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Perky's finding, that an observer confuses his internal imagery with a normally supraliminal stimulus, has been reinterpreted in signal detection language. Two experiments compared a task where the subject was asked concurrently to image items and discriminate signals from noise to a task where the subject only discriminated signals. The subject's criterion was slightly higher in the imaging task and sensitivity (d′) was significantly lower, whether the imaging task was given before or after the discrimination task. As d′ measures signal‐to‐noise ratio, it was suggested that imaging alters d′ because it is a source of neural activity. This activity can be conceptualized as internal noise or as another signal. For some subjects, stimuli were gradually brightened; for others, they were presented briefly at full brightness. Mode of onset did not affect d′, rather d′ depended on brightness summed over time.

Seventeen Ss were told to imagine six colored objects, successively, while facsimiles of the objects were flashed on a screen at increasing levels of intensity. Ten Ss received a placebo which was described as a relaxant. Ss' reported experiences were rated as perceptions of reality or as imagery. Control Ss had an increasing number of reality experiences as the intensity of the stimuli increased. Placebo Ss were relatively unaffected by the level of intensity of the stimulus. Instead the placebo seemed to polarize their reactions into reactors who tended to experience most of the stimuli as part of their imagery and the non-reactors, who tended to identify all six stimuli as perceptions.