RESPONSES OF NEURONS IN THE INFERIOR COLLICULUS OF THE RAT TO INTERAURAL TIME AND INTENSITY DIFFERENCES IN TRANSIENT STIMULI - IMPLICATIONSFOR THE LATENCY HYPOTHESIS
Drf. Irvine et al., RESPONSES OF NEURONS IN THE INFERIOR COLLICULUS OF THE RAT TO INTERAURAL TIME AND INTENSITY DIFFERENCES IN TRANSIENT STIMULI - IMPLICATIONSFOR THE LATENCY HYPOTHESIS, Hearing research, 85(1-2), 1995, pp. 127-141
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.