Surround suppression' is a contrast normalization computation in which the apparent contrast in one location is reduced by contrast in another, spatially-separated region. The two regions must have similar orientation and spatial frequency. Because sub-cortical neurons in the visual pathway have weak orientation tuning, surround-suppression is thought to be a cortical computation but its precise neuronal substrate is still unknown.

In the human visual system, chromatic information is carried in three largely independent pathways until the second or third synapse in V1. If surround suppression mechanisms act after this mixing, we expect contrast in one chromatic pathway to normalize contrast in another. If the site of surround suppression is early in V1, we expect to find relative independence between chromatic surround suppression mechanisms.

We measured the suppressive effects of annular surround regions on central 'probe' regions in a 2AFC contrast detection experiment. The surrounds and probes were defined by either S-cone or pure luminance contrast and could be co-linear or orthogonal to each other. We also measured the detection thresholds of the probes in isolation yielding a total of 12 different conditions. The spatial positions of the probes were cued at all times to reduce the effect of positional uncertainty.

We found powerful, orientation-tuned surround suppression effects when probes and surrounds had the same chromatic contrast. But we found little effect of luminance surrounds on S-cone probes or vice versa. We conclude that surround suppression occurs at the first or second synapse in V1, before multiple chromatic channels are generated.