Masking level difference: an electrophysiological approach

Auditory brainstem responses (ABRs), middle latency responses (MLRs), and slow cortical potentials (SCPs) were registered in normal-hearing adults to trains of low-frequency signals delivered binaurally on a background of a continuous masking noise. Two stimulus conditions, labelled as S0M0 and S pi M0 paradigms, respectively, were systematically compared. In the S0M0 paradigm, both the signals and the masker were in-phase at two ears. In the S pi M0 paradigm, the signals were out-of-phase at two ears, while the masker was in-phase. The psychoacoustic release from masking in S pi M0 vs. S0M0 paradigms was regularly accompanied by an increase in amplitudes and a shortening in peak latencies of the SCPs. In contrast, no differences were evidenced between the S0M0 and the S pi M0 paradigms with respect to the ABRs and the MLRs. Considering the generation loci of the studied electric responses, it is concluded that the binaural psychoacoustic phenomenon, referred to as the masking level difference, is operated primarily at the cortical level.