Detection of combined changes in interaural time and intensity differences: Segregated mechanisms in cue type and in operating frequency range?

Although physiological studies have revealed segregated binaural pathways, namely the medial and lateral superior olives, it is unclear whether the human auditory system has separate mechanisms for different cue types (interaural time and intensity differences; ITD and IID, respectively) and for operating frequency ranges. This study hypothesized "channels" for ITD and IID processing, and examined channel interaction at low and high frequencies based on the signal detection theory. The stimuli were a 125- or 500-Hz tone and a 4-kHz tone amplitude-modulated with a half-wave-rectified 125-Hz sinusoid, presented dichotically with various baseline ITDs and IIDs. The detectability indices, d('), for ITD and IID changes, imposed individually or simultaneously in the same direction, were derived from the results of a forced-choice task. The degree of channel interaction was estimated by comparing d(') for combined cues with those for individual cues. The estimated interaction showed little effect of baseline ITD or IID. The results generally exhibited nonzero interaction, indicating that the cue processes are not completely independent. The interaction was stronger for high frequencies than for low frequencies. The results can be interpreted as indicating the involvement of different binaural mechanisms for different frequency regions.