Classification of response patterns in cochlear nucleus of barn owl: correlation with functional response properties

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Abstract
Response patterns of neurons in the cochlear nuclei of the barn owl (Tyto alba) were studied by obtaining poststimulus time histograms (PSTHs) and interspike interval histograms for the response to short tone bursts at the neuron's characteristic frequency. The observed response patterns can be classified according to the scheme developed for neurons of the mammalian cochlear nuclear complex (22). Neurons of the magnocellular cochlear nucleus (n. magnocellularis), which respond in a phase-locked manner to sinusoidal signals and do not show large increases in spike discharge rate with changes in stimulus intensity (26), have "primarylike" (PSTH) discharge patterns and broad interspike interval histograms. This indicates that magnocellular neurons have irregular firing patterns, with the timing of individual spikes being dependent on the phase of the stimulus waveform. Neurons of the angular cochlear nucleus (n. angularis), which show little or no phase-locking and large increases in spike rate with increasing intensity (26), had almost exclusively "transient chopper" discharge patterns. The interspike interval histograms of these angular units are sharp, indicating that their discharge is very regular. At the onset of the response where the chopper pattern is observed, both discharge regularity and rate-intensity sensitivity are at their maximum levels. Several "onset" units were isolated in the angular cochlear nucleus, but no "pauser" or "buildup" units were seen. Also, all of the units in the angular nucleus had monotonic rate-intensity functions. Thus no neural response patterns typical of mammalian dorsal cochlear nucleus units were observed. The relationship of response pattern type to neural function is discussed in relation to the acoustic cues used by the owl for two-dimensional sound localization. The primarylike, phase-locked discharge of magnocellular units is undoubtedly involved in the analysis of interaural differences in stimulus phase, which the owl uses for horizontal localization. There is strong evidence suggesting that the angular nucleus is involved in processing stimulus intensity information, which is important for determining sound elevation (due to asymmetries in vertical directionality of the owl's external ears). The predominant chopper patterns seen in the angular nucleus suggest that in the owl, this response type is correlated with stimulus intensity processing. Similarities in both anatomy and physiology suggest that the magnocellular nucleus is analogous to the spherical cell or bushy cell population of the anterior division of the mammalian anteroventral cochlear nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)