Transformation of binaural response properties in the ascending auditory pathway: Influence of time-varying interaural phase disparity

Previous studies demonstrated that tuning of inferior colliculus (IC) neuro ns to interaural phase disparity (IPD) is often profoundly influenced by te mporal variation of IPD, which simulates the binaural cue produced by a mov ing sound source. To determine whether sensitivity to simulated motion aris es in IC or at an earlier stage of binaural processing we compared response s in IC with those of two major IPD-sensitive neuronal classes in the super ior olivary complex (SOC), neurons whose discharges were phase locked (PL) to tonal stimuli and those that were nonphase locked(NPL). Time-varying IPD stimuli consisted of binaural beats, generated by presenting tones of slig htly different frequencies to the two ears, and interaural phase modulation (IPM), generated by presenting a pure tone to one ear and a phase modulate d tone to the other. IC neurons and NPL-SOC neurons were more sharply tuned to time-varying than to static IPD, whereas PL-SOC neurons were essentiall y uninfluenced by the mode of stimulus presentation. Preferred IPD was gene rally similar in responses to static and time-varying TPD for all unit popu lations. A few IC neurons were highly influenced by the direction and rate of simulated motion, but the major effect for most IC neurons and all SOC n eurons was a linear shift of preferred IPD at high rales-attributable to re sponse latency. Most IC and NPL-SOC neurons were strongly influenced by IPM stimuli simulating motion through restricted ranges of azimuth; simulated motion through partially overlapping azimuthal ranges elicited discharge pr ofiles that were highly discontiguous, indicating that the response associa ted with a particular IPD is dependent on preceding portions of the stimulu s. In contrast, PL-SOC responses tracked instantaneous IPD throughout the t rajectory of simulated motion, resulting in highly contiguous discharge pro files for overlapping stimuli. This finding indicates that responses of PL- SQC units to time-varying IPD reflect only instantaneous IPD with no additi onal influence of dynamic stimulus attributes. Thus the neuronal representa tion of auditory spatial information undergoes a major transformation as in teraural delay is initially processed in the SOC and subsequently reprocess ed in IC. The finding that motion sensitivity in IC emerges from motion-ins ensitive input suggests that information about change of position is crucia l to spatial processing at higher levels of the auditory system.