Discharge Patterns of Single Units in the Lateral Superior Olive of Decerebrate Cats
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Neurons in the central nucleus of the inferior colliculus (ICC) of decerebrate cats show three major response patterns when tones of different frequencies and sound pressure levels are presented to the contralateral ear. The frequency response maps of type I units are uniquely defined by a narrow V-shaped excitatory area at best frequency (BF) and flanking inhibition at higher and lower frequencies. Units that produce type I maps typically have a high BF (>3 kHz), a high rate of spontaneous activity (~10 spikes/s), and monotonic rate-level curves for BF tones and noise. These units receive ipsilateral inhibition, and display binaural excitatory/inhibitory (EI) interactions. Given this constellation of properties, it has been hypothesized that the contralateral lateral superior olive (LSO) provides the dominant excitatory input to type I units. Most previous studies of the LSO have been made in anesthetized preparations; here, we report on the monaural and binaural response properties of single units in the LSO of decerebrate cats. Monaural responses were classified in the response map scheme, and binaural classification was based on sensitivity to interaural level differences (ILDs). The results reveal that LSO units form a relatively homogeneous population. The response maps of LSO units show V-shaped excitatory tuning and flanking inhibition (in units with spontaneous activity). LSO units have high BFs, low spontaneous rates (> 1 spike/s), monotonic rate-level curves for BF tones and noise, and exhibit IE binaural response properties. These results are largely consistent with prior observations, and support the conjecture that a crossed LSO projection is the primary source of excitation for type I units. Quantitative comparisons between the LSO and ICC unit types suggest that additional excitatory and inhibitory inputs sharpen the tuning and enhance the ILD sensitivity of type I units. Supported by NIDCD grant R01 DC 05161-07.
Response Properties of Single Units in the Lateral Superior Olive of Decerebrate Cats.
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Most studies of the lateral superior olive (LSO) have been conducted in anesthetized preparations. In such preparations, LSO units usually respond to ipsilateral best-frequency tone bursts with a chopper type discharge pattern, characterized by regularly spaced peaks of activity initially time-locked to the stimulus onset. Limited data from one decerebrate cat study suggest, however, that the chopper nature of these responses may be augmented by anesthesia. The goal of the present study was to classify objectively the temporal discharge patterns of single units in the LSO of decerebrate cats. Post-stimulus time histograms obtained at 20 dB re threshold were classified based on a decision tree. Tests in this tree quantify several features of a unitÃs response including reliability of spike activity in the first two peaks of the response, peak to sustained rate ratio, inter-peak gap and discharge regularity as a function of time. Of 32 units recorded, 13 units were classified as primary-like, three as primary-like with notch, six as onset-sustained and 10 as choppers. The first three response types (all associated with cochlear nucleus bushy cells which project to the LSO) fire irregularly. Thus, on the whole, LSO units in a decerebrate preparation are indeed much less regular than those observed in anesthetized preparations. These results suggest that LSO units can follow the temporal patterns of their inputs more faithfully than thought previously, and thereby can transmit monaural temporal information as well as compare effectively instantaneous amplitude fluctuations at the two ears. Supported by NIDCD grant R01 DC 05161.
Relative Importance of Binaural and Monaural Cues to the Directional Sensitivity of Different Unit Types in the Inferior Colliculus.
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Psychophysical experiments have established that three main acoustic cues contribute to the ability to localize sounds in space: interaural differences in time (ITDs) and level (ILDs), and monaural spectral notches (SNs). Initially, these cues are processed in separate brainstem nuclei, with ITDs in the medial superior olive, ILDs in the lateral superior olive, and SNs in the dorsal cochlear nucleus. It has been hypothesized that each of the three major response types (type V, I, and O units) in the central nucleus of the inferior colliculus (ICC) receives dominant inputs from one of these sources (in order), and thus represents a segregated pathway specialized to process one sound localization cue. To test this hypothesis, the responses of individual ICC units to binaural virtual space (full-cue VS) stimuli were compared to those elicited by a variety of partial-cue VS stimuli. Head-related transfer functions (HRTFs) were used to filter broadband noise spectra to synthesize binaural VS stimuli in the frontal field. In response to full-cue VS stimuli, type I units showed excitatory responses in the contralateral hemifield and inhibitory responses in the ipsilateral hemifield. The responses of type O units were dominated by inhibition except for a tuned excitatory response in the contralateral hemifield that followed a diagonal contour from low contralateral to high ipsilateral elevations. Manipulations of the binaural cues suggested that ILD, and not ITD, cues were a strong determinant of the directional sensitivity of both of these unit types. Both unit types showed directional sensitivity to monaural VS stimuli. However, type I unit responses were mainly attributable to the effects of sound level (head shadowing), whereas type O units were more sensitive to SNs. These data show that ICC unit types are not sensitive to one sound localization cue alone, but do show different patterns of cue integration. Supported by NIDCD grant R01 DC 05161-06.
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