Chromatic Gratings Research Articles

543 P120 component of foveal visual evoked potentials encodes luminance contrast in humans

Psychophysical evidence suggests that two major parallel pathways, the parvocellular (P) and the magnocellular (M) pathways, exist in humans. We herein report that responses specific to the P and M systems can be recorded in human visual evoked potentials (VEPs) by using the appropriate stimuli. The onset of isoluminant chromatic (red-green) and high contrast achromatic sinusoidal gratings were used for stimulating the P-system. A chromatic stimulation evoked a characteristic negative wave (N1) with peak latencies around 120 msec. The amplitude showed an inverse U-shaped function as a function of spatial frequency with a peak at 2 c/deg. In contrast, VEPs to achromatic (black-white) gratings showed different spatial frequency characteristics with a peak at 5.3 c/deg. By varying the luminous intensity ratio between the red and green gratings, N1 was found to reach a maximum during isoluminant stimulation. An apparent motion display was used for stimulating the M-system. The speed of alternation (i.e., the interstimulus interval (ISI)) was varied to record both the transient and steady-state VEPs. Transient VEPs showed triphasic waves with the major positive peak (P1) at around 120 ms. Steady-state VEPs were quasi-sinusoidal waveforms, depending on the ISI, and were quite stable across all subjects. There was a also high correlation between the motion threshold and the VEP amplitude. The above observations indicate that characteristic potentials may distinguish between these two parallel visual systems in humans. Thus, the combined use of isoluminant color and high contrast achromatic gratings and an apparent motion display is considered to be useful for evaluating both systems electrophysiologically.

Reaction time to motion onset of luminance and chromatic gratings is determined by perceived speed.

We measured reaction times for detecting motion onset for sinusoidal gratings whose contrast was modulated in either luminance or chromaticity, for various drift rates and contrasts. In general, reaction times to chromatic gratings were slower than to luminance gratings of matched cone contrast, but the difference in response depended critically on both contrast and speed. At high image speeds there was virtually no difference, whereas at low speeds, the difference was pronounced, especially at low contrasts. At high image speeds there was little dependence of reaction times on contrast (for either luminance or colour), whereas at low speeds the dependence was greater, particularly for chromatic stimuli. This pattern of results is reminiscent of those found for apparent speed of drifting luminance and chromatic gratings. We verified the effects of contrast on perceived speed, and went on to show that the effects of contrast on reaction times are totally predictable by the perceived speed of the stimuli, as if it were perceived rather than physical speed that determined reaction times. Our results support that idea of separate systems for fast and slow motion (with separate channels for luminance and colour at slower speeds), and further suggest that apparent speed and reaction times may be determined at a similar stage of motion analysis.

Detection of chromatic and luminance contrast modulation by the visual system.

The data presented in this paper examine the ability of observers to detect a modulation in the contrast of chromatic and luminance gratings as a function of the carrier contrast, duration, and spatial frequency. The nature of the signal underlying this ability is investigated by examining both the paradigm used to make the measurement and the effect of grating masks on performance in the tasks. The results show that observers' ability to discriminate amplitude modulation from an unmodulated carrier is dependent on carrier contrast but only up to approximately 5-8 times carrier-detection threshold. Discrimination is, however, independent of spatial frequency [10-1 cycles per degree (cpd) component-frequency range], carrier color, and, most surprisingly, stimulus duration (1000-30 ms). This set of experiments compliments data from previous papers and assimilates many of the conclusions drawn from this previous data. There is absolutely no evidence for the existence of a distortion product mediating performance under any of the current conditions, and the data seriously question whether the visual system might use such a signal even if it does exist under more extreme conditions than those used here. The evidence suggests that the visual system detects variations in both chromatic and luminance contrast by means of a mechanism operating locally upon the spatial structure of the carrier.

Perceptual, oculomotor, and neural responses to moving color plaids.

Moving plaids constructed from two achromatic gratings of identical luminance contrast are known to yield a percept of coherent pattern motion, as are plaids constructed from two identical chromatic (e.g. isoluminant red/green) gratings. To examine the interactive influences of chromatic and luminance contrast on the integration of visual motion signals, we constructed plaids with gratings that possessed both forms of contrast. We used plaids of two basic types, which differed with respect to the phase relationship between chromatic and luminance modulations (after Kooi et al, 1992 Perception 21 583-598). One plaid type ('symmetric') was made from component gratings that had identical chromatic/luminance phase relationships (e.g. both components were red-bright/green-dark modulation). The second plaid type ('asymmetric') was made from components that had complimentary phase relationships (i.e. one red-bright/green-dark grating and one green-bright/red-dark grating). Human subjects reported that the motion of symmetric plaids was perceptually coherent, while the components of asymmetric plaids failed to cohere. We also recorded eye movements elicited by both types of plaids to determine if they are similarly affected by these image cues for motion coherence. Results demonstrate that, under many conditions, eye movements elicited by perceptually coherent vs noncoherent plaids are distinguishable from one another. To reveal the neural bases of these perceptual and oculomotor phenomena, we also recorded the responses of neurons in the middle temporal visual area (area MT) of macaque visual cortex. Here we found that individual neurons exhibited differential tuning to symmetric vs asymmetric plaids. These neurophysiological results demonstrate that the neural mechanism for motion coherence is sensitive to the phase relationship between chromatic and luminance contrast, a finding which has implications for interactions between 'color' and 'motion' processing streams in the primate visual system.

Multimodality visual evoked potentials in evaluating visual dysfunction in optic neuritis.

Visual information is processed via multiple, parallel channels. The present study examined the clinical feasibility of multimodality visual evoked potentials (VEPs) in optic neuritis. We recorded transient VEPs to 30' checkerboard patterns, chromatic and achromatic sinusoidal gratings and apparent motion, and steady-state VEPs to achromatic gratings in 15 normal controls and 14 patients. VEPs to 30' checks were abnormal in 10 eyes (7 patients); however, considering all five modalities, abnormal responses were seen in 20 eyes (12 patients). Abnormality rates were not equal among the visual stimuli, which thus suggested possible dysfunction of individual subdivisions in the visual pathways. We conclude that use of multimodality VEPs may increase both understanding of the pathophysiology of the visual pathways and diagnostic yield.

The influence of established and new antiepileptic drugs on visual perception

In this study, we investigated visual performance under chronic antiepileptic drug treatment. Patients were under carbamazepine (CBZ) (n = 18), valproic acid (VPA) (n = 9), CBZ and vigabatrin (VGB) (n = 4), CBZ and gabapentin (GBP) (n = 8), and under CBZ and topiramate (TPR) (n = 6), respectively. Seven untreated patients with epilepsy and 42 healthy volunteers served as controls. The test battery comprised the Lanthony-D15-désaturé colour perception test, increment, postadaptation and transient tritanopia (TT) threshold measurements, visual perception threshold assessments for monochromatic and chromatic gratings and gaussian dots, and critical flicker fusion (CFF) tests. No differences were seen between naive patients and healthy controls. Patients under drug treatment always showed alterations of visual perception. Postadaptation and TT thresholds were altered under each drug regimen after short delays between switching off the adaptation light and switching on the blue test light. Threshold elevations were maximum under the combination of CBZ and TPR and lowest under CBZ and GBP. Consistent impairment of the CFF was seen under combined CBZ and TPR whereas VPA as well as combined CBZ and VGB led to ameliorations the mechanisms of which are discussed. The other tests were less sensitive. In conclusion, alterations of visual function were apparent under chronic antiepileptic drug treatment both with established and new agents. However, it may be difficult to distinguish between effects based on specific modes of action and nonspecific retino- and neurotoxicity.

Pattern ERGs from isoluminant gratings; poor selectivity compared with VEPS

The PERGs elicited by low contrast achromatic (luminance-modulated) and chromatic (isoluminant) gratings were studied as a function of the presentation mode (rapid onset, offset, reversal), departure from isoluminance, contrast and spatial frequency; predominantly phasic-type responses were found in all cases. We propose that the PERGs for low and moderate contrast, achromatic and chromatic grating stimuli, are generated by non-linear, transient retinal cells of the magno-system. Parvo retinal cells seem to contribute to the response only at higher chromatic contrasts. The transient nature of the PERG obtained from isoluminant gratings makes it doubtful that these signals represent the activity of colour-related processes.

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.

Spatial Integration of Objectively Equiluminous Chromatic Gratings

Contrast sensitivity functions for achromatic and chromatic gratings tend to be band-pass and low-pass in shape, respectively. Our aim was to test whether spatial integration contributes to the shape difference found at low spatial frequencies. We measured binocular chromatic contrast sensitivity as a function of grating area for objectively equiluminous red - green and blue - yellow chromatic gratings. Chromatic contrast refers to the Michelson contrast of either of the two chromatic component gratings presented in counterphase against the combined background. Grating area ( A) varied from 1 to 256 square cycles ( Af2) at spatial frequencies ( f) of 0.125 – 4.0 cycles deg−1. We used only horizontal gratings at low and medium spatial frequencies to minimise the transverse and longitudinal chromatic aberrations due to ocular optics. At all spatial frequencies studied, chromatic contrast sensitivity increased with grating area. Ac was found to be constant at low spatial frequencies (0.125 – 0.5 cycles deg−1) but decreased in inverse proportion to increasing spatial frequency at 1 – 4 cycles deg−1. Thus, spatial integration depends similarly on spatial frequency for achromatic (Luntinen et al, 1995 Vision Research35 2339 – 2346) and chromatic gratings, and differences in spatial integration do not contribute to the shape difference of the respective contrast sensitivity functions.

The development of chromatic and achromatic contrast sensitivity in infancy as tested with the sweep VEP

Swept-contrast visual evoked potential (VEP) techniques were used to measure the development of contrast sensitivity functions (CSFs) for achromatic and red/green isoluminant chromatic gratings. Subjects were infants of 8, 14, 20 and 32 weeks of age, and adults. Stimuli were 20 deg, 0.3–4 cyc/deg sinusoidal gratings, counterphased at 6 Hz and modulated through white. Achromatic and chromatic CSFs for all ages could be fit simultaneously with a double exponential equation of a common, lowpass shape. Both achromatic and chromatic CSFs exhibited developmental shifts in both sensitivity and spatial scale. From 8 weeks to adulthood, sensitivity increased by 0.64 log units for achromatic gratings and by 0.91 log units for chromatic gratings, yielding an 0.27 log unit larger sensitivity change for chromatic than for achromatic stimuli. Spatial scale shifts were closely similar across achromatic and chromatic CSFs, and were consistent with the factor of about four predicted on the basis of changes in foveal receptor packing density and eye size. The question of uniform vs differential loss of sensitivity for chromatic vs achromatic stimuli at fixed spatial frequencies is discussed.

Detection of Chromatic and Achromatic Patterns at Threshold: RT Evidence of Sustained and Transient Mechanisms

In primates, relatively slow sustained-like physiological mechanisms mediate chromatic processing and rapid transient-like mechanisms mediate low-spatial-frequency achromatic processing. Our objective was to investigate the role of transient and sustained mechanisms in the detection of chromatic and achromatic gratings with the use of a reaction time (RT) paradigm. Following Tolhurst (1975 Vision Research15 1143 – 1149), we constructed RT histograms using near-threshold grating stimuli. Tolhurst showed that, for low-spatial-frequency achromatic patterns, the RT histograms cluster around the onset and the offset of the pattern, indicating transient detection. His onset and offset distributions were consistent with probability theory. With high spatial frequencies, RTs are evenly distributed over the duration of the stimulus presentation, showing the operation of a sustained mechanism. When we used low-spatial-frequency (0.22 cycle deg−1) isoluminant red - green gratings, the RT histograms were unimodal, revealing detection mechanisms with predominantly sustained characteristics, as expected. RT histograms obtained with low-spatial-frequency achromatic stimuli in our study were bimodal. However, when we compared small and large fields, the shape of the RT histogram varied in a manner which could not be accounted for by a simple probabilistic model. This can be rationalised if onsets and offsets are not equally detectable.

Masking Effects Induced by Perceptual Colour Components

Signals from the three types of cone photoreceptors are generally thought to be combined into two opponent-colour components and an achromatic component. Here we have chosen the cardinal directions Ach (achromatic axis), Cr1 (reddish - greenish axis) and Cr2 (bluish - yellowish axis) defined by Krauskopf to build a colour image coding scheme based on features of the human visual system. In order to design the optimal perceptual quantisers, we studied the perception of colour at different spatial frequencies, measuring the effect of a colour masking signal on the perception threshold for achromatic or chromatic sine-wave gratings. Three cases were considered to describe these interactions. First, we measured the masking effect of each colour component by itself. For the two chromatic components, we observed a pedestal effect for low masking contrasts and threshold rises for higher masking contrasts. Second, we studied interactions between the two colour axes. With colour masking signals composed of Cr1 and Cr2 components, thresholds for Cr1 stimuli dropped at low contrasts of the Cr2 component of the masking signal. This effect disappeared for high Cr2 masking contrasts. This was true at all contrasts of the Cr1 component of the masking signal. Thresholds for Cr2 stimuli were not modified by Cr1 masking. Finally, interactions between the chromatic and achromatic components were more complex. We observed a strong modification of the achromatic contrast-sensitivity function when a Cr1 masking sine-wave grating was added. When the achromatic frequency was lower than the colour masking frequency, the threshold rose. However, when the achromatic frequency was higher than the colour masking frequency, there was a significant pedestal effect. The global effect is a shift of the achromatic contrast sensitivity function. With a Cr2 masking signal, no significant modification of the achromatic contrast-sensitivity function was observed.

Effect of Luminance Contrast on the Motion Aftereffect

The effects of luminance contrast and spatial frequency in the transient channel were investigated by making use of the motion aftereffect (MAE) caused by adaptation to a drifting sinusoidal grating. Two experiments were performed. The PSE of the velocity was measured as an index of the MAE. The adapting grating was made to drift at a velocity of 2.28 deg s−1 and its spatial frequency was 0.8, 1.6, or 3.2 cycles deg−1. In the first experiment, the MAE caused by a luminance contrast grating or an equiluminous chromatic grating was measured. In the second experiment, luminance contrast gratings were used to measure the effect of the contrast differences between adapting and test gratings. The largest MAE was observed when a low-luminance-contrast grating or an equiluminous chromatic grating was presented as test stimulus after adaptation to a high-luminance-contrast grating in the low-spatial-frequency condition. Generally, the MAE increased with increasing adapting contrast and with decreasing test contrast or spatial frequency. Little MAE was observed at high test contrasts. The results may be explained by assuming that activity in the sustained channel (or parvocellular pathway) inhibits activity in the transient channel (or magnocellular pathway) owing to the domination of sustained channel activity when the test is a static high-luminance-contrast grating providing much information about position and form.

Motion minima for different directions in color space.

We have used the minimum-motion stimulus of Cavanagh, MacLeod & Anstis [(1987) Journal of the Optical Society of America A, 4, 1428-1438] to examine how signals along different directions in color space interact in motion perception. Stimuli were pairs of counterphasing gratings combined 90 deg out of phase in both space and time and modulated along different color-luminance axes. The axis for one of the gratings was fixed, while the axis for the second was varied so as to null perceived motion in the stimulus. The motion nulls show that observers are sensitive to motion signals carried by each of the cardinal directions of color space [an achromatic axis and L-M and S-(L+M) chromatic axes], but that signals along different cardinal axes are not combined to yield a net direction of motion. Pairing an achromatic and chromatic grating resulted in a motion null regardless of the relative or overall contrast of the two gratings, while the null directions for intermediate axes shifted depending on contrast. This result points to the special status of the luminance and chromatic axes. However, our results do not reveal a special pair of axes within the equiluminant plane. When contrasts along the cardinal axes are scaled for equal multiples of their respective detection thresholds, the L-M and S chromatic contrasts contribute roughly equally to the perceived motion, but are many times weaker than luminance contrast. Moreover, sensitivity to luminance motion is little affected by the presence of chromatic contrast, whereas sensitivity to chromatic motion is strongly masked by either luminance or chromatic contrast. These asymmetric interactions suggest that the motion of the luminance and chromatic components is encoded in qualitatively different ways.

Motion coherence across different chromatic axes.

It has been reported that equiluminant plaid patterns constructed from component gratings modulated along different axes of a cardinal colour space fail to create a coherent impression of two-dimensional motion [Krauskopf and Farell (1990). Nature, 348, 328-331]. In this paper we assess whether this lack of interaction between cardinal axes is a general finding or is instead dependent upon specific stimulus parameters. Type I and Type II plaids were made from sinusoidal components (1 cpd) each modulated along axes in a cardinal colour space and presented at equivalent perceived contrasts. The spatial angular difference between the two components was varied from 5 to 90 deg whilst keeping the Intersection of Constraints (I.O.C.) solution of the pattern constant. Observers were required to indicate the perceived direction of motion of the pattern in a single interval direction-identification task. We find that: (i) When plaids were made from components modulated along the same cardinal axis, coherent "pattern" motion was perceived at all angular differences. As the angular difference between the components decreased in a Type II plaid, the perceived direction of motion moved closer to the I.O.C. solution and away from that predicted by the vector sum. (ii) A plaid made from components modulated along red-green and blue-yellow cardinal axes (cross-cardinal axis) did not cohere at high angular differences (> 30 deg) but had a perceived direction of the fastest moving component. At lower angular differences, however, pattern motion was detected and approached the I.O.C. solution in much the same way as a same-cardinal axis Type II plaid. (iii) A plaid made from a luminance grating and a cardinal chromatic grating (red-green or blue-yellow) failed to cohere under all conditions, demonstrating that there is no interaction between luminance and chromatic cardinal axes. These results indicate that there are conditions under which red-green and blue-yellow cardinal components interact for the purposes of motion detection.

Behavioural and electrophysiological chromatic and achromatic contrast sensitivity in an achromatopsic patient.

In cases of incomplete achromatopsia it is unclear whether residual visual function is mediated by intact striate cortex or results from incomplete lesions to extrastriate cortical visual areas. A patient with complete cerebral achromatopsia was tested to establish the nature of his residual vision and to determine the integrity of striate cortex function. Behavioural contrast sensitivity, using the method of adjustment, and averaged visually evoked cortical potentials were measured to sinusoidally modulated chromatic and achromatic gratings in an achromatopsic patient and a normal observer. Eye movements were measured in the patient using a Skalar infrared monitoring system. The patient's chromatic contrast sensitivity was normal, indicating that despite his dense colour blindness his occipital cortex still processed information about spatial variations in hue. His sensitivity to achromatic gratings was depressed particularly at high spatial frequencies, possibly because of his jerk nystagmus. These behavioural results were reinforced by the nature of visually evoked responses to chromatic and achromatic gratings, in which total colour blindness coexisted with an almost normal cortical potential to isoluminant chromatic gratings. The results show that information about chromatic contrast is present in some cortical areas, and coded in a colour-opponent fashion, in the absence of any perceptual experience of colour.

Human VEPs to isoluminant chromatic and achromatic sinusoidal gratings: separation of parvocellular components.

To extract the responses specific to the parvocellular system (color vision and form perception) in humans, visual evoked potentials (VEPs) in response to the onset of isoluminant chromatic (red-green) and achromatic (black-white) sinusoidal gratings were recorded in 15 young adults. Chromatic stimulation evoked a characteristic negative wave (N1) with peak latencies around 120 msec. The amplitude resembled an inverse U-shaped function against the spatial frequency with a peak at 2 c/deg. In contrast, VEPs to achromatic gratings showed different spatial frequency characteristics with a peak at 5.3 c/deg. With variations in the luminous intensity ratio between the red and green gratings, N1 was seen to respond maximally to isoluminant stimulation. These results indicate that the combined use of isoluminant chromatic and achromatic patterns could stimulate the parvocellular system selectively in humans.

Detection and motion detection in chromatic and luminance beats

Moving beat patterns were made by adding together sinusoidal luminance or chromatic gratings of equal contrasts and different spatial frequencies moving in opposite directions. Lower thresholds of motion (the speed required for discriminating the direction of motion of the moving beat) were measured at a contrast 0.5 log unit above the contrast required for detecting the spatial modulation in the beat pattern. At durations below 0.12 s the direction of motion of beat patterns could not be discriminated, even though beats could be discriminated from pure sinusoids. At longer durations the lower thresholds of motion declined with duration. When expressed in terms of temporal frequency, functions relating lower threshold of motion to duration were superimposed for beats made from color gratings and for beats with carrier frequencies from 1 to 10 cycles/degree. This is consistent with the hypothesis that a common mechanism analyzes the motion in all types of second-order patterns for which the motion can be reduced to be one of spatial contrast modulation of a one-dimensional carrier.

Magnetic and electrical brain responses to chromatic contrast in human

Differences between magnetic responses to red-green chromatic gratings and yellow-black luminance gratings were: (1) response waveforms differed considerably; (2) at some recording sites the chromatic grating response was considerably greater than the sum of responses to the red and green components of the chromatic grating; (3) the latencies of the successive peaks in the response to the onset of chromatic contrast were greater than the latencies of the corresponding peaks in the response to luminance contrast onset; (4) chromatic grating responses were lowpass with respect to spatial frequency while luminance grating responses were bandpass; (5) chromatic grating responses attenuated more steeply with increasing frequency above 2 c/deg than did luminance grating responses. Items (3)–(5) above are consistent with well-known psychophysical findings that contrast sensitivity is lowpass for chromatic gratings and chromatic responses are more sluggish than luminance responses. In subsidiary experiments we found that magnetic responses to red-green and blue-yellow equiluminant gratings had similar waveforms in all six subjects tested, but the topographical distributions were different in three subjects. The results of comparing magnetic and electrical responses to the onset and offset of contrast can be understood in terms of the considerable intersubject variability in the relation between neuroanatomy and cortical function that has been demonstrated by other techniques.

Retinal and cortical evoked responses to chromatic contrast stimuli. Specific losses in both eyes of patients with multiple sclerosis and unilateral optic neuritis.

It is known that colour vision may be altered in optic neuritis. Our aim was to establish whether chromatic and achromatic vision are differentially impaired using stimuli designed to favour the activity of either the magnocellular or the parvocellular stream of the visual pathway. Fourteen patients with a past history of unilateral optic neuritis in the course of multiple sclerosis and 10 age-matched control subjects were included in the study. Patients had relatively good visual acuity in the affected eyes and no gross colour deficits (Ishihara). Stimuli were alternating gratings of low spatial frequency and of different chromaticity along the red-green axis. The psychophysical contrast sensitivity (CS) was measured at 5 Hz as a function of colour ratio [red/(red + green)] to evaluate both the equiluminant point (the colour ratio corresponding to the lowest CS) and the CS for isochromatic, luminance gratings (red-black and green-black). Steady-state (2-24 Hz) and transient pattern electroretinograms (PERGs) and visually evoked potentials (VEPs) were recorded in response to high contrast (90%) stimuli of low spatial frequency (0.3 cycles deg-1) modulated in either pure chromatic contrast (equiluminant red-green) or pure luminance contrast (yellow-black). On average, CSs were reduced (10 dB) in optic neuritis eyes compared with controls for both luminance and chromatic gratings. In the VEPs (both transient and steady-state) amplitude losses and latency delays were far larger for the chromatic VEPs than for the luminance VEPs. Chromatic VEP latency delays were remarkable also in the fellow, clinically normal, eyes. Significant losses were apparent in both the luminance and chromatic PERG. However, the chromatic PERG was comparatively more altered. In agreement with previous reports, selective losses were not apparent at threshold. By contrast, suprathreshold electrophysiological responses displayed a clear dissociation between luminance and colour, suggesting that the parvocellular stream, compared with the magnocellular stream is more impaired in optic neuritis.