PROCESSING OF MODULATION FREQUENCY IN THE DORSAL COCHLEAR NUCLEUS OF THE GUINEA-PIG - AMPLITUDE-MODULATED TONES

The modulation frequency (Fm), particularly high Fm (> 200 Hz), in amp litude modulated (AM) tones can elicit the perception of the periodici ty pitch (Langner, 1992). In this study, single unit responses to the Fms of the sinusoidal AM tones were investigated at 50 to 90 dB SPL. T he recordings were made from the dorsal cochlear nucleus (DCN) of neur oleptic anesthetized guinea pigs with an intact cerebellum The DCN uni ts show a good capability of phase-locking to Fm at 400-1200 Hz. On-S- type II and Pauser/Buildup (P/B) units have a high modulation gain (7. 2-8.3 dB). P/B units can preserve the high modulation gain (5-9 dB) up to 90 dB SPL. The modulation gain exponentially increases with decrea sing modulation depth (Dm) and the phase-locking is detectable even at the Dm as low as 2-5%. The 'central skipping' of the phase-locking pe ak has been found at deep Dms in a few cases. The synchronization is i ndependent of the discharge rate and can remain high even when the res ponses to AM tones are inhibited below the spontaneous activity. Such encoding behaviors over the unit's response area show that the Fm phas e-locking is strong near or at its characteristic frequency (CF). The synchronization index (SI) versus carrier frequency (Fc) curve is simi lar to the inverse shape of tuning curve but more narrowly tuned than the iso-intensity function of pure tones at moderate to high intensity levels. The phase-locking is related to the unit's spontaneous rate ( SR). The average modulation gain of the lower SR (less than or equal t o 2 spikes/s) units is 5 dB higher than that of the higher SR (> 2 spi kes/s) units (8.16 and 2.92 dB, respectively) at 70 dB SPL. These resu lts suggest that AM information is temporally encoded over broad range s of modulation parameters in the DCN and is conveyed by the Fc channe l. Such a timing mechanism can play an important role in processing of complex sounds under normal acoustic conditions.