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Edward Vul, Donald I. A. MacLeod; Color without consciousness: Dynamics of the McCollough effect. Journal of Vision 2006;6(6):419. doi: https://doi.org/10.1167/6.6.419.
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Recent studies have shown that certain aspects of unconscious visual processing have higher spatial and temporal resolution than conscious perception (e.g., He & Macleod, 2001). We investigated whether such differences exist in the temporal resolution of conscious and unconscious cortical color processing. A McCollough effect (McCollough, 1965) is an orientation-contingent color aftereffect: adaptation to (e.g.) red vertical and green horizontal gratings causes subsequent vertical and horizontal gratings to appear greenish and reddish, respectively. Because the adaptation occurs at V1 or later (Humphrey and Goodale, 1998), it is ideal for studying cortical color processing. Humans can consciously discern color alternations up to approximately 16 Hz (Kelly, 1983). We alternated a four-frame sequence of phase-shifted gratings that induced a McCollough effect at color alternation rates up to 50 Hz. At this rate the constituent red and green colors fuse together into a yellow field and can no longer be discerned. Despite the fact that the colors of the inducing stimulus were invisible, a McCollough effect was successfully induced at 50 Hz. These results show that V1 can track color alternations much faster than conscious perception.
Furthermore, we found that the color contrast available to conscious perception is reduced at a much faster rate than that available to McCollough effect mechanisms as the color alternation rate increases. This suggests that color processing in V1 is faster than consciousness not just because of a high contrast threshold, but because extra steps of temporal integration interfere between V1 and visual awareness.
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