[The psychoacoustic phenomenon of masking level differences in the human subject studied by auditory evoked...

Auditory brainstem responses (ABRs), middle latency responses (MLRs), and slow cortical potentials (SCPs) were recorded in normal-hearing adults to trains of low-frequency acoustic signals delivered binaurally against a background of a continuous masking noise. Two stimulus conditions, labelled as binaural homophasic and binaural antiphasic paradigms, respectively, were systematically compared. In the homophasic paradigm both the signals and the masker were in-phase at two ears. In the antiphasic paradigm the signals were 180 degrees out-of-phase at two ears, while the masker was in-phase. The psychoacoustic release from masking in the antiphasic vs. the homophasic paradigm 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 homophasic and the antiphasic 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.