Shared pathways for rod and cone vision.

We have used heterochromatic gratings falling on 10 deg temporal retina to measure the spatial contrast sensitivities of the isolated rod and cone systems in the mesopic range. As the level of illumination was raised within this range, the contrast sensitivity of the rod system increased, reaching a peak of about 50 (and providing an acuity of 6 c/deg) at 20 scot. td, whereupon the rod system began to saturate. Over most of the mesopic range the sensitivity of the cone system was lower than that of the rod system, although it provided better acuity (up to 15 c/deg). Within the range of spatial frequencies capable of exciting both rod and cone systems, a grating that excited only rods was indistinguishable from a grating of the same spatial frequency that excited only cones. Moreover, contrast adaptation to gratings that excited either rods or cones raised threshold for gratings that excited rods or cones. From these results we conclude that signals from rods and cones travel together in pathways subserving the detection of low spatial frequencies, while only signals from cones travel in pathways subserving the detection of high spatial frequencies.