Single Cell Responses to AM Tones of Different Envelops at the Auditory Midbrain

Abstract

Amplitude modulation (AM) and frequency modulation (FM) are important components of complex acoustic signals including speech. Response to AM tone is found at many places of the central auditory system particularly the inferior colliculus (IC) of the midbrain. With a repetitive sinusoidal AM tone, IC cells could respond best to a preferred modulation rate, indicating the rate of amplitude change is an important response determinant. Notably, most spike responses are found during the rising phase of the AM slope, and rarely during its falling phase. However, cells at the auditory cortex are known to respond to a sound source moving away from the animal, and presumably the sound would contain an AM component corresponding to a drop in amplitude. We speculated that the falling phase of an AM tone could evoke cell response at levels below the cortex, and that the response may depend on certain feature of the AM stimulus. In this study we systematically varied the envelop of an AM tone and examined its effects on single cell response at the IC of urethane-anesthetized rats.