Abstract
Fiser, Bex, & Makous (Vision Research, 2003) reported a dissociation between the orientation of contours and their apparent contrast: suddenly changing the orientation of a grating during brief presentations had no effect on their apparent contrast although raising detection thresholds under otherwise identical conditions. Makous and Pulos (IOVS, 1981) reported a similar dissociation between the orientation of contours and their apparent color: during binocular rivalry between a red and black grating and a perpendicular green and black grating, the colors mixed, so that the observer reported seeing alternation between perpendicular yellow and black gratings (not even a hint of a plaid). Here we tested whether the apparent contrasts of gratings also tend to mix during rivalry. We presented perpendicular 1 cpd gratings to fellow eyes within 2 deg fields. The contrasts of the gratings were 75% and 15%. The five observers (four naïve) compared the contrast of an otherwise identical pair of binocular matching gratings to that of the rivalrous gratings in their dominant phase of rivalry. Test and matching gratings were separated vertically by 1 deg. We found that the matches to the 75% grating when dominant did not differ reliably from a veridical match, but observers matched a binocular grating of 21% contrast to a 15% grating in the dominant phase of rivalry. This is reliably greater than the 15% veridical matches (whether all five or just the 4 naïve observers are included). Control observations showed that both monocular and binocular matches made without rivalry did not differ reliably from veridical matches. This demonstrates some averaging of the contrasts of rivalrous gratings, and is further evidence of the separate processing of contrast and contour information by the brain. However, the effect is small, only about 10% of the difference between the grating contrasts. This is to be compared with the complete mixing of the colors of rivalrous gratings.
Supported by U. S. Public Health Service grants EY-4885 and EY-1319 and the Wellcome Trust.