Is Emergent Motion Sensitivity in the Inferior Colliculus a Result of Convergent Inputs?

Abstract

A model of low-frequency sound localization consists of an array of brainstem neurones, acting as coincidence detectors, receiving input from the two ears via axons of different lengths (Jeffress, 1948). When the external delay of the signal is compensated by an appropriate internal delay (the characteristic delay, CD, see Rose et al. 1966; Yin and Ku- wada, 1983), inputs from each ear arrive at the same time. The CD can be measured as the (linear) slope of the plot of the best interaural phase (BP) as a function of the stimulus frequency (the phase plot). CDs that correspond with a peak in the delay function indicate that the delay sensitivity is generated by excitatory inputs from both sides, whilst CDs that correspond with a TROUGH indicate the interaction of excitation from one ear with inhibition from the other. Recordings of single neurones in the medial superior olive (MSO: Goldberg and Brown, 1959; Yin and Chan, 1990; Spitzer and Semple, 1995) have confirmed the existence of binaurally-sensitive, coincidence detector neurones that fire maximally at a particular interaural delay of the stimulus (peak-type). The low-frequency region of the lateral superior olive (LSO) also contains neurones sensitive to interaural- time and -phase (e.g. Goldberg and Brown, 1969; Finlayson and Caspary, 1991; Batra et al., 1995), mediated by excitation from the ipsilateral ear, and a timed inhibition from the contralateral ear (trough-type). The responses of many low-frequency neurones in the inferior colliculus (IC) to interaural time delays are also consistent with a single, fixed delay, or CD. However, for a significant proportion of neurones in the IC, the phase plot is not a single straight line, but is often better fitted by two or more straight-line segments. We hypothesize that such neurones receive convergent input from brainstem. Consistent with this hypothesis, it is possible to suppress the contribution of one input by presenting a tone at its least favourable delay (Palmer et al. this volume), leading to domination of the activity by the remaining input(s). It has also been reported that sensitivity to the direction of sound-source motion is an emergent property of the IC neurones (Spitzer and Semple, 1993), as it is not found in at the level of the MSO (Spitzer and Semple, 1992). In this report, we investigate the possibility that the emergent motion sensitivity results from convergent inputs from lower brainstem neurones onto single neurones in the IC.