Isoluminant Stimuli Research Articles

Temporal response characteristics of the spatiochromatic visual evoked potential: nonlinearities and departures from psychophysics.

Although the visual evoked potential (VEP) for isoluminant stimuli has been characterized in terms of spatiochromatic parameters, temporal tuning along various chromatic directions has received less systematic attention. Additionally, there has been little categorical comparison of psychophysical appearance with VEP responses obtained for temporal variation of these patterns. At appropriate contrasts the VEP's for color axes (LM, S) show a robust and contrast-sensitive temporal tuning peak at 4 Hz. Contrast response functions at 4 Hz for the LM color axis are markedly nonmonotonic. However, there is a clear monotonicity with contrast for VEP latencies along these color axes. The anomalous behavior does not appear to be due to interactions between chromatic signals, to luminance artifact, or to rod intrusion. These anomalies in the temporal characteristics of the chromatic VEP may reflect interactions between chromatic responses and inherent cortical responsivity not linked to psychophysical behavior.

A longitudinal study of visual evoked responses to tritan stimuli in human infants.

The principal goal of this study was to assess the development of the S-cone pathway in human infants. For comparison, development of the L-M opponent pathway was also assessed. Visual evoked responses were measured to chromatic (tritan and red-green) and luminance-modulated (yellow-black) sinusoidal gratings of 0.2 cpd, viewed binocularly at a distance of 40 cm. For each type of chromatic stimulus, responses were recorded at a range of color ratios around the adult isoluminant point, as determined using standard flicker photometry criteria. Presentation was by abrupt pattern-onset with a stimulus duration of 100 ms and an interstimulus interval of 400 ms. Responses were recorded from Oz, referred to Cz, by averaging 30 sweeps. Infants were examined between the ages of 4 weeks and 3 months. Reliable visual evoked responses were recorded to isoluminant tritan stimuli from as early as 4 weeks of age, and to red-green stimuli from 6 weeks. Responses to the luminance-modulated stimuli were evident in all infants at 4 weeks of age (the earliest age examined). Our results provide evidence that the rate of development for pathways mediating information about tritan stimuli, red-green stimuli, and luminance-modulated stimuli is similar.

Hue Scaling of Isoluminant and Cone-specific Lights

Using a hue scaling technique, we have examined the appearance of colored spots produced by shifts from white to isoluminant stimuli along various color vectors in order to examine color appearance without the complications of the combined luminance and chromatic stimulation involved in most previous hue scaling studies, which have used flashes of monochromatic light. We also used spots lying along cone-isolating vectors in order to determine what hues would be reported with a change in activation of only single cone types or of only single geniculate opponent-cell types, an issue of direct relevance to any model of color vision. We find that:1. Unique hues do not correspond either to the change in activation of single cone types or of single geniculate opponent-cell types. This is well known to be the case for yellow and blue, but we find it to be true for red and green as well.2. These conclusions are not limited to the particular white (Illuminant C) used as an adapting background in most of the experiments. Shifts along the same cone-contrast vectors relative to different backgrounds lead to much the same hue percepts, independent of the starting white used.3. The shifts of the perceptual colors from the geniculate axes are in the directions, and close to the absolute amounts, predicted by our [De Valois and De Valois (1993). Vision Research, 33, 1053–1065] multi-stage color model in which we postulate that the S-opponent cells are added to or subtracted from the M- and L-opponent cells to form the four perceptual color systems.4. There are distinct asymmetries with respect to the extent to which various hues within each perceptual opponent system deviate from the geniculate opponent-cell axes. Blue is shifted more from the S−LM axis than is yellow; green is shifted more from the L−M axis than is red. There are also asymmetries in the angular extent of opponent color regions. Blue is seen over a larger range of color vectors than is yellow, and red over a slightly larger range than green.5. Such asymmetries are not accounted for by any model that treats red-green and yellow-blue each as unitary, mirror-image opponent-color systems. Although red and green are perceptually opponent, the red and green perceptual systems do not appear to be constructed in a mirror-image fashion with respect to input from different cone types or from different geniculate opponent-cell types. The same is true for yellow and blue. © 1997 Elsevier Science Ltd. All rights reserved.

Selective stimulation of colour mechanisms: an empirical perspective

Anatomically distinct parvo and magno visual pathways show considerable functional overlap. However, specific stimulation of the most sensitive colour-opponent parvo-neurones is still possible, provided that colour stimuli are verified for selectivity. The authors have shown that gratings of low contrast, low spatial frequency and of restricted spatial content (6 or less spatial cycles) are optimal stimuli for distinguishing between colour-related (tritan and red/green) from achromatic or partly chromatic responses. This is particularly important when recording global responses, such as visual evoked potentials, VEPs. The crucial point is that at low presentation rates (< 2 Hz), colour-related onset VEPs are maximally different from contrast reversal VEPs, thereby reflecting the activity of sustained-type parvo mechanisms. Achromatic onset, offset and reversal VEPs are similar, reflecting mediation by transient-type magno mechanisms. A stringent test of colour-response specificity is to check whether the chromatic reversal VEP has a low-pass temporal tuning curve, since it becomes band-pass when substantial achromatic intrusions are present. Specification of chromatic isoluminant stimuli, e.g. along cardinal axes, does not guarantee their colour-selectivity, if chromatic aberration and variable macular pigmentation changes the chromatic content of the retinal image. It is shown here how chromatic stimuli, namely (1) red/green and (2) purple/green (tritanopic) gratings, can be optimized for selective stimulation of the colour system.

Spatial pooling of contrast in contrast gain control.

We report psychophysical measurements of spatial pooling functions for contrast gain control. We use a nulling technique to measure the dependence of contrast induction on the spatial frequency of a sinusoidal contrast modulation. This dependence on spatial frequency, when transformed, provides the profile of a spatial pooling function. The measured profiles are fitted well by Gaussians. We confirm earlier results that the area over which spatial pooling takes place depends on the scale of the modulated pattern. We also find that pooling functions are similar for achromatic and isoluminant stimuli.

Temporal mechanisms underlying flicker detection and identification for red-green and achromatic stimuli.

We have simultaneously measured detection and temporal frequency identification for both red-green isoluminant and achromatic stimuli over a range of temporal frequencies for two observers. Results show that temporal frequency identification can be made along the temporal frequency dimension for both red-green and achromatic stimuli at contrasts close to detection threshold. In general, temporal frequency identification was better for the achromatic than for the red-green stimuli; however, the level of chromatic identification performance was still sufficient to permit us to reject the notion that the red-green mechanism embodies a single temporal filter. We have developed a model based on signal detection theory that assumes that detection and identification both depend on the properties of the temporal filters underlying each mechanism. From this we have derived putative underlying shapes and sensitivities for the temporal filters of the red-green and achromatic mechanisms that comprise a low-pass and a bandpass filter for red-green color vision and two bandpass filters for luminance vision. Finally, we suggest that the relative perceived slowing of isoluminant stimuli may be accounted for by a common motion analysis subserved by different front-end temporal filters for red-green and achromatic motion signals.

The Binocular Summation of Chromatic Contrast

The level of binocularity possessed by mechanisms sensitive to chromatic contrast is still unclear. Recent studies of stereopsis and chromatic contrast have suggested that stereopsis is maintained at isoluminance, although the contrast sensitivity and disparity ranges of chromatic stereopsis mechanisms are reduced compared to luminance stereopsis mechanisms. Rose, Blake, and Halpern (1988 Investigative Ophthalmology and Visual Science29 283 – 290) hypothesised a link between binocular summation (ie the superiority of binocular detection over monocular detection) and stereopsis. Is this link maintained with heterochromatic isoluminant stimuli? To address this question, the binocular and monocular contrast thresholds for the detection of 0.5 cycle deg−1 Gabor patches were measured. The stimuli possessed different relative amounts of colour and luminance contrast ranging from isoluminance (red/green) to isochrominance (yellow/black) through intermediate values. It was found that, with these stimuli, binocular detection was well modelled by assuming independent mechanisms sensitive to chromatic contrast and luminance contrast. Furthermore, with isoluminant stimuli, levels of binocular summation were above those expected from probability summation between the eyes, thus providing evidence for binocular neural summation within chromatic detection mechanisms. Given that stereoscopic depth identification is impossible at contrast detection threshold with isoluminant heterochromatic stimuli, these results suggest that the link between stereopsis and levels of binocular neural summation may not be a particularly strong one. These results also provide clear evidence for the binocularity of chromatic detection mechanisms.

Orientation Discrimination at Isoluminance

Colour and form are important attributes of the objects in our visual environment. We tested the hypothesis that colour and orientation are processed independently in the visual system. Orientation perception for stimuli defined by luminance is characterised by a decrease in thresholds with increasing stimulus contrast, and by a strong oblique effect: stimuli along the horizontal or vertical axes are discriminated more easily. We determined orientation discrimination thresholds for stationary, slow (1 Hz) or fast (8 Hz) moving sine-wave gratings (1 cycle deg−1) defined by luminance or isoluminant (red — green) contrast. In a 4AFC paradigm, three of four stimulus patches were identical and the fourth differed in orientation, contrast, or in both. When we measured orientation discrimination thresholds as a function of stimulus contrast, thresholds decreased for all stimuli with increasing contrast. At all temporal frequencies the functions relating orientation thresholds to stimulus contrast had similar shapes for luminance and isoluminant gratings. On a cone-contrast metric, thresholds for stationary and slowly moving stimuli were consistently lower for isoluminant compared to luminance stimuli. For fast-moving stimuli orientation thresholds were similar for both kinds of gratings. For both types of stimuli a marked oblique effect was observed. To characterise processing of contrast and orientation completely, we measured simultaneous thresholds for contrast and orientation. The shapes of the resulting two-dimensional threshold contours were similar for luminance and isoluminant gratings, indicating similar rules for combining differences in contrast and orientation. We conclude that processing of isoluminant and luminance stimuli undergoes the same neural processing at least for the low spatial frequencies used here.

Orientation Discrimination for Moving Isoluminant and Luminance Targets

Last year we presented orientation discrimination thresholds for stationary isoluminant red — green and luminance-defined stimuli as a function of spatial frequency and contrast (Wuerger and Morgan, 1995 Perception24 Supplement, 16). The horizontal profile of the stimuli was a Gabor function with a carrier frequency of either 0, 2, or 4 cycles deg−1. Observers made a binary decision (clockwise versus anticlockwise from vertical) and thresholds were defined as the standard deviation of their psychometric function. One main finding was that for carrier frequencies of 2 and 4 cycles deg−1, when stimuli of equal cone contrast are compared, orientation discrimination thresholds for red — green isoluminant stimuli do not differ significantly from the thresholds for luminance-defined stimuli. To further characterise the chromatic mechanisms involved in spatial vision, we assessed orientation discrimination thresholds for isoluminant red — green and luminance-defined Gabor stimuli as a function of the velocity of the moving target. When velocity is increased, orientation discrimination thresholds for isoluminant targets increase more rapidly than the thresholds for luminance targets of identical cone contrasts. We conclude that orientation discrimination is mediated by different chromatic mechanisms with different spatial and temporal sensitivities.

The development of the chromatic visual evoked potential

Evidence exists to suggest that visual function of very young infants is not cortically controlled and may in fact be mediated by a more primitive subcortical system. Visual evoked potentials (VEP) are used widely as a clinical and research tool and in this study the development of the chromatic VEP was investigated. Chromatic vision is a cortical function, hence the presence of a chromatic VEP may suggest the presence of a functioning cortical pathway.Using pattern reversal stimuli of red and green checks, the presence and morphology of the transient chromatic VEP were assessed. The checkerboards reversed in chromaticity and control of the relative brightness (luminance) of the checks allowed the production of a range of stimuli of different red/green luminance ratios. A black/white stimulus was included in all trails for comparison. VEPs to these stimuli were recorded from 36 infants aged between 1 and 14 weeks post‐term. Fourteen of these infants were then assessed on two further occasions to monitor the longitudinal development of the chromatic VEP. All infants tested demonstrated clear VEPs to achromatic stimuli and to those red/green stimuli that contained luminance information. As the luminance information within the stimuli decreased, the morphology of the wave changed with the major component (P1) becoming diminished and delayed. No VEP could be recorded from any infant less than 7 weeks post‐term at time of testing when using an isoluminant stimulus. In the longitudinal study, all of the infants, who initially demonstrated no response to isoluminant stimuli, did so on reaching their eighth post‐term week.These results would suggest that those channels responsible for the processing of chromatic information do not function until the eighth post‐term week. During the period prior to this, visual processing takes place via a colour‐insensitive pathway, probably subcortical.

Binocular rivalry with isoluminant stimuli visible only via short-wavelength-sensitive cones

To test whether the binocular contour rivalry mechanism is tritanopic, we presented isoluminant, rival stimuli visible only via the short-wavelength-sensitive (S) cones. We stimulated only the S cones with violet gratings superimposed on a bright yellow field that adapted the responses of the middle- and long-wavelength-sensitive (M and L) cones. We found that an S-cone grating presented to one eye rivalled with an orthogonal grating presented to the other. Rivalry persisted over a range of luminances and contrasts of the S-cone stimuli, and was greater than could be accounted for by nonrival fading. The spatial spread of rivalry from S-cone stimuli is similar to that for the same stimuli when visible also to the M and L cones (luminance stimuli). We found that an S-cone stimulus would rival with a luminance stimulus, and exploited this to determine the equivalent luminance contrast of S-cone stimuli by putting them in a rivalry competition with luminance stimuli. For rivalry, the equivalent luminance contrast of isoluminant, S-cone stimuli is much less than their S-cone contrast. The existence of rivalry with isoluminant stimuli, along with earlier evidence that such stimuli can support stereopsis, challenges the view that an achromatic channel alone drives certain higher level functions such as depth perception.

Contribution of chromatic mechanisms to the production of small-field optokinetic nystagmus (OKN) in normals and strabismics

The contribution of chromatic mechanisms to motion processing is currently debated. Although OKN as a measure of motion processing has been employed for chromatic stimuli, the contribution of chromatic mechanisms to the production of OKN has not been assessed directly. We measured voluntary-pursuit and involuntary-stare OKN responses to drifting patterns defined by colors modulated in a cone based color space which isolates luminance and color mechanisms. Results show that OKN responses are reduced for isoluminant stimuli, particularly for the stare conditions. The greatest reduction in responses occur for isoluminance stimuli that isolate the S cones. These results suggest that the chromatic mechanisms may have reduced input to the neural substrate that produces involuntary-stare OKN or that stare OKN has different temporal characteristics when stimulated by chromatic and luminance mechanisms. We also measured OKN asymmetries in several convergent strabismics in response to isoluminant and luminance patterns and compared their response to previous reports of horizontal OKN asymmetries in normal neonate infants. The results question the validity of either the modeling of OKN asymmetries in strabismus as incomplete development or the comparison of adult eye movement records evaluated with criteria for OKN with psychophysical forced choice evaluation of ocular drift in infants.

Luminance and color effects on localization of briefly flashed visual stimuli.

Visual localization was studied by flashing small stimuli on a green background and requiring observers to press keys to indicate whether the stimulus appeared to the left or right of fixation. The results suggest that, for small (0.25 deg) briefly flashed (17 ms) stimuli at an eccentric location (10 deg), color contrast is not useable and localization presumably must rely on the magnocellular pathway. When stimulus size and duration were increased at 10-deg eccentricity, isochromatic stimuli could be localized at less than 10% luminance contrast (again suggesting use of the magnocellular high sensitivity luminance-contrast system), but isoluminant color-contrast stimuli could also be localized (suggesting use of the color-contrast sensitive parvocellular system). Thus, the results indicate that, dependent on stimulus conditions, both magnocellular and parvocellular pathways were utilized by normal observers in this localization task.

Pupillary light reflex.

A wealth of new information has recently come to light concerning the pupillary response to various types of visual input. Much of this information is recent, and has either been published in the last year, is in press, or has just been reported at meetings. This new information is important because it is resulting in reexamination of the pupillary input pathways and the clinical role for using pupillary movements to understand the physiology and pathology of the visual system. This review will focus on this new information and its clinical implication. The review is organized in the following sections: 1) the pupillary response to full-field, global light stimuli; 2) "pupil perimetry" or the pupillary response to focal light stimuli; 3) the pupillary response to isoluminant stimuli, eg, color spatial frequency, motion; 4) receptive field organization in the pupillary light reflex pathway.

Infant eye movement asymmetries: Temporal-nasal asymmetry is reversed at isoluminance in 2-month-olds

Tested monocularly with luminance-modulated stimuli, young infants classically show a directional asymmetry of eye movements: optokinetic nystagmus (OKN) and OKN-like eye movements are more reliably elicited by temporo-nasally- (T-N) than by naso-temporally- (N-T) directed stimuli. In the present experiment, 2-month-old infants were tested with isoluminant red-green gratings. A reverse asymmetry was found: at isoluminance, directionally appropriate eye movements were more readily elicited by N-T- than by T-N-directed stimuli. The results suggest the presence of a reverse asymmetry in the ensemble of direction-selective neurons that controls young infants' immature eye movements in response to isoluminant stimuli.

Psychophysical evidence of differential latencies of colour inputs to motion perception

A novel psychophysical observation allows the determination of the relative latencies with which long, middle, and short cone signals provide input to motion perception. It is known that when two spatially displaced isoluminant stimuli in spectrally different colours are simultaneously presented, any temporal lag between the perception of the two will, due to the spatial displacement, cause the perception of apparent motion. The illusion reported here occurs through the inadvertent production of spatial displacement; peripheral observation of the boundary between two differently-coloured neighbouring areas which alternately interchange colours leads, due to transverse chromatic aberration caused by the eye's optics, to the formation of a double boundary on the retina, the serial perceptions of which create the sensation of motion. By offsetting the relative temporal phases of any two colours we have determined the relative magnitude of the latencies with which they provide input to motion perception. In all subjects motion of blue is perceived after that of red, and green is perceived after that of blue. The origins of these latencies are unclear.

Visual electrophysiology to achromatic and chromatic stimuli in premature and full term infants

We have previously reported on the development of black/white pattern reversal VEPs in premature babies of more than 30 weeks post-menstrual age (PMA). Unlike the flash VEP, the pattern reversal VEP shows a similar morphology to that of the full term infant and the major positive component (P1) decreased in latency with increasing PMA. The N1 and N2 components were more likely to be present with increasing maturity. In our present study we are examining the development of the transient chromatic pattern VEP. In order to produce a purely chromatic stimulus it is necessary to remove luminance cues. Based on forced choice preferential looking we developed a method of determining the isoluminant point for infants. Preference was tested for a flickering sinusoidal red and green grating over the uniform field. As sensitivity for chromatic flicker is much poorer than for luminance flicker, sensitivity is expected to be least when the residual luminance variation in the stimulus is at a minimum. The red/green luminance ratio at which this occurs represents the isoluminant point. From this method we found the subjective isoluminant point for infants of 2–3 months of age to be very close to the objective measure of isoluminance. Using this information, pattern reversal VEPs to 20 chromatic red/green and achromatic checks were studied and it would appear that pattern reversal VEPs cannot be obtained to isoluminant stimuli before 7 weeks chronological age.

Contrast dependence of colour and luminance motion mechanisms in human vision.

Conventional views of visual perception propose a colour-blind pathway conveying motion information and a motion-blind pathway carrying colour information. Recent studies show that motion perception is not always colour blind, is partially dependent on attention, can show considerable perceptual slowing around isoluminance and is contrast-dependent. If there is a single motion pathway, receiving luminance and chromatic input, then the dependence of relative perceived velocity on relative stimulus contrast should be the same for both luminance and chromatic targets. Here we provide a distinctive characterization of the motion mechanisms using a robust velocity-matching task. A relative contrast scale allows direct comparison of the performance with luminance and chromatic targets. The results show that the perceived speed of slowly moving coloured targets at isoluminance has a steep contrast dependence. The perceived speed of slowly moving luminance targets shows a much lower contrast dependence. At high speeds the contrast dependence is low for both luminance and isoluminant stimuli, although the behaviour is unlike either of the slow mechanisms. The results suggest two independent pathways that perceive slowly moving targets: one is luminance-sensitive and the other is colour-sensitive. Fast movement is signalled via a single motion pathway that is contrast-invariant and not colour blind.

Color enhances Mach bands detection threshold and perceived brightness.

We have previously argued that unless color and luminance are shown to be processed independently, using isoluminant stimuli may not reveal the full contribution of color to visual functioning. Here we study the interaction of color and luminance in a task, Mach bands detection and perceived brightness, where color by itself is not effective at all. Subjects viewed luminance or color/luminance ramps and had to determine in either case the luminance contrast necessary for detecting Mach bands and, in another experiment, to compare the brightness of the bands in the luminance and in the combined displays. Isoluminant color displays did not generate any Mach bands, but the addition of color to the luminance display lowered Mach bands detection thresholds and enhanced their perceived brightness. It is thus concluded that the failure to perceive Mach bands in an isoluminant display is not indicative of the lack of color contribution to spatial vision but rather indicates that the strong effect that color has on contrast enhancement mechanisms can be revealed if color and luminance are allowed to interact.

Color, contrast sensitivity, and the cone mosaic.

This paper evaluates the role of various stages in the human visual system in the detection of spatial patterns. Contrast sensitivity measurements were made for interference fringe stimuli in three directions in color space with a psychophysical technique that avoided blurring by the eye's optics including chromatic aberration. These measurements were compared with the performance of an ideal observer that incorporated optical factors, such as photon catch in the cone mosaic, that influence the detection of interference fringes. The comparison of human and ideal observer performance showed that neural factors influence the shape as well as the height of the foveal contrast sensitivity function for all color directions, including those that involve luminance modulation. Furthermore, when optical factors are taken into account, the neural visual system has the same contrast sensitivity for isoluminant stimuli seen by the middle-wavelength-sensitive (M) and long-wavelength-sensitive (L) cones and isoluminant stimuli seen by the short-wavelength-sensitive (S) cones. Though the cone submosaics that feed these chromatic mechanisms have very different spatial properties, the later neural stages apparently have similar spatial properties. Finally, we review the evidence that cone sampling can produce aliasing distortion for gratings with spatial frequencies exceeding the resolution limit. Aliasing can be observed with gratings modulated in any of the three directions in color space we used. We discuss mechanisms that prevent aliasing in most ordinary viewing conditions.