Effect of stimulus phase reversal on the 20–35 Hz frequency component of the AEP

Auditory evoked potentials (AEP) have been recorded to investigate the effect of phase reversal of the stimulus presentation. Acquisition of the Middle Latency Response (MLR) for both homophasic and antiphasic cases was obtained by epoch averaging 500 trials of in-phase and 500 trials of out-phase EEG. An epoch was defined as the EEG data between 20 ms to 100 ms after the presentation of stimuli. The MLR was segmented by a trigger signal synchronized to the onset of the stimuli. The homophasic stimulus was a 1000 Hz Blackman windowed pure tone with a duration of 18 ms followed by 200 ms silence. The antiphasic stimulus was identical except for the phase of the stimulus. The stimuli were presented as blocks of 10 antiphasic tones followed by 10 homophasic tones for a total of 1000 tones. The comparison of the MLR responses upon presentation of homophasic and antiphasic stimuli showed there is electrophysiological evidence of binaural processing in the 20-35 Hz dominant frequency component.