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.
Key Words
Auditory evoked potentials - frequency-following response - phase locking - masking
level difference
