Effects of isoluminant-background color on metacontrast and stroboscopic motion: interactions between sustained (P) and transient (M) channels

The effect of smoking on sensory and attentional masking

This study examined the influence of smoking on low-level sensory and higher level visual cognitive abilities. Three groups (nonsmokers, deprived smokers, and nondeprived smokers) of 22 observers were tested using a visual target discrimination task. To assess sensory and visual cognitive differences, a masking task developed by Enns and Di Lollo (1997) was used. In the task, the target was masked by either a contour mask (contour masking) or an object mask (attentional masking by object substitution). The strongest masking effects were found in the group of nondeprived smokers, for both sensory and attentional masking. This pattern of results as well as results in the deprived smokers indicated changes in transient visual processing channels due to the acute actions of nicotine, not mediated by withdrawal relief.

On the use of metacontrast to assess magnocellular function in dyslexic readers

It has been proposed that dyslexia is the result of a deficit in the magnocellular system. Reduced metacontrast masking in dyslexic readers has been taken as support for this view. In metacontrast, a masking stimulus reduces the visibility of a spatially adjacent target stimulus when the target stimulus precedes the masking stimulus by about 30-100 msec. Recent evidence indicates that the latency difference between the magnocellular and parvocellular subcortical pathways is at most 20 msec and may be as small as only 5 msec, or even less. This makes it difficult to attribute the latency in metacontrast to the latency differences between the magnocellular and parvocellular systems. It is therefore problematic to attribute reduced metacontrast masking to a deficit in the magnocellular system.

The influence of color on transient system activity: implications for dyslexia research

Metacontrast and apparent motion experiments designed to utilize transient system resources were adopted to investigate the proposal that transient system activity is differentially influenced by different colored stimuli. The results generally showed no effect of color on transient system activity in either adults or children. However, the predicted pattern of results was demonstrated when contrast rather than color was manipulated in a final metacontrast experiment. We discuss the tenuousness of the proposal that color differentially influences transient activity, exploring its physiological implications and its durability as a theory of transient activity regarding reading-disability research.

Categorical and coordinate spatial processing: more on contributions of the transient/magnocellular visual system

Observers were presented with stimuli consisting of a line and two horizontally separated dots. A categorical spatial task required observers to indicate whether the dots were above or below the line and a coordinate spatial task required observers to indicate whether the line could fit into the space between the two dots. Coordinate (but not categorical) spatial processing was less accurate and took longer with stimuli presented on a red background than with stimuli presented on a green background, even though the background color varied randomly from trial to trial and the viewing screen remained gray between trials. Because the color red attenuates processing in the transient/magnocellular visual system, these results suggest that coordinate spatial processing is dependent on that pathway. Furthermore, such effects do not involve mechanisms of perceptual adaptation that depend on the same color background being present throughout an experiment or for a prolonged period of time. As in earlier experiments, the effects of color condition were the same regardless of which visual field (and hemisphere) received the stimulus information. However, in contrast to the results of earlier experiments, there was no significant interaction of task and visual field.

Recent models and findings in visual backward masking: a comparison, review, and update

Visual backward masking not only is an empirically rich and theoretically interesting phenomenon but also has found increasing application as a powerful methodological tool in studies of visual information processing and as a useful instrument for investigating visual function in a variety of specific subject populations. Since the dual-channel, sustained-transient approach to visual masking was introduced about two decades ago, several new models of backward masking and metacontrast have been proposed as alternative approaches to visual masking. In this article, we outline, review, and evaluate three such approaches: an extension of the dual-channel approach as realized in the neural network model of retino-cortical dynamics (Ogmen, 1993), the perceptual retouch theory (Bachmann, 1984, 1994), and the boundary contour system (Francis, 1997; Grossberg & Mingolla, 1985b). Recent psychophysical and electrophysiological findings relevant to backward masking are reviewed and, whenever possible, are related to the aforementioned models. Besides noting the positive aspects of these models, we also list their problems and suggest changes that may improve them and experiments that can empirically test them.

Strabismic amblyopia. Part 2. Neural processing

This is the second of a two-part survey of current literature concerning strabismic amblyopia. The aim of this review is to bring the optometric community up to date on the status of scientific research into strabismic amblyopia. Part 1 in this series discussed research into strabismic amblyopia from the viewpoint of psychophysical experiments, which investigate both spatial and temporal behavioural deficits accompanying strabismic amblyopia. These include deficits in contrast sensitivity, spatial localisation, fixation, ocular motility, accommodation, crowding, attention, motion perception and temporal processing. Part 2 concerns neural processing in regards to strabismic amblyopia. It discusses current understanding of more fundamental aspects of central processing of visual information and in particular current theories regarding neural sites and mechanisms involved in amblyopia.

Influences of occlusion, color, and luminance on the perception of fragmented pictures

The contribution of the magnocellular (M) pathway to perceptual completion and depth processing was examined by comparing performance under black-and-white conditions with isoluminant color and diffuse red background conditions expected to suppress M pathway activity. Participants identified the repeated figure in pictures where only fragments of the figures were visible. The fragments were presented either alone (unoccluded) or with an occluder (occluded) filling the space between them. Identification with an occluder involved amodally completing the fragments behind it, i.e., depth processing. All unoccluded versions were easy to identify indicating perceptual completion of the fragments was not influenced by suppressing M pathway activity. Black-and-white occluded versions were also easy to identify. The significantly longer identification times for occluded versions under isoluminant and diffuse red background conditions indicates amodal completion of the fragments was hindered when M pathway activity was reduced, supporting the importance of M pathway activity for depth processing.

Effects of background color on the global and local processing of hierarchically organized stimuli

Recent studies have shown that (1) the global precedence effects in processing the hierarchically organized stimulus can be attenuated by eliminating the low spatial frequencies contained in the stimulus and (2) the human magnocellular pathway is responsible for processing low spatial frequencies and the pathway can be attenuated by imposing a red background on the stimulus. In the present study, a reaction-time experiment was conducted to examine the effect of background color of the stimulus to the processing of the hierarchically organized stimulus. The result showed that although the control condition (a green background) produced a prototypical asymmetric global interference, a red background that was equiluminant to the green background produced a symmetrical interference. It was concluded that the human magnocellular pathway is at least partially responsible in producing the global precedence effects.

Spatial processing and hemispheric asymmetry. Contributions of the transient/magnocellular visual system

Right-handed observers were presented with stimuli consisting of a line and two horizontally separated dots. A categorical spatial task required observers to indicate whether the dots were above or below the line, and a coordinate spatial task required observers to indicate whether the line could fit into the space between the two dots. For the coordinate task, reaction time was faster when the stimuli were presented to the left visual field (right hemisphere) than when the stimuli were presented to the right visual field (left hemisphere). The opposite hemispheric asymmetry was obtained for the categorical task. In addition, coordinate spatial processing took longer with stimuli presented on a red background than with stimuli presented on a green background. The opposite hemispheric asymmetry was obtained for the categorical task. In addition, coordinate spatial processing took longer with stimuli presented on a red background than with stimuli presented on a green background. The opposite trend characterized categorical spatial processing. Because the color red attenuates processing in the transient/magnocellular visual pathway, these results suggest that coordinate spatial processing is more dependent on the transient/magnocellular pathway than is categorical spatial processing. However, manipulations of color condition had no effect on visual field (hemispheric) asymmetries, suggesting that the two hemispheres rely on the same visual information and on the same computational mechanisms as each other-although they do not always use that information with equal efficiency.

Visual and language processing disorders are concurrent in dyslexia and continue into adulthood

A recent study by Slaghuis. Lovegrove and Davidson (1994) found that visual and language processing differences were concurrent in a group of preadolescent dyslexic. In the present study, two experiments are reported that investigate the concurrence and continuity of visual and language processing differences in groups of young and adult dyslexics on a measure of visual processing and a measure of phonological coding. The visual processing task in the present experiments was a measure of Ternus apparent movement which was used as an index of the duration of visible persistence. Ternus apparent movement is multistable and provides two mutually exclusive and easily distinguishable percepts for the observer, referred to as 'element' and 'group' movement, that are highly dependent on the temporal interval between frame 1 and frame 2 of the display. The language processing task in the present experiments was a test of phonological coding measured using a non-word test of 100 orthographically legal non-words. The results of the first experiment showed that in comparison to normal readers the young dyslexic participants showed a significant reduction in Ternus 'group movement' and a significant reduction in the pronunciation of non-words. In a second experiment, Ternus apparent movement and performance on the non-word test was measured in groups of adult dyslexic and normal readers in order to investigate whether the visual and language processing differences found in young dyslexics were also present in adult dyslexics. The results showed that adult dyslexics also have a significant reduction in Ternus 'group movement' and a significant reduction in the ability to pronounce non-words similar to that found in the young dyslexic group in Experiment 1. The significant reduction in Ternus 'group movement' in dyslexic participants was explained in terms of an increase in the duration of visible persistence and was shown to be consistent with evidence for a transient system disorder. The combined results show that visual and language processing difference are concurrent in dyslexia and continue into adulthood.

Effects of a red background on magnocellular functioning in average and specifically disabled readers

Two experiments were conducted using metacontrast masking to examine responses in the magno system of adults, average reading adolescents and adolescents with specific reading disability. In Experiment 1 the effects of a red background field on the metacontrast functions of adult subjects were investigated. Results showed that a red, compared to a photometrically matched white background field, significantly attenuated metacontrast magnitude, supporting the interpretation of metacontrast as due to magno system suppression of parvo system responses. The finding of a red background effect was replicated in Experiment 2 with the two adolescent groups. The metacontrast functions of the adolescent groups also differed significantly, with those with specific reading disability exhibiting weaker metacontrast than the average readers. This result is consistent with a deficit in the magno system of individuals with specific reading disability and indicates the continuation of the deficit beyond childhood.

Luminance discrimination, color contrast, and multiple mechanisms

The purpose of this study was to investigate the effect of color contrast on luminance discrimination. Observers were required to indicate the more intense of two stimuli presented briefly in a surround. In some conditions the two stimuli were the same chromaticity as the surround, while in other conditions the stimuli differed in chromaticity from the surround. The luminance of the fixed test stimulus was varied in different conditions over a range from below the surround level to above the surround level. Difference thresholds were proportional to the luminance difference between test and surround over much of the range. However, difference thresholds were higher at low luminance contrasts when the chromatic contrast between the stimuli and the surround was high. Results also indicate that the effects of chromatic contrast may be mediated by local contrast mechanisms, but that the relationship between threshold and luminance contrast is not mediated entirely by these local contrast mechanisms.

Effects of background color on reaction time to stimuli varying in size and contrast: inferences about human M channels

In two experiments we looked at the effects of the color of equiluminant backgrounds on simple reaction time (RT) to increment and decrement spot-stimuli varying in diameter. When comparing both red vs blue and red vs green backgrounds, we found that for the smallest diameter stimuli, RTs tended to be faster with red background; however, as the diameter of the stimuli increased, RTs were faster with the blue or green backgrounds. This trend held only for increment stimuli; no systematic or significant differences between RTs to decrement stimuli presented on red vs either blue or green backgrounds were found. We discuss these results in terms of the effects of diffuse lights of varying wavelength on magnocellular-channel activity.

Dynamic noise backgrounds facilitate target fading

With strict fixation, a small uniform target of medium contrast, placed at 10 deg eccentricity, faded much faster when presented on a dynamic random noise background than on either a static random noise background or a uniform background of the same luminance. Time to first disappearance was between 10 and 16 sec when the background was dynamic, 26 sec when it was static, and 57 sec when it was uniform. Times were shortest for temporal noise frequencies of the background between 3.5 and 15 Hz. These findings are unexpected: the frequent change of pixel contrast at the edge of the target should perceptually enhance the border, make it less susceptible to local adaptation, and prevent fading. Instead, dynamic random noise facilitates, rather than suppresses fading. Three potential mechanisms are discussed: edge perturbation, jerk effect and surround induction.

Metacontrast with masks varying in spatial frequency and wavelength

In a metacontrast masking paradigm, adult subjects were required to detect briefly presented target lines followed at various delays by a flanking mask varying in spatial frequency and wavelength. Detection accuracy, the dependent measure, was recorded as a function of the delay of the mask. The results showed that long wavelength masks produced maximum masking at a relatively short delay, while short wavelength masks produced maximum masking at a relatively long delay. Results are discussed within the framework of transient/sustained (magnocellular/parvocellular) theory of visual processing, and suggest that low spatial frequency channels respond with shorter latency and/or faster rise time to short wavelength stimuli, and high spatial frequency channels respond with greater sensitivity to long wavelength stimuli.