Responses of neurons in the cat primary auditory cortex to sequential sounds

In the natural acoustic environment sounds fre- quently arrive at the two ears in quick succession. The re- sponses of a cortical neuron to acoustic stimuli can be dra- matically altered, usually suppressed, by a preceding sound. The purpose of this study was to determine if the binaural interaction evoked by a preceding sound is involved in sub- sequent suppressive interactions observed in auditory cor- tex neurons. Responses of neurons in the primary auditory cortex (AI) exhibiting binaural suppressive interactions (EO/I) were studied in barbiturate-anesthetized cats. For the major- ity (72.5%) of EO/I neurons studied, the response to a mon- aural contralateral stimulus was suppressed by a preceding monaural contralateral stimulus, but was not changed by a preceding monaural ipsilateral stimulus. For this subset of EO/I neurons, when a monaural contralateral stimulus was preceded by a binaural stimulus, the level of both the ipsilat- eral and the contralateral component of the binaural stimulus influenced the response to the subsequent monaural con- tralateral stimulus. When the contralateral level of the binau- ral stimulus was constant, increasing its ipsilateral level de- creased the suppression of the response to the subsequent monaural contralateral stimulus. When the ipsilateral level of the binaural stimulus was constant, increasing its contralat- eral level increased the suppression of the response to the subsequent monaural contralateral stimulus. These results demonstrate that the sequential inhibition of responses of AI neurons is a function of the product of a preceding binaural interaction. The magnitude of the response to the contralateral stimulus is related to, but not determined by the magnitude of the response to the preceding binaural stimulus. Possible mechanisms of this sequential interaction are discussed.