- -om invisible contrast tn a -j 470. :ion with test 3pth planes. 'tion: border ion Res.. 38: :Adam D.L. bridge, MA. r.J. (2001) .rear proces- Vision Res., -.D. (2003) 1Ue prrrnaly :eived color' 153 1362. rn and its i7 624. symmetries rd the sig- tlc texture -O Annual chromatic Ll Meeting of global rllon J. D.. Defective :r rn hrirl oe lluence of Lst. J. Opt. .{. (lee2) ultaneous rtex: The ngths and rtex: The d cells in rposition. Progress bt Broin Reseurtlt. Vol. 144 ISSN0079-6123 Copl'right e 2004 Elsevier BV. All rightsreserved CHAPTER 1I *Correspondin-e author.Tel. : + 44- 19 1 -33 4-0437 ; Fax: f 44-191-334-3241 ; E-maii: robert.kentridge (rirdurham.ac. uk t)OI:I0. I 0I6 S0i)79-6113(0-1)l:l:101-1 The primacy of chromatrcedge processing in normal and cerebrally achromatopsic subjects R.W. Kentridge*, G.G. Cole and C.A. Heywood Depet"ttllent of Psys:11plagy, Science Laboratories, Sotrth Rocrcl, Dtu'hcnn DHI 3LE, UK Abstract: The local chromatic contrast between surfaces in a visual sceneplays an important role jn theories of color perception. Our studies of cerebral achromatopsia suggest that this contrast signal is computed independently of the more complex processes such as edgeintegration and anchoring. We report a study in which we attempted to determine whether local-contrast signalsalso drove behavior in normal subjects. We sought to reduce the role of edge integration and anchoring by using stimuli whose background varied very gradually in color from top to bottom. The local chromatic contrast ofpatches relative to such backgrounds dependsupon the position at which they are presented. It is therefore possible for patcheswith identical spectralcomposition to have opposite contrasts.We constructed stimuli in which two of three vertically arranged discs had the same contrast while the third had opposite contrast. The stimuii were also constructed so that the contrast-odd disc and one of the other two had identical spectral composition while the third disc had different composition. We used these stimuli in an attentional task where, after a brief delay, a letter discrimination target was presented in the location of one of the discs. Attention should automatically be attracted to the odd disc in such a display. Normal observers were faster at making the letter discrimination when the target appeared at the contrast-odd as opposed to spectrally odd location. We conciude that local chromatic contrast, but not raw spectral composition, is accessible to normal observers at an appropriate stage in visuai processing to drive attention. Introduction that obiect. Perceived color can be thought of as an estimate of surface-reflectance properties. It depends Our normal visual experience of the world is fllled upon a complex relationship between spatial and with color. Color is a property of surfaces, shadows temporal patternsof wavelength and intensity varia- and lights. These colors do not, however, correspond tion in the light reaching the retina that undoubtedly to the physical wavelengths of light striking the retina relies on specialized neurai computations. Some of in as direct a way as might at first be imagined. Color these computations serve simply to disentangle inten- is a distal, not a proximal percept. The perceived sity and wavelength variation in the visual scene. color of objectsin the world is a property of those Others allow the visual system to estimate the com- objects, not of the wavelength of light reflected from position of the iliuminant or to discountthe effects of them to the retina. The light reachingour eyesfrom changes in the illuminant without explicit estimation. an objectis dependent both on the surface-reflectance Signals derived from a visual scene which are properties of the object and the light illurninating invariant under changesof illumination provide a useful starting point for the perception of surface color. The waveiength oflight reflected from surfaces themselves is, of course, not invariant under changes of illumination. If, however, one compares a pair t61