Discrimination for equiluminant chromatic stimuli that vary in L- and
M-cone excitation depends on the chromaticity difference between the test
field and the surrounding area. The current study investigated the effect
of the proximity in space and time of a surround to the test field on
chromatic contrast discrimination. The experimental paradigm isolated
spatial, temporal, and spatial-and-temporal chromatic contrast effects on
discrimination. Chromatic contrast discrimination thresholds were assessed
by a four-alternative spatial forced-choice procedure. Stimuli were either
metameric to the equal energy spectrum, or varied in L-cone activation
along a line of constant S-cone activation. A model based on primate
parvocellular pathway physiology described the data. Spatial and temporal
contrast produced equivalent reductions in chromatic discriminability as
the chromatic difference between the test and surround increased. For all
test chromaticities, discrimination was best in the absence of chromatic
contrast. Chromatic contrast discrimination is determined by either the
spatial or temporal contrast component of the signal.
