Abstract

Rod light adaptation removes a suppressive influence on cone pathways and hence enhances sensitivity to rapid flicker, i.e., suppressive rod-cone interaction (SRCI). Horizontal cell (HC) recordings from cat retina (Pflug and Nelson, 1989) show SRCI to be limited by a mechanism with a space constant of ~160 µm, a value consistent with estimates of cat HCs obtained by conventional procedures. In the present study we measured the space constant for SRCI using psychophysical procedures in humans. We examined the influence of rod stimulating background fields of 0.5 scotopic td illuminance on the minimal illuminance necessary to detect 15-Hz flicker in a slit-shaped flicker probe (10° length centered 7° parafoveally). With one paradigm, we examined the influence of flicker probe width on enhancement of flicker sensitivity (E) induced by large rod backgrounds; alternatively, we determined the influence of a 10° long × 2.4° wide rod background on flicker thresholds for a 7' wide test probe. E decreased as flicker test probe increased in size, or as the rod background slit was displaced from the test slit. Both effects are characterized by a similar space constant (depending on the observer, 100-170 µm), which we suggest to be the space constant for the HC neurosyncytium underlying SRCI.

© 1989 Optical Society of America