Behavioural and electrophysiological binaural masking level difference

The binaural masking level difference (BMLD) is a psychoacoustic phenomenon that demonstrates how we benefit from using both ears when detecting signals in noise, especially when they are presented with an interaural phase difference. The behavioural BMLD is believed to reflect brainstem processing. However, this conflicts with results from past research of transient auditory evoked potentials, which suggest that the BMLD is generated at the cortical level and not at the brainstem or thalamus level. The objective of the present study is to investigate the brainstem and cortical mechanisms underlying the BMLD in humans using the auditory steady-state response (ASSR). A 500-Hz pure-tone, amplitude-modulated (AM) at 80 Hz and 7(orl3)-Hz, was used to elicit brainstem and cortical ASSRs, respectively. The masker was a 200-Hz-wide noise centred on 500 Hz. Eleven adult subjects with normal hearing were tested. Both ASSR (brainstem and cortical) and behavioural thresholds for diotic AM stimuli (when the signal and noise are in phase binaurally: S₀N₀) and dichotic AM stimuli (when either the signal or noise is 180° out-ofphase between the two ears: S[sub π]N₀, S₀N[sub π]) were investigated. ASSR and behavioural BMLDs were obtained by subtracting the threshold for the dichotic stimuli from that for the diotic stimuli, respectively. Effects for modulation rate, signal vs noise phase changes, and behavioural vs ASSR measure on the BMLD were investigated. Behavioural BMLDs (means = 8.5-10.5 dB) obtained are consistent with results from past research. The ASSR results are similar to the pattern of results previously found for the transient auditory brainstem responses and the N1-P2 cortical auditory evoked potentials, such that only the cortical ASSRs (7 or 13 Hz) demonstrate BMLDs (mean = 5.8 dB) but the brainstem ASSRs (80 Hz) (mean = 1.5 dB) do not. This suggests that brain processes underlying the BMLD occur either in a different pathway or beyond the brainstem auditory processing, and are at or before the auditory cortical processing. The ASSR results differ from the previous transient N1-P2 studies, however, in that the cortical ASSRs show a BMLD only when there is a change in the signal interaural phase, but not for changes of noise interaural phase. This suggests that the cortical ASSRs are not directly related to the transient N1-P2 responses, and that they may reflect the output of neural populations which previous research has shown to be insensitive to binaural differences in noise.Medicine, Faculty ofAudiology and Speech Sciences, School ofGraduat