DORSAL COCHLEAR NUCLEUS SINGLE NEURONS CAN ENHANCE TEMPORAL PROCESSING CAPABILITIES IN BACKGROUND-NOISE

Envelope temporal fluctuations are critical for effective processing o f biologically relevant sounds including speech, animal vocalizations, sound-source location and pitch. Amplitude modulation (AM) of sound e nvelopes can be encoded in quiet with high fidelity by some auditory n eurons, including those of the cochlear nucleus. From both neurophysio logical and clinical perspectives, it is important to understand the e ffects of background noise on the processing of AM. To further this go al, single-unit recordings were made from dorsal cochlear nucleus (DCN ) units in urethane-anesthetized chinchillas. All units of this study were classified as pauser/buildup or On-s units according to PSTH resp onse patterns, first spike latencies, and shape of best-frequency (BF) rate-intensity functions. BF pure-tone and AM (10-500 Hz) tone bursts were presented at several sound levels, in quiet and in the presence of a continuous wideband masker. The following was found: (1) DCN unit s can enhance their AM coding relative to quiet in the presence of lou d noise (+14 or +19 dB S/N) and at high signal levels (e.g. 75 dB SPL) ;(2) for the sample of units of the present study, this is usually ach ieved by lowering the average firing rate and increasing the synchrono us (fundamental frequency) response; (3) for some units, the AM coding stays the same or declines in the background noise. The nature of the se findings suggests that part of a DCN unit's abilities to preserve o r enhance AM coding with masking noise results from peripheral operati ng range shifts, whereas part comes from intrinsic circuitry (inhibito ry inputs) or cellular mechanisms (dendritic filtering of sound tempor al features) within the DCN.