Neural Correlates of the Precedence Effect in the Inferior Colliculus: Effect of Localization Cues

2002 ◽  
Vol 87 (2) ◽  
pp. 976-994 ◽  
Author(s):  
R. Y. Litovsky ◽  
B. Delgutte
Keyword(s):  
Inferior Colliculus ◽  
Time Course ◽  
Transfer Functions ◽  
Free Field ◽  
Field Studies ◽  
Suppressive Effect ◽  
Virtual Space ◽  
Precedence Effect ◽  
Excitatory Response ◽  
Accurate Localization

The precedence effect (PE) is an auditory phenomenon involved in suppressing the perception of echoes in reverberant environments, and is thought to facilitate accurate localization of sound sources. We investigated physiological correlates of the PE in the inferior colliculus (IC) of anesthetized cats, with a focus on directional mechanisms for this phenomenon. We used a virtual space (VS) technique, where two clicks (a “lead” and a “lag”) separated by a brief time delay were each filtered through head-related transfer functions (HRTFs). For nearly all neurons, the response to the lag was suppressed for short delays and recovered at long delays. In general, both the time course and the directional patterns of suppression resembled those reported in free-field studies in many respects, suggesting that our VS simulation contained the essential cues for studying PE phenomena. The relationship between the directionality of the response to the lead and that of its suppressive effect on the lag varied a great deal among IC neurons. For a majority of units, both excitation produced by the lead and suppression of the lag response were highly directional, and the two were similar to one another. For these neurons, the long-lasting inhibitory inputs thought to be responsible for suppression seem to have similar spatial tuning as the inputs that determine the excitatory response to the lead. Further, the behavior of these neurons is consistent with psychophysical observations that the PE is strongest when the lead and the lag originate from neighboring spatial locations. For other neurons, either there was no obvious relationship between the directionality of the excitatory lead response and the directionality of suppression, or the suppression was highly directional whereas the excitation was not, or vice versa. For these neurons, the excitation and the suppression produced by the lead seem to depend on different mechanisms. Manipulation of the directional cues (such as interaural time and level differences) contained in the lead revealed further dissociations between excitation and suppression. Specifically, for about one-third of the neurons, suppression depended on different directional cues than did the response to the lead, even though the directionality of suppression was similar to that of the lead response when all cues were present. This finding suggests that the inhibitory inputs causing suppression may originate in part from subcollicular auditory nuclei processing different directional cues than the inputs that determine the excitatory response to the lead. Neurons showing such dissociations may play an important role in the PE when the lead and the lag originate from very different directions.

Receptive Fields and Binaural Interactions for Virtual-Space Stimuli in the Cat Inferior Colliculus

1999 ◽  
Vol 81 (6) ◽  
pp. 2833-2851 ◽  
Author(s):  
Bertrand Delgutte ◽  
Philip X. Joris ◽  
Ruth Y. Litovsky ◽  
Tom C. T. Yin
Keyword(s):  
Inferior Colliculus ◽  
Sound Localization ◽  
Transfer Functions ◽  
Sagittal Plane ◽  
Free Field ◽  
Receptive Fields ◽  
Field Studies ◽  
Contralateral Side ◽  
Virtual Space ◽  
Acoustic Cues

Receptive fields and binaural interactions for virtual-space stimuli in the cat inferior colliculus. Sound localization depends on multiple acoustic cues such as interaural differences in time (ITD) and level (ILD) and spectral features introduced by the pinnae. Although many neurons in the inferior colliculus (IC) are sensitive to the direction of sound sources in free field, the acoustic cues underlying this sensitivity are unknown. To approach this question, we recorded the responses of IC cells in anesthetized cats to virtual space (VS) stimuli synthesized by filtering noise through head-related transfer functions measured in one cat. These stimuli not only possess natural combinations of ITD, ILD, and spectral cues as in free field but also allow precise control over each cue. VS receptive fields were measured in the horizontal and median vertical planes. The vast majority of cells were sensitive to the azimuth of VS stimuli in the horizontal plane for low to moderate stimulus levels. Two-thirds showed a “contra-preference” receptive field, with a vigorous response on the contralateral side of an edge azimuth. The other third of receptive fields were tuned around a best azimuth. Although edge azimuths of contra-preference cells had a broad distribution, best azimuths of tuned cells were near the midline. About half the cells tested were sensitive to the elevation of VS stimuli along the median sagittal plane by showing either a peak or a trough at a particular elevation. In general receptive fields for VS stimuli were similar to those found in free-field studies of IC neurons, suggesting that VS stimulation provided the essential cues for sound localization. Binaural interactions for VS stimuli were studied by comparing responses to binaural stimulation with responses to monaural stimulation of the contralateral ear. A majority of cells showed either purely inhibitory (BI) or mixed facilitatory/inhibitory (BF&I) interactions. Others showed purely facilitatory (BF) or no interactions (monaural). Binaural interactions were correlated with azimuth sensitivity: most contra-preference cells had either BI or BF&I interactions, whereas tuned cells were usually BF. These correlations demonstrate the importance of binaural interactions for azimuth sensitivity. Nevertheless most monaural cells were azimuth-sensitive, suggesting that monaural cues also play a role. These results suggest that the azimuth of a high-frequency sound source is coded primarily by edges in azimuth receptive fields of a population of ILD-sensitive cells.

Spatial Tuning to Virtual Sounds in the Inferior Colliculus of the Guinea Pig

2003 ◽  
Vol 90 (4) ◽  
pp. 2648-2659 ◽  
Author(s):  
Susanne J. Sterbing ◽  
Klaus Hartung ◽  
Klaus-Peter Hoffmann
Keyword(s):  
White Noise ◽  
Inferior Colliculus ◽  
Transfer Functions ◽  
Free Field ◽  
Receptive Fields ◽  
Sound Intensity ◽  
Spatial Location ◽  
Average Diameter ◽  
Maximal Response ◽  
Spatial Tuning

How do neurons in the inferior colliculus (IC) encode the spatial location of sound? We have addressed this question using a virtual auditory environment. For this purpose, the individual head-related transfer functions (HRTFs) of 18 guinea pigs were measured under free-field conditions for 122 locations covering the upper hemisphere. From 257 neurons, 94% responded to the short (50-ms) white noise stimulus at 70 dB sound pressure level (SPL). Out of these neurons, 80% were spatially tuned with a receptive field that is smaller than a hemifield (at 70 dB). The remainder responded omnidirectionally or showed fractured receptive fields. The majority of the neurons preferred directions in the contralateral hemisphere. However, preference for front or rear positions and high elevations occurred frequently. For stimulation at 70 dB SPL, the average diameter of the receptive fields, based on half-maximal response, was less than a quarter of the upper hemisphere. Neurons that preferred frontal directions responded weakly or showed no response to posterior directions and vice versa. Hence, front/back discrimination is present at the single-neuron level in the IC. When nonindividual HRTFs were used to create the stimuli, the spatial receptive fields of most neurons became larger, split into several parts, changed position, or the response became omnidirectional. Variation of absolute sound intensity had little effect on the preferred directions of the neurons over a range of 20 to 40 dB above threshold. With increasing intensity, most receptive fields remained constant or expanded. Furthermore, we tested the influence of binaural decorrelation and stimulus bandwidth on spatial tuning. The vast majority of neurons with a low characteristic frequency (<2.5 kHz) lost spatial tuning under stimulation with binaurally uncorrelated noise, whereas high-frequency units were mostly unaffected. Most neurons that showed spatial tuning under broadband stimulation (white noise and 1 octave wide noise) turned omnidirectional when stimulated with 1/3 octave wide noise.

Physiological Studies of the Precedence Effect in the Inferior Colliculus of the Cat. II. Neural Mechanisms

1998 ◽  
Vol 80 (3) ◽  
pp. 1302-1316 ◽  
Author(s):  
Ruth Y. Litovsky ◽  
Tom C. T. Yin
Keyword(s):  
Inferior Colliculus ◽  
Free Field ◽  
Neural Mechanisms ◽  
Precedence Effect ◽  
Medial Superior Olive ◽  
Mixed Effect ◽  
Source Level ◽  
Physiological Studies ◽  
Interaural Differences ◽  
Lead Location

Litovsky, Ruth Y. and Tom C. T. Yin. Physiological studies of the precedence effect in the inferior colliculus of the cat. II. Neural mechanisms. J. Neurophysiol. 80: 1302–1316, 1998. We studied the responses of neurons in the inferior colliculus (IC) of cats to stimuli known to evoke the precedence effect (PE). This paper focuses on stimulus conditions that probe the neural mechanisms underlying the PE but that are not usually encountered in a natural situation. Experiments were conducted under both free-field (anechoic chamber) and dichotic (headphones) conditions. We found that in free field the amount of suppression of the lagging response depended on the location of the leading source. With stimuli in the azimuthal plane, the majority (84%) of units showed stronger suppression of the lagging response for a leading stimulus placed in the cell's responsive area as compared with a lead in the unresponsive field. A smaller number of units showed stronger suppression for a lead placed in the unresponsive field, and a few showed little effect of the lead location. In the elevational plane, there was less sensitivity of the leading source to changes in location, but for those cells in which there was sensitivity, suppression was always stronger when the lead was in the cell's responsive area. Studies on stimulus locations also were conducted under dichotic conditions by varying the interaural differences in time (ITD) of the leading source. Results were consistent with those obtained in free field, suggesting that ITDs play an important role in determining the amount of suppression that was observed as a function of leading stimulus location. In addition to location and ITD, we also studied the effect of varying the relative levels of the lead and lag as well as stimulus duration. For all units studied, increasing the level of the leading stimulus while holding the lagging stimulus constant resulted in increased suppression. Similar effects of leading source level were observed in azimuth and elevation. The effect of varying the duration of the leading source also showed that longer duration stimuli produce stronger suppression; this finding was observed both in azimuth and elevation. We also compared the suppression observed under binaural and monaural contralateral conditions and found a mixed effect: some neurons show stronger suppression under binaural conditions, others to monaural contralateral conditions, and still others show no effect. The results presented here support the hypothesis that the PE reflects a long-lasting inhibition evoked by the leading stimulus. Five possible sources for the inhibition are considered: the auditory nerve, intrinsic circuits in the cochlear nucleus, medial and lateral nuclei of the trapezoid body inhibition to the medial superior olive, dorsal nucleus of the lateral lemniscus (DNLL) inhibition to the ICC, and intrinsic circuits in the ICC itself.

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.

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

1989 ◽  
Vol 62 (1) ◽  
pp. 144-161 ◽  
Author(s):  
L. H. Carney ◽  
T. C. Yin
Keyword(s):  
Inferior Colliculus ◽  
Time Course ◽  
Low Frequency ◽  
The Other ◽  
Central Nucleus ◽  
Ipsilateral Side ◽  
Interaural Time Differences ◽  
Excitatory Response ◽  
Inhibitory Component ◽  
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)

The Effect of Nonlinear Amplitude Growth on the Speech Perception Benefits Provided by a Single-Sided Vocoder

2019 ◽  
Vol 62 (3) ◽  
pp. 745-757 ◽  
Author(s):  
Jessica M. Wess ◽  
Joshua G. W. Bernstein
Keyword(s):  
Transfer Functions ◽  
Spatial Configuration ◽  
Free Field ◽  
Spatial Separation ◽  
Compression And Expansion ◽  
Interaural Difference ◽  
Interaural Differences ◽  
Single Sided Deafness ◽  
Perceptual Separation ◽  
Loudness Growth

PurposeFor listeners with single-sided deafness, a cochlear implant (CI) can improve speech understanding by giving the listener access to the ear with the better target-to-masker ratio (TMR; head shadow) or by providing interaural difference cues to facilitate the perceptual separation of concurrent talkers (squelch). CI simulations presented to listeners with normal hearing examined how these benefits could be affected by interaural differences in loudness growth in a speech-on-speech masking task.MethodExperiment 1 examined a target–masker spatial configuration where the vocoded ear had a poorer TMR than the nonvocoded ear. Experiment 2 examined the reverse configuration. Generic head-related transfer functions simulated free-field listening. Compression or expansion was applied independently to each vocoder channel (power-law exponents: 0.25, 0.5, 1, 1.5, or 2).ResultsCompression reduced the benefit provided by the vocoder ear in both experiments. There was some evidence that expansion increased squelch in Experiment 1 but reduced the benefit in Experiment 2 where the vocoder ear provided a combination of head-shadow and squelch benefits.ConclusionsThe effects of compression and expansion are interpreted in terms of envelope distortion and changes in the vocoded-ear TMR (for head shadow) or changes in perceived target–masker spatial separation (for squelch). The compression parameter is a candidate for clinical optimization to improve single-sided deafness CI outcomes.

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.

Neural responses to simple simulated echoes in the auditory brain stem of the unanesthetized rabbit

1995 ◽  
Vol 74 (6) ◽  
pp. 2469-2486 ◽  
Author(s):  
D. C. Fitzpatrick ◽  
S. Kuwada ◽  
R. Batra ◽  
C. Trahiotis
Keyword(s):  
Time Course ◽  
Superior Olivary Complex ◽  
Precedence Effect ◽  
Natural Environments ◽  
Complete Suppression ◽  
Sound Waves ◽  
Neural Basis ◽  
Unanesthetized Rabbit ◽  
Echo Threshold ◽  
Time Required

1. In most natural environments, sound waves from a single source will reach a listener through both direct and reflected paths. Sound traveling the direct path arrives first, and determines the perceived location of the source despite the presence of reflections from many different locations. This phenomenon is called the "law of the first wavefront" or "precedence effect." The time at which the reflection is first perceived as a separately localizable sound defines the end of the precedence window and is called "echo threshold." The precedence effect represents an important property of the auditory system, the neural basis for which has only recently begun to be examined. Here we report the responses of single neurons in the inferior colliculus (IC) and superior olivary complex (SOC) of the unanesthetized rabbit to a sound and its simulated reflection. 2. Stimuli were pairs of monaural or binaural clicks delivered through earphones. The leading click, or conditioner, simulated a direct sound, and the lagging click, or probe, simulated a reflection. Interaural time differences (ITDs) were introduced in the binaural conditioners and probes to adjust their simulated locations. The probe was always set at the neuron's best ITD, whereas the conditioner was set at the neuron's best ITD or its worst ITD. To measure the time course of the effects of the conditioner on the probe, we examined the response to the probe as a function of the conditioner-probe interval (CPI). 3. When IC neurons were tested with conditioners and probes set at the neuron's best ITD, the response to the probe as a function of CPI had one of two forms: early-low or early-high. In early-low neurons the response to the probe was initially suppressed but recovered monotonically at longer CPIs. Early-high neurons showed a nonmonotonic recovery pattern. In these neurons the maximal suppression did not occur at the shortest CPIs, but rather after a period of less suppression. Beyond this point, recovery was similar to that of early-low neurons. The presence of early-high neurons meant that the overall population was never entirely suppressed, even at short CPIs. Taken as a whole. CPIs for 50% recovery of the response to the probe among neurons ranged from 1 to 64 ms with a median of approximately 6 ms. 4. The above results are consistent with the time course of the precedence effect for the following reasons. 1) The lack of complete suppression at any CPI is compatible with behavioral results that show the presence of a probe can be detected even at short CPIs when it is not separately localizable. 2) At a CPI corresponding to echo threshold for human listeners (approximately 4 ms CPI) there was a considerable response to the probe, consistent with it being heard as a separately localizable sound at this CPI. 3) Full recovery for all neurons required a period much longer than that associated with the precedence effect. This is consistent with the relatively long time required for conditioners and probes to be heard with equal loudness. 5. Conditioners with either the best ITD or worst ITD were used to determine the effect of ITD on the response to the probe. The relative amounts of suppression caused by the two ITDs varied among neurons. Some neurons were suppressed about equally by both types of conditioners, others were suppressed more by a conditioner with the best ITD, and still others by a conditioner with the worst ITD. Because the best ITD and worst ITD presumably activate different pathways, these results suggest that different neurons receive a different balance of inhibition from different sources. 6. The recovery functions of neurons not sensitive to ITDs were similar to those of ITD-sensitive, neurons. This suggests that the time course of suppression may be common among different IC populations. 7. We also studied neurons in the SOC. Although many showed binaural interactions, none were sensitive to ITDs. Thus the response of this population may not be

Time Course of Cross-Orientation Suppression in the Early Visual Cortex

2009 ◽  
Vol 101 (3) ◽  
pp. 1463-1479 ◽  
Author(s):  
Rui Kimura ◽  
Izumi Ohzawa
Keyword(s):  
Visual Cortex ◽  
Time Course ◽  
Individual Component ◽  
Correlation Method ◽  
Suppressive Effect ◽  
Negative Response ◽  
Reverse Correlation ◽  
Response Onset ◽  
Early Visual Cortex ◽  
Visual Neuron

Responses of a visual neuron to optimally oriented stimuli can be suppressed by a superposition of another grating with a different orientation. This effect is known as cross-orientation suppression. However, it is still not clear whether the effect is intracortical in origin or a reflection of subcortical processes. To address this issue, we measured spatiotemporal responses to a plaid pattern, a superposition of two gratings, as well as to individual component gratings (optimal and mask) using a subspace reverse-correlation method. Suppression for the plaid was evaluated by comparing the response to that for the optimal grating. For component stimuli, excitatory and negative responses were defined as responses more positive and negative, respectively, than that to a blank stimulus. The suppressive effect for plaids was observed in the vast majority of neurons. However, only ∼30% of neurons showed the negative response to mask-only gratings. The magnitudes of negative responses to mask-only stimuli were correlated with the degree of suppression for plaid stimuli. Comparing the latencies, we found that the suppression for the plaids starts at about the same time or slightly later than the response onset for the optimal grating and reaches its maximum at about the same time as the peak latency for the mask-only grating. Based on these results, we propose that in addition to the suppressive effect originating at the subcortical stage, delayed suppressive signals derived from the intracortical networks act on the neuron to generate cross-orientation suppression.

Periodicity coding in the inferior colliculus of the cat. I. Neuronal mechanisms

1988 ◽  
Vol 60 (6) ◽  
pp. 1799-1822 ◽  
Author(s):  
G. Langner ◽  
C. E. Schreiner
Keyword(s):  
Firing Rate ◽  
Inferior Colliculus ◽  
Amplitude Modulation ◽  
Transfer Functions ◽  
Modulation Frequency ◽  
Temporal Structure ◽  
Stimulus Frequency ◽  
Single Units ◽  
Central Nucleus ◽  
Multiple Unit

1. Temporal properties of single- and multiple-unit responses were investigated in the inferior colliculus (IC) of the barbiturate-anesthetized cat. Approximately 95% of recording sites were located in the central nucleus of the inferior colliculus (ICC). Responses to contralateral stimulation with tone bursts and amplitude-modulated tones (100% sinusoidal modulation) were recorded. Five response parameters were determined for neurons at each location: 1) characteristic frequency (CF); 2) onset latency of responses to CF-tones 60 dB above threshold; 3) Q10 dB (CF divided by bandwidth of tuning curve 10 dB above threshold); 4) best modulation frequency for firing rate (rBMF or BMF; amplitude modulation frequency that elicited the highest firing rate); and 5) best modulation frequency for synchronization (sBMF; amplitude modulation frequency that elicited the highest degree of phase-locking to the modulation frequency). 2. Response characteristics for single units and multiple units corresponded closely. A BMF was obtained at almost all recording sites. For units with a similar CF, a range of BMFs was observed. The upper limit of BMF increased approximately proportional to CF/4 up to BMFs as high as 1 kHz. The lower limit of encountered BMFs for a given CF also increased slightly with CF. BMF ranges for single-unit and multiple-unit responses were similar. Twenty-three percent of the responses revealed rBMFs between 10 and 30 Hz, 51% between 30 and 100 Hz, 18% between 100 and 300 Hz, and 8% between 300 and 1000 Hz. 3. For single units with modulation transfer functions of bandpass characteristics, BMFs determined for firing rate and synchronization were similar (r2 = 0.95). 4. Onset latencies for responses to CF tones 60 dB above threshold varied between 4 and 120 ms. Ninety percent of the onset latencies were between 5 and 18 ms. A range of onset latencies was recorded for different neurons with any given CF. The onset response latency of a given unit or unit cluster was significantly correlated with the period of the BMF and the period of the CF (P less than 0.05). 5."Intrinsic oscillations" of short duration, i.e., regularly timed discharges of units in response to stimuli without a corresponding temporal structure, were frequently observed in the ICC. Oscillation intervals were commonly found to be integer multiples of 0.4 ms. Changes of stimulus frequency or intensity had only minor influences on these intrinsic oscillations.(ABSTRACT TRUNCATED AT 400 WORDS)

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