Evidence for separate processing in the human brainstem of interaural intensity and temporal disparities for sound lateralization

Sound lateralization can be induced by interaural intensity disparities (IIDs) or by interaural temporal disparities (ITDs). The purpose of this study was to indicate whether IIDs and ITDs are processed by the same central units that detect interaural disparity in timing of afferent activity. If sound lateralization to intensity and time cues was determined by the same afferent latency disparity detectors in the brainstem, lateralization would be the same, regardless of whether latency disparity was induced by IIDs or ITDs. Moreover, the disparity detectors, and thus their dipole equivalents, would be the same for equal lateralizations, whether induced by IIDs or ITDs. Auditory brainstem evoked potentials (ABEPs) were recorded in response to monaural and binaural clicks, with a variety of IIDs and ITDs. Peak II (proximal auditory nerve activity), peak III (input to the superior olivary complex), and binaural interaction components (BICs) BeI and BeII (binaurally activated upper pons) were identified and their latencies measured. The psychophysical lateralization of the clicks (in cm from vertex) was also measured in response to the same binaural stimuli. The correlations between interaural afferent latency disparities (difference in corresponding peak latencies originating in each ear) and psychophysical click lateralization were calculated. Similarly, the correlations with click lateralization of the BICs equivalent dipole latency as well as orientation change (relative to symmetrical clicks) were determined. A strong correlation with lateralization was found for peaks II and III latency disparities, with steeper slopes for IIDs than for ITDs. Moreover, binaural activity across the same lateralizations differed between IIDs and ITDs. These results, therefore, indicate that interaural time and intensity cues are processed by separate systems in the brainstem, both at the afferent convergence level and after interaural disparities are determined.