Tone-sequence analysis in the auditory cortex of awake macaque monkeys.

The present study analyzed neuronal responses to two-tone sequences in the auditory cortex of three awake macaque monkeys. The monkeys were passively exposed to 430 different two-tone sequences, in which the frequency of the first tone and the interval between the first and the second tone in the sequence were systematically varied. The frequency of the second tone remained constant and was matched to the single-tone frequency sensitivity of the neurons. Multiunit activity was recorded from 109 sites in the primary auditory cortex and posterior auditory belt. We found that the first tone in the sequence could inhibit or facilitate the response to the second tone. Type and magnitude of poststimulatory effects depended on the sequence parameters and were related to the single-tone frequency sensitivity of neurons, similar to previous observations in the auditory cortex of anesthetized animals. This suggests that some anesthetics produce, at the most, moderate changes of poststimulatory inhibition and facilitation in the auditory cortex. Hence many properties of the sequence-sensitivity of neurons in the auditory cortex measured in anesthetized preparations can be applied to neurons in the auditory cortex of awake subjects.