Effects of Imaging on Signal‐to‐noise Ratio, With Varying Signal Conditions

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.