Steven L. Buck

Sensitivity and dynamics of rod signals in H1 horizontal cells of the macaque monkey retina.

We measured the sensitivity, temporal frequency response, latency, and receptive field diameter of rod input to the H1 horizontal cell type in an in vitro preparation of the macaque retina. The H1 cell has both a cone-connected dendritic tree and a long axon-like process that terminates in a rod-connected arbor. We recorded from the H1 cell body where rod signals were distinguished by sensitivity to short wavelength light after dark adaptation. Receptive fields of rod vs. cone mediated responses were coextensive, indicating that the rod signal is transmitted via rod-cone gap junctions. Sensitivity of the H1 cell rod signal was approximately 1 log unit higher than that of the cone signal. Below cone threshold rod signals were temporally low-pass, with a cutoff frequency below 10 Hz. Rod signals became faster and more transient with increasing light levels. We conclude that the H1 cell rod signal is not sensitive in the low scotopic range and, by comparison with the rod signal recorded directly in cones (Schneeweis & Schnapf (1995) Science, 268, 1053-1056), signal transmission across the cone-H1 synapse does not significantly filter the temporal properties of the rod signal.

Rod influence on hue-scaling functions

Rod influence on hue appearance of spectral lights was characterized by comparing the scaling of red, green, yellow, and blue hue sensations for an 8 degrees-diameter, 7 degrees-eccentric test spot under conditions that minimized (cone plateau) and maximized (dark adapted) rod influence at two mesopic light levels (1.5 and 3.0 log scoptic trolands). At the lower light level, the hue-scaling functions showed that rod signals influenced the spectral range and magnitude of all four primary hues. The rod influence could not be characterized as a ubiquitous augmentation or diminution of any hue over the entire spectrum. This constrains models of rod influence on color vision.

Opponent-color models and the influence of rod signals on the loci of unique hues

To investigate how rod signals influence hue perception and how this influence can be incorporated into opponent-color models, we measured the shift of unique-hue loci under dark-adapted conditions compared with cone-plateau conditions. Rod signals produced shifts of all spectral unique hues (blue, green, yellow) but in a pattern that was inconsistent with simple additive combinations of rod and cone inputs in opponent-color models. The shifts are consistent with non-linear models in which rod influence requires non-zero cone signals. Cone-signal strength may modulate or gate rod influence, or rod signals may change the gain of cone pathways.

Infant color vision: A search for short-wavelength-sensitive mechanisms by means of chromatic adaptation

Incremental thresholds for short- and middle-wavelength test spots, projected upon blue and yellow backgrounds, were determined for 2 mth old infants. Three out of four infants tested showed a change in relative sensitivity to 460 and 560 nm test spots with a change in the wavelength composition of the backgrounds, indicating the existence of at least two separately adaptable chromatic mechanisms. When infant spectral sensitivity under yellow adaptation was examined in more detail, however, it did not agree with that of adults tested under comparable conditions. In adults, the yellow background revealed a short-wavelength mechanism (λmax = 440 nm) in isolation, but did not do so in most 2 and 3 mth old infants. The infants who were least sensitive to short wavelengths on an absolute scale also deviated most from adult-like spectral sensitivity. We tentatively interpret this difference between adults and infants as an immaturity of the infants short-wavelength-sensitive mechanism.

Influence of rod signals on hue perception: evidence from successive scotopic contrast

In successive scotopic color contrast, a colored adapting field induces a hue into a successively presented, purely rod-detected test field. To determine the rod influence on hue perception, a comparison was made, for both spectral matches and hue names, between photopic and scotopic color contrast hues produced by the same adapting fields adjusted to each of the four unique hues. Rod signals evoked hues reflecting each direction of both red/green and blue/yellow hue dimensions. Rod signals differentially strengthened blue relative to red or green hue components under some conditions but not under others. No other differential rod influences on hue were found.

Spatial patterns of rod-cone interaction

Abstract The rod system mediated detection of test flashes, but its sensitivity was altered by excitation of the cone system by a concentric background. The magnitude of this rod-cone interaction depended on the diameter of the background field, but in different ways for different observers: large backgrounds caused greater interactions than small backgrounds in two observers, and nearly the opposite spatial pattern occurred in a third. These results were the same with both adjustment and forced-choice procedures, but increasing background illuminance increased the interaction and changed its spatial pattern. We conclude that rod-cone interaction confounds tests of spatial sensitization and contributes to interobserver differences when a red background is used to isolate the rod system. No simple physiological model seems to account for variations in spatial patterns of rod-cone interactions.

Rods affect S-cone discrimination on the Farnsworth–Munsell 100-hue test

Rod influence on hue discrimination was assessed by the Farnsworth-Munsell 100-hue test. Rod influence was taken as the difference in error scores obtained after complete dark adaptation and during the cone plateau at three mesopic (23, 9, 3 td) and one standard (158 td) light level. On the FM 100, rods produced a differential discrimination loss along a tritan axis as compared with a red-green axis without any bias toward a rod confusion axis. Rods appear to impair discrimination mediated by S-cone pathways, which at moderate levels of illumination can differentially elevate tritan errors on the FM 100.

Rod-cone interaction on large and small backgrounds

Abstract The sensitivity of the rod system to a small test flash is reduced when a concentric background excites cones. For most observers, this rod-cone interaction is greater on small backgrounds than on large backgrounds. However, some observers show nearly equal interaction on both small and large backgrounds. For all observers, the magnitude of interaction on large backgrounds increases with increased background illuminance. Thus, increases of background illuminance can reduce the differences among observers, but some differences remain. The residual differences may represent fundamental differences among observers or the effect of some unexamined parameter.

Cone-rod interaction over time and space

Scotopic background stimulation can elevate photopic increment thresholds by more than 2 log units. This cone-rod interaction is greatest on small backgrounds (less than 1 degree diameter), but is found consistently on large backgrounds as well. Interaction develops and disappears quickly as backgrounds are turned on or off, respectively. The onset, and in some cases the offset, of a background stimulus can produce an additional, transitory interaction that augments the interaction that is maintained by continued presentation of the same background. The majority of the present findings lend support to a simple center-surround model of cone-rod interaction: nearby scotopic excitation raises photopic thresholds and more distant scotopic stimulation primarily antagonizes this interaction.

The time-course of rod-cone interaction

The time-course of rod-cone interaction (change of scotopic sensitivity caused by photopic background stimulation) was measured in the presence of briskly exchanged, scotopically matched, 490- and 630-nm background disks. In all conditions, interaction rose and fell quickly with changes of photopic stimulation. When the background was a small 0.6 degree-diameter disk, photopic stimulation produced relatively constant maintained interaction of about 0.6 log units. When the background was a large 7.8 degree-dia disk, photopic stimulation produced larger initial (0.6-1.0 log unit) than maintained (0.2 log unit) interaction. When a 0.6 degree by 7.8 degree annulus was used instead of a background, photopic stimulation produced substantial interaction only at offset, a transitory interaction. Thus, the spatial dependence of transitory interactions differs from that of maintained interaction: transitory interactions can be large even when maintained interaction is small or absent. The results are discussed in terms of a simple center-surround model of rod-cone interaction that unifies both maintained and transient interaction.