Binaural interaction revisited in the cat primary auditory cortex.

The binaural interactions of neurons were studied in the primary auditory cortex (AI) of barbiturate-anesthetized cats with a matrix of binaural tonal stimuli varying in both interaural level differences (ILD) and average binaural level (ABL). The purpose of this study was to determine: 1) the distribution of preferred binaural combinations (PBCs) of a large population of neurons and its relationships with binaural interactions and binaural monotonicity; 2) whether monaural responses are predictive of binaural responses; and 3) whether there is a restricted set of representative binaural stimulus configurations that could effectively classify the binaural interactions. Binaural interactions were often diverse in the matrix and dependent on both ABL and ILD. Compared with previous studies, a higher proportion of mixed binaural interaction type and a lower proportion of EO/I type were found. No monaural neurons were found. Binaural responses often differed from monaural responses in the number of spikes and/or the form of the response functions. The PBCs of the majority of EO and PB neurons were in the contralateral field and midline, respectively. However, the PBCs of EE units were evenly distributed across the contralateral and ipsilateral fields. The majority of the nonmonotonic neurons responded most strongly to lower ABLs, whereas the majority of monotonic neurons responded most strongly to higher ABLs. This study demonstrated that in AI a restricted set of binaural stimulus configurations is not sufficient to reveal the binaural responses properties. Also, monaural responses are not predictive of binaural responses.