Units In Cochlear Nucleus Research Articles

Neural Segregation of Concurrent Speech: Effects of Background Noise and Reverberation on Auditory Scene Analysis in the Ventral Cochlear Nucleus.

Concurrent complex sounds (e.g., two voices speaking at once) are perceptually disentangled into separate "auditory objects". This neural processing often occurs in the presence of acoustic-signal distortions from noise and reverberation (e.g., in a busy restaurant). A difference in periodicity between sounds is a strong segregation cue under quiet, anechoic conditions. However, noise and reverberation exert differential effects on speech intelligibility under "cocktail-party" listening conditions. Previous neurophysiological studies have concentrated on understanding auditory scene analysis under ideal listening conditions. Here, we examine the effects of noise and reverberation on periodicity-based neural segregation of concurrent vowels /a/ and /i/, in the responses of single units in the guinea-pig ventral cochlear nucleus (VCN): the first processing station of the auditory brain stem. In line with human psychoacoustic data, we find reverberation significantly impairs segregation when vowels have an intonated pitch contour, but not when they are spoken on a monotone. In contrast, noise impairs segregation independent of intonation pattern. These results are informative for models of speech processing under ecologically valid listening conditions, where noise and reverberation abound.

Reverberation impairs brainstem temporal representations of voiced vowel sounds: challenging "periodicity-tagged" segregation of competing speech in rooms.

The auditory system typically processes information from concurrently active sound sources (e.g., two voices speaking at once), in the presence of multiple delayed, attenuated and distorted sound-wave reflections (reverberation). Brainstem circuits help segregate these complex acoustic mixtures into “auditory objects.” Psychophysical studies demonstrate a strong interaction between reverberation and fundamental-frequency (F0) modulation, leading to impaired segregation of competing vowels when segregation is on the basis of F0 differences. Neurophysiological studies of complex-sound segregation have concentrated on sounds with steady F0s, in anechoic environments. However, F0 modulation and reverberation are quasi-ubiquitous. We examine the ability of 129 single units in the ventral cochlear nucleus (VCN) of the anesthetized guinea pig to segregate the concurrent synthetic vowel sounds /a/ and /i/, based on temporal discharge patterns under closed-field conditions. We address the effects of added real-room reverberation, F0 modulation, and the interaction of these two factors, on brainstem neural segregation of voiced speech sounds. A firing-rate representation of single-vowels' spectral envelopes is robust to the combination of F0 modulation and reverberation: local firing-rate maxima and minima across the tonotopic array code vowel-formant structure. However, single-vowel F0-related periodicity information in shuffled inter-spike interval distributions is significantly degraded in the combined presence of reverberation and F0 modulation. Hence, segregation of double-vowels' spectral energy into two streams (corresponding to the two vowels), on the basis of temporal discharge patterns, is impaired by reverberation; specifically when F0 is modulated. All unit types (primary-like, chopper, onset) are similarly affected. These results offer neurophysiological insights to perceptual organization of complex acoustic scenes under realistically challenging listening conditions.

Sensitivity of cochlear nucleus neurons to spatio-temporal changes in auditory nerve activity

The spatio-temporal pattern of auditory nerve (AN) activity, representing the relative timing of spikes across the tonotopic axis, contains cues to perceptual features of sounds such as pitch, loudness, timbre, and spatial location. These spatio-temporal cues may be extracted by neurons in the cochlear nucleus (CN) that are sensitive to relative timing of inputs from AN fibers innervating different cochlear regions. One possible mechanism for this extraction is "cross-frequency" coincidence detection (CD), in which a central neuron converts the degree of coincidence across the tonotopic axis into a rate code by preferentially firing when its AN inputs discharge in synchrony. We used Huffman stimuli (Carney LH. J Neurophysiol 64: 437-456, 1990), which have a flat power spectrum but differ in their phase spectra, to systematically manipulate relative timing of spikes across tonotopically neighboring AN fibers without changing overall firing rates. We compared responses of CN units to Huffman stimuli with responses of model CD cells operating on spatio-temporal patterns of AN activity derived from measured responses of AN fibers with the principle of cochlear scaling invariance. We used the maximum likelihood method to determine the CD model cell parameters most likely to produce the measured CN unit responses, and thereby could distinguish units behaving like cross-frequency CD cells from those consistent with same-frequency CD (in which all inputs would originate from the same tonotopic location). We find that certain CN unit types, especially those associated with globular bushy cells, have responses consistent with cross-frequency CD cells. A possible functional role of a cross-frequency CD mechanism in these CN units is to increase the dynamic range of binaural neurons that process cues for sound localization.

Equivalent-rectangular bandwidth of single units in the anaesthetized guinea-pig ventral cochlear nucleus

Frequency-tuning is a fundamental property of auditory neurons. The filter bandwidth of peripheral auditory neurons determines the frequency resolution of an animal’s auditory system. Behavioural studies in animals and humans have defined frequency-tuning in terms of the “equivalent-rectangular bandwidth” (ERB) of peripheral filters. In contrast, most physiological studies report the Q [best frequency/bandwidth] of frequency-tuning curves. This study aims to accurately describe the ERB of primary-like and chopper units in the ventral cochlear nucleus, the first brainstem processing station of the central auditory system. Recordings were made from 1020 isolated single units in the ventral cochlear nucleus of anesthetized guinea pigs in response to pure-tone stimuli which varied in frequency and in sound level. Frequency-threshold tuning curves were constructed for each unit and estimates of the ERB determined using methods previously described for auditory-nerve-fibre data in the same species. Primary-like, primary-notch, and sustained- and transient-chopper units showed frequency selectivity almost identical to that recorded in the auditory nerve. Their tuning at pure-tone threshold can be described as a function of best frequency (BF) by ERB = 0.31 * BF 0.5.

Alterations in the spontaneous discharge patterns of single units in the dorsal cochlear nucleus following intense sound exposure

Electrophysiological recordings in the dorsal cochlear nucleus (DCN) were conducted to determine the nature of changes in single unit activity following intense sound exposure and how they relate to changes in multiunit activity. Single and multiunit spontaneous discharge rates and auditory response properties were recorded from the left DCN of tone exposed and control hamsters. The exposure condition consisted of a 10 kHz tone presented in the free-field at a level of 115 dB for 4 h. Recordings conducted at 5–6 days post-exposure revealed several important changes. Increases in multiunit spontaneous neural activity were observed at surface and subsurface levels of the DCN of exposed animals, reaching a peak at intermediate depths corresponding to the fusiform cell layer and upper level of the deep layer. Extracellular spikes from single units in the DCN of both control and exposed animals characteristically displayed either M- or W-shaped waveforms, although the proportion of units with M-shaped spikes was higher in exposed animals than in controls. W-shaped spikes showed significant increases in the duration of their major peaks after exposure, suggestive of changes in the intrinsic membrane properties of neurons. Spike amplitudes were not found to be significantly increased in exposed animals. Spontaneous discharge rates of single units increased significantly from 8.7 spikes/s in controls to 15.9 spikes/s after exposure. Units with the highest activity in exposed animals displayed type III electrophysiological responses patterns, properties usually attributed to fusiform cells. Increases in spontaneous discharge rate were significantly larger when the comparison was limited to a subset of units having type III frequency response patterns. There was an increase in the incidence of simple spiking activity as well as in the incidence of spontaneous bursting activity, although the incidence of spikes occurring in bursts was low in both animal groups (i.e., <30%). Despite this low incidence, approximately half of the increase in spontaneous activity in exposed animals was accounted for by an increase in bursting activity. Finally, we found no evidence of an increase in the mean number of spontaneously active units in electrode penetrations of exposed animals compared to those in controls. Overall our results indicate that the increase in multiunit activity observed at the DCN surface reflects primarily an increase in the spontaneous discharge rates of single units below the DCN surface, of which approximately half was contributed by spikes in bursts. The highest level of hyperactivity was observed among units having the response properties most commonly attributed to fusiform cells.

Representation of the spectral dominance region of pitch in the steady-state temporal discharge patterns of cochlear nucleus units

Single-unit responses to infinitely iterated rippled noise and wideband noise were recorded from the cochlear nucleus of anesthetized chinchillas. Rippled noises had a fixed delay of 4 ms, and spectral depth was varied by attenuating the delayed version of the noise. Temporal discharge patterns were analyzed using neural autocorrelograms, and responses to rippled noises were compared to wideband noise responses. Chopper units with best frequencies in the range of the first to second harmonics of the rippled noise showed large differences in discharge patterns between rippled noise and wideband noise responses, but chopper units with best frequencies centered at higher harmonics did not show large differences. Consequently, the Chopper group of units showed no evidence of a neural representation of the dominance region. Primarylike units did show a neural representation of dominance that is related to behavioral performance. For Primarylike units with best frequencies around the third to fifth harmonics of the rippled noise, large differences in discharge patterns between rippled noise and wideband noise responses were observed. The results suggest that bushy cells play an important role in processing pitch-related information and should be included as important elements in neural models of periodicity processing.

Neuronal representation of pitch ambiguity

Iterated rippled noise (IRN) is produced by delaying a broadband noise by time d, multiplying by gain g, adding the delayed noise to the original, and repeating this process for n iterations. When g=+1 IRN has a well‐defined pitch at 1/d Hz. If g=‐1 the pitch can be ambiguous. A gain of ‐1 is equivalent to applying a frequency‐independent phase shift φ of π rads to the delayed noise (g=+1 ≡ φ=0). We recorded spike‐trains from single units in the ventral cochlear nucleus in response to IRN with varying φ. Units with high best frequencies represented waveform envelope modulation (independent of φ), however, units in the phase‐locking range of best frequencies represented stimulus fine structure (which varies with φ). Fine structure responders show a gradual transition from a well‐defined peak in the interspike interval distribution at d when φ=0 to two equal‐amplitude peaks flanking d when φ=π, and a gradual shift back to a well‐defined peak at d as φ approaches 2π. Within the dominance region for pitch interspike interval distributions account for psychophysical pitch matches of 1.07/d and 0.94/d Hz for φ=π/2 and 3/2π respectively, as well as the ambiguous pitches of 0.88/d, 1.14/d, and 1/2d Hz heard when φ=π rads.

Rebound depolarization in single units of the ventral cochlear nucleus: A contribution to grouping by common onset?

Simultaneous grouping by common onset time is believed to be a powerful cue in auditory perception; components that start or stop roughly at the same time are judged as far more likely to have originated from the same source. Here we report a simple experiment designed to simulate a complex psychophysical paradigm first described by Darwin and Sutherland [(1984) Grouping frequency components of vowels. When is a harmonic not a harmonic? Quarterly J of Experimental Psychology: Hum Exp Psychol 36(A):193–208]. It is possible to change the perception of the vowel /I/ to /ε/ by manipulating the harmonics around the first formant (F1). Increasing the amplitude of one harmonic around F1 caused the perception of the vowel to change from /I/ to /ε/. Extending the increased component before the vowel could, however, greatly reduce this change. The role of neural adaptation in this effect was questioned by repeating the experiment but this time using a ‘captor’ tone which was switched on with the asynchronous harmonic and off when the vowel started. This time the vowel percept did change in a fashion analogous to the effect of an increase in the amplitude of the fourth harmonic (which is close to F1). This effect was explained by assuming that the captor had grouped with the leading portion of the asynchronous component enabling the remainder of the asynchronous component to be grouped with the remainder of the components. We propose a relatively low-level neuronal explanation for this grouping effect: the captor reduces the neural response to the leading segment of the asynchronous component by activating across-frequency suppression, either from the cochlea, or acting via a wideband inhibitor in the ventral cochlear nucleus. The reduction in neural response results in a release from adaptation with the offset of the captor terminating the inhibition, such that the response to the continuation of that component is now enhanced. Using a simplified paradigm we show that both primary-like and chopper units in the ventral cochlear nucleus of the anesthetized guinea pig may show a rebound in excitation when a captor is positioned so as to stimulate the suppressive sidebands in its receptive field. The strength of the rebound was positively correlated with the strength of the suppression. These and other results are consistent with the view that low-level mechanisms underlie the psychophysical captor effect.

The temporal representation of the delay of dynamic iterated rippled noise with positive and negative gain by single units in the ventral cochlear nucleus

Spike trains were recorded from single units in the ventral cochlear nucleus of the anaesthetised guinea-pig in response to dynamic iterated rippled noise with positive and negative gain. The short-term running waveform autocorrelation functions of these stimuli show peaks at integer multiples of the time-varying delay when the gain is + 1, and troughs at odd-integer multiples and peaks at even-integer multiples of the time-varying delay when the gain is − 1. In contrast, the short-term autocorrelation of the Hilbert envelope shows peaks at integer multiples of the time-varying delay for both positive and negative gain stimuli. A running short-term all-order interspike interval analysis demonstrates the ability of single units to represent the modulated pitch contour in their short-term interval statistics. For units with low best frequency (≲ 1.1 kHz) the temporal discharge pattern reflected the waveform fine structure regardless of unit classification (Primary-like, Chopper). For higher best frequency units the pattern of response varied according to unit type. Chopper units with best frequency ≳ 1.1 kHz responded to envelope modulation; showing no difference between their response to stimuli with positive and negative gain. Primary-like units with best frequencies in the range 1–3 kHz were still able to represent the difference in the temporal fine structure between dynamic rippled noise with positive and negative gain. No unit with a best frequency above 3 kHz showed a response to the temporal fine structure. Chopper units in this high frequency group showed significantly greater representation of envelope modulation relative to primary-like units with the same range of best frequencies. These results show that at the level of the cochlear nucleus there exists sufficient information in the time domain to represent the time-varying pitch associated with dynamic iterated rippled noise.

Contralateral inhibitory and excitatory frequency response maps in the mammalian cochlear nucleus

There is increasing evidence that the responses of single units in the mammalian cochlear nucleus can be altered by the presentation of contralateral stimuli, although the functional significance of this binaural responsiveness is unknown. To further our understanding of this phenomenon we recorded single-unit (n = 110) response maps from the cochlear nucleus (ventral and dorsal divisions) of the anaesthetized guinea pig in response to presentation of ipsilateral and contralateral pure tones. Many neurones showed no evidence of input from the contralateral ear (n = 41) but other neurones from both ventral and dorsal cochlear nucleus showed clear evidence of contralateral inhibitory input (n = 61). Inhibitory response patterns were divided into two groups. In 36 neurones, contralateral tone-evoked inhibition was closely aligned with the ipsilateral excitatory response map (+/- 0.33 octaves) often extending to low stimulus levels. In 25 neurones, higher threshold contralateral inhibitory responses were found, mostly centred at frequencies greater than 0.33 octaves below the ipsilateral excitation. A few neurones (n = 8) exhibited responses consistent with excitatory input from the contralateral ear, which was closely aligned with the ipsilateral excitation, and were found exclusively in the dorsal cochlear nucleus. The latency of the contralateral interaction was, on average, longer than the ipsilateral latency. Interaural level difference curves are similar to other reports from the cochlear nucleus. Our results are consistent with the idea that contralateral interactions arise from a variety of direct and indirect neuronal projections.

Responses of cochlear nucleus units recorded from chinchillas trained to discriminate iterated rippled noise from flat-spectrum noise

Behavioral responses obtained from chinchillas using a stimulus generalization paradigm show that the perception of pitch strength is dependent on the stimuli used for training. When animals are trained to discriminate a cosine-phase harmonic tone complex (COS) from a flat-spectrum, wideband noise (WBN), generalization gradients indicate that animals rely primarily on temporal information in the envelope. Gradients from animals retrained to discriminate infinitely iterated rippled noise (IIRN) from WBN indicate animals rely more on information in the fine structure. Thus, chinchillas learn to use the information in the fine structure when trained with an appropriate stimulus. In order to begin to examine whether there exists a neural correlate of this learning at the level of the cochlear nucleus, single-unit responses to IIRNs and WBN were recorded from three anesthetized chinchillas that had received considerable behavioral training in discriminating IIRNs from WBN. Preliminary data were obtained for 19 single units consisting of primarylike, chopper, and onset response types. All-order interspike-interval histograms obtained for IIRNs and WBN were qualitatively similar to those obtained from naive, untrained animals for each unit type. Quantitative comparisons among units recorded from trained and untrained animals will be presented. [Work supported by NIDCD R01 DC005596.]

Contributions of Aage Møller in the study of the cochlear nucleus

At a time when little was known about processing in the auditory system, Aage Møller 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.

The time course of recovery from suppression and facilitation from single units in the mammalian cochlear nucleus

The responses to two identical, consecutive pure tone stimuli with varying inter-stimulus intervals (Δ ts) were measured for 89 neurons in the cochlear nucleus of the anaesthetised guinea pig. We observed two main effects; either a decrease (suppression) or an increase (facilitation) in response to the second tone followed by an exponential recovery. Response behaviour correlated with the unit type; primary-like, primary-like with notch and transient-chopper units showed a recovery from suppression that was very similar to that already reported in the auditory nerve. For chopper units the strength of the adaptation was correlated with the units regularity of spike discharge; sustained chopper (CS) units showed less suppression than transient choppers. Onset units showed complete suppression at short Δ ts. Pause/Build (PB) units responded with increased activity to the second tone. In contrast to previous studies in the cochlear nucleus the recovery from suppression or facilitation was well described by a single exponential function, enabling us to define a recovery time constant and a maximum suppression/facilitation. There appeared to be a hierarchy in the time constant of recovery with PB and CS units showing the longest recovery times and onset units showing the shortest.

Estimating the similarity of temporal discharge patterns of auditory neurons

Behavioral data obtained from stimulus generalization experiments are often interpreted to imply how similar animal perceptions of test stimuli are to a training stimulus. An appropriate decision variable would be useful for evaluating physiological responses in the context of stimulus generalization tasks. The decision variable P(A) obtained from ROC analysis of spike count distributions is useful for evaluating rate coding schemes [e.g., Relkin and Pelli, J. Acoust. Soc. Am. 82, 1679–1691 (1987)], but it ignores temporal information which may exist in responses to periodic sounds. Neural autocorrelograms show temporal firing patterns following a spike and were generated for cochlear nucleus unit responses to aperiodic, quasiperiodic, and periodic stimuli. Variance from average rate was computed as a function of time for each autocorrelogram; similarity between two autocorrelograms was estimated as the ratio of these variances. A value of 1 indicates that the autocorrelograms are identical, whereas larger values indicate less similarity. Ratios were around 1 if both autocorrelograms were from responses to wideband noise and were greater than 1 if one autocorrelogram was from responses to iterated rippled noises or harmonic complex tones. This ratio may be a useful decision variable for evaluating temporal coding schemes. [Work supported by NIDCD P01 DC00293.]

Type III units in the gerbil dorsal cochlear nucleus may be spectral notch detectors.

Broadband sounds originating in the median plane are thought to be localized by neural processing of spectral notches introduced by the filtering action of the pinnae. Previous studies (Nelken, I., and E. D. Young. J. Neurophysiol. 71:2446-2462, 1994: Spirou, G. A., and E. D. Young. ibid. 66: 1750-1768. 1991) suggested that type IV units in decerebrate cat dorsal cochlear nucleus (DCN) are functional detectors of these spectral notches. Intracellular marking studies by Ding et al. (Ding, J., T. E. Benson, and H. F. Voigt. J. Neurophysiol. 82:3434-3457, 1999) have shown that type III units in gerbil arise from the DCN's principal output neurons, which are thought to have type IV unit properties in cat. A relative paucity of type IV units in the decerebrate gerbil (Davis. K. A., J. Ding, T. E. Benson, and H. F. Voigt. J. Neurophysiol. 75: 1411-1431, 1996) has motivated this study of spectral notch sensitivity in the gerbil DCN. Responses to notch noise stimuli were recorded from 15 gerbil type III units to investigate whether these units may function as spectral notch detectors. For narrow notch noise stimuli, all 15 units showed excitatory responses. For progressively wider notches, the discharge rate of 13/15 units became inhibited. As the maximum limits of notch width were approached, 11/15 units showed some degree of recovery from this inhibition. This response pattern in gerbil type III units possesses the salient features of notch noise responses in cat type IV units and implicates type III units in gerbilline spectral notch detection processes.

Temporal representation of iterated rippled noise as a function of delay and sound level in the ventral cochlear nucleus.

The discharge patterns of single units in the ventral cochlear nucleus (VCN) of anesthetized guinea pigs were examined in response to iterated rippled noise (IRN) as a function of the IRN delay (which determines the IRN pitch) and the IRN sound level. Delays were varied over five octaves in half-octave steps, and sound levels were varied over a 30- or 50-dB range in steps of 5 dB. Neural responses were analyzed in terms of first-order and all-order inter-spike intervals (ISIs). The IRN quasi-periodicity was preserved in the all-order ISIs for most units independent of unit type or best frequency (BF). A deterioration of the temporal all-order code was found, however, when the neural response was influenced by inhibition. The IRN quasi-periodicity was also preserved in first-order ISIs for a limited range of IRN delays and levels. Sustained Chopper units (CS) in the VCN responded with very regular ISIs when the IRN delay corresponded to the unit's chopping period; i.e., the unit showed an increased proportion of intervals corresponding to the IRN delay (interval enhancement) relative to an equal-level, white-noise stimulation. This interval enhancement has a band-pass characteristic with a peak corresponding to the chopping period. Moreover, for CS units in rate saturation, the chopping period, and thus the interval enhancement to the IRN, did not vary with level. Units classified as onset-chopper also show a band-pass interval enhancement to the IRN stimuli; however, they show more level-dependent changes than CS units. Primary-like (PL) units also show level-dependent changes in their ability to code the IRN pitch in first-order intervals. The range of delays where PL units showed interval enhancement was broader and extended to shorter delays. Based on these findings, it is suggested that CS units may play an important role in pitch processing in that they transform a higher-order interval code into a first-order interval place code. Their limited dynamic range together with the preservation of the temporal stimulus features in saturation may serve as a physiological basis for the perceived level independence of pitch.

The responses of single units in the ventral cochlear nucleus of the guinea pig to damped and ramped sinusoids

Human listeners hear an asymmetry in the perception of damped and ramped sinusoids; the partial loudness of the envelope component is greater than the partial loudness of the carrier component for damped sinusoids. Here we show that an asymmetry also occurs in the physiological responses of most units in the ventral cochlear nucleus to these same sounds. The activity elicited by damped sinusoids is mainly restricted to the beginning of each envelope period, which is not the case for ramped sinusoids. This can be quantified by computing the ratio of the tallest bin of the modulation period histogram to the total number of spikes (the peak-to-total ratio, p/ t). Damped sinusoids produce a higher p/ t than ramped sinusoids, which demonstrates physiological temporal asymmetry. It is also the case that ramped sinusoids typically elicit more spikes than damped sinusoids. The physiological asymmetry occurs where the perceptual asymmetry is present. It is maximal at modulation half-lives of 4 and 16 ms, greatly reduced at 1 ms and absent at 64 ms. Different unit types exhibit differing degrees of temporal asymmetry. Onset units produce the greatest p/ t asymmetry, primary-like units produce the least asymmetry and chopper units are in-between. With regard to total spike count, the maximal asymmetry occurs with chopper units. If primary-like units are assumed to reflect the activity in primary auditory nerve fibres, then there is enhancement of temporal asymmetry in the ventral cochlear nucleus by both onset and chopper units.

Purkinje cell degeneration and control mice: responses of single units in the dorsal cochlear nucleus and the acoustic startle response

The cartwheel cell is the most numerous inhibitory interneuron of the dorsal cochlear nucleus (DCN). It is expected to be an important determinant of DCN function. To assess the contribution of the cartwheel cell, we examined the discharge characteristics of DCN neurons and behavioral measures in the Purkinje cell degeneration (pcd) mice, which lack cartwheel cells, and compared them to those of the control mice. Distortion product otoacoustic emissions and auditory brainstem-evoked response thresholds were similar between the two groups. Extracellularly recorded DCN single units in ketamine/xylazine-anesthetized mice were classified according to post-stimulus time histogram (PSTH) and excitatory–inhibitory response area (EI-area) schemes. PSTHs recorded in mouse DCN included chopper, pauser/buildup, onset, inhibited and non-descript types. EI-areas recorded included Types I, II, III, I/III, IV and V. There were no significant differences in the proportions of various unit types between the pcd and control mice. The pcd units had slightly lower thresholds to characteristic frequency tones; however, they had spontaneous rates, thresholds to noise, and maximum driven rates to noise that were similar to those of the control units. Pcd mice had smaller startle amplitudes, but startle latency, prepulse inhibition/augmentation and facilitation by a background tone were comparable between the two groups. From these results, we conclude that DCN function in response to relatively simple acoustic stimuli is minimally affected by the absence of the cartwheel cells. Future studies employing more complex and/or multimodal stimuli should help assess the role of the cartwheel cells.

Responses of cochlear nucleus units in the chinchilla to iterated rippled noises: analysis of neural autocorrelograms.

Temporal encoding of stimulus features related to the pitch of iterated rippled noises was studied for single units in the chinchilla cochlear nucleus. Unlike other periodic complex sounds that produce pitch, iterated rippled noises have neither periodic waveforms nor highly modulated envelopes. Infinitely iterated rippled noise (IIRN) is generated when wideband noise (WBN) is delayed (tau), attenuated, and then added to (+) or subtracted from (-) the undelayed WBN through positive feedback. The pitch of IIRN[+, tau, -1 dB] is at 1/tau, whereas the pitch of IIRN[-, tau, -1 dB] is at 1/2tau. Temporal responses of cochlear nucleus units were measured using neural autocorrelograms. Synchronous responses as shown by peaks in neural autocorrelograms that occur at time lags corresponding to the IIRN tau can be observed for both primarylike and chopper unit types. Comparison of the neural autocorrelograms in response to IIRN[+, tau, -1 dB] and IIRN[-, tau, -1 dB] indicates that the temporal discharge of primarylike units reflects the stimulus waveform fine structure, whereas the temporal discharge patterns of chopper units reflect the stimulus envelope. The pitch of IIRN[+/-, tau, -1 dB] can be accounted for by the temporal discharge patterns of primarylike units but not by the temporal discharge of chopper units. To quantify the temporal responses, the height of the peak in the neural autocorrelogram at a given time lag was measured as normalized rate. Although it is well documented that chopper units give larger synchronous responses than primarylike units to the fundamental frequency of periodic complex stimuli, the largest normalized rates in response to IIRN[+, tau, -1 dB] were obtained for primarylike units, not chopper units. The results suggest that if temporal encoding is important in pitch processing, then primarylike units are likely to be an important cochlear nucleus subsystem that carries the pitch-related information to higher auditory centers.

Intrinsic oscillations and discharge regularity of units in the dorsal cochlear nucleus (DCN) of the barbiturate anesthetized gerbil.

Spike discharge patterns showing intrinsic oscillations (IOs) have been reported in units in the dorsal cochlear nucleus (DCN) of the decerebrate cat. These oscillations are related to the regularity of a unit's discharge rate and may be important for pitch perception. A DCN unit's regularity can be affected by several factors including: synaptic architecture, cell membrane properties, and the auditory nerve discharge rate. Responses to multiple presentations of short-duration tone bursts (200 ms duration, 1 s trial) at the unit's best frequency (BF) at 20 dB re threshold were recorded from 297 units in the DCN of the barbiturate-anesthetized gerbil. Comparisons of unit regularity properties and IO properties are shown. The relative power spectrum (Fourier transform of the autocorrelogram normalized by the average rate) was used to quantify IO properties. Most units (84%) exhibited IOs in their sustained discharge rate. With the exception of Onset units and most bursting units, the mean inter-spike interval was correlated with the IO frequency and the coefficient of variation was correlated with the IO magnitude. These results suggest that stimulus-encoding mechanisms utilizing IOs may depend on the temporal evolution of the units' regularity properties.