VOWEL REPRESENTATIONS IN THE VENTRAL COCHLEAR NUCLEUS OF THE CAT - EFFECTS OF LEVEL, BACKGROUND-NOISE, AND BEHAVIORAL STATE

Single-unit responses were studied in the ventral cochlear nucleus (VC N) of cats as formant and trough features of the vowel /epsilon/ were shifted in the frequency domain to each unit's best frequency (BF; the frequency of greatest sensitivity). Discharge rates sampled with this spectrum manipulation procedure (SMP) were used to estimate vowel rep resentations provided by populations of VCN neurons. In traditional po pulation measures, a good representation of a vowel's formant structur e is based on relatively high discharge rates among units with BFs nea r high-energy formant features and low rates for units with BFs near l ow-energy spectral troughs. At most vowel levels and in the presence o f background noise, chopper units exhibited formant-to-trough rate dif ferences that were larger than VCN primary-like units and auditory-ner ve fibers. By contrast, vowel encoding by primary-like units resembled auditory nerve representations for most stimulus conditions. As is se en in the auditory nerve, primarylike units with low spontaneous rates (SR <18 spikes/s) produced better representations than high SR primar y-like units at all but the lowest vowel levels. Awake cats exhibited the same general response properties as anesthetized cats but larger b etween-subject differences in vowel driven rates. The vowel encoding p roperties of VCN chopper units support previous interpretations that p atterns of auditory nerve convergence on cochlear nucleus neurons comp ensate for limitations in the dynamic range of peripheral neurons.