Examining the standard model of signal detection theory in motion discrimination

We studied a fundamental assumption in signal detection theory that is applied to motion discrimination. Using random-dot motion stimuli of 100% coherence, it is natural to represent the two directions as two normal distributions. Then a same-different task and a forced-choice task should give rise to consistent d' estimates, because both tasks share the same underlying signal detection theory model. To verify this prediction, we used 4 degrees, 8 degrees, and 12 degrees as the angular difference in motion discrimination. In a between-subjects design, we found the predicted result only with the 4 degrees angular difference. With 8 degrees and 12 degrees, the estimated same-different d' was 32% greater than the two-alternative forced-choice d'. In a subsequent within-subject design with counterbalancing, the first half of the data confirmed this finding. Interestingly, there was now within-subject consistency. Namely, the second task's d' was comparable to the first task's, as if the first task's discrimination was carried over to the second task. This carryover effect diminished when the time gap between the two tasks was lengthened.