Phase effects on the middle and late auditory evoked potentials.

A masking level difference (MLD) paradigm was used to investigate the presence of an electrophysiologic correlate of the psychoacoustic MLD in the middle and late auditory evoked potentials. In experiment 1, middle latency potentials were recorded in six normal-hearing young adults using vertex and temporal electrode montages. Tone bursts of 500 Hz presented in SoNo and Spi No conditions produced no threshold differences that were consistent with an MLD. In experiment 2, late latency potential (P2) thresholds to 500-Hz tone bursts under various phase conditions in noise were measured and compared to behavioral thresholds from the same stimuli. Ten subjects provided behavioral and P2 thresholds to SoNo, S pi No, So N pi S pi N pi, SmNo, and SmNm conditions. The rank order of behavioral and P2 thresholds and MLDs was consistent with previous literature on the behavioral MLD. Cortical contributions were confirmed as necessary for the production of the electrophysiologic MLD and, by analogy, the psychoacoustic MLD.