Contrast-dependent red-green balance shifts depend on S-cone activity

Previous research from our lab has established that red-green-balanced yellow targets become greenish-brown as surround luminance increases, while red-green-balanced brown targets become reddish-yellow as surround luminance decreases. To help assess the generality and underlying processes of this contrast-dependent red-green hue shift, we investigated red-green hue shifts for target stimuli that appeared achromatic or blue as well as yellow/brown. Results confirmed that the red-green hue shift was largest for yellow/brown targets and was progressively reduced for achromatic and blue targets as target excitation of S cones increased. The magnitude of the hue shift could be predicted by the S/(L+M) excitation of the target when bright white surrounds are used. The hue shift also requires that the target and surround are presented to the same eye, consistent with processing in monocular pathways. Increased S-cone excitation by the surround was associated with red-green hue shifts for all targets equally. Thus, S-cone signals from bright white surrounds might play a role in the contrast-dependent red-green hue shift, but the source of the variation of the magnitude of the hue shift with variations in target S-cone excitation when presented on those surrounds is unknown.

Contrast-dependent red-green hue shift

On bright surrounds, red-green-balanced yellow targets become greenish brown with decreased target luminance, and red-green-balanced brown targets become reddish yellow with increased target luminance. These effects imply luminance- and/or contrast-dependent weighting of M- and L-cone signals in post-receptoral pathways. We show psychophysically that luminance contrast between the surround and the target is the primary determinant of the magnitude of red-green hue shift, requiring surround luminance at least twice the target luminance and increasing with further increases of surround/target contrast. There is a much smaller effect of absolute stimulus luminance, with dimmer stimuli showing slightly larger hue shifts. To evaluate a possible retinal origin of the changes in cone-signal weightings underlying the hue shift, we recorded spike responses from both ON- and OFF-center midget ganglion cells in peripheral primate retina. We found no evidence that the relative strength of L- and M-cone post-receptoral responses changed systematically with change of surround irradiance. Nor was there any systematic difference between ON- and OFF-subtypes. This suggests that the change in cone signal weighting occurs later in the visual system.

Color Vision 2018: Introduction by the feature editors

This feature issue of the Journal of the Optical Society of America A (JOSA A) reflects the basic and applied research interests of members of the color vision community. Most of the articles stem from presentations at the 24th Biennial Symposium of the International Colour Vision Society (ICVS).

Color Constancy in Two-Dimensional and Three-Dimensional Scenes: Effects of Viewing Methods and Surface Texture

There has been debate about how and why color constancy may be better in three-dimensional (3-D) scenes than in two-dimensional (2-D) scenes. Although some studies have shown better color constancy for 3-D conditions, the role of specific cues remains unclear. In this study, we compared color constancy for a 3-D miniature room (a real scene consisting of actual objects) and 2-D still images of that room presented on a monitor using three viewing methods: binocular viewing, monocular viewing, and head movement. We found that color constancy was better for the 3-D room; however, color constancy for the 2-D image improved when the viewing method caused the scene to be perceived more like a 3-D scene. Separate measurements of the perceptual 3-D effect of each viewing method also supported these results. An additional experiment comparing a miniature room and its image with and without texture suggested that surface texture of scene objects contributes to color constancy.

Brown

In this Quick Guide, Steven Buck explains how, uniquely among the bright primary perceptual hues, yellow changes its hue when it appears dark, becoming the colour brown.

Influence of surround proximity on induction of brown and darkness

A bright white surround makes a yellow long-wavelength target look both browner and darker. We explored the parallel between these two types of induction by examining their dependence on the proximity of the bright surround to the target at two different time scales with 27 ms and 1 s stimulus durations. We assessed (a) brown induction by adjustment of target luminance to perceptual brown and yellow boundaries and (b) darkness induction by a successive matching procedure. We found that brown induction is a quick process that is robust even for 27 ms stimuli. For darkness induction, there was a strong, spatially localized surround proximity effect for the 27 ms stimuli and much weaker proximity effect for the 1 s stimuli. For brown induction, proximity effects were generally weaker but still showed relatively stronger localized proximity effects for 27 ms stimuli than for 1 s stimuli. For these stimuli, darkness induction predicts the relative pattern but not the magnitudes of brown induction. Both brown and darkness inductions show the operation of quick, spatially localized processes that are apparently superseded by other processes for extended stimulus presentations.

Color vision: introduction by the feature editors

This feature issue of the Journal of the Optical Society of America A (JOSA A) reflects the basic and applied research interests of members of the color vision community. Most of the articles stem from presentations at the 23rd Biennial Symposium of the International Colour Vision Society (ICVS).

Dichoptic perception of brown

Two experiments assessed mechanisms underlying brown induction by presenting a foveal target disk and concentric annular surround stimuli that varied in contrast relative to larger backgrounds. Stimuli were presented under monocular, binocular, and dichoptic viewing conditions. Observers adjusted the luminance of the target disk to a criterion brown level. We found evidence for at least two separate mechanisms for brown induction: one mechanism that is dependent on physically contiguous contrast and operates in monocular pathways and another mechanism that responds to high luminance contrast anywhere in the visual field and can operate after convergence of signals from the two eyes.

No effects of surround complexity on brown induction

A yellow stimulus turns brown when it is made sufficiently darker than its surroundings. Most previous studies have used simple contiguous surround stimuli to induce brown, so we know little about how brown induction may be controlled by more distant and more complex surround features. We begin to address this issue by varying the complexity of two configurations of achromatic surround stimuli. It was shown that the area most immediately contiguous to the test stimulus has strong effects on brown induction. More importantly, we found that neither the number of surround features nor the distribution of light in the surround region had an effect on brown induction, as long as the overall size of the surround region remained constant. Instead, we found that brown induction depended on the total amount of light in the constant-size surround region, regardless of how that light was distributed. This potentially distinguishes the mechanisms of brown induction from those of brightness induction.

Luminance-dependent long-term chromatic adaptation

There is theoretical and empirical support for long-term adaptation of human vision to chromatic regularities in the environment. The current study investigates whether relationships of luminance and chromaticity in the natural environment could drive chromatic adaptation independently and differently for bright and dark colors. This is motivated by psychophysical evidence of systematic difference shifts in red-green chromatic sensitivities between contextually bright- versus dark-colored stimuli. For some broad classes of scene content, consistent shifts in chromaticity are found between high and low light levels within images. Especially in those images in which sky and terrain are juxtaposed, this shift has direction and magnitude consistent with the observed psychophysical shifts in the red-green balance between bright and dark colors. Taken together, these findings suggest that relative weighting of M- and L-cone signals could be adapted, in a luminance-dependent fashion, to regularities in the natural environment.

Color vision: Introduction by the feature editors

This feature issue of the Journal of the Optical Society of America A (JOSA A) stems from the 22nd Biennial Symposium of the International Colour Vision Society (ICVS) and reflects the basic and applied research interests of members of the color vision community. A profile is included of the 2013 Verriest Medal recipient.

Dark versus bright equilibrium hues: rod and cone biases

Equilibrium (unique) red, green, blue, and yellow stimuli look bright in a black surround, but they look dark in a bright white surround, and yellow changes to brown. We investigated differences in equilibrium-hue chromaticity between bright and dark hues to reveal changes in weighting of cone and rod signals. The largest, most consistent shifts were found between yellow and brown, with equilibrium-brown chromaticity shifted toward red compared to equilibrium yellow at both photopic and mesopic levels. Also, at mesopic levels, rod influence reversed for most observers from a green bias for yellow to a red bias for brown. Bright/dark differences for blue, green, and red were much smaller and/or less consistent. Thus, shifts of cone and rod hue biases between bright and dark hues are most prominent in L-M-cone pathways, especially those activated by yellow and brown stimuli.

Rod hue biases for foveal stimuli on CRT displays

Signals from rod photoreceptors bias (shift) the hues determined by cone photoreceptors for extrafoveal mesopic stimuli, creating green, blue, and red rod hue biases at long, middle, and short wavelengths, respectively. The fovea contains far fewer rods and S cones but may not be immune to rod hue biases. Here, we determine the biases found for mesopic foveal stimuli presented on a CRT display. The rod green bias was observed at unique yellow for all but one observer with 2° tests and persisted for most observers with 0.5° tests. The rod red bias typically seen at unique blue in extrafoveal studies was not apparent for either size of foveal test stimulus, and it was sometimes replaced by a rod green bias. The rod blue bias typically seen at unique green and unique red in extrafoveal studies was weak on average and inconsistent for both sizes of foveal test stimuli. Thus, small mesopic foveal stimuli permit rod influence on M- and L-cone color pathways but disadvantage rod influence on S-cone pathways, perhaps because of the sparseness of foveal S-cones. However, some observers did show idiosyncratic foveal rod hue biases that do not follow the general trends.

Rod hue biases produced on CRT displays

Studies of rod hue biases using monochromatic stimuli have shown that rod stimulation can shift the balance of hues at mesopic light levels. We found that the CRT display produced all three previously identified rod hue biases, which shifted the loci of all four unique hues at low mesopic light levels. Rod hue biases occurred at 2.6 cd/m(2) for some observers but not at 26 cd/m(2). At optimal light levels below 0.5 cd/m(2), rod hue biases varied among observers but generally (1) enhanced green versus red at unique yellow and sometimes at unique blue, (2) enhanced blue versus yellow at both unique green and unique red, and (3) enhanced red versus green at unique blue. Rod hue biases persisted for some observers even for smaller foveal stimuli.

A re-examination of local contrast in multiple schedules

Pigeons were presented with multiple schedules of alternating 90-sec components. When components in which grain was never presented alternated with components in which grain was presented on a variable-interval schedule, the average rate of responding in the variable-interval components increased, showing overall positive behavioral contrast. Unlike previous reports, this study found that the response rates for all birds increased toward the end of the variable-interval components as training proceeded. This increase in local response rate disappeared when the multiple schedule was composed solely of variable-interval components and reappeared when the variable-interval components were again alternated with extinction. This finding cannot be predicted or explained by recent theories of behavioral contrast based on autoshaping, and thus questions their sufficiency. We suggest that this local response-rate increase results from the predictable change from high to low density of reinforcement at the end of the fixed-duration component. Thus, the present effect apparently illustrates a different type of interaction between components of a multiple schedule than that described by previous theories of contrast. In a given procedure, either or both types of interaction may occur; neither provides a complete account of behavioral contrast.

Do rods influence the hue of foveal stimuli?

To understand the generality and mechanisms of previously reported rod hue biases, we examined whether they are present for small foveal stimuli by comparing the wavelengths of the three spectral unique hues under dark-adapted and flash-bleached conditions. Rod green bias (shift of unique yellow) and rod blue bias (shift of unique green) were found for some observers with 1 degrees -diameter foveal stimuli, the size most likely to stimulate rods. Smaller stimuli (0.2 degrees and 0.6 degrees diameter), which were least likely to stimulate rods, produced no large or consistent differences between dark-adapted and bleached conditions. This suggests that rod hue biases result from the local stimulation of rods by light, not from remote suppression by dark-adapted, unstimulated rods, and not from bleaching light artifacts.

Foveal and extra-foveal influences on rod hue biases

Green, blue and short-wavelength-red rod hue biases are strongest and most reliable with large, dimly-mesopic, extra-foveal stimuli but tend to diminish when stimuli are confined to a small area of the central fovea. This study explores how the stimulation of foveal and extra-foveal areas interact in determining rod hue biases, and whether large stimuli are as effective for revealing rod hue biases when foveally centered as when eccentrically centered. We assessed rod influence by measuring wavelengths of unique green and unique yellow (with 1-s duration, 1 log scot td stimuli and a staircase procedure) under bleached and dark-adapted conditions. We measured unique hues with foveally centered 2 degrees - and 7.4 degrees -diameter disks, a 7.4 degrees (outer) x 2 degrees (inner) diameter annulus, and a 7 degrees -eccentric, 7.4 degrees -diameter disk. The rod green bias (shift of unique yellow locus) was typically <10 nm and remained fairly constant across spatial configurations, indicating no special foveal influence. The rod blue bias (shift of unique green) varied more among observers and spatial configurations, reaching up to 47 nm. However, stimuli covering the fovea typically produced no rod blue bias. Thus, the present results add differences in spatial dependence (i.e., foveal/extra-foveal interaction) between green and blue rod biases to previously demonstrated differences (e.g., differences in amount of light level dependence, in time course and in the spectral range influenced by each bias).

Generality of rod hue biases with smaller, brighter, and photopically specified stimuli

This study tests the generality of previously demonstrated rod hue biases (red and blue biases at shorter wavelengths and a green bias at longer wavelengths) that cause the loci of the three spectral unique hues to shift to longer wavelengths. We found rod hue biases for 2-deg targets to be generally similar in magnitude and light-level dependence to those observed for 7.4-deg targets (the size most often studied) when measured at 7-deg eccentricity. The largest effects for both test sizes occurred at the lowest light levels tested, 1 log scotopic troland. All three rod hue biases were found with 0.6-deg targets, but were not reliably measurable at the lowest light levels and were reduced in magnitude and consistency across observers. The largest rod hue biases all occurred at the same scotopic light level, which corresponds to different photopic light levels for the three hue biases, because of differences in photopic and scotopic spectral sensitivity. This suggests that no single photopic light level will produce such large effects for all three rod hue biases. Finally, when the rod influence on a specific unique-hue locus was measured using photopically (rather than scotopically) constant stimuli, rod hue biases were still found but were more variable in magnitude and incidence across observers. We conclude that the rod hue biases we have previously described can be found with smaller stimuli, at somewhat higher light levels, and under photopically constant conditions, although our prior conditions tend to produce larger, more reliable rod hue biases.

Time-dependent changes of rod influence on hue perception

Hue-naming was used in conjunction with a probe-flash procedure to determine the time-course of rod-mediated effects on hue appearance across the spectrum. Two types of rod influence on hue are distinguishable on the basis of differences in both spectral specificity and time course of effect: (1) a "faster" rod influence enhances green relative to red and (2) a "slower" rod influence enhances short-wavelength red relative to green and blue relative to yellow. The results show that there are separable rod hue biases that operate over different time courses and that the overall rod influence on hue appearance depends importantly on the temporal properties of the stimuli, presumably because rods interact in different ways with different portions of the neural pathways that mediate human 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.

Broad spectrum antimicrobial activity of a new multi-purpose disinfecting solution

PURPOSE

There has been increasing awareness of the need for more powerful contact lens disinfectants, ideally having low toxicity and allergenicity to ocular tissue. Opti-Free Express with ALDOX Multi-Purpose Disinfecting Solution (MPDS) was recently marketed as a new multi-purpose disinfecting solution for soft contact lenses. MPDS contains two antimicrobial agents, polyquaternium-1 and myristamidopropyl dimethylamine, to broaden the range of antimicrobial activity. The purpose of this study was to evaluate the antimicrobial activity of MPDS against microorganisms that have been reported as contact lens and lens case contaminants and causative agents of microbial keratitis.

METHODS

MPDS was challenged with high numbers of gram-positive and gram-negative bacteria, yeast, mold, and Acanthamoeba. Several lots of product were challenged with at least three separate inocula of each isolate. After exposure to the disinfecting solution, a sample was removed from the product and plated for survivors by suitable recovery methods. The plates were incubated and the number of survivors was enumerated.

RESULTS

The results showed that MPDS was bactericidal, fungicidal and amoebicidal against an extensive variety of environmental contaminants of lens care accessories and ocular pathogens. MPDS was capable of killing a broad spectrum of microorganisms. The log reduction exceeded the primary criteria for disinfecting products and produced a notable reduction of other clinical and environmental isolates as well.

CONCLUSIONS

Opti-Free Express MPDS contains antimicrobial agents that can kill an extensive spectrum of many different types of microorganisms that may contaminate contact lenses and solutions.

Methods used to evaluate the effectiveness of contact lens care solutions and other compounds against Acanthamoeba: a review of the literature

PURPOSE

The purpose of this paper is to review the literature concerning the methods used to evaluate contact lens care solutions against Acanthamoeba. Acanthamoeba keratitis is a potential threat, with 85% of the cases being reported in contact lens wearers.

METHODS

Several studies from the published literature that evaluated contact lens disinfectants were reviewed. The variables included test organism, strain and morphology, growth conditions, inoculum preparation, inoculation method, test solutions and concentration, contact time, neutralization, recovery, quantitation method, and viability determination of survivors. The methods used to test Acanthamoeba against the disinfectants were compared and contrasted.

RESULTS

After a thorough review of methods used to test Acanthamoeba, it was found that there is great variability in the methods used to evaluate contact lens disinfectants. The majority of the studies used A.castellanii and A.polyphaga cysts grown axenically in PYG medium containing cations at about 30 degrees C and the inoculum contained about 1.0 x 10(5) cells/mL. Inactivation media or centrifugation of cells was used to neutralize test samples. Quantitation was performed in most studies and viability was checked in all studies. The disinfectants tested most often were PHMB, hydrogen peroxide, thimerosal, and chlorhexidine.

CONCLUSIONS

After reviewing the studies presented here it can be concluded that an effective method for testing Acanthamoeba against contact lens disinfectants would include A.castellanii or A.polyphaga grown axenically in PYG containing cations and a concentration of organisms high enough to adequately measure kill, a neutralization step, recovery and quantitation of organisms followed by a viability check of survivors.

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.

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 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.

Amoebicidal activity of a preserved contact lens multipurpose disinfecting solution compared to a disinfectinn/neutralisation peroxide system

The amoebicidal activity of a contact lens multipurpose disinfecting solution (MPDS) containing polyquaternium-1 and myristamidopropyl dimethylamine was compared to a disinfection/neutralisation peroxide system against Acanthamoeba castellanii and Acanthamoeba polyphaga trophozoites and cysts. A quantitative microtitre method was used to evaluate the solutions. The MPDS showed similar amoebicidal activity to the disinfection/neutralisation peroxide system against the trophozoites of both species and equal or more rapid activity against the cysts of both species.

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.

A quantitative method to evaluate neutralizer toxicity against Acanthamoeba castellanii

A standard methodology for quantitatively evaluating neutralizer toxicity against Acanthamoeba castellanii does not exist. The objective of this study was to provide a quantitative method for evaluating neutralizer toxicity against A. castellanii. Two methods were evaluated. A quantitative microtiter method for enumerating A. castellanii was evaluated by a 50% lethal dose endpoint method. The microtiter method was compared with the hemacytometer count method. A method for determining the toxicity of neutralizers for antimicrobial agents to A. castellanii was also evaluated. The toxicity to A. castellanii of Dey-Engley neutralizing broth was compared with Page's saline. The microtiter viable cell counts were lower than predicted by the hemacytometer counts. However, the microtiter method gives more reliable counts of viable cells. Dey-Engley neutralizing medium was not toxic to A. castellanii. The method presented gives consistent, reliable results and is simple compared with previous methods.

Partial additivity of rod signals with M- and L-cone signals in increment detection

Test additivity experiments revealed the combination rules for increment detection by rods and either M- or L-cone-dominated mechanisms isolated by means of chromatic adaptation (Stiles' pi 4 and pi 5, respectively). Increment thresholds were measured for single test wavelengths detected by each mechanism. Pairs of test wavelengths were then superimposed, and increment thresholds were measured for simultaneous detection by both rod and cone mechanisms. The observed degree of additivity was corrected (reduced) to compensate for the partial detection by each mechanism of both test wavelengths in the combined stimuli. We find that subthreshold rod signals are partially additive with subthreshold signals from both M- and L-cones. The degree of additivity is high and similar for both M- and L-cones: less than the ideal prediction of linear addition, but greater than that predicted by either probability summation of independent mechanisms or orthogonal vector addition.

Cone pathways and the pi 0 and pi 0' rod mechanisms

The field-adaptation properties of two scotopic (rod) mechanisms, pi 0 and pi 0', were measured to test a two-pathway model that associates the fast temporal properties of pi 0' with the processing of rod signals by early cone pathways, possibly including cone photoreceptors, and the sluggish temporal properties of pi 0 with processing of rod signals by classical rod pathways. This model predicts that cone stimulation will differentially affect the flicker sensitivity of pi 0' compared to pi 0. Both rod mechanisms are seen in double-branched flicker-threshold-vs-intensity (FTVI) curves measured with a 15-Hz, square-wave-modulated, rod-detected test stimulus. We show that the position of the upper branch (pi 0') shifts relative to the lower branch in response to changes of background wavelength, indicating that different receptor types regulate sensitivity of pi 0 and pi 0'. Field spectral sensitivity (FSS) functions for pi 0 closely match the scotopic spectral sensitivity function, indicating that only rods adapt pi 0 under these conditions. In contrast, fitting of FSS functions for pi 0' required a combination of cone and rod spectral sensitivity functions. The relative adaptational effect of cone stimulation compared to rod stimulation increases with background light level: at highest levels, cone stimulation has more influence than rod stimulation. Test additivity experiments assessed the degree of additivity between cones and rods to ensure that the pi 0' branch did not result from sub-threshold summation between receptor mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantification of monocular optokinetic nystagmus asymmetries and motion perception with motion-nulling techniques

When tested monocularly, strabismic and amblyopic subjects often show asymmetries of optokinetic nystagmus (OKN), with OKN being more readily elicited by temporal-to-nasal than by nasal-to-temporal stimulus motion. We tested five visually normal subjects and ten strabismic and/or amblyopic subjects by use of motion-nulling stimuli, which consisted of superimposed temporal-to-nasal and nasal-to-temporal sinusoidal-grating components with a summed contrast of 100%. Both the direction of OKN and the subject's perceived direction of motion (PDM) were tested. Most normal subjects showed symmetrical OKN and PDM, but a rightward OKN bias was observed in one of the visually normal subjects. Temporal-to-nasal eye-movement biases were seen in most strabismic and amblyopic subjects, whereas PDM biases were smaller and less frequent. The primary purpose of this study was to demonstrate the feasibility of quantifying OKN and PDM asymmetries in a diverse group of visually abnormal adults by use of the motion-nulling technique. Application of this technique to larger and more homogeneous clinical populations may contribute to the continued differentiation and characterization of variants of the visual disorders associated with strabismus and amblyopia and with other defects of binocular vision.

Rod-cone interaction in monocular but not binocular pathways

Photopic background stimulation elevates scotopic increment thresholds (rod-cone interaction) at moderate background levels when both test and concentric disk-background stimuli enter the same eye (monocular condition) but not when they enter different eyes (dichoptic condition). Only when background levels are made extremely high is there any measurable dichoptic interaction, and this interaction does not resemble that observed monocularly. Rod-cone interaction, as usually studied, is a property of monocular pathways in human vision.

Determinants of the spatial properties of cone-rod interaction

Photopic increment thresholds can be elevated by 0.2-1.9 log units, depending on the diameter of a concentric scotopic background. This cone-rod interaction displays spatial properties that resemble the spatial sensitization (Westheimer effect) observed in the isolated scotopic and photopic systems. This raises the possibility that the spatial properties of the interaction are determined by the same mechanisms or pathways that determine the spatial properties of either scotopic or photopic vision. When annulus backgrounds are used, the spatial properties of interaction match those of the scotopic system but not the photopic system. When disk backgrounds are used, the spatial properties of interaction match those of neither photopic nor scotopic systems. Thus, under some conditions, the scotopic visual system alone is sufficient to determine the spatial properties of cone-rod interaction. Under other conditions, additional complications arise. The results are discussed in terms of the center-surround model that has previously been applied to cone-rod interaction.

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.

Background visibility and increment thresholds

Increment thresholds for small test flashes measured on superimposed backgrounds sometimes differed depending on whether an observer was instructed to keep the background subjectively visible or (Troxler) faded. The magnitude and direction of the threshold change varied among observers. For some observers, the magnitude of the effect of such instructions also varied with background size, psychophysical method, and displacement of test from background. How the instructions and their presumed association with different phenomenological states affect thresholds is not known. However, these results show that the magnitude of effects and the shapes of psychophysically measured functions can be affected by the instructions followed by the observer, whether self-imposed or imposed by the experimenter.

Visibility of borders: separate and combined effects of color differences, luminance contrast, and luminance level

The strength of the border between two regions was assessed by measuring the tendency of the border to disappear during a 5-min fixation period. We measured the total time the border was judged visible when subjects viewed two hemifields with different chromaticities (six wavelength pairs), five luminance contrasts, or with differences in both, at five levels of luminance. For a homochromatic field, border visibility increased linearly with the logarithm of the luminance contrast, regardless of the particular wavelength of the field. For fields with only chromatic differences, border visibility increased linearly with the logarithm of the tritanopic-purity difference (the relative activity of only the long- and middle-wavelength-sensitive cones). For all borders, visibility increased with an increase in luminance level. Thus borders formed by the combination of chromatic differences and luminance contrasts are more visible than borders that have the same chromatic difference or the same luminance contrast alone. For any chromatic difference, calculations of a luminance contrast that would yield the same border visibility were made. These equivalent luminance-contrast functions for border visibility provide a metric for chromatic differences across a range of luminances that allows chromatic and luminance-contrast sensitivities to be compared in a nonarbitrary way. Relative to chromatic differences, luminance contrast is less effective in maintaining border visibility at lower luminance levels. When the borders combine both chromatic differences and luminance contrasts, a root-mean-square model can be used to account for the data. This is consistent with the idea that chromatic and luminance systems make independent contributions to the visibility of borders.