The effects of spatial filtering and contrast reduction on visual search times in good and poor readers

From intermediate shape-centered representations to the perception of oriented shapes: response to commentaries

In this response paper, we start by addressing the main points made by the commentators on the target article's main theoretical conclusions: the existence and characteristics of the intermediate shape-centered representations (ISCRs) in the visual system, their emergence from edge detection mechanisms operating on different types of visual properties, and how they are eventually reunited in higher order frames of reference underlying conscious visual perception. We also address the much-commented issue of the possible neural mechanisms of the ISCRs. In the final section, we address more specific and general comments, questions, and suggestions which, albeit very interesting, were less directly focused on the main conclusions of the target paper.

The Measurement of Eye Movements in Mild Traumatic Brain Injury: A Structured Review of an Emerging Area

Mild traumatic brain injury (mTBI), or concussion, occurs following a direct or indirect force to the head that causes a change in brain function. Many neurological signs and symptoms of mTBI can be subtle and transient, and some can persist beyond the usual recovery timeframe, such as balance, cognitive or sensory disturbance that may pre-dispose to further injury in the future. There is currently no accepted definition or diagnostic criteria for mTBI and therefore no single assessment has been developed or accepted as being able to identify those with an mTBI. Eye-movement assessment may be useful, as specific eye-movements and their metrics can be attributed to specific brain regions or functions, and eye-movement involves a multitude of brain regions. Recently, research has focused on quantitative eye-movement assessments using eye-tracking technology for diagnosis and monitoring symptoms of an mTBI. However, the approaches taken to objectively measure eye-movements varies with respect to instrumentation, protocols and recognition of factors that may influence results, such as cognitive function or basic visual function. This review aimed to examine previous work that has measured eye-movements within those with mTBI to inform the development of robust or standardized testing protocols. Medline/PubMed, CINAHL, PsychInfo and Scopus databases were searched. Twenty-two articles met inclusion/exclusion criteria and were reviewed, which examined saccades, smooth pursuits, fixations and nystagmus in mTBI compared to controls. Current methodologies for data collection, analysis and interpretation from eye-tracking technology in individuals following an mTBI are discussed. In brief, a wide range of eye-movement instruments and outcome measures were reported, but validity and reliability of devices and metrics were insufficiently reported across studies. Interpretation of outcomes was complicated by poor study reporting of demographics, mTBI-related features (e.g., time since injury), and few studies considered the influence that cognitive or visual functions may have on eye-movements. The reviewed evidence suggests that eye-movements are impaired in mTBI, but future research is required to accurately and robustly establish findings. Standardization and reporting of eye-movement instruments, data collection procedures, processing algorithms and analysis methods are required. Recommendations also include comprehensive reporting of demographics, mTBI-related features, and confounding variables.

On the use of spatial frequency to isolate contributions from the magnocellular and parvocellular systems and the dorsal and ventral cortical streams

Many authors have claimed that suprathreshold achromatic stimuli of low and high spatial frequency can be used to separate responses from different entities in the visual system. Most prominently, it has been proposed that such stimuli can differentiate responses from the magnocellular and parvocellular systems. As is reviewed here, investigators who have examined stimulus specificity of neurons in these systems have found little difference between magno- and parvocellular cells. It has also been proposed that spatial frequency can be used to selectively activate the "magnocellular-dorsal stream". The present review indicates that cells in Area MT of the dorsal stream do prefer very low spatial frequencies. However, the review also shows that cells in Area V4 of the ventral stream respond, not only to relatively high spatial frequencies, but also to low frequency stimuli. Thus, low spatial frequencies cannot be relied upon to selectively activate the dorsal stream.

Use of the Hermann grid illusion in the measurement of contrast perception in dyslexia

We measured contrast thresholds for perception of the Hermann grid illusion, using different contrast polarities and mean luminances, in dyslexics and non-dyslexics. Both groups of subjects gave significantly lower thresholds with grids having dark squares and light paths, but there was no significant threshold difference between groups. Perceived strength of illusion was also measured in grids at suprathreshold contrast levels. Dyslexics perceived the illusion to be significantly stronger than non-dyslexics when the grid had light paths and low luminance.

Developmental disorders of vision

This review of developmental disorders of vision focuses on only a few of the many disorders that disrupt visual development. Given the enormity of the human visual system in the primate brain and complexity of visual development, however, there are likely hundreds or thousands of types of disorders affecting high-level vision. The rapid progress seen in developmental dyslexia and WMS demonstrates the possibilities and difficulties inherent in researching such disorders, and the authors hope that similar progress will be made for congenital prosopagnosia and other disorders in the near future.

Coloured overlays and their effects on reading speed: a review

Coloured overlays can reduce symptoms of visual stress and improve reading speed. These benefits are reliable and are not attributable simply to placebo effects. Five percent of children in mainstream education read at least 25% more quickly with an overlay, provided they have chosen the colour. The suboptimal design of children's text and the high level of classroom lighting may be partly responsible.

The effect of contrast on reading speed in dyslexia

Contrast coding has been reported to differ between dyslexic and normal readers. Dyslexic readers require higher levels of contrast to detect sinewave gratings for certain spatiotemporal conditions, and dyslexic readers show faster visual search at low contrast. We investigated whether these differences in early contrast coding generalize to reading performance by measuring reading speed as a function of text contrast for dyslexic children and adults and for age-matched controls. Contrast affected reading performance of dyslexic and normal readers similarly. For both groups, reading speed was relatively constant between 100 and 2% contrast, and decreased rapidly below 2% contrast. This pattern of results held true for both children and adults, for text with and without sentence context, across a range of character sizes, and for reading aloud and reading silently. We conclude that earlier findings of group differences in contrast effects on grating detection or visual search tasks do not generalize to reading.

Magnocellular visual function and children's single word reading

Recent research has shown that reading disabled children find it unusually difficult to detect flickering or moving visual stimuli, consistent with impaired processing in the magnocellular visual stream. Yet, it remains controversial to suggest that reduced visual sensitivity of this kind might affect children's reading. Here we suggest that when children read, impaired magnocellular function may degrade information about where letters are positioned with respect to each other, leading to reading errors which contain sounds not represented in the printed word. We call these orthographically inconsistent nonsense errors "letter" errors. To test this idea we assessed magnocellular function in a sample of 58 unselected children by using a coherent motion detection task. We then gave these children a single word reading task and found that their "letter" errors were best explained by independent contributions from motion detection (i.e., magnocellular function) and phonological awareness (assessed by a spoonerism task). This result held even when chronological age, reading ability, and IQ were controlled for. These findings suggest that impaired magnocellular visual function, as well as phonological deficits may affect how children read.

Psychophysics of reading--XVI. The visual span in normal and low vision

The visual span in reading is the number of characters that can be recognized at a glance. The shrinking visual span hypothesis attributes reading deficits in low vision, and slow reading in normal vision at low contrast, to a reduction in the visual span. This hypothesis predicts that reading time (msec/word) becomes increasingly dependent on word length as text contrast decreases. We tested and confirmed this prediction using the rapid serial visual presentation (RSVP) method. Estimates of the visual span ranged from about 10 characters for high-contrast text to less than two characters for low-contrast text. Eye-movement recordings showed that longer reading times at low contrast are partitioned about equally between prolonged fixation times and an increased number of saccades (presumably related to a reduced visual span). RSVP measurements for six out of seven low-vision subjects revealed a strong dependence of reading time on word length, as expected from reduced visual spans.

Analysis of perceptual confusions between nine sets of consonant-vowel sounds in normal and dyslexic adults

It is widely accepted that most developmental dyslexics perform poorly on tasks which assess phonological awareness. One reason for this association might be that the early or "input" phonological representations of speech sounds are distorted or noisy in some way. We have attempted to test this hypothesis directly. In Experiment 1, we measured the confusions that adult dyslexics and controls made when they listened to nine randomly presented consonant-vowel (CV) segments [sequence: see text] under four conditions of increasing white noise masking. Subjects could replay stimuli and were under no obligation to respond quickly. Responses were selected with a computer mouse from a set of nine letter-strings, corresponding to the auditory stimuli, presented on a VDU. While the overall pattern of confusions made by dyslexics and controls was very similar for this stimulus set, dyslexics confused [sequence: see text] significantly more than did controls. In Experiment 2, subjects heard each stimulus once only and were forced to respond as quickly as possible. Under these timed conditions, the pattern of confusions made by dyslexics and controls was the same as before, but dyslexics took longer to respond than controls. The slower responses of dyslexics in Experiment 2 could have arisen because: (a) they were slower at processing the auditory stimuli than controls, (b) they had worse visual pattern memory for letter strings than controls, (c) they were slower than controls at using the computer mouse. In Experiments 3, 4 and 5 subjects carried out control tasks which eliminated each of these possibilities and confirmed that the results from the auditory tasks genuinely reflected subjects' speech perception. We propose that the fine structure of dyslexics' input phonological representations should be further explored with this confusion paradigm by using other speech sounds containing VCs, CCVs and VCCs.

On a failure to replicate: methodologically close, but not close enough. A response to hogben et al

Williams, Brannan and Lartigue (1987) (Clinical Vision Science, 1, 367-371) reported that poor readers took significantly longer to search letter arrays for a target than did good readers. In addition, they reported that blurring the letter arrays leads to faster search times for poor readers and a loss of the significant differences between the groups seen with unblurred displays. In a recent attempt to replicate these findings, Hogben et al. (1996) (Vision Research, 36, 1503-1507) found no differences in search rates between good and poor readers using unblurred arrays, and no differences in search rate between the groups when blurred arrays were used. In the present article, we have compared these two research efforts, and a third paper on the same topic, with regard to methodological factors in an attempt to understand how these two different results could occur. It is our belief that the letter spacing employed in the two studies may account for the difference and should be the focus of future studies of the original effect.