Temporal encoding and transmitting of amplitude and frequency modulations in dorsal cochlear nucleus

Amplitude- and frequency-modulated (AM and FM, respectively) tones have been considered as simplified models of natural sounds. The responses 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 not any f(m) physical peak in spectrum. In the present study, we approached these issues by recording the phase-locked responses of the dorsal cochlear nucleus (DCN) units in guinea pigs to different AM and FM tones. For AM noise tones without the spectral cues of f(m), the unit's discharges still phase-locked to the envelope cycles, but it was generally weaker than to sinusoidal AM (SAM) tones. At 50% modulation depth (d(m)), the mean modulation gains of Pauser/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 courses of frequency changes, and it could be separated and changeable corresponding to the frequency increasing and decreasing. There were differences 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-locking to the SFM tones than to the SAM tones. The phase-locking to the possible demodulated f(m) components was further examined with different carrier frequencies (f(c)) and pure tones. The DCN units showed pooror no responses to modulation tones out of their response areas even in the low characteristic frequency (CF) units, but the low-CF units had clear phase-locking to pure tones at the similar f(m) ranges. The pure-tone phase-locking had a band-pass shape different from the low-pass shape of the auditory nerve fibers. These data suggest that the modulation phase-locking in the DCN units may be based on the temporal modulation cues and transmitted in the carrier place. The temporal integration of modulation information over the unit's response area as an across-frequency temporal processing model was discussed for modulation enhancement in the CN units.