We studied the sensitivity of cells in the medial superior olive (MSO) of the anesthetized cat to variations in interaural phase differences (IPDs) of low-frequency tones and in interaural time differences (ITDs) of tones and broad-band noise signals. Our sample consisted of 39 cells histologically localized to the MSO. All but one of the cells had characteristic frequencies <3 kHz, and 79% were sensitive to ITDs and IPDs. More than one-half (56%) of the cells responded to monaural stimulation of either ear, and both the binaural and monaural responses were highly phase locked. All of the cells that were sensitive to IPDs and monaurally driven by either ear responded in accord with that predicted by the coincidence model of Jeffress, as judged by comparison of the phases at which the monaural and binaural responses occurred. The optimal IPDs were tightly clustered between 0.0 and 0.2 cycles. Most cells exhibited facilitation of the response at favorable ITDs and inhibition at unfavorable ITDs compared with the monaural responses. Cells in the MSO exhibited characteristic delay, as judged by a linear relationship between the mean interaural phase and stimulating frequency. Characteristic phases were clustered near 0 indicating that most cells responded maximally when the two input tones were in phase. With the use of binaural beat stimulus we found no differential selectivity for either the direction or speed of interaural phase changes. The cells were also sensitive to ITDs of broad-band noise signals. The ITD curve in response to broad-band noise was similar to that predicted by the composite curve, which was calculated by linearly summating the tonal responses over the frequencies in the response area of the cell. Most (93%) of the peaks of the composite curves were between 0 and +400 μs, corresponding to locations in the contralateral sound field. Moreover, computer cross correlations of the monaural spike trains were similar to the ITD curve generated binaurally for both correlated and uncorrelated noise signals to the two ears. Thus our data suggest that the cells in the MSO behave much like cross-correlators. By combining data from different animals and locating each cell on a standard MSO, we found evidence for a spatial map of ITDs across the anterior-posterior (A-P) axis of the MSO. Low ITDs, near 0 μs, were located near the anterior pole, and more positive ITDs, corresponding to delays of the ipsilateral stimulus, were mapped at more posterior locations. Some general response characteristics, such as spontaneous activity, shape of the poststimulus time histogram, tuning curves, response areas, and rate-level curves, were also examined. Generally, the response characteristics in response to ipsilateral and contralateral stimulation were similar. The phase-locking ability of cells in the MSO, as measured by the synchronization coefficient, was consistently over that seen in the auditory nerve. We compared the monaural and binaural response properties of cells in the MSO with that in the central nucleus of the inferior colliculus (ICC). Although many similarities were seen, there were also differences that presumably reflected additional processing at the level of the ICC.
