Metacontrast with masks varying in spatial frequency and wavelength

Further Examination of the Pulsed- and Steady-Pedestal Paradigms under Hypothetical Parvocellular- and Magnocellular-Biased Conditions

The pulsed- and steady-pedestal paradigms were designed to track increment thresholds (ΔC) as a function of pedestal contrast (C) for the parvocellular (P) and magnocellular (M) systems, respectively. These paradigms produce contrasting results: linear relationships between ΔC and C are observed in the pulsed-pedestal paradigm, indicative of the P system's processing, while the steady-pedestal paradigm reveals nonlinear functions, characteristic of the M system's response. However, we recently found the P model fits better than the M model for both paradigms, using Gabor stimuli biased towards the M or P systems based on their sensitivity to color and spatial frequency. Here, we used two-square pedestals under green vs. red light in the lower-left vs. upper-right visual fields to bias processing towards the M vs. P system, respectively. Based on our previous findings, we predicted the following: (1) steeper ΔC vs. C functions with the pulsed than the steady pedestal due to different task demands; (2) lower ΔCs in the upper-right vs. lower-left quadrant due to its bias towards P-system processing there; (3) no effect of color, since both paradigms track the P-system; and, most importantly (4) contrast gain should not be higher for the steady than for the pulsed pedestal. In general, our predictions were confirmed, replicating our previous findings and providing further evidence questioning the general validity of using the pulsed- and steady-pedestal paradigms to differentiate the P and M systems.

Examining Increment thresholds as a function of pedestal contrast under hypothetical parvo- and magnocellular-biased conditions

Theoretically, the pulsed- and steady-pedestal paradigms are thought to track contrast-increment thresholds (ΔC) as a function of pedestal contrast (C) for the parvocellular (P) and magnocellular (M) systems, respectively, yielding linear ΔC versus C functions for the pulsed- and nonlinear functions for the steady-pedestal paradigm. A recent study utilizing these paradigms to isolate the P and M systems reported no evidence of the M system being suppressed by red light, contrary to previous physiological and psychophysical findings. Curious as to why this may have occurred, we examined how ΔC varies with C for the P and M systems using the pulsed- and steady-pedestal paradigms and stimuli biased towards the P or M systems based on their sensitivity to spatial frequency (SF) and color. We found no effect of color and little influence of SF. To explain this lack of color effects, we used a quantitative model of ΔC (as it changes with C) to obtain Csat and contrast-gain values. The contrast-gain values (i) contradicted the hypothesis that the steady-pedestal paradigm tracks the M-system response, and (ii) our obtained Csat values indicated strongly that both pulsed- and steady-pedestal paradigms track primarily the P-system response.

Spectral Overlays for Reading Difficulties: Oculomotor Function and Reading Efficiency Among Children and Adolescents With Visual Stress

This study analyzed the effects of spectral overlays on ocular motility during reading among a clinical group of children and adolescents experiencing visual–perceptual distortions of text. We reviewed the records of 323 eye-hospital patients diagnosed with visual stress and divided this participant sample into two age-based cohorts: children ( n =  184; Mean [ M] age = 10.1, standard deviation [ SD]  =  1.3 years) and adolescents ( n =  139; M age = 14.6, SD = 1.5 years). We used a Visagraph III Eye-Movement Recording System to record ocular motor efficiency while reading with and without spectral overlays, and we examined the following parameters: (a) Fixations, (b) Regressions, (c) Span of Recognition, (d) Reading Rate, (e) Relative Efficiency, and (f) Comprehension. Our results showed that using one or some combination of 10 participant-selected spectral overlays immediately and significantly ( p <  .001) reduced the number of Fixations and Regressions per 100 words, while there were significant ( p <  .001) gains in positive factors such as Span of Recognition, Reading Rate, Relative Efficiency, and Comprehension. Our findings indicate that spectral filtering can be an effective tool for helping many young patients who experience visual–perceptual distortions while reading. Future expanded research employing eye-tracking technology is clearly needed.

Connectivity and signal intensity in the parieto-occipital cortex predicts top-down attentional effect in visual masking: an fMRI study based on individual differences

Top-down attention affects even the early stages of visual processing. For example, several studies have reported that instructions prior to the presentation of visual stimuli can both enhance and reduce visual masking. The finding that top-down processing influences perceptual processing is called the attentional effect. However, the magnitude of the attentional effect differs between individuals, and how these differences relate to brain activation remains to be explained. One possibility would be that activation intensity predicts the magnitude of the attentional effect. Another possible explanation would be that effective connectivity among activated areas determines the attentional effect. In the present study, we used structural equation modeling to analyze individual differences in the attentional effect on visual masking, in relation to the signal and connectivity strength of activated brain regions prior to presentation of the visual stimuli. The results showed that signal intensity was positively correlated with attentional effect in the occipital areas, but not in fronto-parietal areas, and the effect was also positively correlated with connective efficiency from the right intraparietal sulcus (IPS) to the bilateral fusiform gyrus (GF). Furthermore, a higher degree of effective connections from the right IPS to the GF led to greater neural activity in the GF. We therefore propose that the effective modulator in the parietal areas and strong activation in the visual areas together and in cooperation predict higher attentional effects in visual processing.

Metacontrast masking suggests interaction between visual pathways with different spatial and temporal properties

We examined the spatiotemporal characteristics of metacontrast using sinusoidal grating stimuli as the target and mask for quantitative comparison with the functional properties of the visual cortex. The magnitude of metacontrast effects depended on the stimulus features such as the orientation and spatial frequency of the target and mask. The characteristics of metacontrast dynamically changed depending on the stimulus onset asynchrony (SOA). At short SOAs (0 to approximately 40 ms), metacontrast exhibited a high stimulus feature specificity and a low contrast sensitivity, whereas at long SOAs ( approximately 40 to 80 ms), metacontrast exhibited a low stimulus feature specificity and a high contrast sensitivity. We suggest that metacontrast is explained by the interaction between two parallel visual pathways: one with a low contrast sensitivity and a high feature specificity, and the other with a high contrast sensitivity and a low feature specificity.

Effects of background color on detecting spot stimuli in the upper and lower visual fields

Participants were required to detect spot stimuli briefly presented to the upper, central, or lower visual fields. The stimuli were presented either on a green or a red background. Results showed that reaction time (RT) was shorter for the lower visual field (LVF) compared to the upper visual field (UVF). Furthermore, this LVF advantage was significantly reduced in the red background condition compared to the green one. A red light is known to suppress activity of the magno-dominated stream. Therefore, the LVF advantage in RT can be explained as resulting from the biased representation of the magno-dominated stream in the LVF.

The magnocellular visual system and schizophrenia: what can the color red tell us?

Previous research has suggested that genetic loading for schizophrenia is related to a dysfunctional magnocellular (M) subcortical visual pathway-responsible for processing movement and location. However, data substantiating this mechanism remains inconclusive. The present study used a novel technique to selectively suppress the M system in order to investigate the impact of genetic loading for schizophrenia on its functioning. A visual backward masking task was administered to 28 healthy first-degree relatives of persons with schizophrenia and 31 healthy controls. The task was administered on both a red and neutral background, as diffuse red light has been shown to selectively suppress the M system in basic vision research. On a location condition of backward masking, controls demonstrated reduced accuracy on the red compared to the neutral background. In contrast, relatives did not display differential performance between the two backgrounds. The differential effect on the two groups appears to be attributable to a difference in activity of the M pathway. Performance in the relatives was consistent with the notion of a hyperactive M pathway.

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.

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.

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.

Achromatic visual backward masking of colored stimuli in type I diabetes

On a visual backward masking task using color stimuli with an achromatic patterned mask, we compared the masking performances of 3 Type I diabetics with those of 9 participants in a control group. Analysis indicated that the diabetics show a marked decrement in performance with blue stimuli and a lesser decrement with red stimuli. Suggestions for further theoretical and parametric studies are discussed.

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.

Visual attention modulates metacontrast masking

How does the human visual system 'bind' different fragments in the visual scene to create enduring representations of objects? A visual illusion known as 'metacontrast' or backward masking provides compelling evidence that perception is not instantaneous and that it occurs sequentially in distinct stages. If a solid white target square is displayed for 50 ms in a tachistoscope, switched off, and followed by a 50 ms display of two flanking mask squares, remarkably, subjects report seeing only the two flanking squares: the first square is simply not 'seen'. By plotting the magnitude of masking as a function of the delay between the target and mask (the stimulus onset asynchrony), one can obtain a characteristic 'U'-shaped function with optimum masking occurring at about 50 ms, and no masking with synchronous target and mask presentations or at delays higher than 300 ms. The illusion is also highly sensitive to elementary stimulus dimensions such as colour, orientation and spatial frequency, and it has been suggested that it is based on 'low level' autonomous visual mechanisms rather than cognitive processes. Here we describe a novel visual stimulus that demonstrates that metacontrast can be strongly modulated by 'top down' influence such as voluntary visual attention.

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.

Visual information decoding deficits in schizophrenia

Schizophrenic and control subjects were tested on two-flash fusion (TFF) and visual backward masking (VBM) tasks in a repeated measures design. Each subject was tested in a single session. Both tasks used the same equipment and stimuli. There was no difference between the groups in their ability to detect the presence of two separate stimuli in the TFF task. Schizophrenic subjects did require longer interstimulus intervals (ISI) than control subjects to accurately report one of the two targets in the VBM task. Analysis of individual targets reveals that the VBM deficit is a function of the similarity of the target and mask. The more feature detail discrimination necessary, the longer an ISI is required in VBM. The data are interpreted as supporting the conclusion that since the groups did not differ in their performance of the TFF task, which would also have been affected by a sensory abnormality, the deficit in VBM must be explained by reference to a higher level of information processing. The VBM deficit is a failure to decode the target stimulus, and is not simply a function of abnormalities due to an overactive transient channel system.