This project will study how speech stimuli are presented in the ventral cochlear nucleus using parallel physiological and psychophysical studies in a rodent with low frequency thresholds near that of humans, chinchilla laniger. The representation of consonants, vowels, concurrent vowels and vowels after noise damage of the cochlea will be studied in the three principal response categories of units, primary-like, onset and chopper. Particular features of the acoustic signal will be differentially represented in the responses of different classes of units. We hypothesize that phonetic features are robustly encoded in neural responses of units at the level of the cochlear nucleus: Individual cell groups covey different spectro-temporal features of complex spectra to higher centers: primary-like neurons code fine spectral features and preserve time-place information; chopper neurons rate code spectral features such as formants and phase lock to the glottal frequency; onset neurons both code voice pitch and the contribute to segmentation of some speech by virtue of their rapid adaptation. Simultaneous representation of signal features in different unit types is a possible outcome. Ideal observer computational stimulations will be used to quantify the relationship between single- unit activity and behavioral performance. Structure/function studies will employ single-cell labeling after characterization to correlate cell morphology with physiology and projection patterns from anteroventral cochlear nucleus. The main questions are whether globular/bushy cells can respond with PL/N/PL/O/L patterns, whether octopus cells can respond with both O/I and O/L patterns, and whether there is a morphological correlate of the C/S/C/T dichotomy. Responses to virtual-space stimuli, concurrently used for studies in the auditory brainstem and cortex, will also be recorded in order to learn more about the differential functions of these unit categories to sounds of localization significance.
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