Normal binaural hearing allows the auditory system to determine the direction and distance of sound sources and to detect certain sounds at much lower intensity levels. Different stimuli may have different impact on binaural processing and may generate different brain responses. The mechanism by which this occurs is poorly understood.
Time averaged EEG responses of normal hearing subjects to repeated stimuli were analyzed. The stimuli, 500 Hz Blackman windowed pure tones, were presented as homo-phasic or anti-phasic and were also mixed with various noise conditions. Auditory evoked potentials (AEP) were obtained by averaging 500 trials of in-phase and 500 trials of out-phase of each EEG epoch.
The results show that the amplitude of the dominant frequency component in the 20 - 50 Hz range of the middle latency response of the AEP was larger for the anti-phasic condition than for the homo-phasic condition. The normalised amplitude differences were larger when the stimuli were embedded in noise resulting in a higher mean value of the normalized amplitude difference than for noise free stimuli. These results are likely to relate to binaural masking level difference which finds that the detection of a signal in a background noise is easier when the signal has a different inter-aural phase difference than the noise.