Processing of interaural time and intensity differences in the cat inferior colliculus.

1. Binaural neurones were recorded in the central nucleus of the cat inferior colliculus and were stimulated with tone and noise bursts. Closed field sound systems were used to produce independent interaural time (ITD) and intensity (IID) differences. Particular attention was paid to high frequency (above 2 kHz) cells. 2. Three main types of binaural neurone were found: High frequency excitatory-inhibitory neurones (EI cells), excited by input from the contralateral ear and inhibited by ipsilateral input, high frequency excitatory-excitatory cells (EE cells), excited by inputs from either ear and low frequency cells sensitive to interaural phase differences (IPD cells). 3. The EI cells had characteristics similar to those of IE cells in the contralateral lateral superior olive. They were sensitive to envelope ITDs (most cells) and IIDs (all cells) favouring the contralateral ear. The response of these cells increased with increasing contra lead ITDs or contra loud IIDs up to values well outside the physiological range. 4. Low frequency binaural cells were sensitive to interaural phase differences (IPDs). The peak response was often in the contralateral physiological range and the response was unaffected by IIDs. 5. Many high frequency EE cells were sensitive to envelope ITDs. These units were relatively unaffected by IID. Although the ITD sensitivity of these cells was generally less than that of the IPD cells, the peak response of the ITD curve was also often in the contralateral physiological range. 6. Some of the high frequency EI and EE cells were sensitive to ongoing time differences (OTDs) in white noise signals, i.e. they showed ITD response curves to carrier only shifted noise bursts. 7. The EI cells often showed recovery from inhibition at large ipsilateral lead. This tendency was increased as the sound pressure level on the inhibitory side was lowered and by the use of click stimuli. Similarly, cycles of suppression could be seen to follow excitation in some EE cells. The time course of these effects was in the order of hundreds of microseconds. 8. Binaural characteristics (degree of ITD, IID or OTD sensitivity) showed considerable interunit variation within each cell type. These variations were also affected by signal type (tone or noise bursts) and did not appear to be correlated with best frequency, nature of the tuning curve or PSTH type. We suggest that the time course of the inhibitory and excitatory effects at each unit (and its interaction with the signal type) determines the type of ITD response and that this time course varies from cell to cell.