Spatial relations of flicker signals in the two rod pathways in man.

1. Flicker signals originating from the human rod photoreceptors seem to have access to two retinal pathways: one slow and sensitive, the other fast and insensitive. The phase lag between signals in the two pathways grows monotonically with frequency, reaching 180 deg near 15 Hz. 2. At 15 Hz, destructive interference between the slow and the fast signals can cause two related phenomena: (i) a suprathreshold intensity region--the perceptual null--within which the perception of flicker vanishes, and (ii) a double branching of the 15 Hz rod-detected flicker threshold versus intensity (TVI) curve. 3. Here we investigate the effect of changing target size on these phenomena in normal human observers. We find that the double-branched flicker TVI curve and the perceptual null are found for all targets larger than 2 deg in diameter. For smaller diameter targets, however, neither the lower branch of the double-branched flicker TVI curve nor the null are found. 4. While this might suggest that the slow rod signals are selectively disadvantaged by the use of small targets, phase measurements relative to a cone standard reveal that the slow signals are always present. For targets < or = 2 deg in diameter, however, they remain below detection threshold because of destructive interference with the fast rod signals. Thus, for small targets, the perceptual null is not absent, but has merged with (and therefore obliterated) the lower branch of the double-branched flicker TVI function. 5. This situation could arise if decreasing the target size causes a parallel reduction in the sensitivities of both pathways, rather than a selective reduction in the sensitivity of either one. Our findings are therefore consistent with a model in which the large-scale spatial organization of the two rod pathways is roughly similar.

The spectral properties of the two rod pathways.

Psychophysical and electroretinographic observations in normal and achromat observers suggest that rod flicker signals have access to at least two retinal pathways: one (pi 0), slow and sensitive, predominating at scotopic luminance levels; the other (pi'0), fast and insensitive, predominating at mesopic ones. We have measured steady-state flicker detection sensitivities on background fields ranging from 430 to 640 nm in normal observers. Our results suggest that cone signals can reduce the sensitivity of pi'0, but have comparatively little effect on pi 0. The pi'0 field sensitivities derived from these measurements have been fitted with linear combinations of the scotopic luminosity function, V' lambda, the M-cone spectral sensitivity function, M lambda, and the L-cone function, L lambda. These fits demonstrate a clear cone influence on pi'0, but they cannot tell us unequivocally whether the influence is from the M-cones, from the L-cones or from both. Accordingly, we made similar measurements in dichromats, who lack one of the two longer wavelength cone types. These measurements revealed an L-cone influence on pi'0 in the deuteranope and an M-cone influence in the protanope. This suggests that both cone types can affect the sensitivity of pi'0. The finding that the steady-state cone signals reduce the sensitivity of pi'0 but have little effect on pi 0 could suggest that pi'0 signals travel through a faster cone pathway (with its own gain control at which both rod and cone signals can reduce rod threshold), while pi 0 signals travel through a separate rod pathway. However, it could simply reflect the fact that pi'0 predominates at higher luminances than pi 0 where the cone excitation level is inevitably greater. To examine the influence of the cones on pi 0 more closely, we: (i) produced transient cone excitation by alternating rod-equated 480 and 679 nm fields; and (ii) extended our steady-state measurements to include deep-red backgrounds of 650 and 680 nm. Both experiments revealed a small, but measurable influence of the cones on pi 0.

Temporal and spatial summation in the human rod visual system.

1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the fovea with 520 nm targets of varying size and duration. Measurements were made under rod-isolation conditions in two normal observers and in a typical, complete achromat observer who has no cone-mediated vision. The purpose of these experiments was to determine how the temporal and spatial summation of rod-mediated vision changes with light adaptation. 2. The absolute threshold and the rise in increment threshold with background intensity depend upon target size and duration, but the psychophysically estimated dark light of the eye (the hypothetical light assumed to be equivalent to photoreceptor noise) does not. 3. The rise in increment threshold for tiny (10 min of arc), brief (10 ms) targets approaches the de Vries-Rose square-root law, varying according to the quantal fluctuations of the background light. The slope of the rod increment threshold versus background intensity (TVI) curves in logarithmic co-ordinates is about 0.56 +/- 0.04 (when cones are not influencing rod field adaptation). For large (6 deg) and long (200 ms) targets, a maximum slope of about 0.77 +/- 0.03 is attained. 4. The steeper slopes of the rod-detected TVI curves for large, long targets implies some reduction in temporal or spatial summation. In fact, the change in summation area is much more critical: under conditions where only the rod system is active the TVI curve slope is independent of target duration, suggesting that temporal summation is practically independent of background intensity. 5. The rise in threshold also depends on the wavelength of the background field in the normal observer but not in the achromat, confirming reports that the field adaptation of the rods is not independent of the quantal absorptions in the cones. The cone influence is most conspicuous on long-wavelength backgrounds and is found for all target sizes and durations, but is greater for large and long targets than for the other conditions.

The field adaptation of the human rod visual system.

1. Incremental thresholds were measured in a retinal region 12 deg temporal from the fovea with a target of 200 ms in duration and 6 deg in diameter superimposed on background fields of various intensities and wavelengths. Measurements were made under rod-isolation conditions in five normal observers and in a typical, complete achromat observer who had no cone function. 2. The rise in threshold with background intensity changes with background wavelength in the normal trichromat observers. On 450, 520 and 560 nm backgrounds the average slope in logarithmic co-ordinates (0.78 +/- 0.04, S.D.) is similar to that found for the achromat--whose slope is independent of background wavelength (0.79 +/- 0.03)--but on a 640 nm background it more nearly approaches Weber's law (0.91 +/- 0.02). This indicates that the sensitivity of the rods to an incremental target is not determined by quantal absorptions in the rods alone but by quantal absorptions in both the rods and the cones. 3. Rod incremental thresholds were also measured in various colour-blind observers lacking one or more of the cone classes: a blue-cone monochromat, four deuteranopes and a protanope. For the blue-cone monochromat, like the achromat, the slope of the increment threshold curve is constant with background wavelength. For the deuteranopes and the protanope, like the normal, the slope increases with wavelength. The protanope, however, shows a smaller increase in slope, consistent with the lower sensitivity of his cones to long-wavelength light. 4. The dependence of the field adaptation of the rods on the cones was confirmed by field-mixture experiments, in which the incremental threshold was measured against bichromatic backgrounds, and in silent substitution experiments, in which backgrounds equated for their effects on either the cones or the rods but not both were instantaneously substituted for one another.

Contingent aftereffects: lateral interactions between color and motion.

A randomly dotted yellow disk was rotated at a speed of 5 rpm, alternating in direction every 10 sec. Its change in direction of rotation was paired with a change in surround color, which was either red or green. After 15 min of exposure, observers reported vivid motion aftereffects contingent on the color of both the stationary disk and the surround, even though during adaptation only motion or color was associated with either alone. In further experiments, it was established that a change in color (or direction of motion) of the disk could be associated with a change in direction of motion (or color) of the surround. Such lateral effects were found even when a wide (5 degree) annulus was introduced between the disk and the surround during adaptation and testing. Furthermore, the aftereffects generalized to the annulus, which was not associated with either color or motion during adaptation. However, when the disk alone was adapted to color and motion, no generalization to the surround was found (and vice versa), suggesting that the effects are not produced by adaptation of large receptive fields or by scatter of light within the eye. The results appear to conflict with the ideas that contingent aftereffects are confined to the adapted area of the retina and that they are built up by links between single-duty neurones, and with an extreme view of the segregation of color and motion early in human vision.