Imperceptibly rapid contrast modulations processed in cortex: Evidence from psychophysics.

Rapid fluctuations in contrast are common in our modern visual environment. They arise, for example, in a room lit by a fluorescent light, when viewing a CRT computer monitor and when watching a movie in a cinema. As we are unconscious of the rapid changes, it has been assumed that they do not affect the operation of our visual systems. By periodically reversing the contrast of a fixed pattern at a rapid rate we render the pattern itself, as well as the modulations, invisible to observers. We show that exposure to these rapidly contrast-modulated patterns alters the way subsequent stationary patterns are processed; patterns similar to the contrast-modulated pattern require more contrast to be detected than dissimilar patterns. We present evidence that the changes are cortically mediated. Taken together, our findings suggest that cortical stages of the visual system respond to the individual frames of a contrast-reversed sequence, even at rates as high as 160 frames per second.