Effect of reduced accommodative response on isoluminance stereothreshold

A previous study has shown that stereothreshold varies as a function of the luminance difference between a target and its background. On the average, the stereothreshold at the isoluminance point is three times higher than when there is maximum luminance difference between the target and the background. One of the possible reasons for this finding is the effect of less than optimum accommodative response for isoluminant targets. This possibility has been investigated in this study. The results show that the suboptimal accommodative response cannot explain the high stereothreshold at isoluminance.     

Chromatic and luminance difference contribution to stereopsis

Stereoacuity has been measured at various luminance difference levels and also at the isoluminance point, where the target and the background have equal luminances. The results of the experiment show that acuity at isoluminance is three times worse than the value obtained when there is maximum contrast between target and background. possible reasons for these findings were investigated. Results of control experiments show that neither chromatic aberration nor defective accomodative response could be responsible for this finding.     

Luminance and chromatic contributions to a hyperacuity task: Isolation by contrast polarity and target separation

Vernier thresholds are known to be elevated when a target pair has opposite contrast polarity. Polarity reversal is used to assess the role of luminance and chromatic pathways in hyperacuity performance. Psychophysical hyperacuity thresholds were measured for pairs of gratings of various combinations of luminance (Lum) and chromatic (Chr) contrast polarities, at different ratios of luminance to chromatic contrast. With two red–green gratings of matched luminance and chromatic polarity (+Lum+Chr), there was an elevation of threshold at isoluminance. When both luminance and chromatic polarity were mismatched (−Lum−Chr), thresholds were substantially elevated under all conditions. With the same luminance contrast polarity and opposite chromatic polarity (+Lum−Chr) thresholds were only elevated close to isoluminance; in the reverse condition (−Lum+Chr), thresholds were elevated as in the −Lum−Chr condition except close to equiluminance. Similar data were obtained for gratings isolating the short-wavelength cone mechanism. Further psychophysical measurements assessed the role of target separation with matched or mismatched contrast polarity; similar results were found for luminance and chromatic gratings. Comparison physiological data were collected from parafoveal ganglion cells of the macaque retina. Positional precision of ganglion cell signals was assessed under conditions related to the psychophysical measurements. On the basis of these combined observations, it is argued that both magnocellular, parvocellular, and koniocellular pathways have access to cortical positional mechanisms associated with vernier acuity.     

A new technique for estimating chromatic isoluminance in humans and monkeys

Current approaches to the problem of equating different colors for luminance (chromatic isoluminance) rely upon human reports of perceptual events that are reduced at some luminance ratio. In this report, a technique is described that evokes a vivid percept of motion of a textured pattern only at isoluminance. Furthermore, in both humans and monkeys, the moving stimulus produces a striking optokinetic response in the same direction as the perceived motion. If used in this manner, the technique can provide an estimate of chromatic isoluminance in a variety of species and be used to corroborate a human subject's perceptual judgement.     

Occurrence of express saccades under isoluminance and low contrast luminance conditions.

Saccadic reaction times (SRTs) of three human subjects were analyzed. The gap paradigm was used (i.e. fixation point offset precedes target onset) to obtain high proportions of express saccades (i.e. saccades of extremely short reaction times) in the SRT distributions. In one set of experiments, the luminance of the (red) saccade target was varied from brighter to darker than the (green) background including an isoluminance condition. Express saccades were obtained in response to pure color contrast stimuli with about the same frequency and reaction time as to stimuli with both color and luminance contrast. In a second experiment, the luminance contrast of a white target on a white background was lowered below 10%. Again the number of express saccades was not reduced. Thus, in contrast to other perceptual phenomena the visual neural mechanisms underlying the generation of express saccades are not affected by isoluminance nor low contrast luminance.     

Interactions between colour and luminance contrast in the perception of motion

It has been demonstrated widely that at isoluminance moving chromatic stimuli are seen to be stationary or moving more slowly than their luminance counterparts. We have examined the effect on perceived velocity of adding luminance contrast to an isoluminant chromatic stimulus. We show that moving luminance contrast 'captures' colour so that a combined colour and luminance stimulus is seen moving as a unified percept. However, in the presence of colour contrast, significantly higher levels of luminance contrast are required to achieve a veridical velocity than for monochromatic stimuli with only luminance contrast. We show that this interactive effect between colour and luminance contrast cannot be fully explained by a threshold masking of luminance by colour contrast. The effect suggests that a breakdown in the veridical perception of velocity should be expected for colours with a wide range of associated luminance contrasts and not just for those at the point of isoluminance.     

Perception of random-dot symmetry and apparent movement at and near isoluminance

There have been conflicting reports on whether apparent movement in random-dot kinematograms is abolished at isoluminance. The present results suggest that it is, provided that dynamic (uncorrelated) surrounds are used, and the subject has to report the shape of the target rather than the presence of movement in an isolated portion of the target. On the other hand, perception of random-dot symmetry is still possible at isoluminance. The reason for this difference appears to be the need for exact-position information in movement but not symmetry perception. Control experiments suggest that the effects are not due to artefacts such as chromatic aberration in the eye.     

Visual evoked potentials elicited by chromatic motion onset.

Visually Evoked Potentials (VEPs) were recorded in response to the onset of chromatic and luminance motion gratings of 1 cpd and luminance 40 cd m(-2) subtending a 7 degrees field. At slow speeds (< or =2 cycles s(-1)) the motion onset response exhibits a clear amplitude minimum at isoluminance. Over the Michelson contrast range tested (0.05-0.75) the chromatic response at 2 cycles s(-1) possesses a linear response function compared to the saturating function of the luminance response and the contrast dependency of the former is a factor of 5-6 times greater than for the latter. These differences are suggestive of different neural substrates for the chromatic and luminance motion VEPs at slow speeds. At 10 cycles s(-1) the chromatic motion onset VEP exhibits no amplitude minimum at isoluminance and becomes more like its luminance counterpart in terms of its saturating contrast response function. Furthermore, the contrast dependency of the chromatic and luminance responses differs by only a factor of 1.6 at this faster rate. These findings are consistent with the idea of separate motion mechanisms that operate at fast and slow speeds, the latter having separate channels for colour and luminance motion.     

Heterochromatic Fusion Nystagmus: its use in estimating chromatic equiluminance in humans and monkeys.

The use of chromatic patterns that are equated for luminance has become increasingly popular in psychophysical and neurophysiological studies of visual processing. The currently available techniques for equating different colors for brightness rely upon human reports of perceptual events that are reduced at some luminance ratio. We report here the results of a study using a technique we have recently developed that produces a vivid and compelling motion percept only at isoluminance. That is, unlike previous methods, this technique relies upon a perceptual event (motion) that actually becomes more salient at isoluminance. We also observed that the optokinesis generated by the moving pattern mirrors the perceptual reports at all luminance ratios. If used in this manner, the technique can provide an estimate of chromatic isoluminance in a variety of species and can be used to corroborate a human subject's perceptual experience.     

Motion at isoluminance: discrimination/detection ratios for moving isoluminant gratings

Subjects viewed a 2.3 x 2.3 deg patch of a moving 1.3 c/deg, 3.75 Hz sinusoidal grating, centered 1.8 deg from fixation. Two-alternative forced-choice contrast thresholds were measured along the luminance axis and 10 chromatic axes at isoluminance for three tasks: detection (D), form discrimination (F), and discrimination of upward from downward motion (M). F/D threshold ratios averaged approx. 1:1 on all axes. M/D ratios were approx. 1:1 on the luminance axis, but varied from 3:1 to indeterminately large with chromatic axis at isoluminance. We conclude that under the present conditions there are large, highly specific losses of direction-of-motion information at isoluminance. The results imply the existence of chromatic channels that are labeled for form but not for direction of motion at threshold. The pattern and significance of variations in M/D ratios within the isoluminant plane is also discussed.     

Comparative monocular contributions to brightness in relation to abnormal stereothreshold

This study was designed to compare brightness contributions of the two monocular inputs to the binocular visual system at three different levels of luminance adaptation in two groups of 15 subjects each. The only known difference between these groups was stereothreshold, 50 to 70 sec arc in one group vs. 40 sec arc or less in the other group. The group with elevated threshold was found to have a significantly larger average difference (13.5%, p less than 0.002) in monocular brightness contributions, between the right and left eyes, when compared to the group with lower stereothresholds (4% mismatch). The results also indicate that neither group showed a significant variation in mismatch as adapting luminance was altered by a factor of four times (0.6 log units). Finally, no clear relation was found between eye dominance and the eye requiring higher illuminance to attain the dichoptic brightness match.     

Geometric-optical illusions at isoluminance

The idea of a largely segregated processing of color and form was initially supported by observations that geometric-optical illusions vanish under isoluminance. However, this finding is inconsistent with some psychophysical studies and also with physiological evidence showing that color and luminance are processed together by largely overlapping sets of neurons in the LGN, in V1, and in extrastriate areas. Here we examined the strength of nine geometric-optical illusions under isoluminance (Delboeuf, Ebbinghaus, Hering, Judd, Müller-Lyer, Poggendorff, Ponzo, Vertical, Z?llner). Subjects interactively manipulated computer-generated line drawings to counteract the illusory effect. In all cases, illusions presented under isoluminance (both for colors drawn from the cardinal L-M or S-(L+M) directions of DKL color space) were as effective as the luminance versions (both for high and low contrast). The magnitudes of the illusion effects were highly correlated across subjects for the different conditions. In two additional experiments we determined that the strong illusions observed under isoluminance were not due to individual deviations from the photometric point of isoluminance or due to chromatic aberrations. Our findings show that our conscious percept is affected similarly for both isoluminance and luminance conditions, suggesting that the joint processing for chromatic and luminance defined contours may extend well beyond early visual areas.     

Infants' spontaneous color preferences are not due to adult-like brightness variations

In the present work, we explore the perceptual bases of infants' spontaneous looking preferences among isoluminant chromatic stimuli (Bornstein, 1975). Three experiments were conducted. In Experiment 1, adult subjects made brightness matches between a white standard and each of six isoluminant chromatic stimuli. The classic variations of brightness with chromaticity were found. In Experiment 2, 12-week-old infants' spontaneous looking preferences were measured for white lights of different luminances. Preference increased with increasing luminance, suggesting that brightness differences are sufficient to create looking preferences among isochromatic stimuli. In Experiment 3, infants' preferences were tested for each of the six chromatic stimuli paired against white, at both isoluminance and (adult) isobrightness. All chromatic stimuli were preferred to white, and the pattern of preferences was similar for both isoluminance and isobrightness conditions. It is concluded that hue and/or saturation, rather than brightness, control infants' spontaneous looking preferences among chromatic stimuli.     

The contribution of color to global motion processing

This study investigates the contribution of color vision to global motion. We present evidence demonstrating that performance on a global motion task attains similar levels for both types of chromatic (L/M-cone opponent and S-cone opponent) and luminance stimuli at suprathreshold contrasts. We show, however, that the motion thresholds for isoluminant chromatic stimuli are luminance based, on the grounds that they are masked by luminance noise but robust to chromatic noise. Detection thresholds, on the other hand, are chromatic in origin (masked by chromatic but not luminance noise), indicating that there is no luminance artifact in the stimulus. We suggest that for color vision at isoluminance the global motion task is based on the integration of many local, luminance-based signals.     

Chromatic assimilation: spread light or neural mechanism?

Chromatic assimilation is the shift in color appearance of a test field toward the appearance of nearby light. Possible explanations of chromatic assimilation include wavelength independent spread light, wavelength-dependent chromatic aberration and neural summation. This study evaluated these explanations by measuring chromatic assimilation from a concentric-ring pattern into an equal-energy-white background, as a function of the inducing rings' width, separation, chromaticity and luminance. The measurements showed, in the s direction, that assimilation was observed with different inducing-ring widths and separations when the inducing luminance was lower or higher than the test luminance. In general, the thinner the inducing rings and the smaller their separation, the stronger the assimilation in s. In the l direction, either assimilation or contrast was observed, depending on the ring width, separation and luminance. Overall, the measured assimilation could not be accounted for by the joint contributions from wavelength-independent spread light and wavelength-dependent chromatic aberration. Spatial averaging of neural signals explained the assimilation in s reasonably well, but there were clear deviations from neural spatial averaging for the l direction.     

Development of chromatic and luminance detection contours using the sweep VEP

This study measured the development of contrast-sweep VEP thresholds to a range of chromatic and luminance stimuli. Subjects were 14-32 week-old infants (n=21) and three adults. Stimuli were 1 c/d sine gratings reversed at 5.6 Hz. Chromaticity was varied from the L-M axis to an achromatic axis. VEP thresholds when plotted in L- and M-cone contrasts showed that: (1) VEP thresholds did not consistently locate the psychophysical isoluminance match under the same stimulus conditions; (2) About 50% of the data were described by independent chromatic and luminance mechanism, however, thresholds were limited by the cone contrast of the stimulus, phase cancellation between visual mechanisms, and the proper sampling of thresholds in L- and M-cone contrast space and; (3) No significant changes occurred in VEP detection contours across ages but suprathreshold VEP amplitudes suggested complex developmental changes. Data from previous studies showed similar results.     

Chromatic input to cells of the magnocellular pathway: mean chromaticity and the relative phase of modulated lights

If the relative phase of red and green modulated lights is changed, at low temporal frequencies the response of cells of the magnocellular (MC) pathway has been found to be minimal not to counterphase, chromatic modulation (as expected of a luminance mechanism) but shifted to some phase intermediate between luminance and chromatic modulation. The results could only be modeled by assuming interaction between achromatic and chromatic inputs to MC cells. The 'phase shift' resembled that seen with psychophysical threshold measurements using the same stimuli. Psychophysical results also showed that the phase shift is dependent on the chromaticity of a background. The results reported here show that the direction of the phase shift in MC cells is reversed by changing the background from long to short wavelengths and is consistent with psychophysical observations. Cell behavior was again modeled by assuming vector summation of achromatic and chromatic inputs. The reversal of phase-shift direction requires a reversal in polarity of the chromatic input. The underlying physiological mechanism may involve summation of chromatic signals of opposite polarity; if the relative size of these signals depends on the background, this may determine the direction of phase shift.     

The effect of the density of background elements on the conspicuity of objects

Engel's concept of conspicuity is formalised to define a conspicuous object as one that will, for a given background, be seen with certainty within a short observation time regardless of the location of the object in relation to the line of sight. Thus for a defined level of probability of seeing and given observation time, the angle of eccentricity at which the defined level of probability of seeing occurs becomes the measure of the conspicuity of the target. Using this measure the conspicuity of disc targets has been determined when the targets are presented in complex backgrounds made up of random arrays of discs. A first experiment determined thresholds for discrimination of size differences and luminance differences for two discs since this was regarded as the elementary task in the detection of the target within a complex background made up of elements of the same shape. In the main experiment the task was the detection of a target disc located in a background consisting of randomly arranged discs. The target disc differed from the background discs in either size or luminance or both. The background array had three densities of background element. When the target discs differed in luminance from the background discs an increase in background density made the target disc less conspicuous but changing background density had no effect on the conspicuity of target discs that differed in size from the background discs. It was found that the task of detecting a luminance difference when there were only two discs (first experiment) was easier than detecting a target disc in the random arrays. However, the detection of a disc of different size in the random arrays appears to be no more difficult than the elementary task of comparing size differences between two discs. This suggests that the visual processes for detecting a target in a complex background by size difference are different from those for detecting the target by luminance difference.     

Isoluminant stimuli may not expose the full contribution of color to visual functioning: spatial contrast sensitivity measurements indicate interaction between color and luminance processing.

Visual performance is greatly impaired when tested with heterochromatic isoluminant stimuli. It is thus concluded that the chromatic system contribution to many visual tasks is limited. We suggest that unless color and luminance are shown to be processed independently, such experiments do not demonstrate shortcomings of the chromatic system but rather the inadequacy of using isoluminant stimuli for isolating that system. We hypothesize that color vision has evolved not only to encode color per se but also to enhance luminance-based visual processing, so that for color information to be fully effective, luminance as well as chromatic variations should be present in the stimulus. The hypothesis was tested by studying the contribution of color to spatial vision. The human contrast sensitivity function (CSF) was studied using luminance, isoluminance (color) and combined luminance/color sinusoidal gratings. It is found that luminance contrast sensitivity is enhanced when luminance contrast is accompanied by color contrast and vice versa. The nature of the interaction is best described by an additive single analyzer model. Color opponent cells which respond to both chromatic and achromatic stimuli may be identified as the analyzer.     

Luminance mechanisms mediate the motion of red-green isoluminant gratings: the role of "temporal chromatic aberration"

In this paper we use a dynamic noise-masking paradigm to explore the nature of the mechanisms mediating the motion perception of drifting isoluminant red-green gratings. We compare contrast thresholds for the detection and direction discrimination of drifting gratings (1.5 cpd), over a range of temporal frequencies (0.5-9 Hz) in the presence of variable luminance or chromatic noise. In the first experiment, we used dynamic luminance noise to show that direction thresholds for red-green grating motion are masked by luminance noise over the entire temporal range tested, whereas detection thresholds are unaffected. This result indicates that the motion of nominally isoluminant red-green gratings is mediated by luminance signals. We suggest that stimulus-based luminance artifacts are not responsible for this effect because there is no masking of the detection thresholds. Instead we propose that chromatic motion thresholds for red-green isoluminant gratings are mediated by dynamic luminance artifacts that have an internal, physiological origin. We have termed these "temporal chromatic aberration". In the second experiment, we used dynamic chromatic noise masking to test for a chromatic contribution to red-green grating motion. We were unable to find conclusive evidence for a contribution of chromatic mechanisms to the chromatic grating motion, although a contribution at very high chromatic contrasts cannot be ruled out. Our results add to a growing body of evidence indicating the presence of dynamic, internal luminance artifacts in the motion of chromatic stimuli and we show that these occur even at very low temporal rates. Our results are compatible with our previous work indicating the absence of a chromatic mechanism for first order (quasi-linear) apparent motion [Vision Res. 40 (2000) 1993]. We conclude that previous conclusions based on the motion of chromatic red-green gratings should be reassessed to determine the contribution of dynamic luminance artifacts.