RESPONSES OF INFERIOR COLLICULUS NEURONS TO TIME-VARYING INTERAURAL PHASE DISPARITY - EFFECTS OF SHIFTING THE LOCUS OF VIRTUAL MOTION

1. Motion of sound sources results in temporal variation of the binaur al cues for sound localization. We evaluated the influence of virtual motion on neural tuning to one of these cues, interaural phase dispari ty (IPD). Responses to dichotic stimuli were recorded from single unit s in the inferior colliculus of the anesthetized cat and gerbil (Merio nes unguiculatus). Static IPDs were generated by presenting dichotic t one pairs with a constant phase offset maintained for the duration of the stimulus. Time-varying IPDs were generated by simultaneously prese nting a pure tone to one ear and a phase-modulated tone to the other e ar. Sets of time-varying stimuli consisted of modulations through part ially overlapping ranges of IPD, corresponding to movement of a sound source through partially overlapping arcs in the horizontal plane. 2. In agreement with previous results, neuronal discharge was typically a peaked function of static IPD resulting from both binaural facilitati on at favorable IPDs and binaural suppression at unfavorable IPDs. Res ponses to time-varying IPD stimuli appeared to be shaped by the same f acilitative and inhibitory mechanisms that underlie static IPD tuning. Modulation toward the peak of binaural facilitation increased the pro bability of discharge, and modulation toward the peak of binaural supp ression decreased the probability of discharge. However, it was also c lear that IPD tuning could be significantly altered by the temporal co ntext of the stimulus. For the vast majority of units in response to m odulation through partially overlapping ranges of IPD the discharge ra te profiles were generally nonoverlapping. This shift in IPD tuning in duced by the virtual motion reflects the fact that the binaural intera ction associated with a given IPD depends on the recent history of sti mulation. In addition, modulation in opposite directions through the s ame range of IPDs often elicited asymmetric responses. These nonlinear ities imply that most inferior colliculus neurons do not unambiguously encode a specific IPD, but instead may encode small changes of IPD oc curring virtually anywhere within their receptive fields. In a few cas es modulation through overlapping ranges of IPD elicited contiguous re sponse profiles, indicating that for these units responses were determ ined entirely by instantaneous IPD. 3. The nonlinearity of responses t o time-varying IPD stimuli could not be attributed to monaural entrain ment to the phase-modulated signals, did not depend on the phase modul ation waveform, and occurred irrespective of which ear received the ph ase-modulated signal. Responses were similar in cats and gerbils, sugg esting that the underlying mechanisms are common to binaural processin g in diverse mammalian species. 4. The consistent shifts in IPD tuning displayed by most neurons in our sample suggests that sensitivity to dynamic spatial cues is a general property of neurons in the inferior colliculus. A measure was developed to quantify the magnitude of the s hifts in IPD tuning across a large sample of units. This index of moti on sensitivity was distributed continuously across the sample of units and was independent of frequency tuning, preferred static IPD, and sh arpness of tuning to static IPD. 5. The initial neural encoding of IPD is believed to occur through a process of coincidence detection or cr oss-correlation at the superior olivary complex. The present finding t hat IPD tuning in the inferior colliculus is so dependent on the dynam ic context suggests that the output of the original cross-correlator m ust be modified in the ascending auditory pathway. These data reveal t hat within the inferior colliculus the neural representation of IPD, a nd consequently sound location, is influenced by movement of a sound s ource.