The primacy of chromatic edge processing in normal and cerebrally achromatopsic subjects.
The local chromatic contrast between surfaces in a visual scene plays an important role in theories of color perception. Our studies of cerebral achromatopsia suggest that this contrast signal is computed independently of the more complex processes such as edge integration and anchoring. We report a study in which we attempted to determine whether local-contrast signals also 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 of patches relative to such backgrounds depends upon the position at which they are presented. It is therefore possible for patches with identical spectral composition 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 stimuli 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 conclude that local chromatic contrast, but not raw spectral composition, is accessible to normal observers at an appropriate stage in visual processing to drive attention.
Kentridge
R W
RW
Department of Psychology, Science Laboratories, South Road, Durham DH1 3LE, UK. robert.kentridge@durham.ac.uk
Cole
G G
GG
Heywood
C A
CA
D000328:Adult; D000704:Analysis of Variance; D001927:Brain Diseases / Q000150:complications* / Q000503:physiopathology; D016022:Case-Control Studies; D003118:Color Perception; D003117:Color Vision Defects / Q000209:etiology* / Q000503:physiopathology*; D015350:Contrast Sensitivity; D006801:Humans; D010775:Photic Stimulation / Q000379:methods; D011930:Reaction Time; D014793:Visual Cortex / Q000502:physiology* / Q000503:physiopathology