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 col liculus (ICC) of kittens 4-43 days of age were studied using sinusoida lly amplitude-modulated (AM) tones delivered monaurally or binaurally via sealed and calibrated earphones. The carrier frequency of the AM s ignal was set to the CF of the neuron. CFs ranged from 2-26 kHz. Durin g the about first 2 weeks of postnatal life, ICC neurons responded to sound with periodic bursts of activity. In response to AM tones, disch arges of ICC neurons at all ages studied were phase-locked to the enve lope of the modulation waveform over a wide range of stimulus level an d modulation depth. A linear relationship, independent of SPL, was fou nd between the average phase of discharge on the modulation cycle and modulation frequency. The slope of the line represents a time delay, w hich 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 sys tematic change in average phase of discharge with changing stimulus le vel or modulation depth, although the number of spikes evoked and the temporal pattern of the spikes within a modulation cycle could vary. T he sensitivity function relating spike synchrony or spike count to mod ulation frequency was typically band-pass in nature. The most effectiv e modulation frequency (MEMF) was, on average, 15 Hz, far below that r eported for adult cat ICC cells. When AM tones were delivered binaural ly, the discharge was a periodic function of the interaural phase diff erence of the stimulus envelopes. The results indicate that prior to t he time the cochlea is able to respond to most environmental sounds, m onaural and binaural circuits involving the ICC faithfully transmit in formation pertaining to amplitude-modulated signals in the rate and ti ming of their discharges. During the next several weeks, when neural t hresholds fall to adult levels, ICC circuits are activated by amplitud e modulated sounds at levels encountered in the normal acoustic enviro nment even though they are restricted to modulation frequencies below those encoded by the adult.