Chromatic and luminance contributions to a hyperacuity task.

Displacement thresholds with incremental chromatic and luminance edges were measured on different backgrounds. Above 3% luminance contrast, thresholds were always similar. At luminance contrasts below 3%, luminance edges could not be detected, but chromatic edges were still visible. At these low contrasts displacement thresholds for chromatic edges increased to a high level. We interpret these data in terms of multiple mechanisms; above 3% contrast a luminance mechanism determines thresholds, but when, at lower contrasts, chromatic mechanisms support detection, they also support the spatial task. Physiological data were consistent with the different mechanisms originating at the retinal ganglion cell level.