TEMPORAL ENCODING AND TRANSMITTING OF AMPLITUDE AND FREQUENCY MODULATIONS IN DORSAL COCHLEAR NUCLEUS

Amplitude- and frequency-modulated (AM and FM, respectively) tones hav e been considered as simplified models of natural sounds. The response s of auditory neurons can phase-lock to the modulation frequency (f(m) ). The encoding and transmitting of such modulation phase-locking are interesting since there is no any f(m) physical peak in spectrum. In t he present study, we approached these issues by recording the phase-lo cked responses of the dorsal cochlear nucleus (DCN) units in guinea pi gs to different AM and FM tones. For AM noise tones without the spectr al cues of f(m), the unit's discharges still phase-locked to the envel ope cycles, but it was generally weaker than to sinusoidal AM (SAM) to nes. At 50% modulation depth (d(m)), the mean modulation gains of Paus er/ Buildup (P/B) units (n = 7) to AM noise tones was -0.61 dB whereas they had a 6.48 dB mean to SAM tones. Similar to the case of AM tones , phase-locking to sinusoidal FM (SFM) tones represented the time cour ses of frequency changes, and it could be separated and changeable cor responding to the frequency increasing and decreasing. There were diff erences between the phase-locking to SAM and SFM tones in an identical unit. Both ON and type I/III units tended to have stronger phase-lock ing to the SFM tones than to the SAM tones. The phase-locking to the p ossible demodulated f(m) components was further examined with differen t carrier frequencies (f(m)) and pure tones. The DCN units showed poor or no responses to modulation tones out of their response areas even in the low characteristic frequency (CF) units, but the low-CF units h ad clear phase-locking to pure tones at the similar f(m) ranges. The p ure-tone phase-locking had a band-pass shape different from the low-pa ss shape of the auditory nerve fibers. These data suggest that the mod ulation phase-locking in the DCN units may be based on the temporal mo dulation cues and transmitted in the carrier place. The temporal integ ration of modulation information over the unit's response area as an a cross-frequency temporal processing model was discussed for modulation enhancement in the CN units.