Responses of cochlear nucleus units in the chinchilla to iterated rippled noises: Analysis of neural autocorrelograms

Responses of cochlear nucleus units in the chinchilla to iterated rippled n oises: analysis of neural autocorrelograms. J. Neurophysiol. 81: 2662-2674, 1999. Temporal encoding of stimulus features related to the pitch of itera ted rippled noises was studied for single units in the chinchilla cochlear nucleus. Unlike ether periodic complex sounds that produce pitch, iterated rippled noises have neither periodic waveforms nor highly modulated envelop es. Infinitely iterated rippled noise (TIRN) is generated when wideband noi se (WBN) is delayed (tau) attenuated, and then added to (+) or subtracted f rom (-) the undelayed WBN through positive feedback. The pitch of IRN[+, ta u, -1 dB] is at 1/r, whereas the pitch of IIRN[-, tau, -1 dB] is at 1/2 tau . Temporal responses of cochlear nucleus units were measured using neural a utocorrelograms. Synchronous responses as shown by peaks in neural autocorr elograms that occur at time lags corresponding to the IIRN tau can be obser ved for both primarylike and chopper unit types. Comparison of the neural a utocorrelograms in response to IIRN[+, tau, -1 dB] and IIRN[-, tau, -1 dB] indicates that the temporal discharge of primarylike units reflects the sti mulus waveform fine structure, whereas the temporal discharge patterns of c hopper units reflect the stimulus envelope. The pitch of IIRN[+/-, tau, -1 dB] can be accounted for by the temporal discharge patterns of primarylike units but not by the temporal discharge of chopper units. To quantify the t emporal responses, the height of the peak in the neural autocorrelogram at a given time lag was measured as normalized rate. Although it is well docum ented that chopper units give larger synchronous responses than primarylike units to the fundamental frequency of periodic complex stimuli, the larges t normalized rates in response to IIRN[+, tau, -1 dB] were obtained for pri marylike units, not chopper units. The results suggest that if temporal enc oding is important in pitch processing, then primarylike units are likely t o be an important cochlear nucleus subsystem that carries the pitch-related information to higher auditory centers.