Under most conditions, increasing the intensity of a flickering light makes the flicker more conspicuous. For a light flickering at 15 times per second, however, increasing the intensity can cause the flicker to disappear before reappearing again at higher intensities [ Vision Res. 29, 1539,( 1989)]. This flicker disappearance or null is also evident in human electrophysiological recordings at the same intensity levels. These results point to a duality within the rod visual pathway, in which flicker signals travel through a slow and a fast pathway and then recombine at a later stage. At 15 Hz the slow rod flicker signals are delayed by half a cycle relative to the fast signals. Thus, when the two signals are recombined, they destructively interfere and diminish the perception of flicker. The dual-pathway interpretation is supported by both electroretinographic and psychophysical evidence showing a phase difference of half a cycle between 15-Hz rod signals just below and just above the null region. These effects are apparent not only in the normal observer but also in an achromat observer who lacks functioning cone vision.
© 1991 Optical Society of America
Original Manuscript: September 6, 1990
Revised Manuscript: May 10, 1991
Manuscript Accepted: May 14, 1991
Published: October 1, 1991
Andrew Stockman, Lindsay T. Sharpe, Eberhart Zrenner, and Knut Nordby, "Slow and fast pathways in the human rod visual system: electrophysiology and psychophysics," J. Opt. Soc. Am. A 8, 1657-1665 (1991)