Responses of single neurons in cat auditory cortex to time-varying stimuli: linear amplitude modulations.

Single neurons in the auditory cortex of anesthetized cats were examined quantitatively for their sensitivity to the sound pressure level of characteristic frequency (CF) tone pulses, and to 6 dB, linear modulations in the amplitude of a continuous CF carrier tone. The direction and rate of amplitude modulation (AM), and the carrier level on which it was imposed, were manipulated parametrically. Studied with amplitude modulations, the majority of neurons responded only to intensity increments. The minimum carrier level upon which an amplitude modulation was able to evoke spike discharges was typically comparable to the tone pulse threshold SPL. For many neurons, an "intensity increment response area", i.e., the domain of AM rate and carrier level conjunctions within which a 6 dB AM was able to evoke discharges, could be delimited. For many neurons, preferred rate of AM drifted from high to low with increases in the carrier level on which the modulation was imposed. The most vigorous responses to AM stimuli often occurred when the carrier levels were associated with the rising slope or the peak of the tone pulse rate intensity function. It may be possible to understand the general form of AM response areas in terms of short-term adaptation, the disposition of excitatory and inhibitory tone pulse response areas, and the spectra of the AM stimuli used.