ENCODING OF AMPLITUDE-MODULATED TONES BY NEURONS OF THE INFERIOR COLLICULUS OF THE KITTEN

Responses of single neurons of the central nucleus of the inferior colliculus (ICC) of kittens 4-43 days of age were studied using sinusoidally amplitude-modulated (AM) tones delivered monaurally or binaurally via sealed and calibrated earphones. The carrier frequency of the AM signal was set to the CF of the neuron. CFs ranged from 2-26 kHz. During the about first 2 weeks of postnatal life, ICC neurons responded to sound with periodic bursts of activity. In response to AM tones, discharges of ICC neurons at all ages studied were phase-locked to the envelope of the modulation waveform over a wide range of stimulus level and modulation depth. A linear relationship, independent of SPL, was found between the average phase of discharge on the modulation cycle and modulation frequency. The slope of the line represents a time delay, which was highly correlated with the first-spike latency to tone onset, and hence with the age of the animal. The mean effective phase of the discharge remained relatively constant with age. There was little systematic change in average phase of discharge with changing stimulus level or modulation depth, although the number of spikes evoked and the temporal pattern of the spikes within a modulation cycle could vary. The sensitivity function relating spike synchrony or spike count to modulation frequency was typically band-pass in nature. The most effective modulation frequency (MEMF) was, on average, 15 Hz, far below that reported for adult cat ICC cells. When AM tones were delivered binaurally, the discharge was a periodic function of the interaural phase difference of the stimulus envelopes. The results indicate that prior to the time the cochlea is able to respond to most environmental sounds, monaural and binaural circuits involving the ICC faithfully transmit information pertaining to amplitude-modulated signals in the rate and timing of their discharges. During the next several weeks, when neural thresholds fall to adult levels, ICC circuits are activated by amplitude modulated sounds at levels encountered in the normal acoustic environment even though they are restricted to modulation frequencies below those encoded by the adult.