A diffusion model account of masking in two-choice letter identification

The diffusion model developed by R. Ratcliff (1978, 1981, 1985, 1988) for 2 -choice decisions was applied to data from 2 letter identification experime nts. In the experiments, stimulus letters were displayed and then masked, a nd the stimulus onset asynchrony between letter and mask was manipulated to vary accuracy from near chance to near ceiling. A standard reaction time p rocedure was used in one experiment and a deadline procedure in the other. Two hypotheses about the effect of masking on the information provided to t he decision process were contrasted: (a) The output of perception to the de cision process varies with time, so that the information used by the decisi on process rises and falls, reflecting the stimulus onset and mask onset. ( b) The output of perception to the decision is constant over time, reflecti ng information integrated over the time between the stimulus and mask onset s. The data were well fit by the diffusion model only with the assumption o f constant information over time.