Binaural interaction in low-frequency neurons in inferior colliculus of the cat. IV. Comparison of monaural and binaural response properties.

We studied the monaural and binaural response properties of 82 low-frequency inferior colliculus (IC) neurons that display a clear sensitivity to changes in interaural phase. Most cells (60%) are excited by sound delivered to either ear, the remainder being excited only by stimulation of one ear; 70% of the neurons receive their stronger or sole excitatory input from the contralateral ear. A monotonic relation between spike discharge and sound pressure level (SPL) is seen in 65% of the monaural response areas, i.e., the range of stimulus frequencies and intensities effective in eliciting a response, while 30% show a nonmonotonic response pattern. In 33% of the cases there is a significant shift in the most effective frequency as a function of SPL. Most discharge patterns are classified as sustained (69%) and the remainder as onset. However, there is considerable variability within these patterns and often two types of discharges are present at different points in the same response area of a single cell. The sustained responses show a broad range of latencies, while onset patterns show a tighter distribution and shorter first spike latencies. Thus, IC neurons showing sensitivity to changes in interaural phase can differ in laterality preferences, response area characteristics, discharge patterns, and latency parameters. Given the diversity of inputs to the IC from lower brain stem structures, this heterogeneity is not surprising. For most neurons excited by stimulation to either ear, the characteristic frequencies, discharge patterns, and first spike latencies are similar, suggesting that the monaural inputs to a binaural cell are of the same type. A neuron's most effective frequencies at a particular SPL for monaural and binaural stimulation are, in general, the same. In some cases a neuron's monaural and binaural response areas can show remarkable similarities, suggesting that certain monaural features are intimately related to the binaural response. In 18% of the IC cells, phase locking to the monaural stimulating frequency is seen. When both inputs are phase locked, a simple coincidence model can predict the interaural phase or delay at which the maximal binaural discharge occurs.