Simultaneous tone on tone masking : an investigation of monaural and dichotic overshoot

Transient elevations in the threshold of a signal presented within milliseconds of onset and Just prior to the offset of a masker are called "overshoot." Overshoot was investigated by manipulating three independent variables frequency of the signal, temporal delay of the signal, and mode of stimuli presentation (monaural or dichotic). The masker was always a 60 dB SPL, 250 msec, 1000 Hz pure tone, while the 10 msec signal was one of 8 pure tone frequencies centered around the masker from 400 to 1600 Hz. The signal onset was delayed by one of 4 temporal intervals: +2, +20, +125, and +238 msec. Thresholds were taken for both dichotic and monaural presentation of stimuli. A 2x4x8 Repeated Measures design was used, and an ANOVA provided the statistical analysis. Significant main effects for all three independent variables were found, as well as a significant frequency by mode of presentation interaction. The results confirmed previous research on the effects of signal temporal delay and mode of presentation of stimuli. The absence of frequency selective characteristics of overshoot in the dichotic condition conflicted with research by Zwislocki et al. (1967, 1968). The results were discussed in terms of Scholl's neural sharpening hypothesis