The extrafoveal retina if the human eye contains both rod and cone photoreceptor systems. Once thought to function independently, rod and cone systems have been shown in recent years to interact under many experimental conditions. However, the role of these rod-cone interactions in human vision is not well understood. Interactions between rods and cones become of particular importance in ocular disorders such as retinitis pigmentosa and cone dystophy, in which visual losses in the peripheral field may be due only to alterations within rod or cone mechanisms, but also to abnormalities in rod-cone interactions. The research proposed in this application will investigate the properties of an unusual type of rod-cone interaction that has recently been discovered. Experiments have shown that in the dark-adapted eye, the unstimulated rods surrounding a flickering test stimulus substantially reduce the flicker sensitivity of the cone system at high temporal frequencies. The specific aims of the research are first, to measure the spatial and chromatic properties of this rod-cone flicker interaction in normal individuals; second, to determine whether the interaction is abnormal in certain types of color vision deficiencies, as suggested by preliminary studies; and third, to determine the locus of the interaction within the visual system by studying the interaction in selected patients with visual disorders in which the underlying pathological changes are known to occur at specific levels of the visual system. Flicker sensitivity will be measured using either a Maxwellian-view optical system (normal subjects) or a Tubinger perimeter (color deficient subjects amd patients with visual disorders). The long-term objective is to determine the mechanisms by which rods and cones interact in the detection of flicker in the normal human visual system. If the mechanism underlying peripheral flicker detection can be identified, then abnormalities in flicker detection, such as these that have been observed in patients with ocular disorders, will provide useful information concerning the pathological changes that accompany these disorders.