Detection thresholds were obtained for a circularly-symmetric Gabor pr
ofile and Craik-Cornsweet profiles presented on a large white adapting
field. These stimuli possessed peak spatial power between 1 and 6 c/d
eg. Their contrast was represented in an L, M and S cone contrast spac
e. Detection thresholds were obtained for many vectors close to specif
ic but theoretically important planes within this space. These data we
re fitted with a model comprising independent mechanisms, each a weigh
ted sum of cone contrasts. The fit revealed a chromatic mechanism driv
en by Delta L/L - Delta M/M with no S cone input. Within cone contrast
space, this mechanism was more sensitive than both a luminance mechan
ism with little S cone input but considerable variation in relative L
to M cone input, and a blue-yellow chromatic mechanism.