Human Frequency-Following Responses to Binaural Masking Level Difference Stimuli

 

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Binaural masking level difference is the behavioral threshold difference between a diotic condition (SoNo) and a dichotic condition with a 180° interaural phase delay of either the signal (SπNo) or the masker (SoNπ). Threshold disparity is partially related to coincidence-detecting units in the medial superior olive that are sensitive to low-frequency binaural stimuli with interaural phase differences. Previous surface evoked potential studies report significant latency and amplitude differences to SπNo stimuli with respect to SoNo stimuli in the P1-N1 auditory event related potential, but no study has reported physiologic masking level differences in a brain stem evoked potential. The human frequency-following response (FFR) represents activity from low-frequency, phase locking neural units in the upper brainstem. Unmasked FFRs to 500 Hz tone bursts and masked FFRs using a 1.5 kHz low-pass masker were recorded from nine normal-hearing adult subjects. Significant reduction in FFR amplitude occurred in the SoNo condition, re the So condition, with masker intensities near the psychoacoustic SoNo masking level. Significant FFR amplitude recovery was observed for both the SoNπ and SπNo conditions. These results support the role of phase-locked neural activity in brainstem mechanisms involved in perceptual masking release.