Rod-cone interaction in form detection.

Using a Wright colorimeter, absolute threshold, absolute form threshold and specific form threshold were measured during long-term dark adaptation in the extrafoveal retina. The specific form threshold was found to fall markedly at about the cone-rod break but thereafter rose steeply. Furthermore, during the rod phase of the dark adaptation the form percept of the small, slender rectangular test field changed qualitatively from a line or rectangle to a circular field at all mesopic intensities. The results indicate that light signals from rods may both facilitate and suppress cone-mediated information about form, and that the rod system may completely dominate the perception of form several log units above the absolute dark-adapted cone threshold when the eye is dark adapted.