The perception of moving comets at high retinal illuminance levels: a rod-cone interaction effect.

A small circular target of high retinal illuminance level can have a comet-like appearance when presented moving continuously with a speed as low as 0.2 deg/s. This perceived lengthening of the circular target increases with the speed of movement and is only observed for target presentations outside the foveal region. Data on the parametric properties of the "comet effect" are presented together with related results on the time-course of recovery of retinal sensitivity following brief exposure to intense stimuli. Measurement of target spectral irradiance levels which are just sufficient to yield the comet effect suggests that the lengthening of the circular target reflects a rod-cone interaction and therefore it may be due to unsuppressed, saturated rod responses at high retinal illuminance levels. The restriction of the comet effect to areas outside the foveal region is used to produce spatial maps of what appears to be the rod-free area of the retina. A model simulation by means of a computational approach shows that the predicted appearance of the moving target matches very closely the experimental observations on the comet effect. Model predictions based on psychophysical estimates of comet length for the stimulus conditions of these experiments yield an overall response time for the rod system of some 600 ms.