Effects of ipsilateral and contralateral precursors on overshoot

Overshoot is defined as the decrease in threshold as a brief signal is moved from the beginning to near the temporal center of a longer duration, broadband noise masker. Overshoot can be reduced when another noise (a precursor) is presented just prior to the masker. The purpose of the present investigation was to follow up on a recent psychophysical study which showed that overshoot could be reduced by a precursor presented to the ear contralateral to that receiving the masker and signal. The signal was a 20-ms, 4000-Hz tone that was presented at the beginning or in the temporal center of a 400-ms broadband noise masker. In the first experiment, a 200-ms broadband precursor was presented either to the ipsilateral or to the contralateral ear. The ipsilateral precursor reduced overshoot for all ten subjects, but the contralateral precursor reduced overshoot for only four of the ten subjects. In a supplementary experiment, the contralateral precursor failed to reduce overshoot in a new group of five subjects, both when tested with supra-aural headphones and with insert earphones. In the second experiment, the four subjects who showed an effect of the contralateral precursor in experiment 1 were tested under conditions where the bandwidth of the precursor was manipulated, resulting in either a narrow-band precursor centered at 4000 Hz, a low-band precursor with energy primarily below 4000 Hz, or a high-band precursor with energy primarily above 4000 Hz. There was a tendency for the effectiveness of the ipsilateral and contralateral precursors to be affected similarly (though to different degrees) by changes in the spectral content of the precursor. These results suggest that the effect of the contralateral precursor is not due to a timing cue, and that the processing underlying the effectiveness of ipsilateral and contralateral precursors may be largely the same.