Perky's 1910 observation that mental images were indistinguishable from external stimuli is considered as two issues: (1) description of images which apparently resemble the unidentified stimulus, has been given, but not satisfactorily measured; (2) poorer detection of ambiguous signals while experiencing images, has been consistently demonstrated. While distraction may play some role, the critical finding was with sensitivity ( d′), which was highest in a simple discrimination task, intermediate when Ss were told both to describe imagery and detect the signals, and poorest when they lacked information that signals might be present during imaging and relied on retrospective judgments (Perky replication). In the intermediate task, alerting Ss to the signals by having them project the slides themselves, did not alter sensitivity or bias (Lx).
The effect of spreading depression on a simple discrimination task
