PRE-EXPOSURE TO CONTRAST ASYMETRICALLY COMPRESSES THE ACHROMATIC AXIS OF COLOR SPACE ((R.D. Beer and D.I.A. Macleod)) Department of Psychology, U.C. San Diego, La Jolla, CA 92093-0109

Purpose. Is it possible to selectively compress the response to contrast along half-axes in color space? Webster and Mollon (1991) showed that pre-exposure to a field modulated in contrast along one axis in color space selectively reduces perceived contrast along that axis. Further experiments (Wise and MacLeod, ARVO 1995) showed a half-axis selective loss in contrast sensitivity to chromatic pulses after pre-exposure to a uniform field. In these experiments, we test whether chromatic or achromatic contrast pre-exposure can selectively (i.e. asymmetrically) compress complementary half-axes in color space. Methods. We assessed selective compression using a perceptual bisection of intervals along the bright-dark and red-green axes of color space. Tests, viewed against a gray background, were either a triplet of disks, or triplets of pinstripes. Pre-exposure was to disks, or to drifting pinstripes, respectively. Subjects set one of the disks or pinstripes to bisect the interval defined by the other two. Results and Conclusions. 1) Pre-exposure to achromatic disks or pinstripes produced clear shifts in bisection settings in the direction indicating asymmetrical compression. This result may reflect separately-adapting contrast-sensitive pathways, one sensitive to light increments, one sensitive to decrements. 2) Although the experiment conducted with intervals along the red-green axis does demonstrate size-selective chromatic adaptation, it does not indicate half-axis compression. Separately-adapting contrast pathways may only be present for achromatic contrast. Supported by NIH grant EY01711. None.