Rate-level functions of neurons in the inferior colliculus of cats measured with the use of free-field sound stimuli

1991 ◽  
Vol 65 (2) ◽  
pp. 383-392 ◽  
Author(s):  
L. Aitkin
Keyword(s):  
Firing Rate ◽  
Inferior Colliculus ◽  
Dynamic Range ◽  
Discharge Rate ◽  
Free Field ◽  
Peak Level ◽  
Stimulus Level ◽  
Sound Level ◽  
Acoustic Stimuli ◽  
Rate Level

1. The responses as a function of stimulus level of 125 single units in the inferior colliculus of anesthetized cats were studied with the use of free-field acoustic stimuli. 2. The characteristic frequency (CF; frequency at which threshold was lowest) of each unit was determined, and stimuli were presented from one of three speaker positions: 45 degrees contralateral to the midline, midline, and 45 degrees ipsilateral to the midline. 3. For each unit a variety of stimulus levels was presented at CF, and the total spike count was summed for 20 stimuli at each level. If time permitted, a similar series of levels of noise was presented. 4. Four classes of rate-level (RL) functions were observed. Monotonic increases in firing rate were observed in 10% of units stimulated with CF stimuli and 57% of units studied with noise. Nonmonotonic RL functions, for which firing first increased and then declined to less than 50% of the peak level, were observed in 61% of units responding to CF tones and in 10% responding to noise. Plateau functions, with shapes lying between these, accounted for 19% of CF responses and the remaining units excited by noise. Some very complex shapes that could not be categorized into the above groups were seen in the remaining 10% of the units responding to CF stimuli. 5. The RL functions of units studied with both noise and CF tones could belong to different classes; commonly, nonmonotonic RL functions to tones were associated with monotonic RL functions to noise. The noise thresholds averaged 10 dB, some 10-20 dB less sensitive than those to CF stimuli. 6. For the vast majority of both noise and tone responses, stimuli from the contralateral location were more effective than those from the other two positions in terms of a lower threshold, higher peak discharge rate, and, for nonmonotonic units, a lower sound level at which the function became nonmonotonic (turnover point). 7. The turnover points of nonmonotonic functions at any given CF could be spread broadly but, overall, tended to be concentrated between -6 and 44 dB. 8. The dynamic ranges (range of levels over which firing rate increased) were larger for monotonic and plateau functions than for nonmonotonic functions, which had dynamic ranges less than 45 dB. The median dynamic range for units stimulated with CF tones was 20 dB and for noise stimuli, 40 dB.(ABSTRACT TRUNCATED AT 400 WORDS)

Duration Selectivity of Neurons in the Inferior Colliculus of the Big Brown Bat: Tolerance to Changes in Sound Level

2005 ◽  
Vol 94 (3) ◽  
pp. 1869-1878 ◽  
Author(s):  
Thane Fremouw ◽  
Paul A. Faure ◽  
John H. Casseday ◽  
Ellen Covey
Keyword(s):  
Inferior Colliculus ◽  
Stimulus Duration ◽  
Dynamic Range ◽  
Signal Amplitude ◽  
Limited Range ◽  
Stimulus Level ◽  
Sound Level ◽  
Broad Dynamic Range ◽  
Big Brown Bat ◽  
Spike Latency

At and above the level of the inferior colliculus (IC), some neurons respond maximally to a limited range of sound durations, with little or no excitatory response to durations outside of this range. Such neurons have been termed “duration tuned” or “duration selective.” In this study we examined the effects of varying signal amplitude on best duration, width of tuning, and first spike latency of duration tuned neurons in the IC of the big brown bat, Eptesicus fuscus. Response areas as a function of stimulus duration and intensity took a variety of forms, including open (V-shaped), narrow and level tolerant (U-shaped), or closed (O-shaped). The majority (82%) of duration tuned neurons had narrow U-shaped or O-shaped duration response areas. Those with narrow U-shaped response areas retained their duration tuning across a broad dynamic range, ≤50 dB above threshold, whereas those with O-shaped response areas were narrowly tuned to both stimulus duration and amplitude. For about one-half (55%) of the neurons with either a U- or O-shaped response areas, best duration (BD) changed by <1 ms across the range of suprathreshold amplitudes tested. Changes in BD most often took the form of a shift to slightly shorter durations as stimulus level increased. For the majority (65%) of U- and O-shaped neurons, 50% width of duration tuning changed by <2 ms with increasing amplitude. Latency of response at BD remained stable across changes in sound level, suggesting that the relative strengths of excitatory and inhibitory inputs to duration tuned neurons remain in balance over a wide dynamic range of sound pressure levels.

Regularity of Firing of Neurons in the Inferior Colliculus

1997 ◽  
Vol 77 (6) ◽  
pp. 2945-2965 ◽  
Author(s):  
Adrian Rees ◽  
Ali Sarbaz ◽  
Manuel S. Malmierca ◽  
Fiona E. N. Le Beau
Keyword(s):  
Guinea Pig ◽  
Characteristic Feature ◽  
Firing Rate ◽  
Inferior Colliculus ◽  
Coefficient Of Variation ◽  
Stimulus Level ◽  
The Other ◽  
Central Nucleus ◽  
Spike Discharge ◽  
Primary Type

Rees, Adrian, Ali Sarbaz, Manuel S. Malmierca, and Fiona E. N. Le Beau. Regularity of firing of neurons in the inferior colliculus. J. Neurophysiol. 77: 2945–2965, 1997. The spike discharge regularity of 254 tonically firing units in the inferior colliculus (IC) of the anesthetized guinea pig was studied in response to tones presented at best frequency (BF) to the ear contralateral to the recorded IC. Regularity of firing was measured by calculating the coefficient of variation (CV) as a function of time over the course of a unit's response. Two hundred and fifteen units (56 under urethan and 159 under chloralose anesthesia) in the central nucleus of the IC (CNIC) were studied in detail. In response to tones at 15–25 dB above threshold, 80% of units in the urethan sample fired regularly (CV < 0.5) during their sustained response, and 46% were highly regular (CV ≤ 0.35). For chloralose the values were 68% and 23%, respectively. Units recorded under urethan were significantly more regular than those recorded under chloralose. For units in the sample with a measurable onset CV, 63% were regular and 44% highly regular under urethan, and 73% were regular and 54% highly regular under chloralose. The units' peristimulus time histogram (PSTH) patterns were classified into subdivisions of four categories: choppers [9%: chop-sustained (Cs), chop-onset (Co)]; pausers [42%: pauser-chop-sustained(P/Cs), pauser-chop-onset (P/Co), pauser-no-chop]; on-sustained(43%: primary-type, L-type, h-type); and sustained (6%). The presence of chopping was a reliable predictor of regularity: Cs and P/Cs units were highly regular throughout their response, whereas Co and P/Co units were highly regular at onset and became less regular. Some units in the other PSTH categories were highly regular despite the absence of chopping, and units with virtually identical PSTHs showed very different sustained CVs. Regularity was measured as a function of firing rate in 71 units. In 23%, regularity remained constant when firing rate changed with stimulus level. Forty-six percent fired more regularly as firing rate increased, 8% fired less regularly, and 23% of units showed no consistent relationship between CV and firing rate. Regularity did not correlate with the neurons' frequency response areas or BFs. Regular firing was also found in a smaller sample of units recorded in cortices surrounding the CNIC. We conclude that regular firing is a characteristic feature of most neurons in the IC. Regularity is a specific feature correlated with four PSTH types (Cs, Co, P/Cs, and P/Co). Other PSTH types may or may not exhibit regularity.

Level dependence of cochlear nucleus onset unit responses and facilitation by second tones or broadband noise

1995 ◽  
Vol 73 (1) ◽  
pp. 141-159 ◽  
Author(s):  
I. M. Winter ◽  
A. R. Palmer
Keyword(s):  
Cochlear Nucleus ◽  
Discharge Rate ◽  
Nerve Fibers ◽  
Sound Level ◽  
Spike Timing ◽  
Broadband Noise ◽  
Sound System ◽  
Rate Level ◽  
Sound Levels ◽  
Level Function

1. The responses of onset units in the cochlear nucleus of the anesthetized guinea pig have been measured to single tones, two-tone complexes, and broadband noise (BBN; 20-kHz bandwidth). The onset units were subdivided into three groups, onset-I (OnI), onset-L (OnL), and onset-C (OnC), on the basis of a decision tree using their peristimulus time histogram (PSTH) shape and discharge rate in response to suprathreshold best-frequency (BF) tone bursts. 2. PSTHs were constructed from responses either to single tones at a unit's BF or to BBN as a function of level. When sufficient sustained activity could be elicited from the unit, arbitrarily defined as > 100 spikes/s, a coefficient of variation (CV) was calculated; the majority were characterized by a CV that was similar to transient chopper units (0.35 < CV < 0.5). First spike latency decreased monotonically with increasing sound level. For the majority of onset units, the first spike timing was very precise. 3. BF rate-level functions recorded from OnL and OnC units did not show any signs of discharge rate saturation at the highest sound levels we have used (100-115 dB SPL). No systematic relationship was observed between the threshold at BF and the shape of the rate-level function. BBN rate-level functions were typically characterized by higher discharge rates than in response to BF tones. However, for OnI units and a minority of other onset units, there was little difference in the shape of their rate-level functions in response to BF tones or BBN. 4. The threshold of most onset units to BBN was similar to the threshold to a BF tone that had similar overall root-mean-square (RMS) energy. The BBN threshold was, on average, 5.5 dB greater than the BF threshold. This result contrasts with that found in auditory-nerve fibers recorded in the same species, with the use of an identical sound system, where the threshold to BBN was, on average, 19.4 dB higher. The mean threshold difference between BBN and BF tones for a population of chopper units recorded in the same series of experiments was 17.7 dB. The relative thresholds to BBN and BF tones indicated that the bandwidths near the onset units' BF threshold were broader than could be estimated with the use of single tones. Ten units were characterized by bimodal response areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Coding for Auditory Space in the Nucleus of the Brachium of the Inferior Colliculus in the Ferret

1997 ◽  
Vol 78 (5) ◽  
pp. 2717-2731 ◽  
Author(s):  
Jan W. H. Schnupp ◽  
Andrew J. King
Keyword(s):  
Inferior Colliculus ◽  
Free Field ◽  
Sound Level ◽  
Fine Tuning ◽  
Threshold Condition ◽  
Two Dimensional ◽  
Auditory Space ◽  
Sound Levels ◽  
Response Profiles ◽  
Near Threshold

Schnupp, Jan W. H. and Andrew J. King. Coding for auditory space in the nucleus of the brachium of the inferior colliculus in the ferret. J. Neurophysiol. 78: 2717–2731, 1997. The nucleus of the brachium of the inferior colliculus (BIN) projects topographically to the deeper layers of the superior colliculus (SC), which contain a two-dimensional map of auditory space. In this study, we have used broadband stimuli presented in the free field to investigate how auditory space is represented in the BIN of the ferret. Response latencies and temporal firing patterns were comparable with those in the SC, and both properties showed some variation with stimulus location. We obtained spatial response profiles at two sound levels (5–15 and 25–35 dB above unit threshold). A large proportion of azimuth profiles (41% in the suprathreshold condition, 80% in the near-threshold condition) presented a single peak, indicating that they were tuned to single regions in space. For some of these units, the preferred speaker position varied considerably with sound level. The remaining units showed predominantly either broad “hemifield” or spatially ambiguous “bilobed” response profiles. At suprathreshold sound levels, the preferred azimuths of the tuned cells were ordered topographically along the rostrocaudal axis of the BIN, although this representation is considerably more scattered than that in the SC. In contrast to the SC, we observed no systematic variation in the distribution of near-threshold best azimuths, which were instead concentrated around the interaural axis in the contralateral hemifield. The azimuth tuning of individual units in the BIN was generally broader at both sound levels than that in the SC. Many units also were tuned for the elevation of the sound source (48% for supra-, 77% for near-threshold stimulation), but there was no evidence for topographic order in the distribution of preferred elevations within the BIN. These results suggest that the BIN sends inputs to the SC that are already selective for sound azimuth and elevation and that show some degree of topographic order for sound azimuth. These inputs then presumably are sharpened and their topography refined by a mechanism that is likely to involve convergence of other inputs and activity-dependent fine tuning of terminal connections, to result in a precise two-dimensional map of auditory space in the SC.

Contribution of GABA- and glycine-mediated inhibition to the monaural temporal response properties of neurons in the inferior colliculus

1996 ◽  
Vol 75 (2) ◽  
pp. 902-919 ◽  
Author(s):  
F. E. Le Beau ◽  
A. Rees ◽  
M. S. Malmierca
Keyword(s):  
Firing Rate ◽  
Inferior Colliculus ◽  
Discharge Rate ◽  
Glycine Receptor ◽  
Stimulus Onset ◽  
Sustained Response ◽  
Response Patterns ◽  
Gamma Aminobutyric Acid ◽  
Temporal Response ◽  
Sustained Activity

1. To determine the contribution of inhibition to the generation of the temporal response patterns of neurons in the inferior colliculus (IC), the effects of iontophoretically applied gamma-aminobutyric acid (GABA), glycine, and the GABAA and glycine receptor antagonists, bicuculline and strychnine were studied on 121 neurons in the IC of urethan-anesthetised guinea pig. 2. The neurons temporal discharge patterns were classified into six categories on the basis of their peristimulus time histograms (PSTHs). 1) Onset units fired at the stimulus onset and could be divided into two subtypes: narrow (1-2 spikes only) or broad (response lasting up to approximately 30 ms). 2) Pauser units had a precisely timed onset peak separated from a lower level of sustained activity by either a marked reduction or complete cessation of firing. 3) Chopper units had three or more clearly defined peaks near stimulus onset or evidence of regularly spaced peaks over the duration of the stimulus. 4) Onset-chopper units had three clearly defined peaks at onset but no sustained firing. 5) On-sustained units had a clearly defined single onset peak followed by a lower level of sustained activity. 6) Sustained units fired throughout the stimulus, but lacked an onset peak. 3. Iontophoretic application of GABA and glycine produced a dose-dependent reduction in firing rate in 76% (42/55) and 79% (11/14) of units, respectively. Application of bicuculline or strychnine increased the discharge rate in 91% (64/70) and 94% (16/17) of neurons, respectively. 4. The effects of bicuculline and strychnine on PSTH class were studied in detail on 70 neurons. Changes in discharge rate were accompanied by changes in PSTH in 49% (34/70) of neurons tested with bicuculline and 41% (7/17) tested with strychnine. Pauser units were the most affected with 69% changing their PSTH class, but some units in all PSTH classes, except the chopper group, exhibited changes in PSTH pattern after application of bicuculline. The majority of units (approximately 50%) that changed PSTH pattern in the presence of bicuculline became chopper units. Units of all PSTH classes could become choppers, but the proportion of units showing this change was dependent on the unit's control response pattern. All seven units that changed PSTH class with strychnine also became choppers. Changes in PSTH, including the appearance of a chopper pattern, did not depend on either a unit's control discharge rate or the magnitude of the change in discharge rate induced by the antagonists. 5. Bicuculline and strychnine had no significant effect on latency for units in the chopper, onset-chopper, onset, pauser, and on-sustained groups. A few sustained and unclassified units that had long predrug latencies did show marked reductions in latency when tested with bicuculline. 6. The majority of units did not fire spontaneously, and neither bicuculline or strychnine produced a significant increase in spontaneous rate. 7. In many units, the changes in firing rate did not occur equally over the duration of the response. Firing rates at the onset and in the last quarter of the sustained response were compared. Three effects of bicuculline and strychnine were observed. For 80% of units the largest change in firing rate occurred in the sustained response, while in 14% of units the change was greatest at onset.

scholarly journals Response Growth With Sound Level in Auditory-Nerve Fibers After Noise-Induced Hearing Loss

2004 ◽  
Vol 91 (2) ◽  
pp. 784-795 ◽  
Author(s):  
Michael G. Heinz ◽  
Eric D. Young
Keyword(s):  
Hearing Loss ◽  
Hair Cell ◽  
Auditory Nerve ◽  
Outer Hair Cell ◽  
Dynamic Range ◽  
Nerve Fibers ◽  
Sound Level ◽  
Broadband Noise ◽  
Noise Induced Hearing Loss ◽  
Rate Level

People with sensorineural hearing loss are often constrained by a reduced acoustic dynamic range associated with loudness recruitment; however, the neural correlates of loudness and recruitment are still not well understood. The growth of auditory-nerve (AN) activity with sound level was compared in normal-hearing cats and in cats with a noise-induced hearing loss to test the hypothesis that AN-fiber rate-level functions are steeper in impaired ears. Stimuli included best-frequency and fixed-frequency tones, broadband noise, and a brief speech token. Three types of impaired responses were observed. 1) Fibers with rate-level functions that were similar across all stimuli typically had broad tuning, consistent with outer-hair-cell (OHC) damage. 2) Fibers with a wide dynamic range and shallow slope above threshold often retained sharp tuning, consistent with primarily inner-hair-cell (IHC) damage. 3) Fibers with very steep rate-level functions for all stimuli had thresholds above approximately 80 dB SPL and very broad tuning, consistent with severe IHC and OHC damage. Impaired rate-level slopes were on average shallower than normal for tones, and were steeper in only limited conditions. There was less variation in rate-level slopes across stimuli in impaired fibers, presumably attributable to the lack of suppression-induced reductions in slopes for complex stimuli relative to BF-tone slopes. Sloping saturation was observed less often in impaired fibers. These results illustrate that AN fibers do not provide a simple representation of the basilar-membrane I/O function and suggest that both OHC and IHC damage can affect AN response growth.

Topographic representation of auditory space in the superior colliculus of adult ferrets after monaural deafening in infancy

1994 ◽  
Vol 71 (1) ◽  
pp. 182-194 ◽  
Author(s):  
A. J. King ◽  
D. R. Moore ◽  
M. E. Hutchings
Keyword(s):  
Superior Colliculus ◽  
Free Field ◽  
Receptive Fields ◽  
Spatial Location ◽  
Stimulus Level ◽  
Sound Level ◽  
Lateral Side ◽  
Auditory Space ◽  
Sound Levels ◽  
Auditory Representation

1. We have investigated the role of monaural cues provided by the outer ear in the construction of a map of auditory space in the superior colliculus. Single-unit recordings were made from the superior colliculus of adult ferrets that were deprived of binaural inputs by surgically ablating the ipsilateral cochlea on postnatal day 21 or 24. 2. The spatial response properties of auditory units in the deeper layers of this nucleus were studied using white-noise bursts presented under free-field conditions in an anechoic chamber. The thresholds of the units recorded in the monaural ferrets were not significantly different from those recorded in the superior colliculus of normal adult ferrets. In both groups the unit thresholds varied by 30-50 dB in each region of the superior colliculus. 3. In normal and monaural ferrets the elevation tuning tended to be sharper than the azimuth tuning. At sound levels of approximately 10 dB above threshold the auditory units recorded in both groups of animals were tuned to a specific region of space that was restricted in azimuth and elevation. The spatial location at which the maximum response was obtained (auditory best position) varied topographically in azimuth along the rostrocaudal axis of the nucleus and in elevation along the mediolateral axis. 4. The azimuthal distribution of best positions associated with each recording location in the superior colliculus of the monaural ferrets and the alignment between this dimension of the auditory map and that of the visual map in the overlying superficial layers were no different from those found at corresponding near-threshold sound levels in normal ferrets. 5. Elevation spatial selectivity was examined in a smaller sample of units. Although elevation best positions shifted downward from the medial to the lateral side of the nucleus in both normal and monaural ferrets, we found that the topography of the auditory representation and its alignment with the visual representation were statistically different in the two groups of animals. 6. Increasing the sound level does not affect the representation of auditory space in normal ferrets. However, when the stimulus level presented to monoaural ferrets was increased, the receptive fields either expanded so that the responses were no longer tuned to any particular region of space, or the responses remained tuned but exhibited a marked shift in the value of the best position.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasticity of response properties of inferior colliculus neurons following acute cochlear damage

1996 ◽  
Vol 75 (1) ◽  
pp. 171-183 ◽  
Author(s):  
J. Wang ◽  
R. J. Salvi ◽  
N. Powers
Keyword(s):  
Inferior Colliculus ◽  
Discharge Rate ◽  
Excitatory Response ◽  
Response Properties ◽  
Tuning Curves ◽  
Rate Level ◽  
Discharge Rates ◽  
Inhibitory Circuit ◽  
Threshold Tuning ◽  
High Sound

1. The discharge patterns of 40 neurons in the central nucleus of the inferior colliculus (ICC) of the chinchilla were evaluated before and after acute cochlea trauma from intense tone exposure. Single-unit recordings were obtained from neurons in the ICC contralateral to the sound-stimulated ear. Cochlear trauma was induced with a short-duration (15-25 min), high-intensity pure tone (95-115 dB SPL) at a frequency above the neuron's characteristic frequency (CF). The aim of the traumatizing exposure was to damage the peripheral sensory receptors associated with frequencies above the neuron's CF. The damage was expected to attenuate inputs to the neural circuits responsible for activating the inhibitory sidebands above CF. 2. Three types of frequency-threshold tuning curves were observed before the exposure: 1) open V tuning curves (55%) that became wider as sound intensity increased; 2) level-tolerant tuning curves (37.5%) that maintained a very narrow bandwidth even at high sound intensities; and 3) upper-threshold tuning curves (7.5%) in which excitatory responses were elicited at low and moderate intensities, but not at high intensities. The traumatizing exposure caused a dramatic widening of level-tolerant (80% of sample) and upper-threshold tuning curves (100% of sample) at high sound intensities but caused almost no change in the low-threshold tip of the tuning curves. By contrast, tuning curves with an open V configuration were generally unaffected (approximately 90% of sample) by the traumatizing exposure. 3. Discharge rate-level functions in the ICC were of two general types: 1) strongly nonmonotonic (60%) or 2) saturating, monotonic (40%). The traumatizing exposure caused a significant increase in the suprathreshold discharge rates in 70% of all neurons studied. Among the neurons with strongly nonmonotonic discharge rate-level functions, 93% showed a significant increase in discharge rate. 4. The poststimulus time histograms (PSTH) to tone bursts were of three main types: 1) onset, 2) pauser, and 3) sustained responders. The traumatizing exposure had almost no effect on the PSTH of onset or sustained responders. However, pause PSTH frequently (75%) showed a significant decrease in the pause duration and an increase in the sustained discharge rate following the pause after the exposure. 5. The results suggest that the response properties of neurons with extremely narrow tuning curves and nonmonotonic discharge rate-level functions are shaped by an inhibitory circuit that is activated by frequencies above the high-frequency flank of the tuning curve. This inhibitory circuit modifies the excitatory response in the following ways: 1) it narrows the excitatory response area at suprathreshold intensities particularly at frequencies below CF, 2) it alters the shape of the discharge rate-level function by suppressing the discharge rates at suprathreshold intensities, and 3) it modifies the temporal discharge pattern of the pause PSTH by suppressing neural activity that occurs after the onset response. The effects of these inhibitory inputs can be reduced (disinhibition) by damaging the sensory cells in the inner ear that activate this circuit. The exact locus of the inhibitory circuit(s) is unknown but may involve inhibitory inputs located at the level of the cochlear nucleus and/or at levels up to the inferior colliculus.

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

1986 ◽  
Vol 55 (2) ◽  
pp. 280-300 ◽  
Author(s):  
T. C. Yin ◽  
J. C. Chan ◽  
D. R. Irvine
Keyword(s):  
Inferior Colliculus ◽  
Discharge Rate ◽  
Low Frequency ◽  
Central Peak ◽  
Stimulus Level ◽  
Central Nucleus ◽  
Interaural Phase ◽  
Wideband Noise ◽  
Pure Tones ◽  
Phase Differences

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)

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