Two experiments on the temporal boundaries for the nonlinear additivity of masking.

The present paper describes the results from two experiments which explored the temporal boundary between overlapping and nonoverlapping maskers and its effects on the additivity of masking. In the first experiment, detection thresholds for a short-duration 1000-Hz signal were measured in the presence of two equal-duration broadband maskers which varied in degree of temporal overlap. Following complete overlap of maskers, the temporal separation of masker onsets was systematically varied to create conditions ranging from partially overlapping simultaneous masking to combined forward and backward masking. The signal was always temporally centered between the onset of the first masker and the offset of the second masker. Nonlinear additivity of masking occurred for the majority of subjects when maskers and signal did not overlap, whereas linear additivity resulted for all subjects when the maskers and signal overlapped. In the second experiment, two separate forward maskers were used so that masker/masker overlap could be manipulated independent from masker/signal overlap. Maskers were changed gradually from temporally overlapping, or concurrent, forward maskers to sequential forward maskers. Results for all subjects showed nonlinear additivity for all combined-masker conditions. Together, these two experiments indicated that nonlinear masking additivity is observed when the signal does not overlap temporally with the maskers. However, when maskers and signal overlap temporally (and spectrally), linear additivity is observed.