Formant coding in the cochlear nucleus—Is there a place for time?

Abstract

Ever since Ohm and Seebeck, hearing theory has pondered whether “place” or “time” is the dominant means by which acoustic information is encoded in the ear. Over the past decade it has become increasingly clear that, at the level of the auditory nerve, both place and timing mechanisms participate in the representation of complex sounds. As a consequence, attention is now focused on the cochlear nucleus (CN), to which the auditory nerve projects. Within this nuclear complex occurs a remarkable degree of physiological diversity of potential significance to speech coding. The present survey reviews the temporal response properties of the major CN cell types to voiced CV syllables and the potential role played by each in the encoding of speech. All of the major response classes—primarylike, onset, chopper, onset‐chopper, and pauser‐buildup—are capable of synchronizing to the fundamental frequency or first formant. However, the range of acoustic conditions over which they do so, as well as the precision and upper frequency limits of phase locking, appreciably differ. Of particular interest are the onset‐choppers of the posteroventral division, which temporally encode the lower formants and f0 with a degree of precision unrivaled by other CN cells or auditory‐nerve fibers. Their superior performance is probably a consequence of integrating afferent activity spanning a broad tonotopic range. The majority of CN units retain their ability to temporally encode low‐frequency information in all but the most intense masking noise, consistent with the hypothesis that phase locking provides an effective means of reliably encoding informationally significant elements under acoustically uncertain conditions. [Work supported by NIDCD.]