The role of across-frequency processes in dichotic listening conditions

In the bandwidening experiment with a diotic noise masker, an apparently wider critical bandwidth has often been reported when a dichotic signal (Spi) is used instead of a diotic signal (So). Two competing across-channel processes were proposed to account for this apparently wider critical bandwidth: (i) A detrimental across-channel effect reducing the binaural masking-level difference (BMLD) for broadband maskers and (ii) a beneficial integration of information across channels for narrowband maskers. The two hypotheses result in different predictions of the BMLD in the notched-noise experiment: According to the first hypothesis, the change in BMLD with notch width is determined by the level-dependence of the BMLD for a narrowband masker centered at the signal frequency, whereas the second hypothesis predicts that it is determined by the level-dependence of the BMLD for a broadband masker. To test the hypotheses, masked thresholds of a diotic or dichotic 500-Hz signal were measured for a diotic notched-noise masker as a function of notch width. In addition, thresholds were measured for a diotic broadband and narrowband masker as a function of masker level. The data indicate that neither of the two hypotheses is able to predict the continuous decrease in the BMLD as the notch width increases.