Intensity discrimination under forward and backward masking: role of referential coding.

The present experiments investigated the hypothesis that listeners can code intensity by reference to proximal stimuli in order to improve intensity discrimination performance in conditions of nonsimultaneous masking. The experiments used 30-ms tone bursts as the masker, pedestal, and "proximal burst." The masker level was 80 dB, the pedestal level was 50 dB. In the first experiment the silent interval between the masker and the pedestal was varied. Surprisingly, in both forward and backward masking situations, the Weber fraction decreased as the silent interval was decreased from 100 to 12.5 ms. This is consistent with the referential coding hypothesis: At short intervals performance improves because the level of the pedestal is coded by reference to the proximal masker. In a further set of experiments, the silent interval was 100 ms and an additional proximal burst was presented either 12.5 ms before or 12.5 ms after the pedestal. The proximal burst produced a substantial decrease in the Weber fraction, but only when it was close in frequency to the pedestal, and with a higher intensity. The results are consistent with the auditory system having the ability to produce a robust intensity measure by reference to proximal signals. These findings also provide further evidence that the mid-level elevation in forward masking is not solely the result of processes operating at the level of the auditory nerve.