Biomedical subjects

T C Yin

Publications and source records attributed to T C Yin.

At least 19 recordsLinked to original sources

PubMed1995

Envelope coding in the lateral superior olive. I. Sensitivity to interaural time differences.

1. Interaural level differences (ILDs), created by the head and pinna, have long been known to be the dominant acoustic cue for azimuthal localization of high-frequency tones. However, psychophysical experiments have demonstrated that human subjects can also lateralize complex high-frequency sounds on the basis of interaural time differences (ITDs) of the signal envelope. The lateral superior olive (LSO) is one of two pairs of binaural nuclei where the primary extraction of binaural cues for sound source location occurs. "IE" cells in LSO are inhibited by stimuli to the contralateral and excited by stimuli to the ipsilateral ear, and their response rate is therefore dependent on ILD. Anatomic specializations in the afferent pathways to the LSO suggest that this circuit also has a function in the detection of timing cues. We hypothesized that, besides ILD sensitivity, the IE property also conveys a sensitivity to ITDs of amplitude-modulated (AM) tones and could provide the physiological substrate for the psychophysical effect mentioned above. 2. In extracellular recordings from binaural LSO cells in barbiturate-anesthetized cats, response rate was a periodic function of ITDs of AM stimuli, i.e., all cells displayed ITD sensitivity. Binaural responses were smaller than responses to stimulation of the ipsilateral ear alone and were minimal when the envelopes in both ears were in-phase or nearly so. There was good correspondence between responses to ITDs and to dynamic interaural phase differences (IPDs), created by a difference in the envelope frequency to the two ears. Qualitatively, the responses were consistent with the outcome of an IE operation on temporally structured inputs. 3. To compare the relative importance of ILD and ITD, responses to combinations of the two cues were obtained. Despite robust ITD sensitivity in all binaural LSO cells encountered, the changes in response rate that would occur in response to naturally occurring ITDs were small in comparison with the changes expected for naturally occurring ILDs. The main limitation on ITD sensitivity was a steep decline in average discharge rate as the modulation frequency exceeded several hundred Hertz. 4. ITD sensitivity was also present to broadband stimuli, again with minimal rates occurring near 0 ITD. The sensitivity depended in a predictable fashion on the passband of filtered noise and was absent to binaurally uncorrelated noise bands. In response to clicks, ILDs interacted with ITD in a complicated fashion involving amplitude and latency effects. 5. Three low-characteristic frequency (CF) LSO cells were encountered that were IE and showed ITD sensitivity to the fine structure of low-frequency stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1994

Enhancement of neural synchronization in the anteroventral cochlear nucleus. I. Responses to tones at the characteristic frequency.

1. Encoding temporal features of the acoustic waveform is an important attribute of the auditory system. Auditory nerve (AN) fibers synchronize or phase-lock to low-frequency tones and transmit this temporal information to cells in the anteroventral cochlear nucleus (AVCN). Phase-locking in the AVCN is usually reported to be similar to or weaker than in the AN. We studied phase-locking in axons of the trapezoid body (TB), which is the output tract of the AVCN, and found, to our surprise, that most TB axons exhibited enhanced synchronization compared with AN fibers. 2. Responses from axons in the TB of the cat were obtained with horseradish peroxidase (HRP)- or Neurobiotin-filled micropipettes or metal microelectrodes. A series of short tone bursts at increasing sound pressure level (SPL) was presented at the characteristic frequency (CF) of the fiber and phase-locking was quantified with the vector strength R at each SPL. For each fiber the maximum R value (Rmax) was then determined. 3. Low-frequency fibers in the TB showed very precise phase-locking: Rmax values could approach 0.99. For the majority of fibers (33/44, 75%) with CF < 700 Hz, Rmax was > or = 0.9 and therefore higher than is ever observed in the AN. We define such fibers as "high-sync." Most of these fibers also entrained to the stimulus, i.e., they fired a precisely timed action potential to almost every stimulus cycle. Some fibers showed perfect entrainment, with maximum discharge rates equaling the stimulus frequency. 4. To exclude the possibility that stimulus paradigms or acoustic and recording equipment were the source of this enhancement, we obtained additional data on low-frequency AN fibers using the same experimental protocol as in our TB experiments. These AN data agree well with published reports. 5. The morphological class of some of the cells studied was identified on the basis of anatomic features revealed by intra-axonal injection of HRP or Neurobiotin. Labeled low-CF axons (N = 7), which were all high-sync, originated from AVCN bushy cells: five were globular and two were spherical bushy cell axons. 6. Spontaneous rate of high-sync fibers covered a range from 0 to 176 spikes/s but were biased toward low values (mean 16 spikes/s). Responses to broadband clicks and sinusoidally amplitude-modulated signals provided additional evidence of improved timing properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1994

Enhancement of neural synchronization in the anteroventral cochlear nucleus. II. Responses in the tuning curve tail.

1. Discharges of neurons in the peripheral auditory system contain information about the temporal features of acoustic stimuli. Phase-locking of neurons in the anteroventral cochlear nucleus (AVCN) is usually reported to be less robust than in auditory nerve (AN) fibers, which provide their major input. In a companion paper we reported that some cells in AVCN of the cat show enhanced phase-locking compared with the AN when stimulated at the frequency to which they are most sensitive [characteristic frequency (CF)]. We called neurons "high-sync" when they showed vector strengths (R, a measure of phase-locking) > or = 0.9. Here we report phase-locking properties to stimuli at frequencies below CF. 2. Horseradish peroxidase-filled glass micropipettes or metal microelectrodes were inserted into the trapezoid body (TB), which is the large output tract of the AVCN. Acoustically driven fibers were classified on the basis of the shape of the poststimulus time (PST) histograms to short tone bursts at CF. We then presented low-frequency tones of increasing SPL and determined the maximum R value at 500 Hz (R500) for each fiber. Using the same experimental protocol we studied phase-locking in the ANs of two animals because maximal R values at the tuning curve tail have not been reported for AN fibers. 3. Although phase-locking in AN fibers is usually assumed to be independent of CF, we found that fibers with CF > 2 kHz tended to have higher R500 values than fibers with CF < or = 2 kHz. Moreover, R500 was > or = 0.9 in 20% (42 of 196) of the fibers studied and could be as high as 0.95. This population of fibers was defined as having "high-sync tails" and consisted almost entirely of fibers with low or medium spontaneous rate. 4. High-CF TB fibers stimulated at 500 Hz showed very high phase-locking. High-sync tails (R500 > or = 0.9) were found in 41 of 70 TB fibers. For a subset of these fibers (1/3 in total: 23 of 70) phase-locking was higher than is ever observed in the AN (R500 > or = 0.95); these fibers were defined as showing synchronization "enhancement." Virtually all fibers showing synchronization enhancement had primary-like-with-notch (PLN) PST histograms. Chopper and primary-like fibers showed high-sync tails for CFs > 3 kHz. 5. Synchronization filter functions were obtained for high-CF AN fibers by determining maximum synchronization for a range of stimuli below CF.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1994

Physiological correlates of the precedence effect and summing localization in the inferior colliculus of the cat.

The precedence effect (PE) describes an illusion produced when two similar sounds are delivered in quick succession (interclick delays of 2-8 msec) from sound sources at different locations so that only a single sound is perceived. The localization of the perceived sound is dominated by the location of the leading sound. If the delays are very short (< 1-2 msec), summing localization occurs and a phantom source is perceived whose location is toward the leading sound. The purpose of these experiments was to look for physiological correlates of the precedence effect and summing localization by recording from single neurons in the inferior colliculus of the anesthetized cat. Click stimuli were delivered under two different situations: over headphones in dichotic experiments and through two speakers in an anechoic room in free-field studies. In the latter case the cat was placed midway between the speakers and a single click stimulus was delivered to each speaker with variable interclick delays (ICDs). Most cells, under both dichotic and free-field conditions, exhibited a form of the precedence effect in which the response to the lagging click was suppressed when ICDs were short. The suppression of the lagging click, or echo, was measured by recovery curves, which plotted the response of the lagging click as a function of ICD. There was considerable variability in the recovery curves from different cells: the ICDs at which the recovery reached 50%, which is a measure of the echo threshold for the cell, ranged from 1 to 100 msec with a median of 20 msec. Human psychophysical experiments report echo thresholds for clicks ranging from 2 to 8 msec. If we assume that absolute echo threshold is determined by the cells with shortest recovery curves, then the thresholds for single cells are in accord with the psychophysical results. The possible sites of generation of the echo suppression are also considered. Changes in the relative level of the leading and lagging clicks produced the expected shifts in the recovery curves. With short ICDs in the summing localization range (between about +/- 2 msec) cells also showed responses consonant with the human psychophysical result that the sound source is localized to a phantom image between the two speakers and toward the leading one. The location of the image varied systematically with the relative levels or ICDs of the clicks.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗

PubMed1993

Projections of physiologically characterized spherical bushy cell axons from the cochlear nucleus of the cat: evidence for delay lines to the medial superior olive.

Bushy cells in the anteroventral cochlear nucleus (AVCN) receive their principal excitatory input from the auditory nerve and are the primary source of excitatory input to more centrally located brainstem auditory nuclei. Despite this pivotal position in the auditory pathway, details of the basic physiological information being carried by axons of these cells and their projections to more central auditory nuclei have not been fully explored. In an attempt to clarify these details, we have physiologically characterized and anatomically labeled individual axons of the spherical bushy cell (SBC) class of the cat AVCN. The characteristic frequencies (CFs) of our injected SBC population are low, all less than 12 kHz and primarily (83%) less than 3 kHz, while their spontaneous activity is comparatively high (mean of 59 spikes/sec). In response to short tone bursts at CF, low CF (< 1 kHz) SBC units can phase-lock better than auditory nerve fibers. SBCs with CFs above 1 kHz have primary-like responses at all stimulus levels and can show robust phase-locking to an off-CF, 500 Hz tone. When compared with our previously reported population of labeled globular bushy cells (GBC; Smith et al., 1991, J. Comp. Neurol. 304:387-407), some similarities and differences are apparent in both physiological response properties and axonal projection pattern. GBCs show no low frequency bias in CFs, have lower spontaneous rates, and the high CF units exhibit a primary-like-with-notch response at high stimulus levels as a consequence of a very well timed onset component. Low CF, GBC short tone responses are indistinguishable from those of SBCs. Anatomically, the axons of SBCs cross the midline in the dorsal component of the trapezoid body and typically innervate the medial superior olive (MSO) on both sides, the ipsilateral lateral superior olive (LSO), and the contralateral ventral nucleus of the lateral lemniscus (VNLL). The projections to the contralateral, but not the ipsilateral MSO, show a rostral to caudal delay line configuration, similar to the scheme first proposed by Jeffress (1948, J. Comp. Psychol. 41:35-39). The form of this delay line is consistent with the topographic map of interaural time delays reported by Yin and Chan (1990, J. Neurophysiol. 64:465-488). Projections to the ipsilateral LSO often take an indirect route. In contrast, GBC axons travel in the ventral component of the trapezoid body, never innervate the MSO, rarely innervate the ipsilateral LSO, and always innervate the contralateral medial nucleus of the trapezoid body. The terminal specializations of both SBC and GBC axons contain round vesicles.

Animals↗

PubMed1992

Visual response properties of neurons in the middle and lateral suprasylvian cortices of the behaving cat.

The visual response properties of cells in the middle (MS) and lateral (LS) suprasylvian cortices were studied in alert cats, which were trained to fixate a spot of light and maintain fixation when a second test light was introduced in the midst of fixation. This second light served to test for visual sensitivity, and it could be moved at different speeds in any direction under computer control. Over half of the cells exhibited a visual response. With a small spot of light, most cells were directionally selective and responded better to a moving spot than to a stationary one. In some cases movements of the spot in the non-preferred direction revealed an inhibitory process. The visual receptive fields were large and often extended into the ipsilateral hemifield, though the centers of the receptive fields were usually in the contralateral field. We used Fourier analysis to quantify directional selectivity and compared these results to other commonly used measures of directional selectivity. Compared to cells in MS, there was a higher incidence of visual cells in LS and the visual cells were more directional. We also made comparisons between our results and those found in anesthetized cats and awake monkeys.

Anesthesia, General↗

PubMed1992

Visuomotor interactions in responses of neurons in the middle and lateral suprasylvian cortices of the behaving cat.

We studied visuomotor processing in the middle (MS) and lateral suprasylvian (LS) cortices of the alert cat by making single cell recordings while the cat was working in a behavioral task requiring visual fixation and visually guided eye movements. We found responses with three different components: visual sensory, saccade-related motor, and fixation. Some cells exhibited purely visual responses and all of their activity during visuomotor tasks could be attributed to the sensory aspects of the task. Other cells showed no sensory response properties, but discharged in relation to the saccadic eye movements that the cat made to visual targets. A smaller number of fixation cells displayed increased discharge when the cat fixated a target light and usually only when that target was in a particular region of the visual field. These response components could be present in a variety of combinations in different cells, of which the largest proportion combined visuomotor responses and could take five general forms: simple visuomotor, saccadic enhanced, visually triggered movement (VTM), enhanced VTM, and disenhanced. Simple visuomotor responses had both a visual and saccade-related component. Saccadic enhanced responses had a visual response to the appearance of a spot in the cell's receptive field that became enhanced when the cat subsequently made a saccade to that spot. The VTM responses were synchronized better to the visual stimulus than to the saccade, but they also exhibited properties expected of motor responses. The last two classes of visuomotor responses were rare: one we termed enhanced VTM and the other disenhanced. Cells could combine different visuomotor response components or even sensory, saccade-related and fixation responses in different combinations for different directions of eye movements. Generally, the timing of the saccade-related responses occurred too late to play a role in the initiation of saccades: most (83%) saccade-related responses occurred between 40 ms before to 80 ms after the onset of the eye movement. Cells of all different types could be found in both the MS and LS areas, though in general the responses in LS were more sensory in nature while those in MS were more closely related to the eye movement. About a quarter of the cells were unresponsive during any aspect of our tasks.

Animals↗

PubMed1992

Responses to amplitude-modulated tones in the auditory nerve of the cat.

Sinusoidally amplitude-modulated (AM) tones are frequently used in psychophysical and physiological studies, yet a comprehensive study on the coding of AM tones in the auditory nerve is lacking. AM responses of single auditory-nerve fibers of the cat are studied, systematically varying modulation depth, frequency, and sound level. Synchrony-level functions were nonmonotonic with maximum values that were inversely correlated with spontaneous rate (SR). In most fibers, envelope phase-locking showed a positive gain. Modulation transfer functions were uniformly low pass. Their corner frequency increased with characteristic frequency (CF), but changed little for CFs above 10 kHz. The highest modulation frequencies to which phase locking occurred were more than 0.8 oct lower than the highest frequencies to which phase locking to pure tones occurs. Cumulative, or unwrapped, phase increased linearly with modulation frequency: The slope was inversely related to CF, and slightly higher than group delays reported for pure tones. High SR, low CF fibers showed the poorest envelope phase locking. In some low CF fibers, phase locking increased at high levels, associated with "peak-splitting" phenomena. Changes in average rate due to modulation were small, and could be enhancement or suppression.

Acoustics↗

PubMed1991

Projections of physiologically characterized globular bushy cell axons from the cochlear nucleus of the cat.

We made intraaxonal recordings from 30 individual globular bushy cell axons in the trapezoid body of the cat using HRP-filled glass microelectrodes. With subsequent HRP injection, we determined their axonal projection patterns. For cells with characteristic frequencies (CFs) above 3 kHz, short-tone peristimulus time histograms (PSTHs) at CF were typically primarylike at low tone intensities and primarylike with notch (PLN) or onset with low sustained activity (OL) at higher stimulus levels. Cells with CFs between 1 and 3 kHz showed the same response features with the spikes in the sustained region of the response phase-locked to the stimulus tone. Cells with CFs below 1 kHz showed phase-locked PSTHs with exceptionally high levels of synchrony compared to eighth nerve fibers with comparable CFs. This exceptional phase-locking was also noted when cells with CFs of 1-3 kHz were presented with tones below 1 kHz. Although the globular bushy cell axons were not completely filled from the soma of origin to terminal fields in the contralateral brainstem, a number of consistent anatomical features were distinguished in the population. All but one of the myelinated axons crossed the midline in the middle, large fiber component of the trapezoid body. Ipsilaterally, the axon always gave off from one to four collateral branches whose major targets were the posterior periolivary nucleus (PPO) and the lateral nucleus of the trapezoid body (LNTB). Minor termination sites for ipsilateral collateral branches were the dorsolateral periolivary nucleus (DLPO) and the lateral superior olive (LSO). Contralaterally the axon gave rise to one or two calyces of Held in the medial nucleus of the trapezoid body (MNTB). Three other major collateral branches arose from the contralateral axon and innervated a consistent set of areas. One headed caudally to innervate an area just ventromedial to the facial nucleus. Another followed the sixth nerve dorsally to innervate the dorsomedial periolivary nucleus (DMPO). A third collateral headed rostrally toward the ventral nucleus of the lateral lemniscus (VNLL), giving off occasional small sidebranches. Although each injected axon gave rise to a collateral that innervated the MNTB, it did not necessarily give rise to all three of the other collateral branches.

Action Potentials↗

PubMed1991

Dendritic and axonal morphology of HRP-injected neurons in the inferior colliculus of the cat.

The dendritic and axonal morphology of neurons in the inferior colliculus of the cat was investigated after intracellular injection of HRP, in vivo. All injected axons gave off local collaterals, and most showed a widespread distribution and lacked a specific orientation. In contrast, the dendrites of injected neurons were distinguished by their degree of orientation and the direction of the longest axis of orientation. Dendrites showed a high, moderate, or low degree of orientation. Most highly oriented cells had their longest axis in the rostrocaudal direction with fewer in the mediolateral direction. In the central nucleus, only the rostrocaudally oriented cells correspond to the disc-shaped cells identified in Golgi preparations. Unlike most cells in our sample, the two cells that were disc-shaped had axons that were parallel to the orientation of the dendritic tree. In the dorsal cortex, rostrocaudally oriented cells also were found, but they had unoriented axons. In both the central nucleus and dorsal cortex, cells with a mediolateral axis of orientation or no specific orientation correspond to stellate cells and had axons with widespread local collaterals. These results suggest that an extensive network of local axon collaterals may contribute to neural processing within the inferior colliculus. In the central nucleus, local axons may establish connections within or across the fibrodendritic laminae. In the dorsal cortex, the local and afferent axons may form a complex reticular network. Finally, some injected cells had axons terminating locally and also entering the brachium of the inferior colliculus. This suggests that cells in the inferior colliculus may function as both interneurons and projection neurons.

Animals↗

PubMed1990

Interaural time sensitivity in the inferior colliculus of the albino cat.

Anatomical studies of the Creel albino cat have demonstrated a pronounced atrophy of cells in the medial superior olive, a structure thought to be important for the detection of interaural time differences (ITDs). We looked for physiological abnormalities in the binaural interaction of cells in three albino cats by recording from single cells in the central nucleus of the inferior colliculus to ITDs of tones and noise. We found that the sensitivity to ITDs of tones and noise was somewhat diminished in the albino cats as compared to normally pigmented cats, though this deficit was only evident when a population of cells was examined. The range of sensitivity of individual cells for both tones and noise was the same in albinos and pigmented animals. Our anatomical measurements showed a smaller reduction in cross-sectional area of cells in the medial superior olive than that reported earlier, and the cell bodies in the medial superior olive of the albinos were less elongated than in normal cats.

Albinism↗

PubMed1990

Interaural time sensitivity in medial superior olive of cat.

1. We studied the sensitivity of cells in the medial superior olive (MSO) of the anesthetized cat to variations in interaural phase differences (IPDs) of low-frequency tones and in interaural time differences (ITDs) of tones and broad-band noise signals. Our sample consisted of 39 cells histologically localized to the MSO. 2. All but one of the cells had characteristic frequencies less than 3 kHz, and 79% were sensitive to ITDs and IPDs. More than one-half (56%) of the cells responded to monaural stimulation of either ear, and both the binaural and monaural responses were highly phase locked. All of the cells that were sensitive to IPDs and monaurally driven by either ear responded in accord with that predicted by the coincidence model of Jeffress, as judged by comparisons of the phases at which the monaural and binaural responses occurred. The optimal IPDs were tightly clustered between 0.0 and 0.2 cycles. Most cells exhibited facilitation of the response at favorable ITDs and inhibition at unfavorable ITDs compared with the monaural responses. 3. Cells in the MSO exhibited characteristic delay, as judged by a linear relationship between the mean interaural phase and stimulating frequency. Characteristic phases were clustered near 0 indicating the most cells responded maximally when the two input tones were in phase. With the use of the binaural beat stimulus we found no differential selectivity for either the direction or speed of interaural phase changes. 4. The cells were also sensitive to ITDs of broad-band noise signals. The ITD curve in response to broad-band noise was similar to that predicted by the composite curve, which was calculated by linearly summating the tonal responses over the frequencies in the response area of the cell. Most (93%) of the peaks of the composite curves were between 0 and +400 microseconds, corresponding to locations in the contralateral sound field. Moreover, computer cross correlations of the monaural spike trains were similar to the ITD curve generated binaurally for both correlated and uncorrelated noise signals to the two ears. Thus our data suggest that the cells in the MSO behave much like cross-correlators. 5. By combining data from different animals and lcoating each cell on a standard MSO, we found evidence for a spatial map of ITDs across the anterior-posterior (A-P) axis of the MSO.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1989

Responses of low-frequency cells in the inferior colliculus to interaural time differences of clicks: excitatory and inhibitory components.

1. We studied extracellular responses of low-frequency cells in the central nucleus of the inferior colliculus (ICC) to interaural time differences (ITDs) of clicks and compared their responses to ITDs of noise and tones. Most cells that displayed sensitivity to ITDs of clicks responded cyclically as a function of ITD with central peaks and troughs at the same ITDs as in response to noise. The positions of these peaks and troughs also matched those predicted from tonal ITD curves. Thus over the range of physiologically relevant ITDs, the binaural cells in the ICC showed similar sensitivity to ITDs of tones, noise, and clicks. 2. The transient nature of the response to a click allowed association of individual discharges with either the ipsilateral or contralateral stimulus when the binaural stimulus included a large ITD. We studied the influence of the click presented to one side on responses to the click presented to the other side. By examining responses to clicks with large ITDs, ranging from 2 to 3 up to 200 ms, we could identify both excitatory and inhibitory components in response to binaural clicks. 3. For many cells, there was evidence for a short-lasting excitation arising from one or both inputs of the binaural stimulus. Inhibitory interactions could also be demonstrated over a large range of ITDs. Long-lasting, late inhibitory components arose from both contralateral and ipsilateral inputs. In 87% of cells that were driven by the contralateral input, a late inhibitory component originating from the ipsilateral side was detected. In all cells that were driven by the ipsilateral side, a late inhibitory contralateral component was detected. This late inhibition of the excitatory response to one side by a leading stimulus to the other side could be evoked even when the leading stimulus was not effective in evoking an excitatory response. 4. Some cells also exhibited an early inhibitory component that preceded the excitation. An early contralateral inhibition was detected in 44% of cells that were driven by the ipsilateral input, whereas an early ipsilateral component was detected in 17% of cells driven by the contralateral input. 5. We confirmed hypotheses about the laterality and time course of the inhibitory and excitatory components by introducing interaural level differences (ILDs) into the binaural clicks and thus varying the strengths of the different components. 6. Inhibitory components may play a role in shaping the sensitivity of individual cells to ITDs of stimuli other than clicks; they were also apparent in responses to noise.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1988

Temporal coding of resonances by low-frequency auditory nerve fibers: single-fiber responses and a population model.

1. We recorded responses of low-frequency auditory nerve fibers (characteristic frequency (CF) less than 3 kHz) in the cat to resonant stimuli with varied natural frequencies, damping coefficients, and sound pressure levels. Responses to resonances were synchronized to frequencies lying between the peak frequency of the stimulus spectrum and a frequency near the fiber's CF. The frequency of the dominant synchrony in the response varied systematically as a function of the stimulus parameters. 2. More lightly damped resonances, which have sharp spectral peaks, elicited synchrony closer to the peak frequency, whereas the broader peaks of more highly damped resonances elicited synchrony closer to the fiber's CF. Thus as the stimulus was varied from an undamped tone to a highly damped transient, the dominant component of the synchronized response moved from the peak frequency of the stimulus toward the CF of the fiber. The trajectory of the dominant component varied as a function of stimulus level, with higher levels resulting in synchrony biased toward the peak of the stimulus spectrum over a wider range of damping. 3. The frequency tuning and synchronization characteristics of a fiber, along with the stimulus parameters, determined the temporal properties of its response to complex stimuli. Using reverse correlation (revcor) filters to characterize the tuning and synchronization of auditory nerve fibers, we were able to predict the temporal properties of responses to resonant stimuli. 4. A parametric model was fit to measured revcor functions derived from responses of auditory nerve fibers to wideband noise. In this way, a bank of model revcor filters was developed based on our population of measured filters. 5. The filter bank was used to model the response of a population of auditory nerve fibers to resonances. Temporal patterns present in the response of a population of fibers encoded the parameters of resonant stimuli. 6. The model revcor filter bank provided a means of studying temporal response patterns of the population of fibers to other complex sounds. 7. The output of the population model is a representation of the temporal information provided by the auditory periphery to the central nervous system; thus it provides a potentially useful tool for testing hypotheses concerning the processing of temporal information by the central auditory system.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1987

Effects of interaural time delays of noise stimuli on low-frequency cells in the cat's inferior colliculus. II. Responses to band-pass filtered noises.

1. We studied cells in the central nucleus of the inferior colliculus of the cat that were sensitive to interaural time delays (ITDs) in order to evaluate the influence of the stimulus spectrum of noise signals. Stimuli were sharply filtered low-, high-, and band-pass noise signals whose cutoff frequencies and bandwidths were systematically varied. The responses to ITDs of these noise signals were compared with responses obtained to ITDs of broadband noise and pure tones. 2. The discharge rate in response to band-pass noise as a function of ITD was usually a cyclic function with decreasing peak amplitudes at longer ITDs. The reciprocal of the mean interval between adjacent peaks indicated how rapidly the response rate varied with ITD and was termed the response frequency (RF). This RF was approximately equal to the median frequency of the stimulus spectrum filtered by the cell's sync-rate curve, which was the product of the synchronization to interaural phase and the discharge rate plotted against frequency. This suggests that the RF was determined by all the spectral components in the stimulus that fell within the frequency range in which the cell's response was synchronized. The contribution of each component was proportional to the sync-rate for that frequency. 3. The central peak of the ITD function usually fell within the physiological range of ITDs (+/- 400 microseconds). The location of this peak did not vary significantly with changes in stimulus spectrum by comparison with responses to tones of different frequency. Its shape also remained constant, except for a decrease in width when high-frequency components within the range of the sync-rate curve were added to the stimulus. A few cells responded with a minimal discharge instead of a maximal near-zero ITD, and this central minimum had similar properties as the central peak. The amplitude of the secondary peaks of the ITD function decreased as the stimulus bandwidth that overlapped the sync-rate curve broadened. 4. The sum of the ITD functions to two band-pass signals was similar to that of a broadband signal whose spectrum was composed of the sum of the band-pass spectra. 5. From these binaural responses we could make inferences about the response characteristics of the monaural inputs to binaural neurons. We then verified these predictions by studying responses of low-frequency trapezoid body fibers to band-pass noises.

Acoustic Stimulation↗

PubMed1987

Effects of interaural time delays of noise stimuli on low-frequency cells in the cat's inferior colliculus. III. Evidence for cross-correlation.

1. We tested the coincidence, or cross-correlation, model of Jeffress, which proposes a neuronal mechanism for sensitivity to interaural time differences (ITDs) in low-frequency cells in the central nucleus of the inferior colliculus (ICC) of the cat. Different tokens of Gaussian noise stimuli were delivered to the two ears. We studied the neural responses to changes in ITDs of these stimuli and examined the manner in which the binaural cells responded to them. All of our results support the idea that the central binaural neurons perform an operation very similar to cross-correlation on the inputs arriving from each side. These inputs are transformed from the actual acoustic signal by the peripheral auditory system, and these transformations are reflected in the properties of the cross-correlations. 2. The responses to ITDs of identical broadband noise stimuli to the two ears varies cyclically as a function of ITD at a frequency close to the best frequency of the neuron. This cyclic response is a consequence of the narrowband filtering of the wideband acoustic signal by the auditory nerve fibers. To examine the effects of using stimuli to the two ears that were correlated to each other to different degrees, we generated pairs of noises. Each pair consisted of one standard noise, which was delivered to one ear, and a linear sum of two standard uncorrelated noises, which was delivered to the other ear. The responses of 34 neurons in the ICC to ITDs of noises with variable interaural coherence were examined. When partially correlated noises were delivered, there was a positive and approximately linear relationship between the degree of modulation of the response as a function of ITD and interaural coherence. The degree of modulation was measured by the synchronization coefficient, or vector strength, over one period of the ITD curve. 3. We examined the effects of altering the interaural phase relationships of the input noise stimuli. The phase of the noise stimuli was changed by digitally filtering the standard noise so that only a phase delay was imposed. The responses to ITDs with differing interaural phase relationships were then studied by delivering a phase-shifted noise to one ear and the standard noise to the other. The ITD curves in response to phase-shifted noise were shifted by about the same amount as the shift of the stimulus; the shift of the response was measured with respect to the case with identical noises to the two ears.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1986

Effects of interaural time delays of noise stimuli on low-frequency cells in the cat's inferior colliculus. I. Responses to wideband noise.

We examined the responses of low-frequency neurons in the central nucleus of the inferior colliculus (ICC) of the cat to interaurally delayed, wideband noise stimuli. The stimuli were pseudorandom noise signals that were generated digitally with a nominal bandwidth of 60-4,000 Hz. We also compared the responses to noise with those obtained from interaural phase differences of pure tones. We studied 144 neurons with characteristic frequencies below 2.5 kHz. Eighty-five percent (85%) of these were sensitive to changes in both interaural time differences (ITDs) of noise and interaural phase differences of pure tones, only 2% were sensitive to one stimulus but not the other, and the remainder were insensitive to both stimuli. For most cells the discharge rate was modulated in an approximately cyclic fashion by changes in ITDs of the wideband noise stimuli. The maximal spike counts often occurred near zero ITD, and there was considerable variability in the nature of the cycling, though it usually disappeared for ITDs greater than +/- 4,000 microseconds. The position of the central peak was usually (65%) within the physiologically relevant range of +/- 400 microseconds, and most (80%) occurred at positive ITDs, which corresponded to delays to the ipsilateral stimulus. In general, the shapes of the responses were not affected by changes in stimulus level above threshold. As long as identical noises were delivered to both ears, the responses were not sensitive to the particular noise stimulus used. When uncorrelated noises were delivered to the two ears, there was no sensitivity to ITDs. Composite curves were computed by linear summation of the responses to ITDs of pure tones at frequencies spaced at equal intervals throughout each cell's response area. The shapes of composite curves were similar to the responses of the same cell to ITDs of wideband noise stimuli. The positions of the central peaks of these two functions were highly correlated (r = 0.91, slope = 0.97). The values of characteristic delay and characteristic phase computed from the tonal responses were found to be good indicators of the shapes of the noise delay curves. Characteristic phases (CPs) near zero were associated with noise delay curves symmetric about the central peak, CPs near 0.5 cycles with those symmetric about the trough, while CPs between 0 and 0.5 or between 0.5 and 1.0 had noise delay curves that were asymmetric with a prominent trough to the left or right, respectively, of the central peak.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1985

Responses of neurons in the cat's superior colliculus to acoustic stimuli. I. Monaural and binaural response properties.

Using extracellular electrodes we studied acoustic responses in the superior colliculus (SC) of the barbiturate-anesthetized cat. Pure tonal stimuli were delivered through sealed and calibrated earphones and were presented either monaurally or binaurally with interaural intensity differences (IIDs) and interaural time differences (ITDs). Acoustically sensitive cells were found in the intermediate and deep layers of the SC throughout its rostrocaudal and mediolateral extent. Most cells (80%) discharged only at stimulus onset; the rest had more complex firing patterns. For 88% of our sample the mean first-spike latency measured at 20 dB above threshold ranged between 6 and 16 ms. The sharpness and threshold intensity of the frequency tuning curves varied widely. In the SC, the average characteristic frequency and threshold intensity were higher than in other auditory brain stem nuclei. Neurons whose characteristic frequency was low were never sharply tuned. The probability of response decreased when the repetition rate at which the stimuli were delivered increased. The mean stimulus interval at which spike count reached 50% of maximum was 360 ms. Most (83%) of the cells discharged only to monaural stimulation of the contralateral ear, 7% responded to tones applied to either ear and only 1% to only ipsilateral input. The remaining cells responded only to stimulation of both ears. With binaural stimuli, most neurons (80%) could be shown to receive input from both ears. Seventy percent of the binaural cells showed predominant binaural inhibition (BI), 25% binaural facilitation (BF), and 5% a more complex mixture. Because the majority of SC neurons had high characteristic frequencies, we examined their responses to IIDs. The spike count vs. IID functions of BI cells were monotonic and sigmoidal, those of BF cells were nonmonotonic and bell-shaped. The slopes and horizontal positions of the curves varied among neurons. IIDs favoring the contralateral ear were the most effective. For a given cell, increasing the mean binaural level extended the range of IIDs that evoked maximal discharge. A small number of cells was sensitive to physiologically significant interaural time differences of low-frequency tones or the envelopes of amplitude-modulated, high-frequency tones.

Animals↗