Electrophysiologic Measures For Binaural Masking Level Difference

Abstract

This study evaluates the effect of phase change on auditory steady-state (ASSR), onset CAEP and ACC responses. The aim was to determine how phase change is encoded at lower levels of the auditory system and then processed at the cortical level. The test subjects were 23 young adults with normal hearing. ASSRs and stimulus-onset CAEPs were first recorded in a control condition using a 800 ms duration 500 Hz tone, amplitude modulated at 80 Hz, presented binaurally at 1/s. In the test condition, the phase of the 500 Hz tone was inverted 180° in one ear at 400 ms to evoke the ACC. Broad-band noise was introduced at an -5 dB SNR and the test condition repeated. Stimulus levels were varied to find the threshold of the ACC. Perceptual binaural masking level differences (BMLD) for a 500 Hz tone were also determined. At the phase change, ASSR amplitudes showed no difference between the in-phase and on-phase condition, and the ACC was 1.5-2.0 times larger than the onset CAEP response. This indicates amplification of the phase-change information in the ascending auditory pathway. These findings contribute to the development of an electrophysiological index for BMLD.