Stimulus selection in adaptive psychophysical procedures.

In adaptive psychophysical procedures, the stimulus should be presented at a relatively high level rather than near the middle of the psychometric function, which is often defined as the "threshold" value. For some psychometric functions, the optimal stimulus placement level produces 84% to 94% correct responses in a two-alternative forced-choice task. This result is disquieting because the popular two-down one-up rule tracks a relatively low percentage of correct responses, 70.7%. Computer simulations and a variety of psychometric functions were used to confirm the validity of this analysis. These simulations also demonstrate that the precise form of the psychometric function is not critical in achieving the high efficiencies. Finally, data from human listeners indicate that the standard deviation of threshold estimates is indeed larger when the stimulus presented on each trial is at a stimulus level corresponding to 70.7% rather than 94% correct responses.