Expedited communication Neural activity associated with binaural processes for the perceptual segregation of pitch Blake W. Johnson * , Michael Hautus, Wes C. Clapp Department of Psychology, Private Bag 92019, University of Auckland, Auckland, New Zealand Abstract Objective: We measured late cortical potentials in a psychophysical procedure for binaural unmasking of a dichotically-embedded pitch. Methods: Late-latency auditory evoked potentials were measured from 128 recording channels in 13 healthy subjects. Control stimuli consisted of 500 ms segments of broadband acoustic noise presented identically to both ears via earphones, evoking a perception of noise localized in the centre of the head. Dichotic pitch stimuli were created by introducing a dichotic delay to a narrow frequency region of the same noise segments, and resulted in a perception of both the centrally-located noise and a right-lateralized pitch. Results: Both stimuli evoked late auditory event-related potentials (ERPs) characterized by a P1-N1-P2 complex of waves between 60 and 180 ms after stimulus onset. ERPs associated with the control and dichotic pitch stimuli showed no amplitude differences for the P1 and N1 waves. ERPs to dichotic pitch stimuli became significantly more negative beginning at a latency around 150 ms, an effect that was maximal between 210 and 280 ms. Topographic mapping showed that this late negativity was lateralized to the left hemisphere. Conclusions: The late negative wave elicited by the dichotic pitch stimulus reflects neural processing that is dependent upon binaural fusion within the auditory system. Significance: The dichotic pitch paradigm may provide a useful tool for the electrophysiological assessment and study of the temporal processing capabilities of the auditory system. This paradigm may also be useful for the study of binaural mechanisms for the perceptual segregation of concurrent sound sources. q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Dichotic pitch; Auditory evoked potential; Interaural time difference; Sound localization; Auditory perception; Brain mapping 1. Introduction Slight differences in the timing of acoustic signals reaching the two ears can be utilized by the auditory system to segregate and localize sound sources. Intracranial and lesion studies in animals have shown that interaural timing differences (ITDs) are first extracted at the level of the superior olivary complex of the brain-stem (Masterton and Imig, 1984), and are utilized at higher levels of the auditory system including auditory cortex (Jenkins and Merzenich, 1984). In the human, it is possible to measure neurophy- siological responses to ITDs with evoked potentials. Because binaural fusion of ITDs involves very precise computations of interaural timing relationships, these binaurally-evoked potentials can potentially evaluate very fine-grained temporal processing capabilities in auditory structures of both normal individuals, and those with known or suspected temporal processing deficits underlying problems in hearing or language. Several psychophysical procedures that index our sensi- tivity to ITDs have been adapted for use in evoked potential studies. One method employs abrupt interaural time changes in acoustic streams, such as continuous noise (Halliday and Callaway, 1978; McEvoy et al., 1990) or long trains of clicks (Ungan et al., 1989), causing lateralization shifts of the sound image and evoking late cortical responses similar to, but with later latencies than, the P1-N1-P2 responses to onsets of acoustic stimuli. Because the acoustic streams are designed so they contain no detectable monaural changes, these time-shift evoked potentials must result solely from binaural processes in the auditory system, and their prolonged latencies indicate that these restricted acoustic cues require longer processing times. A second psychoacoustic approach uses stimuli that produce a binaural masking level difference (MLD). With this method, 1388-2457/$30.00 q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/S1388-2457(03)00247-5 Clinical Neurophysiology 114 (2003) 2245–2250 www.elsevier.com/locate/clinph * Corresponding author. Tel.: þ 64-9-373-7599; fax: þ64-9-373-7450. E-mail address: b.johnson@auckland.ac.nz (B.W. Johnson).