Interaural time and level differences: integrated or separated processing?

The processing of internal differences in time (IDT) and sound pressure level (IDL) was studied by using the mismatch negativity auditory evoked potential (MMN), which is a probe of pre-attentive auditory sensory memory. In a passive oddball experiment, subjects were reading in a book while they were presented with a standard stimulus (P = 0.88) having no IDTs or IDLs and three different deviant stimuli revealing an IDT, IDL, or both IDT and IDL. The different deviants elicited MMNs of comparable latencies indicating that memory representations of the IDTs and IDLs have been established. The MMN amplitudes to the IDT-IDL deviant were larger than those to changes in either IDT or IDL only. Moreover, the time-courses, amplitudes, and topographies of the MMNs to the IDT-IDL deviants were very similar to the sum of the MMNs elicited by the IDT and IDL deviants. These findings suggest that the representations of the binaural location cues were (at least partly) processed in parallel. It is argued that separate azimuth representations exist for IDT and IDL at a cortical level.