Neural encoding of single-formant stimuli in the ventral cochlear nucleus of the chinchilla

Responses of the principal unit types in the ventral cochlear nucleus of the chinchilla were studied with a single-formant stimulus set that covered fundamental frequency (f0) from 100 Hz to 200 Hz and formant center frequency (F1) from 256 to 782 Hz. Temporal coding for f0 and F1 was explored for 95 stimulus combinations of f0 (n = 5) and F1 (n = 19) in primarylike, onset and chopper unit categories. Several analyses that explored temporal coding were employed including: autocorrelation, interspike interval analysis, and synchronization to each harmonic of f0. In general, the representation of f0 is better in onset and chopper units than in primarylike units. Nearly all units in the cochlear nucleus showed a gain in phase locking to the envelope (f0) of the single-formant stimulus relative to the auditory nerve. The fundamental is represented directly in neural discharges of units in the cochlear nucleus with an interval code (also Cariani and Delgutte, 1996; Rhode, 1995). The formant is represented in the temporal domain in primarylike units, though some chopper and onset units also possess the ability to code F1 through discharge synchrony. Onset-I units, which are associated with the octopus cells, exhibited the strongest phase locking to f0 of any unit types studied. The representation of f0 and F1 in the temporal domain is weak or absent in some units. All-order-interspike interval distributions computed for populations of units show preservation of temporal coding for both f0 and F1. Results are in agreement with earlier amplitude modulation studies that showed nearly all cochlear nucleus unit types phase lock to the signal envelope better than auditory nerve fibers over a considerable range of signal amplitudes.