Slow and fast pathways in the human rod visual system: electrophysiology and psychophysics

Abstract

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.

An extended 15 Hz ERG protocol (1): the contributions of primary and secondary rod pathways and the cone pathway

“…At higher flash strengths, the perception of flicker reappears and becomes more pronounced [8][9][10][11][12]. In later studies, this disappearance of flicker perception was found to correlate with a minimum in the amplitude versus flash strength curve of 15 Hz flicker electroretinograms (ERGs) [13,14]. The minimum is accompanied by a phase shift of approximately 180°.…”

“…Furthermore, the phenomenon could also be recorded in subjects with complete achromatopsia (patients with no cone function) [13,21]. However, there are also indications in the literature, which imply that cone pathway contributions (cone signals that travel either via ON cone bipolar cells to ON ganglion cells or via OFF cone bipolar to OFF ganglion cells) are present in the responses just above the minimum [8,10,13]. Moreover, the stimuli normally applied in ERG studies do not suppress cone activity.…”

The spectral properties of the two rod pathways

“…The squares in both panels represent the lowest amplitude at which flicker can just be seen measured as a function of intensity; the upright and inverted triangles (in the 14 Hz data only), the lower and upper limits, respectively, of the null region within which suprathreshold flicker is no longer seen. At 14 Hz, a null is found because the signals from 7~~ and X; are approximately out of phase Stockman et al, 1991). At 8 Hz, no null is present because the signals are only approx.…”

“…Psychophysical data (Conner & MacLeod, 1977;Conner, 1982;Sharpe, Stockman & MacLeod, 1989) and electroretinographic (ERG) recordings (Stockman, Sharpe, Zrenner & Nordby, 1991) in the normal and achromat observer demonstrate that rod flicker signals have access to both a slow and a fast retinal pathway. The slower pathway [labeled a,, after the notation of Stiles (1978)] is more sensitive and dominates from absolute threshold up to low mesopic levels.…”

“…180 deg out of phase with one another near 15 Hz . To explain the restricted range of the null, we assume that the intensity dependencies of x0 and ~6 differ, such that the former predominates at intensities below the null and the latter at intensities above the null; the two being approximately equal at intensities within the null (Conner, 1982;Stockman et al, 1991).…”