Characteristics of unit responses at successive levels of the auditory pathway to time-dependent acoustic stimuli

Abstract

Various characteristics of afferent impulsation arising in different parts of the auditory system — the cochlear nuclei, inferior colliculi, and auditory cortex — in response to amplitude- and frequency-modulated stimuli were compared. Along the path from the cochlear nuclei to the inferior colliculus and auditory cortex the number of neurons describing the temporal structure of the envelope of the amplitude-modulated stimulus in the pattern of spike activity decreases sharply where-as the number of neurons responding only to certain phases of the envelope increases, the number of spikes in the discharge of neurons at higher levels of the auditory pathway decreases, and the range of repetition of the modulation rhythms narrows on account of restriction on both the high-frequency and the low-frequency sides. The selectivity of the responses to a combination of stimulus parameters such as the carrier frequency and rhythm of amplitude modulation increases. The specificity of unit responses at higher levels of the auditory system increases in response to certain combinations of parameters of frequency-modulated sounds such as the frequency range in the stimulus and the direction and rate of frequency modulation. On the basis of this investigation some general tendencies for the transformation of impulsation and successive levels of the auditory pathway are indicated.