William Lovegrove

A theoretical and experimental case for a visual deficit in specific reading disability

Abstract We present three lines of evidence indicating a low-level visual deficit in a large percentage of specifically-disabled readers. This research is presented within the theoretical framework of spatial frequency analysis, in particular, the framework of transient and sustained subsystems. a. Measures of visible-persistence duration as a function of spatial frequency in normal and specifically-disabled readers show that disabled readers have a different pattern of temporal processing across spatial frequencies. These differences disappear when transient system activity is reduced. b. On measures of pattern-contrast sensitivity specifically-disabled readers are less sensitive than controls at low spatial frequencies, but equal or more sensitive at high spatial frequencies. c. On measures of temporal-contrast sensitivity disabled readers are less sensitive than controls at all flicker rates with the greatest differences being at the higher temporal frequencies. These three sources of evidence indicate...

Reading disability: Spatial frequency specific deficits in visual information store.

Abstract Visual information store durations for sine-wave gratings were measured in normal and disabled readers. At low spatial frequencies, disabled readers have significantly longer store durations; at medium spatial frequencies, the two groups do not differ; and at high spatial frequencies, disabled readers have shorter store durations.

Flicker Contrast Sensitivity in Normal and Specifically Disabled Readers

Temporal contrast sensitivity for counterphase flicker was determined for specifically disabled and normal readers to investigate whether the two groups differ in the functioning of their transient systems. In experiment 1, temporal contrast sensitivity was measured over a range of temporal frequencies with a spatial frequency of 2 cycles deg−1. Disabled readers were less sensitive than the control subjects at all temporal frequencies. In experiment 2, temporal contrast sensitivity was measured at a temporal frequency of 20 Hz over a range of spatial frequencies. Disabled readers were less sensitive than the controls at all spatial frequencies, with the differences between the groups increasing as spatial frequency increased. Both these findings are interpreted as supporting the hypothesis of a transient-system deficit in the visual systems of disabled readers.

Contrast sensitivity functions and specific reading disability

Contrast sensitivity functions for normal and specifically disabled readers were measured in two experiments. Each study showed that specifically disabled readers and controls differ in the pattern of relative sensitivity across spatial frequencies. Both studies provide evidence of differences between normal and disabled readers on measures of visual mechanisms fundamental to the reading process.

The effects of field size and luminance on contrast sensitivity differences between specifically reading disabled and normal children

Contrast sensitivity functions were measured for normal and specifically disabled readers in experiments which varied either field size or luminance level. The nature of the differences between groups was unaffected by field size, implying that spatial summation in specifically disabled readers is normal. Increasing luminance, however, increased the range over which disabled readers are less sensitive than normal readers. The results are discussed in terms of possible differences in underlying mechanisms.

Low-frequency filtering and the processing of local-global stimuli

The role of low-spatial-frequency information in the processing of global stimuli made up of local elements was examined. After selective removal of low spatial frequencies two major changes occurred in the pattern of results. First, response times to global stimuli were significantly slower and the usual speed advantage of global over local processing was lost. Second, when processing local features the usual decrease in response speed when the local and global letters are not the same (consistency effect) was not obtained. These effects could not be explained by changes in error rate, by contrast variation resulting from the process of filtering, or by loss of visual sensitivity due to greater eccentricity of global images.

The effect of spatial frequency and contrast on visual persistence

The visual persistence of sinusoidal gratings of varying spatial frequency and contrast was measured. It was found that the persistence of low-contrast gratings was longer than that of high-contrast stimuli for all spatial frequencies investigated. At higher contrast levels of 1 and 4 cycles deg−1 gratings, a tendency for persistence to be independent of contrast was observed. For 12 cycles deg−1 gratings, however, persistence continued to decrease with increasing contrast. These results are compared with recently published data on other temporal responses, and are discussed in terms of the different properties of sustained and transient channels.

The effect of stimulus duration on the persistence of gratings

The persistence of gratings varying in spatial frequency and exposure duration was measured using a stimulus-blank alternation method. Persistence was found to lengthen with increasing spatial frequency and to shorten with increasing exposure duration. For each spatial frequency, persistence decreased linearly with a slope of approximately —.75 as duration increased for short stimulus durations. For longer stimulus durations, the rate of decline in persistence with increasing duration was reduced, the slope being approximately —.13. The stimulus duration at which the change in slope of the persistence-duration relationship occurred was shown to increase with increasing spatial frequency and was approximately equivalent to the critical duration for each spatial frequency. The data were consistent with an interpretation of persistence in terms of a temporal integration component and a second, possibly cortically located, component.

Measuring Visual Discomfort

A two-parameter Rasch Rating Scale model was developed to measure visual discomfort. Initially it was found that participants reporting frequent severe headache, reading difficulties of a visual nature, and short effective reading times experienced more severe visual discomfort. The validity of this measurement instrument was tested in four experiments. In Experiments 1 and 2 reports of unpleasant somatic and perceptual side-effects or ratings of unpleasantness were obtained for low, moderate, and high scorers on the Visual Discomfort Scale. It was found that those with higher scores reported a greater number of unpleasant side-effects and rated square-wave patterns across the spatial frequency range, and a letter pattern as more unpleasant and distorted to view than those with low scores on the scale. In Experiments 3 and 4 these subjective findings were extended to performance. Efficiency was measured using a copying and reading task. It was found that those obtaining high scores on the Visual Discomfor...

Measures of phonological short-term memory and their relationship to vocabulary development

Vocabulary scores, word span, nonword repetition, rhyme detection, and articulation rate measures were obtained for a group of 30 preschool children with a mean age of 4;11. After allowing for age and intelligence, it was found that the best predictors of vocabulary were memory span, rhyme detection, and nonword repetition, accounting for 19%. 15%, and 13% of the variance, respectively. Of these children, 28 were tested again on the same measures 13 months later. At the later stage, memory span and rhyme scores again predicted vocabulary to a significant extent, but nonword repetition score did not. Of the measures taken initially, memory span and rhyme detection significantly predicted later vocabulary scores, whereas nonword repetition just failed to reach significance. Cross-lagged correlations showed (hat memory span and rhyme detection on the first test predicted later vocabulary after partialing out initial vocabulary scores. However, initial vocabulary scores did not predict later memory span (or rhyme detection) after partialing out the initial span (or rhyming) scores. Articulation rate was not significantly related to vocabulary at either age. The results are interpreted as indicating that the phonological store of working memory (required by word span, rhyme detection, and nonword repetition, but independent of articulation rates) contributes to vocabulary development.