Rate representations of formant frequencies in the anteroventral cochlear nucleus

Abstract

Features of the spectrum of a complex stimulus may be encoded as a profile of discharge rate versus best frequency (cochlear place) across the population of auditory‐nerve fibers. Such profiles have been shown to represent the formant frequencies of steady‐state vowels and consonant‐vowel syllables at low and moderate stimulus levels. At high stimulus levels, rate saturation and two‐tone suppression degrade the representation in the population of high‐spontaneous rate (SR) fibers, but not in the low‐SR population. Therefore, the central auditory system could produce a representation that is robust across stimulus level by “selectively listening” to its high‐SR auditory‐nerve inputs at low stimulus levels and to its low‐SR inputs at high levels. This possibility has been explored by measuring rate profiles across populations of cells in the anteroventral cochlear nucleus (AVCN). Specifically, the population of chopper units in the AVCN maintains a robust rate representation of vowel spectra across the entire range of levels tested. At low levels, rate profiles for these units are similar to those of their high‐SR auditory‐nerve inputs and at high levels, where the high‐SR inputs are saturated, they maintain at least as good a spectral representation as do the low‐SR inputs. New data and modeling results that relate to the mechanisms underlying this selective listening process will be considered. [Work supported by an NIDCD Award of Excellence.]