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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.

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