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Abstract:

Binaural Evoked Potentials in Young Normal Hearing Adults: Objective Measures of the Binaural System C. Vercammen, , J. Wouters, A. van Wieringen, T. Francart – ExpORL, Dept. Neurosciences, KU Leuven Belgium Normal hearing participants integrate input from two ears. This allows them to understand speech (in noise) or attend to one listener in the presence of others, even in acoustically challenging conditions. The binaural system accounts for sound localization as well, by processing and integrating differences in phase (low frequencies) or intensity (high frequencies) between sounds arriving at the left and right ear. The goal of the present study is to determine objective measures for binaural processing, thereby replicating and extending work by Ross et al. [(2008) A Novel Type of Auditory Responses: Temporal Dynamics of 40-Hz Steady-State Responses Induced by Changes in Sound Localization. Journal of Neurophysiology, 100, 1265-1277)]. Neural temporal coding is investigated by means of multiple-electrode Auditory Steady State Responses (ASSRs) and change responses detected in the EEG time domain and spectrum. Young normal hearing participants listened passively to sinusoidal amplitude-modulated tones. Individual stimuli were 4 seconds in duration, with a phase shift in the sound carrier of -90° in the left and +90° in the right ear – resulting in an overall phase shift of 180° – after two seconds (slow protocol) or recurring every 400 milliseconds (fast protocol). ASSRs to different modulation frequencies (20, 40 and 80 Hz) were used to tap into different neural generators and phase responses to the IPD change were expected to elicit a short distortion in the synchronization of the ASSRs. Carrier frequencies ranging from 500 up to 1500 Hz were used, showing an upper level for phase response detection. The electrophysiological results of the 40 Hz modulation frequency were similar to the ones reported by Ross et al. [(2008) A Novel Type of Auditory Responses: Temporal Dynamics of 40-Hz Steady-State Responses Induced by Changes in Sound Localization. Journal of Neurophysiology, 100, 1265-1277)]. Preliminary data show similar findings for a 20 and 80 Hz modulation frequency. The 20 Hz modulation frequency is believed to tap into more cortical sources than the 40 Hz modulation frequency. This is particularly interesting as it is believed to represent phonemic rate in speech. The 80 Hz modulation frequency reflects brainstem responses.