It is well known that prolonged observation of a dynamic visual pattern raises the contrast threshold for a subsequently presented static pattern. We found that if the post-adaptation test was presented gradually, so that its onset transient was weak, the test pattern was undetectable even at high contrast. Although the smooth-onset patterns were invisible, they caused apparent shifts in the orientation and contrast of neighboring stimuli, indicating the implicit processing of the target features. However, this strong aftereffect was not obtained if the target grating drifted rapidly or was onset abruptly. These results suggest that when human observers become less sensitive to transients in stimuli due to dynamic adaptation, they cannot consciously perceive sluggish stimuli containing weak transients. This is consistent with the notion that the visual system cannot prompt a conscious awareness of a single stimulus unless triggered by enough transient or temporally salient signals.
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