Rod-cone dependence of saccadic eye-movement latency in a foveating task.

This study examines the relations between some well known oculomotor functions (saccades) and well known retinal physiology (dark adaptation): it deals with the overall latency versus target luminance functions, with the underlying rod and cone latency-luminance functions, and with the synergistic interaction between these latency functions for mesopic targets. Saccadic latency was measured to small lit targets presented at 10 deg retinal eccentricity in complete darkness. Target luminance and wavelength were varied. Additional measurements were made during dark adaptation or on backgrounds, or at different retinal eccentricities. Luminance matched stimuli and Palmer's (1968) equivalent luminance transformation were also used. Latency is determined by an achromatic luminance mechanism that receives substantial rod inputs above the cone threshold. Latencies for pure rod or pure cone inputs increase rapidly as target luminance decreases. For the rods this latency increase appears to represent the waiting time for the 140 or so photons (lambda = 507 nm) that are required for a saccade. Errors in direction occur at scotopic luminances, or at low photopic luminances when only cones are functioning.