Effect of target size on the fast and slow rod pathways

Abstract

Psychophysical and electroretinographic observations in normal observers and achromats (who lack cone vision) suggest that rod signals have access to two retinal pathways: one, slow and sensitive; the other, fast and insensitive. The two are revealed by double-branched rod-detected flicker threshold vs intensity (ftvi) curves, and by a suprathreshold intensity region (below cone detection)—the perceptual null—within which the sensation of 15-Hz flicker is canceled. The cause of the null is believed to be destructive interference between signals conveyed by the two pathways. Here we report that the break in the 15-Hz ftvi curve and the perceptual null is not found, in either the normal or achromat observer when the flickering target is ≤2° in diameter. Phase measurements (relative to a cone standard) indicate that the faster rod pathway mediates flicker detection for targets of ≤2°, not because the signals from the slow pathway are severely diminished but because destructive interference between the slow and fast rod flicker signals keeps the slow signals from ever exceeding threshold. There is therefore no need to invoke differences in spatial integration between the two pathways to explain the large difference in the ftvi curves.