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

Suppression and Contrast Normalization in Motion Processing

Sensory neurons are activated by a range of stimuli to which they are said to be tuned. Usually, they are also suppressed by another set of stimuli that have little effect when presented in isolation. The interactions between preferred and suppressive stimuli are often quite complex and vary across neurons, even within a single area, making it difficult to infer their collective effect on behavioral responses mediated by activity across populations of neurons. Here, we investigated this issue by measuring, in human subjects (three males), the suppressive effect of static masks on the ocular following responses induced by moving stimuli. We found a wide range of effects, which depend in a nonlinear and nonseparable manner on the spatial frequency, contrast, and spatial location of both stimulus and mask. Under some conditions, the presence of the mask can be seen as scaling the contrast of the driving stimulus. Under other conditions, the effect is more complex, involving also a direct scaling of the behavioral response. All of this complexity at the behavioral level can be captured by a simple model in which stimulus and mask interact nonlinearly at two stages, one monocular and one binocular. The nature of the interactions is compatible with those observed at the level of single neurons in primates, usually broadly described as divisive normalization, without having to invoke any scaling mechanism.SIGNIFICANCE STATEMENT The response of sensory neurons to their preferred stimulus is often modulated by stimuli that are not effective when presented alone. Individual neurons can exhibit multiple modulatory effects, with considerable variability across neurons even in a single area. Such diversity has made it difficult to infer the impact of these modulatory mechanisms on behavioral responses. Here, we report the effects of a stationary mask on the reflexive eye movements induced by a moving stimulus. A model with two stages, each incorporating a divisive modulatory mechanism, reproduces our experimental results and suggests that qualitative variability of masking effects in cortical neurons might arise from differences in the extent to which such effects are inherited from earlier stages.

Masking with faces in central visual field under a variety of temporal schedules

With a few exceptions, previous studies have explored masking using either a backward mask or a common onset trailing mask, but not both. In a series of experiments, we demonstrate the use of faces in central visual field as a viable method to study the relationship between these two types of mask schedule. We tested observers in a two alternative forced choice face identification task, where both target and mask comprised synthetic faces, and show that a simple model can successfully predict masking across a variety of masking schedules ranging from a backward mask to a common onset trailing mask and a number of intermediate variations. Our data are well accounted for by a window of sensitivity to mask interference that is centered at around 100 ms.

Changes in the visual-evoked P1 potential as a function of schizotypy and background color in healthy young adults

Research has suggested a hypoactive visual magnocellular (M) pathway in individuals with schizophrenia-spectrum disorders and traits, along with a unique response of this pathway to red light. As these abnormalities only appear in a subset of these samples, they may reflect unknown subtypes with unique etiologies and corresponding neuropathologies. The P1 transient visual-evoked component has been found to be influenced by M-pathway activity; therefore, the current study assessed the P1 component in response to a 64% contrast checker stimulus on white, red, and green background conditions. The sample consisted of 28 undergraduate participants (61% male) who endorsed a continuous range of total scores from the Schizotypal Personality Questionnaire (SPQ). Participants with higher total SPQ scores had a reduced P1 mean amplitude with the white (baseline) background, which was primarily related to the SPQ Magical Thinking subscale score. In addition, while participants with lower total SPQ scores showed the expected reduction in P1 amplitude to the red (vs. green) background, participants with higher total SPQ scores showed no change, which was primarily related to the SPQ Ideas of Reference subscale. This differential change to the red background remained after covarying for the P1 amplitude to the green background, thus representing a relatively independent effect. Further confirmation of these early visual processing relationships to particular clusters of symptoms in related psychiatric samples may assist in revealing unique, currently unknown, subtypes of particular psychiatric disorders such as schizophrenia. This can direct treatment efforts toward more homogeneous neuropathology targets.

The backward masking red light effect and schizotypy: the influence of sex

Previous research has shown a unique effect of red light on visual processing related to both schizophrenia and positive schizotypy. The current study examined whether this effect is influenced by sex in a more broadly-defined schizotypy sample. A location backward masking (BM) task with red, green, and gray backgrounds was administered to 34 undergraduate students (59% female) with a high score on the Schizotypal Personality Questionnaire (SPQ) and 38 students (50% female) with a low score. Results revealed that the group by color interaction was significant for the male participants, while it did not approach significance in the females. The male schizotypy participants showed a significant decrease in BM accuracy to the red (vs. green) background, while the male control participants showed a non-significant mean increase in accuracy. A decrease in accuracy to the red background in the male schizotypy participants was related to a higher score on the Social Anxiety subscale of the SPQ. Findings suggest that the previously reported schizophrenia red light effect is limited to males when examining a SPQ-defined sample, and appears to be primarily related to negative schizotypy symptoms. The red light effect continues to show promise as a new endophenotype for schizophrenia-spectrum disorders.

A potential qualitative endophenotype for schizophrenia: backward masking response to red light

Past research with unaffected relatives of individuals with schizophrenia has suggested a new qualitative endophenotype for schizophrenia that involves a unique change in visual processing response to red light. The current study provides the first report of this "red light effect" in individuals with schizophrenia (N=15), compared with nonpsychiatric controls (N=16), using a location backward masking by pattern paradigm with red and green background conditions. Analyses revealed a statistically significant group difference in the overall change in accuracy to a red background. Controls tended to show an increase in accuracy with the red (compared with green) background, although the medium effect size was not statistically significant in the small sample. In contrast, participants with schizophrenia showed a statistically significant decrease in accuracy with the red background. Results support recent reports which have suggested that a unique change in visual processing in response to red light may represent a new endophenotype for schizophrenia. This effect is unique from most existing endophenotypes in that it represents a distinct qualitative performance pattern rather than simply poorer performance relative to a comparison group.

The effect of red light on backward masking in individuals with psychometrically defined schizotypy

INTRODUCTION

Previous research has suggested that individuals with schizophrenia and their relatives show a change in backward masking performance with a red background that is in the opposite qualitative direction as that found in nonpsychiatric controls. The present study examines this effect in individuals with psychometrically defined schizotypy to explore the potential of this effect to be a useful new qualitative endophenotype for schizophrenia-spectrum traits.

METHODS

The Abbreviated Youth Psychosis At-Risk Questionnaire was used to screen a large number of undergraduates for schizotypy symptoms. A sample of 23 participants scoring high on this measure were compared to a sample of 26 controls on a location backward masking task that was presented on both red and green backgrounds.

RESULTS

Consistent with findings in patients with schizophrenia, the participants reporting a high number of schizotypy features showed a decrease in performance to the red (compared to green) background and the controls showed a nonsignificant increase in performance--although this finding was limited to the stimulus-onset asynchrony (SOA) value that approximated the SOA with the largest effect size in the previous schizophrenia study (69 ms).

CONCLUSIONS

Although limited to one SOA, results extend earlier findings approximating this SOA to include a psychometrically defined schizotypy sample.

Metacontrast, target recovery, and the magno- and parvocellular systems: A perspective

In metacontrast a masking stimulus reduces the visibility of an adjacent target stimulus. This effect has been interpreted in terms of magno-/parvocellular interactions. It has also been found that a second masking stimulus, which precedes the primary mask by about 90 ms reduces the masking effect. This reduction, which is termed “target recovery,” has been hypothesized to reflect parvocellular inhibition of the magnocellular system. However, this is problematic because the time course of this effect is much larger than would be expected from magno-/parvocellular interactions. For this and other reasons, it is difficult to understand metacontrast in terms of magno- and parvocellular mechanisms.

Directional motion contrast sensitivity in developmental dyslexia

The present study compared the perception of visual motion in two dyslexia classification schemes; the [Boder, E. (1973). Developmental dyslexia: a diagnostic approach based on three atypical reading-spelling patterns. Developmental Medicine and Child Neurology, 15, 663-687.] dyseidetic, dysphonetic and mixed subgroups and [Williams, M. J., Stuart, G. W., Castles, A., & McAnally, K. I. (2003). Contrast sensitivity in subgroups of developmental dyslexia. Vision Research, 43, 467-477.] surface, phonological and mixed subgroups by measuring the contrast sensitivity for drifting gratings at three spatial frequencies (1.0, 4.0, and 8.0 c/deg) and five drift velocities (0.75, 3.0, 6.0, 12.0, and 18.0 cyc/s) in a sample of 32 children with dyslexia and 32 matched normal readers. The findings show that there were no differences in motion direction perception between normal readers and the group with dyslexia when dyslexia was taken as a homogeneous group. Motion direction perception was found to be intact in the dyseidetic and surface dyslexia subgroups and significantly lowered in both mixed dyslexia subgroups. The one inconsistency in the findings was that motion direction perception was significantly lowered in the [Boder, E. (1973). Developmental dyslexia: a diagnostic approach based on three atypical reading-spelling patterns. Developmental Medicine and Child Neurology, 15, 663-687.] dysphonetic subgroup and intact in the [Williams, M. J., Stuart, G. W., Castles, A., & McAnally, K. I. (2003). Contrast sensitivity in subgroups of developmental dyslexia. Vision Research, 43, 467-477.] phonological subgroup. The findings also provide evidence for the presence of a disorder in sequential and temporal order processing that appears to reflect a difficulty in retaining sequences of non-meaningful auditory and visual stimuli in short-term working memory in children with dyslexia.

Increasing reading speed by using colours: issues concerning reliability and specificity, and their theoretical and practical implications

By using techniques for precision ophthalmic tinting, individuals who report perceptual distortion of text can often find a colour of illumination that eliminates the distortions and increases reading speed. Most individuals choose green or blue hues, but there is considerable variability. We investigated how specific the colour has to be to obtain optimal reading speed. Individuals who habitually wear coloured filters for reading were asked to read text illuminated by coloured light (without using their filters). Reading speed was measured repeatedly with light of different colours. The colour (chromaticity) at which reading was fastest was consistent from one test session to the next. It was different from one individual to another, but highly specific for each individual: departures of colour from optimum by about 6 JNDs eliminated most of the speed advantage conferred by the optimal colour. It was difficult to attribute the consistency and specificity simply to familiarity with the tint or immediate memory for the colour of illumination. A consecutive sample of 1000 tint prescriptions was analysed numerically. For most prescriptions the variation in chromaticity with different types of lighting was not such as to remove all the potential benefit of the tint, as judged from a model of the effect of chromaticity on reading speed. The exceptions were the few tints that were weakly saturated or purple in colour. Across participants, reading speed was not consistently related to the scotopic energy, to the energy captured by any cone class, or to opponent colour processes. The reading was generally slowest with white light, and not with the colour complementary to the optimum. Explanations in terms of magnocellular deficits and cortical hyperexcitability are briefly discussed.

The effect of disrupting the human magnocellular pathway on global motion perception

The purpose of this study was to demonstrate the effect of human magnocellular (M)-pathway disruption on global motion perception. Coherence thresholds for global motion direction discrimination in random dot patterns were determined at slow and moderate dot speeds: (1) after adaptation to full-field sinusoidal flicker or a steady gray field, and (2) on a red or a gray background. Adaptation to flicker and a red background increased motion coherence thresholds relative to the gray baseline conditions at both dot speeds. Physiological studies have shown that M cells in the retina and LGN are inhibited by red light and are a main contributor to flicker perception in monkeys. Therefore, our results suggest that interference with processing in the subcortical M pathway disrupts higher-level motion integration.

Isoluminant stimuli and red background attenuate the effects of transient spatial attention on temporal resolution

The effects of transient spatial attention on temporal resolution were recently studied and compared with attentional effects on spatial resolution. It was found that in contrast to the attentional enhancement of spatial resolution, transient attention impairs temporal resolution. To account for these findings a novel attentional mechanism was suggested. This attentional mechanism facilitates parvocellular neurons at the attended location, which in turn inhibit the activity of magnocellular neurons at the same location [Yeshurun & Levy, Psychol. Sci. 14 (3) (2003a) 225]. The goal of the present study was to replicate the attentional decrement in temporal resolution (Experiment 1), and perform direct tests of this 'parvo facilitation-magno inhibition' hypothesis. The employment of isoluminant stimuli (Experiments 2a and 2b) or a red background (Experiment 3) ensured that the parvo system was the main system mediating performance. Consequently, any parvo-magno inhibitory processes elicited by the attentional mechanism should only have a minor effect on performance. As predicted, these manipulations either significantly attenuated or completely eliminated the attentional decrement in temporal resolution. These findings provide direct support to the hypothesis that attention favors parvocellular over magnocellular neurons.

On the use of red stimuli to isolate magnocellular responses in psychophysical experiments: A perspective

Neurophysiological recordings have shown that activity of magnocellular neurons may be reduced by red backgrounds. This has led some researchers to use red light, or red filters, in attempts to determine the magnocellular contribution to psychophysical tasks. This requires that red light not affect parvocellular neurons, or at least that it is possible to control for the effect on the parvocellular system by using other colors. The present report investigates these assumptions by calculating the effect of red, green, and blue filters on the three cone pigments and on the four parvocellular color-opponent cell mechanisms. It is found that a red filter has a large effect on the long- and middle-wavelength cone pigments and on the red–green color-opponent mechanisms. A green filter, on the other hand, has little effect. A blue filter has a fairly pronounced effect but this effect is distinctly different from that of the red filter. These results indicate that one ought not rely upon red light to isolate magnocellular activity in psychological experiments. The results also indicate that it is difficult to use colors other than red to control for the effect of this color on the parvocellular system.

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.

Can the magnocellular pathway read? Evidence from studies of color

A review of the neurophysiological literature suggests that the magnocellular pathway has adequate spatial-frequency and contrast sensitivity to perceive text under normal contrast conditions (>10%) and also is suppressed by red light. Results from three experiments involving color and reading show that red light impairs reading performance under normal luminance contrast conditions. However in a fourth experiment, isoluminant color text, designed to selectively activate the parvocellular pathway, is easier to read under red light. These discrepant results suggest that the magnocellular pathway is the dominant visual pathway for text perception. Implications for reading models and developmental dyslexia are discussed.

A biochemical analysis of people with chronic fatigue who have Irlen Syndrome: speculation concerning immune system dysfunction

This study investigated the biological basis of visual processing disabilities in adults with Chronic Fatigue Syndrome. The study involved 61 adults with symptoms of Chronic Fatigue Syndrome who were screened for visual processing problems (Irlen Syndrome) and divided into two groups according to the severity of symptoms of Irlen Syndrome. Significant variations were identified in blood lipids and urine amino and organic acids of the two groups, which may be indicative of activation of the immune system due to some infective agent. It was suggested that metabolic profiling may help the development of more valid diagnostic categories and allow more investigation of immune system dysfunction as a possible causal factor in a range of learning and behaviour disorders.

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.

All developmental dyslexic subtypes display an elevated motion coherence threshold

PURPOSE

Psychophysical studies indicate that many dyslexics have a motion-processing deficit. The purpose of this study was to determine whether elevated motion coherence thresholds correlate with the specific dyslexic subtypes as defined by the Boder classification scheme.

METHODS

Twenty-one dyslexics (seven dyseidetics, six dysphonetics, and eight dysphoneidetics) and 19 age- and gender-matched controls participated in the study. The dyslexics were identified by an exclusionary approach and then subtyped with the Adult Dyslexia Test or the Dyslexia Determination Test. Motion coherence thresholds were determined with random dot kinematograms composed of signal dots and noise dots. Signal dots moved either left or right on each trial, whereas noise dots moved in random directions. The percentage of dots that comprised the signal was varied randomly on each trial (0 to 21% in 3% increments). Subjects guessed the direction of signal dot motion on each trial (two-alternative forced-choice task). A 75% correct threshold was determined with a Weibull equation fit to the psychometric function.

RESULTS

All three dyslexic subtypes had elevated motion coherence thresholds (t-test; dyseidetics p = 0.01, dysphonetics p = 0.039, dysphoneidetics p = 0.048).

CONCLUSION

Motion-coherence deficits are not correlated with a specific dyslexic subtype, but, rather, are common to all subtypes. However, some individuals in each of the dyslexic subtypes were found to have normal motion coherence thresholds, suggesting that other factors must be considered to predict the motion sensitivity deficits found in dyslexia.

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.

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.

Visual search performance in dyslexia

According to the magnocellular theory of dyslexia, otherwise intelligent children may fail to learn to read because of abnormalities in the magnocellular layers of the lateral geniculate nucleus (mLGN). If this were the case, one would predict that dyslexic subjects who show a deficit on low-level psychophysical tasks which tax the magnocellular system would also have deficits on higher-level visual tasks which do not rely on the properties of mLGN cells but depend upon the functioning of areas whose main inputs originate in the mLGN. In other words, magnocellular deficits should be traceable at later stages of visual processing. One area where such later processing is thought to occur is the posterior parietal cortex, damage to which impairs function on some classes of visual search. To test this hypothesis, we tested two groups of dyslexic subjects and a group of non-dyslexic controls on a range of visual search tasks. One group of dyslexic subjects had elevated motion coherence thresholds, a sign of deficits at the early levels (e.g. mLGN) of visual processing, and the other group had normal motion coherence thresholds. If the magnocellular deficits extended to the parietal cortex, it follows that the subjects with elevated motion coherence thresholds should have deficit in visual search, whereas those with normal motion coherence thresholds should not. The dyslexics with a motion coherence deficit were also impaired on serial visual search tasks but not on a parallel search. The dyslexics with normal motion coherence performance were unimpaired on visual search. The deficit was expressed as an elevation in reaction times, but there was no difference between the groups either in error rates or in the way the tasks were ranked according to difficulty. The results suggest that those dyslexics who have visual problems related to magnocellular functions also have visual-attentional problems related to the functions of areas such as the parietal cortex, which are dominated by inputs originating in the magnocellular LGN.

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.

Large repulsion, but not attraction, tilt illusions occur when stimulus parameters selectively favour either transient (M-like) or sustained (P-like) mechanisms

A vertical test grating appears tilted away from a surrounding inducing grating which is 15 degrees from vertical (repulsion effect) but towards an inducer 75 degrees from vertical (attraction effect). This is the tilt illusion (TI) and similar effects occur when inducing and test stimuli are presented successively (tilt after-effect or TAE). When it was reported [Wolfe, J. (1984). Vision Research, 24, 1959-1964] that large repulsion TAEs occurred with short test flashes, Wolfe postulated that either there are distinct mechanisms which process brief and longer duration stimuli; or that there are distinct mechanisms which are not primarily concerned with duration but are differentially responsive to temporal parameters, amongst several others. Other evidence that TI attraction effects are not modulated by test flash duration resulted in an hypothesis that repulsion and attraction effects are mediated by transient and sustained mechanisms, respectively [Wenderoth, P., van der Zwan, R., & Johnstone, S. (1989). Perception, 18, 715-728]. We demonstrate that large repulsion TIs can be induced when parameters other than duration are manipulated, including contrast and spatial frequency but that these parameters fail to modulate attraction TIs. These results are consistent with some previous hypotheses regarding the origin of repulsion and attraction effects and with Wolfe's latter hypothesis but do not support the view that the two effects are processed, respectively, by transient and sustained mechanisms.

Neuronal asymmetries in primary visual cortex of dyslexic and nondyslexic brains

Dyslexic brains exhibit histologic changes in the magnocellular (magno) cells of the lateral geniculate nucleus, and consistent with these changes, dyslexics demonstrate abnormal visually evoked potentials and brain activation to magno-specific stimuli. The current study was aimed at determining whether these findings were associated with changes in the primary visual cortex with the prediction that magno components of this cortex would be affected. We measured cross-sectional neuronal areas in primary visual cortex (area 17) in dyslexic and nondyslexic autopsy specimens. There was a significant interaction between hemispheres and diagnostic category; ie, nondyslexic brains had larger neurons in the left hemisphere, whereas dyslexic brains had no asymmetry. On the other hand, cell layers associated with magno input from the lateral geniculate nucleus did not show consistent changes in dyslexic brains. Thus, there is a neuronal size asymmetry in favor of the left primary visual cortex in nondyslexics that is absent in dyslexic brains. This is yet another example of anomalous expression of cerebral asymmetry in dyslexia similar to that of the planum temporale, which in our view reflects abnormality in circuits involved in reading.

Spatio-temporal contrast sensitivity, coherent motion, and visible persistence in developmental dyslexia

Three experiments measured spatio-temporal contrast sensitivity, coherent motion, and visible persistence in a single group of children with developmental dyslexia and a matched control group. The findings were consistent with a transient channel disorder in the dyslexic group which showed a reduction in contrast sensitivity at low spatial frequencies, a significant reduction in sensitivity for coherent motion, and a significantly longer duration of visible persistence. The results were also examined by classifying the dyslexic group into dyseidetic, dysphonetic, and mixed (dysphoneidetic) subgroups. There were no differences between the control and dyseidetic groups in contrast sensitivity, in coherent motion and in visible persistence. In comparison to the control group, the mixed (dysphoneidetic) dyslexic subgroup was found to have a significant reduction in contrast sensitivity at low spatial frequencies, a significant reduction in sensitivity for coherent motion, and a significantly longer duration of visible persistence. In comparison to the control group, the dysphonetic group only showed a reduction in contrast sensitivity at low spatial frequencies. Comparisons between the dyseidetic, dysphonetic and mixed dyslexic subgroups showed that there were no substantive differences in contrast sensitivity, coherent motion, and visible persistence. The results support the proposal and findings by Borsting et al. (Borsting E, Ridder WH, Dudeck K, Kelley C, Matsui L, Motoyama J. Vis Res 1996;36:1047-1053) that a transient channel disorder may only be present in a dysphoneidetic dyslexic subgroup. Psychometric assessment revealed that all the children with dyslexia appear to have a concurrent disorder in phonological coding, temporal order processing, and short-term memory.