Neural sensitivity to dynamic binaural cues: Human electroencephalogram and chinchilla single-unit responses

Animals encounter dynamic binaural timing information in broadband sounds such as speech and background noise due to moving sound sources, self motion, or reverberation. Most physiological studies of interaural time delay (ITD) or interaural correlation (IAC) sensitivity have used static stimuli; neural sensitivity to dynamic ITD and IAC is rarely systematically addressed. We used a system-identification approach using maximum-length sequences (MLS) to characterize neural responses to dynamically changing ITDs and IACs in broadband sounds. Responses were recorded from humans (electroencephalogram; EEG) and from single neurons in terminally anesthetized chinchillas (auditory nerve fibers; ANFs). Chinchilla medial superior olive (MSO) responses were simulated based on binaural coincidence from recorded ANF spike times in response to left- and right-channel input. Estimated ITD and IAC transfer functions were low-pass, with corner frequencies in the range of hundreds of Hz. Human EEG-based transfer functions, likely reflecting cortical responses, were also low-pass, but with much lower corner frequencies in the region of tens of Hz. Human behavioral detection of dynamic IAC extended beyond 100 Hz consistent with the higher brainstem limits. On the other hand, binaural unmasking effects were only evident for low-frequency ITD/IAC dynamics in the masking noise.