Parabolic achromatic color matching functions: Dependence on incremental and decremental luminance.

Either the brightness or lightness of a disk surrounded by an annulus is characterized in the most general case by a parabolic function of the annulus luminance when plotted on a log-log scale. This relationship has been modeled with a theory of achromatic color computation based on edge integration and contrast gain control [J. Vis.10, 1 (2010)1534-736210.1167/10.14.40]. We tested predictions of this model in new psychophysical experiments. Our results support the theory and reveal a previously unobserved property of parabolic matching functions that depends on the disk contrast polarity. We interpret this property in terms of a neural edge integration model incorporating data from macaque monkey physiology that indicates different physiological gain factors for incremental and decremental stimuli.

Using equiluminance settings to estimate the cardinal chromatic directions for individuals.

Color information is processed by the retina and lateral geniculate along principal dimensions known as the cardinal directions of color space. Normal differences in spectral sensitivity can impact the stimulus directions that isolate these axes for individual observers and can arise from variation in lens and macular pigment density, photopigment opsins, photoreceptor optical density, and relative cone numbers. Some of these factors that influence the chromatic cardinal axes also impact luminance sensitivity. We modeled and empirically tested how well tilts on the individual's equiluminant plane are correlated with rotations in the directions of their cardinal chromatic axes. Our results show that, especially for the SvsLM axis, the chromatic axes can be partially predicted by luminance settings, providing a potential procedure for efficiently characterizing the cardinal chromatic axes for observers.

Predicting color matches from luminance matches.

Color vision and spectral sensitivity vary among individuals with normal color vision; thus, for many applications, it is important to measure and correct for an observer's sensitivity. Full correction would require measuring color and luminance matches and is rarely implemented. However, luminance matches (equiluminance settings) are routinely measured and simple to conduct. We modeled how well an observer's color matches could be approximated by measuring only luminance sensitivity, since both depend on a common set of factors. We show that lens and macular pigment density and $L/M$L/M cone ratios alter equiluminance settings in different ways and can therefore be estimated from the settings. In turn, the density variations can account for a large proportion of the normal variation in color matching. Thus, luminance matches may provide a simple method to at least partially predict an observer's color matches without requiring more complex tasks or equipment.

Effects of eccentricity on color contrast.

Using near-threshold stimuli, human color sensitivity has been shown to decrease across the visual field, likely due in part to physiological differences between the fovea and periphery. It remains unclear to what extent this holds true for suprathreshold stimuli. The current study used suprathreshold contrast matching to examine how perceived contrast varies with eccentricity along the cardinal axes in a cone-opponent space. Our data show that, despite increasing stimulus size in the periphery, the LM axis stimuli were still perceived as reduced in contrast, whereas the S axis perceived contrast was observed to increase with eccentricity.

Evaluation of the Waggoner Computerized Color Vision Test.

PURPOSE: Clinical color vision evaluation has been based primarily on the same set of tests for the past several decades. Recently, computer-based color vision tests have been devised, and these have several advantages but are still not widely used. In this study, we evaluated the Waggoner Computerized Color Vision Test (CCVT), which was developed for widespread use with common computer systems. METHODS: A sample of subjects with (n = 59) and without (n = 361) color vision deficiency (CVD) were tested on the CCVT, the anomaloscope, the Richmond HRR (Hardy-Rand-Rittler) (4th edition), and the Ishihara test. The CCVT was administered in two ways: (1) on a computer monitor using its default settings and (2) on one standardized to a correlated color temperature (CCT) of 6500 K. Twenty-four subjects with CVD performed the CCVT both ways. Sensitivity, specificity, and correct classification rates were determined. RESULTS: The screening performance of the CCVT was good (95% sensitivity, 100% specificity). The CCVT classified subjects as deutan or protan in agreement with anomaloscopy 89% of the time. It generally classified subjects as having a more severe defect compared with other tests. Results from 18 of the 24 subjects with CVD tested under both default and calibrated CCT conditions were the same, whereas the results from 6 subjects had better agreement with other test results when the CCT was set. CONCLUSIONS: The Waggoner CCVT is an adequate color vision screening test with several advantages and appears to provide a fairly accurate diagnosis of deficiency type. Used in conjunction with other color vision tests, it may be a useful addition to a color vision test battery.

Precise toxigenic ablation of intermediate cells abolishes the "battery" of the cochlear duct.

The extracellular potential of excitable and nonexcitable cells with respect to ground is ∼0 mV. One of the known exceptions in mammals is the cochlear duct, where the potential is ∼80-100 mV, called the endocochlear potential (EP). The EP serves as the "battery" for transduction of sound, contributing toward the sensitivity of the auditory system. The stria vascularis (StV) of the cochlear duct is the station where the EP is generated, but the cell-specific roles in the StV are ill defined. Using the intermediate cell (IC)-specific tyrosinase promoter, under the control of diphtheria toxin (DT), we eliminated and/or halted differentiation of neural crest melanocytes after migration to the StV. The ensuing adult transgenic mice are profoundly deaf. Additionally, the EP was abolished. Expression of melanocyte early marker and Kir4.1 in ICs precedes the onset of pigment synthesis. Activation of DT leads to loss of ICs. Finally, in accord with the distinct embryology of retinal pigmented cells, transgenic mice with toxigenic ablation of neural crest-derived melanocytes have intact visual responses. We assert that the tyrosinase promoter is the distinct target for genetic manipulation of IC-specific genes.

Physiological correlates of watercolor effect.

The watercolor effect is a visual illusion that manifests itself as a combination of long-range color spreading and figure-ground organization. The current study uses behavioral and physiological measures to study the watercolor effect. We utilize a novel technique of measuring the cortical response of the illusion using the visual evoked potential (VEP). To this end, three experiments were done to investigate the contributions of luminance and hue to the magnitude of the illusion. Results of both VEP and behavior indicate a marked decrease in the -S (yellow) direction in illusion magnitude compared to the +S (blue) illusion, even though these colors were previously matched for perceptual salience.

The locus of color sensation: cortical color loss and the chromatic visual evoked potential.

Color losses of central origin (cerebral achromatopsia and dyschromatopsia) can result from cortical damage and are most commonly associated with stroke. Such cases have the potential to provide useful information regarding the loci of the generation of the percept of color. One available tool to examine this issue is the chromatic visual evoked potential (cVEP). The cVEP has been used successfully to objectively quantify losses in color vision capacity in both congenital and acquired deficiencies of retinal origin but has not yet been applied to cases of color losses of cortical origin. In addition, it is not known with certainty which cortical sites are responsible for the generation of the cVEP waveform components. Here we report psychophysical and electrophysiological examination of a patient with color deficits resulting from a bilateral cerebral infarct in the ventral occipitotemporal region. Although this patient demonstrated pronounced color losses of a general nature, the waveform of the cVEP remains unaffected. Contrast response functions of the cVEP are also normal for this patient. The results suggest that the percept of color arises after the origin of the cVEP and that normal activity in those areas that give rise to the characteristic negative wave of the cVEP are not sufficient to provide for the normal sensation of color.

Contrast adaptation reveals increased organizational complexity of chromatic processing in the visual evoked potential.

Results from psychophysics and single-unit recordings suggest that color vision comprises multiple stages of processing. Postreceptoral channels appear to consist of both a stage of broadly tuned opponent channels that compare cone signals and a subsequent stage, which includes cells tuned to many different directions in color space. The chromatic visual evoked potential (crVEP) has demonstrated chromatic processing selective for cardinal axes of color space. However, crVEP evidence for higher-order color mechanisms is lacking. The present study aimed to assess the contribution of lower- and higher-order color mechanisms to the crVEP by using chromatic contrast adaptation. The results reveal the presence of mechanisms tuned to intermediate directions in color space in addition to those tuned to the fundamental cardinal axes.

Tests of a functional account of the Abney effect.

The Abney effect refers to changes in the hue of lights as they are desaturated. Normally the purity is varied by desaturating with a fixed spectrum. Mizokami et al. [J. Vis.6, 996 (2006)] instead varied purity by using Gaussian spectra and increasing their bandwidth. Under these conditions the hues of lights at short and medium wavelengths tended to remain constant and thus were tied to a fixed property of the stimulus such as the spectral peak, possibly reflecting a compensation for the spectral filtering effects of the eye. Here we test this account more completely by comparing constant hue loci across a wide range of wavelengths and between the fovea and periphery. Purity was varied by adding either a fixed spectrum or by varying the spectral bandwidth, using an Agile Light Source capable of generating arbitrary spectra. For both types of spectra, hue loci were approximated by the Gaussian model at short and medium wavelengths, though the model failed to predict the precise form of the hue changes or the differences between the fovea and periphery. Our results suggest that a Gaussian model provides a useful heuristic for predicting constant hue loci and the form of the Abney effect at short and medium wavelengths and may approximate the inferences underlying the representation of hue in the visual system.

ERP evidence of visualization at early stages of visual processing.

Recent neuroimaging research suggests that early visual processing circuits are activated similarly during visualization and perception but have not demonstrated that the cortical activity is similar in character. We found functional equivalency in cortical activity by recording evoked potentials while color and luminance patterns were viewed and while they were visualized with the eyes closed. Cortical responses were found to be different when imagining a color pattern vs. imagining a checkerboard luminance pattern, but the same when imagining a color pattern (or checkerboard pattern) vs. seeing the same pattern. This suggests that early visual processing stages may play a dynamic role in internal image generation, and further implies that visual imagery may modulate perception.

Task-dependent contrast gain in anomalous trichromats.

Anomalous trichromacy is a form of color vision deficiency characterized by the presence of three cone types, but with shifted spectral sensitivities for L or M cones, causing a red-green color deficiency. However, long-term adaptation to this impoverished opponent input may allow for a more normal color experience at the suprathreshold level ("compensation"). Recent experimental evidence points to the presence of compensation in some tasks. The current study used threshold detection, suprathreshold contrast matching, and a reaction-time task to compare contrast coding in normal and anomalous observers along the cardinal cone-opponent axes. Compared to color normals, anomals required more L-M contrast, but not S contrast, to detect stimuli and to match an achromatic reference stimulus. Reaction times were measured for several contrast levels along the two cone-opponent axes. Anomals had higher overall reaction times, but their reaction-time versus contrast functions could be matched to those of controls simply by scaling contrast by the detection thresholds. Anomalous participants were impaired relative to controls for L-M stimuli in all three tasks. However, the contrast losses were three times greater for thresholds and reaction times than for suprathreshold matches. These data provide evidence for compensation in anomalous trichromats, but highlight the role that the experimental task plays in revealing it.

Attentional shifts have little effect on the waveform of the chromatic onset VEP.

Attention is important for sufficient performance on many visual tasks. This has been shown using achromatic steady-state and pattern-reversal VEPs. Waveform characteristics typically attenuate when attending to distractor stimuli and ignoring VEP stimuli. Chromatic pattern-onset responses have not been tested under conditions of selective attention: as they can be used in clinical settings to test color vision, it is important to know what effects attentional shifts would have on this response. In the present study chromatic pattern-onset VEPs were recorded using spatially divided and spatially contiguous VEP and distractor stimuli. VEP stimuli were 1 cycle.deg(-1) horizontal sine wave patterns (onset mode 100 ms on/400 ms off) used to selectively modulate the L-M and S-(L+M) visual pathways. Distracter stimuli were letters. Subjects attended to either the letters or the gratings and pressed a button when a predetermined stimulus appeared. In Experiment one, VEP and distractor stimuli were superimposed and spatially contiguous. In Experiment two, stimuli were presented to different hemifields. No significant changes in waveform amplitude and latency were found between VEP and distractor attention conditions for either visual pathway. For the chromatic pattern-onset response, modulation of attention does not change responses either with spatially contiguous or spatially separate selective attention manipulations. Consequently, it may not be necessary to monitor attention during recording of this response.

Changes in chromatic pattern-onset VEP with full-body inversion.

PURPOSE: Intra-ocular pressure (IOP) increases to double that of its normal level under full-body inversion, in part simulating high IOPs found in glaucoma and ocular hypertension. Inversion also simulates negative g-forces experienced in aerobatic maneuvers such as those produced during aerial combat. Studies using achromatic pattern-reversal visual evoked potentials (VEPs) have shown losses in response amplitude when subjects are inverted and IOP is increased. In other studies, chromatic, pattern-onset VEPs have been shown to be a sensitive and objective indicator of ocular and systemic pathology. Thus, chromatic pattern-onset VEPs may also show changes in amplitude and/or latency when subjects are subjected to full-body inversion. METHODS: In this study we employed chromatic, pattern-onset VEPs to determine if there were changes in response latency for the different visual pathways with healthy subjects during full-body inversion. Stimuli were 1 cpd horizontal sine-wave patterns presented in an onset mode (100 ms on/400 ms off). Subjects (n = 7) were tested at low to medium contrast levels, which were subjectively equated across chromatic pathways (S, LM, and achromatic). Patterns were presented using LCD goggles. RESULTS: All subjects showed a small but statistically significant increase in latency in the large negative component (CII) for both L - M and S - (L + M) pathways during inversion. The achromatic pathway also showed a statistically significant increase in latency to the positive component (CI) during inversion. CONCLUSIONS: These results demonstrate that changes in ocular and/or systemic physiology during full-body inversion can result in increased latencies of chromatic and achromatic pattern-onset VEPs.

Mechanisms contributing to increment threshold and decrement threshold spectral sensitivities.

The shape of the human spectral sensitivity function depends on how it is measured. In the increment threshold (IT) technique, sensitivity is typically measured as the inverse of threshold for detection of increments of monochromatic light presented for relatively long durations on achromatic pedestals. Spectral sensitivity functions derived from IT techniques have long been used to reveal contribution from opponent color channels. Although IT functions have been studied extensively, little attention has been given to functions derived from decrement thresholds (DT), partly due to technical challenges of producing appropriate stimuli. Comparison of IT and DT spectral sensitivities may be of interest because there are known asymmetries in the visual system between on- and off-pathways and between increment and decrement responses within these pathways. Consequently, spectral sensitivity functions obtained using DT measures may reveal a different complement of contributing mechanisms than those that produce IT functions. We report here that IT and DT derived spectral sensitivities were essentially identical over much of the visible spectrum. However, decrement sensitivity was slightly greater than increment sensitivity in the shorter wavelengths at modest light levels. This difference was not present at higher light levels, implicating rod pathways as a possible source of the difference. In sum, it appears that under conditions shown to reveal strong contribution from opponent mechanisms, decrement functions are either 1) determined by a similar complement of spectrally opponent mechanisms as those that define increment spectral sensitivities or 2) that the present conditions are insensitive to underlying asymmetries.

Contour adaptation reduces the spreading of edge induced colors.

Brief exposure to flickering achromatic outlines of an area causes a reduction in the brightness contrast of the surface inside the area. This contour adaptation to achromatic contours does not reduce surface contrast when the surface is chromatic (the saturation or colorimetric purity of the surface is maintained). In addition to reducing the brightness of physical luminance contrast, contour adaptation also reduces (or even reverses) the illusory brightness contrast seen in the Craik-O'Brien-Cornsweet illusion, in which two physically identical grey areas appear different brightness because of a sharp luminance edge separating them. Chromatic color spreading illusions also occur with chromatic inducing edges, and an unanswered question is whether contour adaptation can reduce the perceived contrast of illusory color spreading from edges, even though it cannot reduce the perceived contrast of physical surface color. The current studies use a color spreading illusion known as the watercolor effect in order to test whether illusory color spreading is affected by contour adaptation. The general findings of physical achromatic contrast being reduced and chromatic contrast being robust to contour adaptation were replicated. However, both illusory brightness and color were reduced by contour adaptation, even when the illusion edges only differed in chromatic contrast with each other and the background. Additional studies adapting to chromatic contours showed opposite effects on illusory color contrast than achromatic adaptation.

Depth Without a Surface: Observations From a "Finger Spinner" Depth Illusion.

A trending novelty toy that is spun between the fingers induces a striking depth illusion from specular reflections. Further examination of the phenomenon suggests that when surface features are obscured by spinning, depth from disparity of reflections is enhanced.

Nonlinearities in color coding: compensating color appearance for the eye's spectral sensitivity.

Most wavelengths change hue when mixed with white light. These changes, known as the Abney effect, have been extensively studied to characterize nonlinearities in the neural coding of color, but their potential function remains obscure. We measured the Abney effect in a new way--by varying the bandwidth of the spectrum rather than mixing with white--and this leads to a new interpretation of the role of nonlinear responses in color appearance. Because of the eye's limited spectral sensitivity, increasing the bandwidth of a spectrum changes the relative responses in the three classes of cone receptor and thus would change hue if the percept were tied to a fixed cone ratio. However, we found that hue is largely independent of bandwidth and thus constant for a constant peak wavelength for stimuli with Gaussian spectra. This suggests that color appearance is compensated for the eye's spectral filtering, and that this compensation embodies specific perceptual inferences about how natural spectra vary. When a wavelength is instead diluted with white light--which does not bias the cone ratios--then the same compensation predicts changes in hue because the "right" response is made to the "wrong" stimulus. This model generates constant hue loci that are qualitatively consistent with measures of the Abney effect and provides a novel functional account of such effects in color appearance, in which postreceptoral responses are adjusted so that constant hue percepts are tied to consistent physical properties of the environment rather than consistent physiological properties such as the cone ratios.

Electrophysiological analysis of the effects of ginkgo biloba on visual processing in older healthy adults.

Several studies have tested the efficacy of ginkgo biloba using compromised visual systems and have found improvement in vision. We measured functional changes in the visual system of older, healthy adults to see if ginkgo extract EGb 761 would increase performance in the normal visual system. Two electrophysiological measures were taken during baseline, placebo, and treatment conditions: visual evoked potentials were used to assess changes in low-level functioning of the visual pathways, and P300 recognition responses were measured to assess higher order processing. No significant effect was found in the lower level visual pathways. However, when using regression analysis across age to assess higher order functioning, an improvement was found. The results suggest that the higher order processing stages, which may be influenced by cognition, decline more rapidly than do lower level processing stages in healthy adults as a function of age, and that the use of ginkgo biloba extract may improve the functioning of this system.

Differential aging of chromatic and achromatic visual pathways: behavior and electrophysiology.

Aging of visual pathways was measured psychophysically and physiologically in subjects aged 20-89 years. Contrast thresholds for the chromatic pathways increased with age, but there were no significant changes for thresholds of the achromatic pathway at low spatial frequencies. For visual evoked potential (VEP) responses, again only the chromatic pathways were significantly affected by age. Age-related changes in chromatic responses were not apparent when stimuli were adjusted by age-related contrast thresholds. This suggests that the chromatic and achromatic visual pathways age differently and that the VEP accurately and objectively reflects behavioral changes with age.