1810 Effect of interaural time difference on neural activity patterns of the guinea pig auditory cortex observed by optical recording

Abstract

Long-latency responses to bidirectional reversals in the laterality of a sound image were recorded from vertex with left and right earlobe references simultaneously. Laterality reversal stimuli with regular ISIs of 3 sec were obtained by alternating between +1 msec and −1 msec the interaural delay (ID) of dichotically presented 70 c/sec continous click trains. Monaural presentation of this temporal modulation did not evoke any potential or any sensation whereas, when dichotically presented, the same ID modulation was perceived as laterality reversals. Evoked potentials from 9 right-handed volunteers with normal hearing were evaluated. The recorded laterality reversal auditory evoked potentials (LRAEPs) consisted, characteristically, of a negative and a positive wave with average peak latencies of 160 msec and 260 msec, respectively. Statistical analysis indicated significant dependence of the response parameters on the direction of the sound image shifts, but the laterality of the earlobe reference did not prove to be important. The appropriateness of the stimulus parameters employed was discussed from the viewpoint of dynamic systems analysis. By contrasting with the monaural auditory off-responses recorded from the same subjects, it was demonstrated that the laterality reversal responses we obtained did not seem to be non-specific ‘rest-to-motion’ responses to any change in the acoustic background. Similarities and differences between these laterality reversal responses and various ERP components in the same latency range were also discussed. It is concluded that the LRAEPs introduced in this communication can be used in assessing the functional integrity and studying the dynamics of the neurophysiological system that accomplishes sound lateralization using cues of interaural time disparity (ITD).