The representation of stimulus azimuth by high best-frequency azimuth-selective neurons in the central nucleus of the inferior colliculus of the cat

1987 ◽  
Vol 57 (4) ◽  
pp. 1185-1200 ◽  
Author(s):  
L. M. Aitkin ◽  
R. L. Martin
Keyword(s):  
Inferior Colliculus ◽  
Discharge Rate ◽  
Median Plane ◽  
Central Nucleus ◽  
High Discharge ◽  
Discharge Rates ◽  
Tonal Stimuli ◽  
Anesthetized Cat ◽  
The Way

The responses to changes in stimulus azimuth of 220 high best-frequency (BF) (greater than 3 kHz) units in the central nucleus of the inferior colliculus of the anesthetized cat were studied with BF tones (220 units) and noise stimuli (84 units). By this means we hoped to gain some insights into the way the azimuthal locations of high BF stimuli were represented in the inferior colliculus. For each unit the discharge rate was determined for stimuli located along a plane tilted at 20 degrees above the horizontal. This plane was chosen to optimize pinna directionality. Locations in the frontal field were sampled in 10-20 degree steps around a 170 degree arc. These measurements were repeated at a number of different stimulus intensities until the directional properties of the unit became clear. Units for which the functions relating discharge rate to azimuth for a given stimulus showed a clear feature (peak or border), the azimuthal location of which varied little with intensities between 20 and 40 dB above threshold, were defined as being azimuth selective for that stimulus. Only 13% of units were azimuth selective for BF tones, whereas 44% were selective for noise. Many azimuth functions for selective units were of the plateau-shaped type for which relatively high discharge rates occurring at most contralateral azimuths declined steeply to near zero and remained low for most ipsilateral azimuths. These plateau-shaped functions were most common for tonal stimuli. Other functions showed a fixed azimuth of maximum firing (best azimuth); these were more common for noise than for tonal stimuli. Detailed azimuth functions for both tone and noise stimuli were measured for 63 units. Some exhibited the same kind of azimuth function to both stimuli. However, 18 units were azimuth selective to noise but not to tones. The borders of plateau-shaped functions obtained using both noise and tonal stimuli were concentrated within 20 degrees of the median plane. Very few units had borders that spanned peripheral ipsilateral or contralateral azimuths. Although the best azimuths of some noise azimuth functions were observed to lie at these peripheral azimuths, the majority occurred around 20 degrees contralateral to the median plane. The recording sites for units were related to a three-by-three matrix of rostrocaudal and mediolateral locations across the central nucleus. Units that were azimuth selective to noise were distributed fairly evenly throughout the central nucleus, whereas units azimuth selective to tones formed highest proportions rostrally.(ABSTRACT TRUNCATED AT 400 WORDS)

Binaural processing of sound pressure level in the inferior colliculus

1987 ◽  
Vol 57 (4) ◽  
pp. 1130-1147 ◽  
Author(s):  
M. N. Semple ◽  
L. M. Kitzes
Keyword(s):  
Inferior Colliculus ◽  
Sound Pressure ◽  
Spatial Information ◽  
Discharge Rate ◽  
Excitatory Input ◽  
Central Nucleus ◽  
Intensity Difference ◽  
Inhibitory Input ◽  
Directional Information ◽  
Discharge Rates

The central auditory system could encode information about the location of a high-frequency sound source by comparing the sound pressure levels at the ears. Two potential computations are the interaural intensity difference (IID) and the average binaural intensity (ABI). In this study of the central nucleus of the inferior colliculus (ICC) of the anesthetized gerbil, we demonstrate that responses of 85% of the 97 single units in our sample were jointly influenced by IID and ABI. For a given ABI, discharge rate of most units is a sigmoidal function of IID, and peak rates occur at IIDs favoring the contralateral ear. Most commonly, successive increments of ABI cause successive shifts of the IID functions toward IIDs favoring the ipsilateral ear. Neurons displaying this behavior include many that would conventionally be classified EI (receiving predominantly excitatory input arising from one ear and inhibitory input from the other), many that would be classified EE (receiving predominantly excitatory input arising from each ear), and all that are responsive only to contralateral stimulation. The IID sensitivity of a very few EI neurons is unaffected by ABI, except near threshold. Such units could provide directional information that is independent of source intensity. A few EE neurons are very sensitive to ABI, but are minimally sensitive to IID. Nevertheless, our data indicate that responses of most EE units in ICC are strongly dominated by excitation of contralateral origin. For some units, discharge rate is nonmonotonically related to IID and is maximal when the stimuli at the two ears are of comparable sound pressure. This preference for zero IID is common for all binaural levels. Many EI neurons respond nonmonotonically to ABI. Discharge rates are greater for IIDs representative of contralateral space and are maximal at a single best ABI. For a subset of these neurons, the influence arising from the ipsilateral ear is comprised of a mixture of excitation and inhibition. As a consequence, discharge rates are nonmonotonically related not only to ABI but also to IID. This dual nonmonotonicity creates a clear focus of peak response at a particular ABI/IID combination. Because of their mixed monaural influences, such units would be ascribed to different classes of the conventional (EE/EI) binaural classification scheme depending on the binaural level presented. Several response classes were identified in this study, and each might contribute differently to the encoding of spatial information.(ABSTRACT TRUNCATED AT 400 WORDS)

Inferior colliculus. I. Comparison of response properties of neurons in central, pericentral, and external nuclei of adult cat

1975 ◽  
Vol 38 (5) ◽  
pp. 1196-1207 ◽  
Author(s):  
L. M. Aitkin ◽  
W. R. Webster ◽  
J. L. Veale ◽  
D. C. Crosby
Keyword(s):  
Inferior Colliculus ◽  
Physiological Responses ◽  
Cell Types ◽  
Central Nucleus ◽  
Low Frequencies ◽  
High Frequencies ◽  
The Common ◽  
Adult Cat ◽  
Tonal Stimuli ◽  
Anesthetized Cat

The responses of 150 units in the central (ICC), pericentral (ICP), and external nuclei (ICX) of the inferior colliculus of the anesthetized cat were studied in relation to their tuning characteristics and binaural responses to tonal stimuli. Units in ICC were characterized by sharp tuning and binaural responses, while those in ICP and ICX were frequently very broadly tuned with a poorly defined best frequency. Nonetheless, in the latter nuclei a tendency existed for tonotopic organization to occur with high frequencies located externally and low frequencies at the margins of the central nucleus. Tuning measurements were hampered by the common occurrence of habituation in the discharges of single units in ICP and, to a lesser extend, ICX. The majority of units in ICP could be differentiated from those in ICX by their monaural input. Speculations were advanced linking anatomical cell types to physiological responses in the three nuclei and into the possible functional significance of the different behavior of units to tonal stimuli.

scholarly journals Physiological Studies of the Precedence Effect in the Inferior Colliculus of the Cat. I. Correlates of Psychophysics

1998 ◽  
Vol 80 (3) ◽  
pp. 1285-1301 ◽  
Author(s):  
Ruth Y. Litovsky ◽  
Tom C. T. Yin
Keyword(s):  
Inferior Colliculus ◽  
Free Field ◽  
Median Plane ◽  
Stimulus Level ◽  
Central Nucleus ◽  
Precedence Effect ◽  
Discharge Rates ◽  
Physiological Studies ◽  
Echo Suppression ◽  
Maximal Suppression

Litovsky, Ruth Y. and Tom C. T. Yin. Physiological studies of the precedence effect in the inferior colliculus of the cat. I. Correlates of psychophysics. J. Neurophysiol. 80: 1285–1301, 1998. The precedence effect (PE) is experienced when two spatially separated sounds are presented with such a brief delay that only a single auditory image at or toward the location of the leading source is perceived. The responses of neurons in the central nucleus of the inferior colliculus (ICC) of cats were studied using stimuli that are known to elicit the PE, focusing on the effects of changes in stimulus conditions that a listener might encounter in a natural situation. Experiments were conducted under both free-field (anechoic chamber) and dichotic (headphones) conditions. In free field, the PE was simulated by presenting two sounds from different loudspeakers with one sound delayed relative to the other. Either click or noise stimuli (2- to 10-ms duration) were used. Dichotically, the same conditions were simulated by presenting two click or noise pairs separated by an interstimulus delay (ISD) with interaural time differences (ITDs) imposed separately for each pair. At long ISDs, all neurons responded to both leading and lagging sources as if they were delivered alone. As the ISDs were shortened, the lagging response became suppressed. The ISD of half-maximal suppression varied considerably within the population of neurons studied, ranging from 2 to 100 ms, with means of 35 and 38 ms for free field and dichotic conditions, respectively. Several correlates of psychophysical findings were observed in ICC neurons: suppression was usually stronger with lower overall stimulus level and longer duration stimuli. Suppression also was compared along the azimuth and elevation in free field by placing the lagging source at (0°,0°), which is common to both axes, and the leading sources at locations along either plane that generated similar discharge rates. All neurons that showed suppression along the azimuth also did so in the elevation. In addition, there was a high correlation in the ISD of half-maximal suppression along the two planes ( r = 0.87). These findings suggest that interaural difference cues, which are robust along the horizontal axis but minimal in the median plane, are not necessary for neural correlates of the PE to be manifested. Finally, single-neuron responses did not demonstrate a correlate of build-up of suppression, a phenomenon whereby echo suppression accumulates with ongoing stimulation. This finding adds credibility to theories about the PE that argue for a “higher order” component of the PE.

Single-Unit Responses in the Inferior Colliculus of Decerebrate Cats II. Sensitivity to Interaural Level Differences

1999 ◽  
Vol 82 (1) ◽  
pp. 164-175 ◽  
Author(s):  
Kevin A. Davis ◽  
Ramnarayan Ramachandran ◽  
Bradford J. May
Keyword(s):  
Frequency Response ◽  
Inferior Colliculus ◽  
Free Field ◽  
Central Nucleus ◽  
Type I ◽  
Discharge Rates ◽  
Ipsilateral Stimulation ◽  
Interaural Level Differences ◽  
Type V ◽  
Decerebrate Cats

Single units in the central nucleus of the inferior colliculus (ICC) of unanesthetized decerebrate cats can be grouped into three distinct types (V, I, and O) according to the patterns of excitation and inhibition revealed in contralateral frequency response maps. This study extends the description of these response types by assessing their ipsilateral and binaural response map properties. Here the nature of ipsilateral inputs is evaluated directly using frequency response maps and compared with results obtained from methods that rely on sensitivity to interaural level differences (ILDs). In general, there is a one-to-one correspondence between observed ipsilateral input characteristics and those inferred from ILD manipulations. Type V units receive ipsilateral excitation and show binaural facilitation (EE properties); type I and type O units receive ipsilateral inhibition and show binaural excitatory/inhibitory (EI) interactions. Analyses of binaural frequency response maps show that these ILD effects extend over the entire receptive field of ICC units. Thus the range of frequencies that elicits excitation from type V units is expanded with increasing levels of ipsilateral stimulation, whereas the excitatory bandwidth of type I and O units decreases under the same binaural conditions. For the majority of ICC units, application of bicuculline, an antagonist for GABAA-mediated inhibition, does not alter the basic effects of binaural stimulation; rather, it primarily increases spontaneous and maximum discharge rates. These results support our previous interpretations of the putative dominant inputs to ICC response types and have important implications for midbrain processing of competing free-field sounds that reach the listener with different directional signatures.

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

1987 ◽  
Vol 58 (3) ◽  
pp. 543-561 ◽  
Author(s):  
J. C. Chan ◽  
T. C. Yin ◽  
A. D. Musicant
Keyword(s):  
Inferior Colliculus ◽  
Time Delays ◽  
Discharge Rate ◽  
Low Frequency ◽  
Central Peak ◽  
Broadband Noise ◽  
Central Nucleus ◽  
Rate Curve ◽  
Median Frequency ◽  
Band Pass

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.

scholarly journals Assessing watershed hydrological response to climate change based on signature indices

2021 ◽  
Author(s):  
Atiyeh Fatehifar ◽  
Mohammad Reza Goodarzi ◽  
Seyedeh Sima Montazeri Hedesh ◽  
Parnian Siahvashi Dastjerdi
Keyword(s):  
Climate Change ◽  
Discharge Rate ◽  
Baseline Period ◽  
Future Climate Change ◽  
Hydrological Response ◽  
High Discharge ◽  
Discharge Rates ◽  
Segment Volume ◽  
The Mean ◽  
Hydrological Signatures

Abstract Due to the fact that one of the important ways of describing the performance of basins is to use the hydrological signatures, the present study is to investigate the effects of climate change using the hydrological signatures in Azarshahr Chay basin, Iran. To this end, Canadian Earth system model (CanESM2) is first used to predict future climate change (2030–2059) under two Representative Concentration Pathways (RCP2.6 and RCP8.5). Six signature indices were extracted from flow duration curve (FDC) as follows: runoff ratio (RR), high-segment volume (FHV), low-segment volume (FLV), mid-segment slope (FMS), mid-range flow (FMM), and maximum peak discharge (DiffMaxPeak). These signature indices act as sorts of fingerprints representing differences in the hydrological behavior of the basin. The results indicate that the most significant changes in the future hydrological response are related to the FHV and FLV and FMS indices. The BiasFHV index indicates an increase in high discharge rates under RCP8.5 scenario, compared to the baseline period and the RCP2.6 scenario, as well. The mean annual discharge rate, however, is lower than the discharge rate under this scenario. Generally, for the RCP8.5 scenario, the changes in the signature indices in both high discharges and low discharges are significant.

Temporal resolution of neurons in cat inferior colliculus to intracochlear electrical stimulation: effects of neonatal deafening and chronic stimulation

1995 ◽  
Vol 73 (2) ◽  
pp. 449-467 ◽  
Author(s):  
R. Snyder ◽  
P. Leake ◽  
S. Rebscher ◽  
R. Beitel
Keyword(s):  
Electrical Stimulation ◽  
Inferior Colliculus ◽  
Temporal Resolution ◽  
Transfer Functions ◽  
Discharge Rate ◽  
Pulse Frequency ◽  
Acoustic Stimulation ◽  
Acute Experiment ◽  
Sustained Response ◽  
Central Nucleus

1. Cochlear implants have been available for > 20 yr to profoundly deaf adults who have lost their hearing after acquiring language. The success of these cochlear prostheses has encouraged the application of implants in prelingually deaf children as young as 2 yr old. To further characterize the consequences of chronic intracochlear electrical stimulation (ICES) on the developing auditory system, the temporal-response properties of single neurons in the inferior colliculus (IC) were recorded in deafened anesthetized cats. 2. The neurons were excited by unilateral ICES with the use of a scala tympani stimulating electrode implanted in the left cochlea. The electrodes were modeled after those used in cochlear implant patients. Responses of 443 units were recorded extracellularly in the contralateral (right) IC with the use of tungsten microelectrodes. Recordings were made in three groups of adult animals: neonatally deafened/chronically stimulated animals (192 units), neonatally deafened/unstimulated animals (80 units), and adult-deafened/prior normal-hearing animals (171 units). The neonatally deafened cats were deafened by multiple intramuscular injections of neomycin sulfate and never developed demonstrable hearing. Most of the deafened, chronically stimulated animals were implanted at 6 wk of age and stimulated at suprathreshold levels for 4 h/day for 3-6 mo. The unstimulated animals were implanted as adults at least 2 wk before the acute physiological experiment and were left unstimulated until the acute experiment was conducted. Prior-normal adults were deafened and implanted at least 2 wk before the acute experiment. 3. IC units were isolated with the use of a search stimulus consisting of three cycles of a 100-Hz sinusoid. Most units responded to sinusoidal stimulation with either an onset response or a sustained response. Onset units were the predominant unit found in the external nucleus, whereas sustained units were found almost exclusively in the central nucleus. The temporal resolution of sustained response units was measured with the use of pulse trains of increasing frequency and calculating the discharges/pulse. 4. The range of electrical pulse frequencies to which IC units responded in a temporally synchronized manner was comparable with that produced by acoustic stimulation. The discharge rate/pulse-versus-pulse frequency transfer functions of IC units were uniformly low-pass, although they varied widely in their cutoff frequencies. This variation in pulse response was partially correlated with the unit's response to sinusoids. Most onset neurons responded only to pulse frequencies below 20 pulses per second (pps). Most sustained units responded best to pulse frequencies < 100 pps, and most ceased to respond to pulse frequencies > 300 pps.(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)

Periodicity coding in the inferior colliculus of the cat. II. Topographical organization

1988 ◽  
Vol 60 (6) ◽  
pp. 1823-1840 ◽  
Author(s):  
C. E. Schreiner ◽  
G. Langner
Keyword(s):  
Inferior Colliculus ◽  
Frequency Band ◽  
Characteristic Frequency ◽  
Frequency Tuning ◽  
Discharge Rate ◽  
Modulation Frequency ◽  
Central Nucleus ◽  
Response Threshold ◽  
Tuning Curves ◽  
Auditory Nucleus

1. The topographical distributions of single-unit and multiple-unit responses to amplitude-modulated tones--and to other relevant parameters of simple tonal stimuli--were defined across the main frequency representational gradient and within narrow frequency ranges represented in "frequency band laminae" in the principal midbrain auditory nucleus, the central nucleus of the inferior colliculus (ICC), in adult, barbiturate-anesthetized cats. 2. Responses to amplitude-modulated tones with the carrier set at the characteristic frequency (CF) of recorded neurons were obtained at many ICC locations in each experiment. The best modulation frequency (BMF) of neurons was defined at each site as that modulation frequency producing the highest neural discharge rate. Encountered BMFs ranged from approximately 10 to 1,000 Hz. A significant range of BMFs were recorded for neurons with any given characteristic frequency. BMF ranges varied as a systematic function of CF and of ICC recording depth. 3. Recorded BMFs were distributed topographically within functionally defined ICC frequency band laminae. Highest BMFs were found clustered in an ICC sector roughly between the middle and lateral third of its frequency band laminae. Progressively lower BMFs were recorded with increasing distance across the laminae in any direction away from the highest-BMF cluster. That is, "iso-BMF contours" were arrayed concentrically around the highest-BMF region. 4. Within frequency band laminae centered at approximately 3 and 12 kHz, quality factors (Q10 dBS) of frequency tuning curves were found to be between 0.8 and 8. Q10 dB values were distributed topographically within given frequency band laminae. Responses with narrow tuning curves (high Q10 dB values) were clustered in the middle third of the mediolateral extent of laminae; sharpness of tuning declined systematically away from this focus of highest Q10 dB values. The center of this distribution did not coincide with the center of the BMF distribution within the same lamina. 5. For neurons at greater than 90% of the ICC loci studied in these experiments, onset latencies to CF tones defined approximately 60 dB above response threshold fell within a range between 5 and 18 ms. Across a given frequency band lamina, onset latencies varied systematically, with longest response latencies recorded medially, and progressively shorter latencies recorded progressively more laterally. 6. Binaural interaction types were systematically distributed within frequency-band laminae. A cluster of excitatory-excitatory (EE) was seen, covering approximately one-third of the mapped area.(ABSTRACT TRUNCATED AT 400 WORDS)

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