Chromatic spatial summation at equiluminance

The effect of spatial summation on chromatic detection at suprathreshold le vels for the red-green and yellow-blue chromatic opponent mechanisms has be en studied using simple reaction time (RT) as a measurement of the response . We varied the target size in the experiments from 8 min of arc in diamete r to 10 deg, and the hue-substitution paradigm was adopted to generate only chromatic changes in the cone input, Equiluminant stimuli were distributed along three tritan and three red-green confusion lines. Three reference st imuli were chosen as both reference and adapting stimuli, in this case to d etermine whether the chromatic adaptation affected the spatial summation. T he results showed that the mean value of RT at a constant cone-input variat ion. either of the L-2M channel or of the S-(L+M) channel, against the targ et size decreased until a stimulus size was reached. the RT being constant for greater sizes. The spatial summation was fulfilled for a maximum target Size which was less for the L-2M channel than for the S-(L+M) channel, thi s size decreasing as the cone-input variation rose. Furthermore, there was no spatial summation for high-enough variations in the cone input. In addit ion, the chromatic mechanisms appear to organize their spatial integration with the adapting level.