Effects of ipsilateral and contralateral precursors on the temporal effect in simultaneous masking with pure tones.

In tone-on-tone masking, thresholds often decrease as the onset of the signal is delayed relative to the onset of the masker, especially when the frequency of the masker is higher than the frequency of the signal. This temporal effect was studied here by using a tonal "precursor," whose offset preceded the onset of the tonal masker (and signal). Under the right conditions, the precursor can reduce or eliminate the temporal effect by decreasing the threshold for a signal at masker onset, presumably for the same reason that the threshold decreases as a signal is delayed relative to the onset of a masker. In the present study, the frequency of the signal was 4000 Hz, and the frequency of the masker and precursor was typically 5000 Hz. In experiment 1, the precursor was presented to the ear receiving the masker and signal (ipsilateral precursor); in experiment 2, it was presented to the opposite ear (contralateral precursor). The results from experiment 1 can be summarized as follows: the ipsilateral precursor (a) reaches its maximum effectiveness (in reducing the temporal effect) for precursor durations of 200-400 ms; (b) is ineffective once the delay between its offset and the onset of the masker reaches about 50-100 ms; (c) is generally ineffective when its level is 10 or more dB lower than the level of the masker, but is effective when its level is equal to or greater than the level of the masker; and (d) becomes progressively less effective as its frequency is either increased or decreased relative to the frequency of the masker. The results from experiment 2 can be summarized simply by stating that the contralateral precursor is ineffective in reducing the temporal effect. These results suggest that the effect of the precursor may be mediated peripherally.