cochlea, and resemble those reported previously by Wilson and Evans (1983). They demonstrate longitudinal tonotopicity with a gradient of approximately 3.6 mm/octave. When allowances are made for the nonlinear mechanisms previously demonstrated in active hook region preparations (Cooper and Rhode, 1992), the data are also consistent with the tonotopic map derived from the intracellular dye-filling studies of Liberman (1982).

A composite model of the auditory periphery, based upon a unique analysis technique for deriving filter response characteristics from cat auditory-nerve fibers, is presented. The model is distinctive in its ability to capture a significant broadening of auditory-nerve fiber frequency selectivity as a function of increasing sound-pressure level within a computationally tractable time-invariant structure. The output of the model shows the tonotopic distribution of synchrony activity of single fibers in response to the steady-state vowel [e] presented over a 40-dB range of sound-pressure levels and is compared with the population-response data of Young and Sachs (1979). The model, while limited by its time invariance, accurately captures most of the place-synchrony response patterns reported by the Johns Hopkins group. In both the physiology and in the model, auditory-nerve fibers spanning a broad tonotopic range synchronize to the first formant (F1), with the proportion of units phase-locked to F1 increasing appreciably at moderate to high sound-pressure levels. A smaller proportion of fibers maintain phase locking to the second and third formants across the same intensity range. At sound-pressure levels of 60 dB and above, the vast majority of fibers with characteristic frequencies greater than 3 kHz synchronize to F1 (512 Hz), rather than to frequencies in the most sensitive portion of their response range. On the basis of these response patterns it is suggested that neural synchrony is the dominant auditory-nerve representation of formant information under "normal" listening conditions in which speech signals occur across a wide range of intensities and against a background of unpredictable and frequently intense acoustic interference.

At a time when little was known about processing in the auditory system, Aage Moller undertook an extensive investigation of the response properties of cochlear nucleus (CN) neurons. With an excellent background in physiological acoustics and a command of computational techniques he systematically explored neural tuning, rate-level functions, and receptive fields of CN neurons using microelectrode recordings. He chose to employ more natural stimuli than just pure tones and employed a variety of stimuli consisting of tones, clicks, noise, amplitude- and frequency-modulated signals to document both intensity and temporal response characteristics. The response to noise stimuli was quantified using linear systems analysis which was very innovative at that time. By choosing to perform the studies in the white rat rather than cat, he provided important comparative data on this first center of the central auditory system. Over a span of ten years he provided a significant body of observations of CN units properties that has rarely been equaled.

Responses of neurons in the ventral cochlear nucleus (VCN) of anesthetized chinchillas to six synthetic vowel sounds (/a/, /e/, /epsilon/, /i/, /o/ and /u/) were recorded at several intensity levels. Stimuli were synthesized with a fundamental frequency of 100 Hz or 181.6 Hz and had formant values at integer multiples of 100 Hz. Responses came from most neuron types in the VCN (with the exception of onset cells with an I-shaped pattern). Population studies, performed only on primary-like (PL) and chopper neurons, showed that PL neurons provide a better temporal representation than do chopper neurons. At the lowest level of stimulation, all neuron types provide an accurate rate-place representation of vowel spectra. With an increase in stimulus level, the rate-place representation of PL neurons becomes inferior to that of chopper neurons, either sustained choppers or transient choppers.

The dorsal cochlear nucleus receives input from the auditory nerve and relays acoustic information to the inferior colliculus. Its principal cells receive two systems of inputs. One system through the molecular layer carries multimodal information that is processed through a neuronal circuit that resembles the cerebellum. A second system through the deep layer carries primary auditory nerve input, some of which is relayed through interneurons. The present study reveals the morphology of individual interneurons and their local axonal arbors and how these inhibitory interneurons respond to sound. Vertical cells lie beneath the fusiform cell layer. Their dendritic and axonal arbors are limited to an isofrequency lamina. They give rise to pericellular nests around the base of fusiform cells and their proximal basal dendrites. These cells exhibit an onset-graded response to short tones and have response features defined as type II. They have tuning curves that are closed contours (0 shaped), thresholds approximately 27 dB SPL, spontaneous firing rates of approximately 0 spikes/s, and they respond weakly or not at all to broadband noise, as described for type II units. Their responses are nonmonotonic functions of intensity with peak responses between 30 and 60 dB SPL. They also show a preference for the high-to-low direction of a frequency sweep. It has been suggested that these circuits may be involved in the processing of spectral cues for the localization of sound sources.

Responses of the principal unit types in the ventral cochlear nucleus of the chinchilla were studied with a single-formant stimulus set that covered fundamental frequency (f0) from 100 Hz to 200 Hz and formant center frequency (F1) from 256 to 782 Hz. Temporal coding for f0 and F1 was explored for 95 stimulus combinations of f0 (n = 5) and F1 (n = 19) in primarylike, onset and chopper unit categories. Several analyses that explored temporal coding were employed including: autocorrelation, interspike interval analysis, and synchronization to each harmonic of f0. In general, the representation of f0 is better in onset and chopper units than in primarylike units. Nearly all units in the cochlear nucleus showed a gain in phase locking to the envelope (f0) of the single-formant stimulus relative to the auditory nerve. The fundamental is represented directly in neural discharges of units in the cochlear nucleus with an interval code (also Cariani and Delgutte, 1996; Rhode, 1995). The formant is represented in the temporal domain in primarylike units, though some chopper and onset units also possess the ability to code F1 through discharge synchrony. Onset-I units, which are associated with the octopus cells, exhibited the strongest phase locking to f0 of any unit types studied. The representation of f0 and F1 in the temporal domain is weak or absent in some units. All-order-interspike interval distributions computed for populations of units show preservation of temporal coding for both f0 and F1. Results are in agreement with earlier amplitude modulation studies that showed nearly all cochlear nucleus unit types phase lock to the signal envelope better than auditory nerve fibers over a considerable range of signal amplitudes.

A heterodyne laser interferometer was used to observe the sound-evoked displacement patterns of Reissner's membrane and various other structures in the apical turn of the guinea-pig cochlea. Most structures (including the basilar membrane) were similarly tuned, and had best frequencies in the 200-350Hz range. A distinct notch was usually observed approximately 0.7 octaves above the best frequency, and amplitude- and phase-plateaus were observed at higher frequencies. In most other respects, however, the mechanical tuning resembled the frequency-threshold curves of low frequency cochlear nerve fibers. In five reasonably intact, in vivo preparations, the frequency of the mechanical sensitivity notch was intensity-dependent: Compressive nonlinearities were observed above approximately 80 dB SPL on the low-frequency side of the notch, with antagonistically expansive nonlinearities on the high-frequency side. Two-tone suppression was observed in one of these preparations. Stimulus-related baseline position shifts were observed in another in vivo preparation. No such nonlinearities were observed in structurally damaged and/or1 hour post-mortem preparations. However, more robust nonlinearities were observed in all preparations at higher levels of stimulation (e.g.100-110 dB SPL). These high-level nonlinearities diminished only slowly after death, and gave rise to various effects, including time-dependent (i.e. adapting) and severely distorted (e.g. peak-split and/or dc-shifted) responses.

1. The ability of cells in the cochlear nucleus (CN) to encode frequency information in the presence of background noise on the basis of "place/rate" information was investigated by measuring the threshold, magnitude, and extent of lateral suppression in the ventral and dorsal CN of the anesthesized cat. The suppression regions were delineated through the use of "masked" response areas (MRAs). The MRA is a family of isointensity curves derived from the average discharge rate in response to a tone of variable frequency and sound pressure level in the presence of a concurrently presented broadband, quasi-flat-spectrum noise. Tonal stimuli of sufficient intensity are often effective in significantly reducing the average discharge rate of CN neurons over a wide frequency range. 2. Most units in the CN exhibit prominent lateral suppressive sidebands, but the variability in threshold, magnitude, and extent of suppression is large. Primary-like and onset units of the ventral CN manifest the least suppression and have the highest suppression thresholds. Pauser/buildup units in the dorsal division and choppers distributed throughout the CN show the largest amount of suppression and have the lowest suppression thresholds. 3. Auditory nerve fibers manifest some degree of lateral suppression, particularly fibers of low and medium spontaneous rate. However, in few instances are the threshold, magnitude, and extent comparable with that observed among the majority of chopper and pauser/buildup units. For this reason the lateral suppression observed among the latter unit types is unlikely to originate entirely from cochlear processes, but rather is likely to reflect largely neural mechanisms intrinsic to the CN. In contrast, the MRAs of most primary-like and onset units suggest that the suppression behavior of most of these cells originates mostly, if not entirely, in the cochlea and auditory nerve. 4. A primary consequence of lateral suppression is to preserve the sharp frequency selectivity of CN neurons at moderate to high sound pressure levels, particularly in background noise. In this fashion lateral suppressive mechanisms potentially enhance the representation of spectral information on the basis of place/rate information relative to that in the auditory nerve under noisy background conditions. 5. Lateral suppressive mechanisms probably underlie the dynamic range shift seen in the presence of a simultaneously presented noise. This mechanism may be crucial for preserving the ability to perceive signals in a noisy background.

Most of our knowledge concerning auditory signal processing is based on the response of auditory nerve fibers. To a certain degree, these peripheral fibers function as bandpass filters, relaying the spectral analysis performed in the cochlea to higher nervous centers. However, the auditory coding of acoustic signals involves far more than just frequency analysis. For example, the sensation of low pitch associated with musical melody and speech prosody appears to be relatively independent of spectral analysis since different portions of the spectrum can give rise to this percept. The ability to focus upon a single signal among many concurrently presented also involves mechanisms which lie beyond the reach of the auditory periphery. Thus, many properties of acoustic signals are processed in parallel, providing information not only concerning the spectrum per se, but about certain “ecological” aspects of the signal source, including its location, approximate size and trajectory.

Distortion product otoacoustic emissions (DPOAEs) and basilar membrane (BM) vibration were measured simultaneously in the 6-9 kHz region of chinchilla cochleae. BM-Input-Output functions in a two-tone paradigm behaved similarly to DPOAEs for the 2f1-f2 component, nonmonotonic growth with the intensity of the lower frequency primary and a notch in the functions around 60 dB SPL. Ripples in frequency functions occur in both BM and OAE curves as a function of the distortion frequency. Optimum f2/f1 ratios for DPOAE generation are near 1.2. The slope of phase curves indicates that for low f2f1(1.1) the emission source is the place location while for f2f11.1 the relative constancy of the phase function suggests that the place is the nonlinear region of f2, i.e., the wave location. Magnitudes of the DPOAEs increase rapidly above 60 dB SPL suggesting a different source or mechanism at high levels. This is supported by the observation that the high level DPOAE and BM-DP responses remain for a considerable period postmortem.

Differences between cochlear mechanical tuning curves and those of auditory nerve fibers (ANFs) exist. In particular, mechanical transfer functions exhibit a high‐frequency plateau; ANFs frequency threshold curves (FTCs) do not. ANF‐FTCs may have a low‐frequency slope due to a velocity forcing function operating on inner hair cells at low frequencies. Neither basilar membrane velocity nor displacement adequately explain the entire ANF tuning curve. A displacement sensitive interferometer was used to study basilar membrane and spiral limbus mechanics in the 6‐kHz region of the chinchilla cochlea. The spiral limbus vibrates at the same phase as the basilar membrane nearly up to the location’s characteristic frequency. In the plateau region, the limbus appears to vibrate 0 to 20 dB less than the basilar membrane. The basilar membrane/limbus amplitude transfer function has a low‐frequency slope of ∼3 dB/oct at low frequencies and is ∼10 dB lower than the basilar membrane amplitude at 1 kHz. It appears that spiral limbus vibration may contribute to the excitation of the cilia of the inner hair cells. [Work supported by NIDCD grant R01 DC001910.]

Professor Joe Zwislocki is a rare individual who has a strong background in mathematics and hydrodynamics that he applied to the physiological characterization of cochlear function. Realizing the preeminent need for more accurate measurements of cochlear mechanical properties, he undertook the study of properties of the tectorial membrane (TM). His measurements are unique in that they remain the only in vivo measurements of stiffness of the TM. In 1979, he introduced longitudinal effects into cochlear modeling that today has been shown to result in realistic cochlear mechanical transfer curves. A difference between the filter properties seen in auditory nerve fibers and basilar membrane (BM) mechanics led to a leading theory of separate resonances for organ of Corti and TM. He developed a model of how this would affect the shear motion between the TM and reticular lamina. Additional studies were undertaken that substituted recordings in the cells of Hensen for the very difficult direct observation of BM mechanics that epitomize the ingenuity and resourcefulness of Joe Zwislocki.

One of the most challenging dilemmas for theories of speech perception is the lack of invariance between acoustic signal and perception. Due to physical constraints upon articulators, there is a good deal of context‐dependency both in speech production and in the resulting acoustic speech signal. Consequently, the acoustic pattern most closely related to a given speech sound varies dramatically depending on context. Yet, by some means, the perceptual system perceives these unique acoustic events as linguistically equivalent. This phenomenon is observed experimentally as perceptual context effects whereby adjacent speech can modulate perceived identity of a given speech sound. Recent perceptual results suggest that this phenomenon may be governed by general auditory mechanisms [Holt et al., 1996; Lotto et al., 1997; Lotto and Kluender, 1998; Holt, 1999] rather than speech‐specific processes. In the present study, we sought to explore how such context‐dependencies might be encoded. We recorded responses of ventral cochlear nucleus (VCN) neurons of anesthetized chinchillas to nonspeech stimulus targets with adjacent context stimuli that varied in spectral content. Results demonstrate context‐dependent effects of frequency and intensity.

Ever since Ohm and Seebeck, hearing theory has pondered whether “place” or “time” is the dominant means by which acoustic information is encoded in the ear. Over the past decade it has become increasingly clear that, at the level of the auditory nerve, both place and timing mechanisms participate in the representation of complex sounds. As a consequence, attention is now focused on the cochlear nucleus (CN), to which the auditory nerve projects. Within this nuclear complex occurs a remarkable degree of physiological diversity of potential significance to speech coding. The present survey reviews the temporal response properties of the major CN cell types to voiced CV syllables and the potential role played by each in the encoding of speech. All of the major response classes—primarylike, onset, chopper, onset‐chopper, and pauser‐buildup—are capable of synchronizing to the fundamental frequency or first formant. However, the range of acoustic conditions over which they do so, as well as the precision and up...

This paper describes part of the anatomy and physiology of the cochlear nucleus and some of the recent techniques developed to study the structure and function of the cochlear nucleus. The cochlear nucleus is the first region in the brain stem that receives information from the auditory nerve. The use of intracellular markers has made possible a new detail of description that should increase significantly our understanding of how the cochlear nucleus organizes the neural information transmitted to it from the auditory nerve.

We report on the anatomical and physiological features of globular bushy cells in the posterior division of the anteroventral cochlear nucleus based on the characteristics of 20 cells from this population. Each of these cells was recorded from and characterized intracellularly and/or extracellularly, injected with horseradish peroxidase, and studied at the light and/or electron microscopic level. Intracellular records from the vicinity of the globular bushy cell body displayed large, fast synaptic potentials, and in some instances a larger presumed action potential both in silence and during short tone stimulation. Intraaxonal recordings displayed large action potentials in addition to small, fast potentials, which evidence indicates may be the decrementally conducted subthreshold synaptic potentials. Both recording situations indicated that these auditory nerve inputs need not be suprathreshold. Bushy cells with high characteristic frequencies (CFs greater than 3 kHz) typically showed primary-like-with-notch responses to short tones at CF or on-type L responses if the sustained level of discharge after the notch was not as robust. Low-CF bushy cells phase-locked after a well-timed onset spike. Light microscopic anatomy revealed a typically oval cell body giving rise to one or two primary dendrites that branched profusely and an axon that gave off no collaterals within the cochlear nucleus before entering the trapezoid body. Electron microscopic analysis showed a high concentration of large, round, vesicle-containing terminals on the cell body and primary dendrite while the population of terminals on the initial segment and sparsely covered distal dendrites was made up mostly of flat and pleomorphic vesicle-containing terminals.

The Mössbauer technique, which can be used to measure very small velocities, on the order of 0.2 mm/sec, has been used to measure the response of the basilar membrane to tones and clicks in squirrel monkeys. The results verify that there is a mechanical frequency analysis performed in the cochlea and that the traveling wave theory holds true. The resonance curves indicate that the tuning of the basilar membrane is greater than was thought. The basilar membrane in the 7–8 kHz region of the cochlea vibrates nonlinearly at frequencies near the “resonant frequency.” The click response shows that the “tail” of the decaying oscillatory response does not decrease in proportion to click amplitude while the early displacements of the basilar membrane have a nearly linear relationship with click amplitude. These results are in good agreement with the results of the measurements using tones as stimuli. Experiments examining postmortem behavior of the basilar membrane indicate a rapid decrease in the sensitivity of vibration along with a decrease of up to one octave in the “resonant” frequency within a six hour period after the animal's death. The shift in resonant frequency is accompanied by a corresponding shift in the phase characteristic. The low frequency slope of the resonance curve becomes 6 dB/octave exactly as Békésy found while the high frequency slope decreases slightly. Comparison of the mechanical resonance curves with the neural data for single auditory nerve fibers in the squirrel monkey indicates that the exquisite tuning exhibited in the nerve cannot be explained solely on the basis of the mechanical behavior of the basilar membrane.

A set of experiments was conducted using the Mössbauer effect to determine the vibratory characteristics of the basilar membrane, Reissner's membrane, the malleus, incus, and oval window in squirrel monkey. A few measurements were also made in guinea pig in the basal cochlear region. The nonlinear vibration properties of the basilar membrane are described in detail for the midfrequency region in the squirrel monkey. Only in this region have nonlinear effects been observed. A comparison of mechanical and neural data indicates good qualitative agreement.

Experimental phase data from the Mossbauer‐effect measurements of basilar‐membrane vibration are given. The phase lag increased with intensity for frequencies less than best frequency, but was a decreasing function of intensity above best frequency. A series of linear minimum‐phase representations of the basilar membrane’s motion were also calculated; they show similar trends.

The physiology and morphology of fusiform cells in the dorsal cochlear nucleus were studied using extracellular and intracellular recording and intracellular injection of horseradish peroxidase. Fusiform cells displayed a variety of responses to tone pips presented at the characteristic frequency; most often these cells exhibited the pauser/buildup pattern defined in earlier studies. The response pattern of each neuron was dependent on frequency and sound-pressure level. Tone pips evoked short-lasting depolarizations of about 10 mV and long-lasting hyperpolarizations of about 10 mV in cells whose resting potentials were -50 to -65 mV. The time courses of both the excitation and the inhibition depended on frequency and sound-pressure level. Generally the depolarization was sustained for the duration of the tone pip, whereas the hyperpolarization could last as long as 600 ms after the end of the tone pip. Often a neuron exhibited a sustained chopper pattern after microelectrode impalement. This was probably a result of a decrease in membrane potential which altered the relative effectiveness of the excitatory and inhibitory inputs. The large, bitufted fusiform cells had many apical dendrites, which branched one to five times and were covered with spines, and fewer basal dendrites, which exhibited little branching and had few appendages. The morphology of fusiform cells varied systematically as a function of location within the dorsal cochlear nucleus. Response patterns for tone pips were not exclusive to individual cell types as two nonfusiform cells were found to exhibit a buildup pattern. Axons of injected neurons left the nucleus via the dorsal acoustic stria and 14 of 15 had collaterals within the dorsal cochlear nucleus.

We report on the anatomy and physiology of three fusiform cells in the dorsal cochlear nucleus (DCN) of the cat. The extra- and intracellular responses of these cells to pure tones showed features typical of the cell type. Peristimulus time histograms (PSTHs) were usually of the pauser or buildup configuration with chopping behavior noted in certain instances. Intracellular records during stimulus presentations revealed sustained depolarizations for the duration of the tone followed by a prolonged after-hyperpolarization (AHP). On rare occasions, a hyperpolarization corresponding to the pause region of the PSTH was noted. Occasionally, a stimulus-induced depolarization would be maintained after stimulus offset. Rebound excitation was also observed after the AHP. Morphologically, all three cells showed the standard fusiform cell features at the light microscopic level. The cell body gave rise to apical and basal dendritic trees. The apical tree branched frequently and displayed numerous spines distally. The basal tree had fewer branches and fewer, more irregular appendages. The axon originated from the cell body and gave rise to one or more collaterals before leaving the nucleus via the dorsal acoustic stria (DAS). At the electron microscopic (EM) level, the axon collaterals may terminate on a variety of cell types in the DCN, including fusiform cells. Their vesicles are round and the terminals closely resemble many unlabeled terminals seen on the cell body and apical and basal dendrites of our labeled fusiform cells. Terminals containing round vesicles, believed to be eighth nerve terminals, were found, with one exception, only on the basal dendrites. The spine-laden, distal apical dendrites received primarily terminals containing round vesicles, presumed to originate from the unmyelinated axons of granule cells. The cell body and unmyelinated initial segment received mostly terminals containing pleomorphic and flat vesicles, which also made up a large percentage of the dendritic input. Some relevant correlations, between the distribution of synaptic terminals and the observed physiology, may be possible.

The Mossbauer technique has been applied to the measurement of vibration of the basilar membrane in the squirrel monkey cochlea. Both steady‐state and transient responses have been recorded in the 7–8‐kHz locus of the cochlea. The steady‐state response indicates that the basilar membrane vibrates nonlinearly for frequencies of stimulation near or greater than the characteristic frequency. The nonlinearity can be observed at the lowest levels of stimulation, 70–80 dB SPL, for which measurements could be made. The nonlinearity extends to lower frequencies and the basilar membrane transfer function tends to broaden as SPL is increased. Rapid postmortem changes occur in the cochlea: (1) the amplitude of the transfer ratio (basilar membrane/malleus) decreases 10–15 dB over a period of several hours with a downward shift of 1.5–3 kHz in the characteristic frequency of the basilar membrane at a given location; (2) the low‐frequency slope of the transfer ratio settles to 6 dB/octave by 6 h after death; (3) the slope of the phase of the transfer function increases the characteristic frequency decreases; (4) the basilar membrane vibrates linearly after death. The transient response was studied using acoustic clicks of approximately 150‐μ duration, presented in sequences of 100 000 to 400 000. The transient response has an early component which has a fast decay and a second component which has an extremely slow rate of decay and which displays nonlinear behavior. Preliminary results for the vibration of Reissner's membrane in the 13‐kHz region of the cochlea are also reported.