RESPONSES OF NEURONS IN THE INFERIOR COLLICULUS OF THE RAT TO INTERAURAL TIME AND INTENSITY DIFFERENCES IN TRANSIENT STIMULI - IMPLICATIONSFOR THE LATENCY HYPOTHESIS

Although the sensitivity to interaural intensity differences (IIDs) of neurons receiving excitatory - inhibitory binaural input (EI neurons) has been examined in numerous studies, the mechanisms underlying this sensitivity remain unclear. According to the 'latency hypothesis', ne uronal sensitivity to IIDs reflects sensitivity to differences in the timing of ipsilateraI and contralateral inputs that are produced as a consequence of the effects of intensity upon latency. If the latency h ypothesis is correct, a neuron's responses over any given IID range sh ould be predicted by its responses to the interauraI time differences (ITDs) that are 'equivalent' to the IIDs tested, in the sense that the y produce the same changes in the relative timing of inputs. This pred iction from the latency hypothesis was examined by determining the sen sitivity of ET neurons in the inferior colliculus of anesthetized rats to IIDs and ITDs in click stimuli, under conditions that allowed 'equ ivalent' ITDs to be estimated. In approximately 10% of the 41 neurons tested, the IID-sensitivity function was a perfect or near-perfect mat ch to the equivalent-ITD function, indicating that IID sensitivity cou ld be entirely accounted for in terms of sensitivity to intensity-prod uced neural time differences, as asserted by the latency hypothesis. F or the majority of neurons, however, sensitivity to equivalent ITDs ac counted only partially for the characteristics of the IID-sensitivity function; other features of the function in these cases appeared to re flect the operation of an additional factor, most probably the relativ e magnitude of the inputs from the two ears. Although the conclusions are qualified by the fact that one of the assumptions on which the est imation of equivalent ITDs was based was probably not satisfied for so me neurons, the results suggest that intensity-produced changes in bot h the magnitude and the timing of excitatory and inhibitory inputs sha pe the IID sensitivity of most EI neurons.