Binaural interaction in low-frequency neurons in inferior colliculus of the cat. I. Effects of long interaural delays, intensity, and repetition rate on interaural delay function

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Abstract
Detailed, quantitative studies were made of the interaural phase sensitivity of 197 neurons with low best frequency in the inferior colliculus (IC) of the barbiturate-anesthetized cat. The responses of single cells to interaural delays in which tone bursts were delivered to the 2 ears via sealed earphones and the onset of the tone to 1 ear with respect to the other was varied were analyzed. For most (80%) cells the discharge rate is a cyclic function of interaural delay at a period corresponding to that of the stimulating frequency. The cyclic nature of the interaural delay curve indicates that these cells are sensitive to the interaural phase difference. These cells are distributed throughout the low-frequency zone of the IC, but they are less numerous in the medial and caudal zones. Cells with a wide variety of response patterns will exhibit interaural phase sensitivities at stimulating frequencies up to 3100 Hz although at > 2500 Hz the number of such cells decreases markedly. Using dichotic stimuli the cell''s sensitivity to the onset delay and interaural phase independently was studied. The large majority of IC cells respond only to changes in interaural phase, with no sensitivity to the onset delay. A small number (7%) of cells exhibit a sensitivity to the onset delay as well as to the interaural phase disparity, and most of these cells show an onset response. The effects of changing the stimulus intensity equally to both ears or of changing the interaural intensity difference on the mean interaural phase were studied. While some neurons are not affected by level changes, others exhibit systematic phase shifts for both average and interaural intensity variations, and there is a continuous distribution of sensitivities between these extremes. A few cells also showed systematic changes in the shape of the interaural delay curves as a function of interaural intensity difference, especially at very long delays. These shifts can be interpreted as a form of time-intensity trading. A few cells demonstrated orderly changes in the interaural delay curve as the repetition rate of the stimulus was varied. Some of these changes are consonant with an inhibitory effect that occurs at stimulus offset. The responses of the neurons show a strong bias for stimuli that would originate from the contralateral sound field; 77% of the responses display mean interaural phase angles that are less than 0.5 of a cycle, which are delays to the ipsilateral tone. When converted to equivalent interaural delays, 59% of the delays fall between .+-. 300 .mu.s and 71% are between .+-. 400 .mu.s.

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