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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.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.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 that disabled readers have a deficient transient system.On the other hand, a fourth series of experiments investigated sustained system functioning in the two groups and failed to show any differences between them. It is argued that the existing confusion over visual deficits in disabled readers may be related to our findings of a transient but not a sustained system deficit. Further analyses indicate that these differences do not result from failing at reading, are present before reading instruction commences and occur in approximately 75% of subjects tested. An attempt is made to relate these findings to existing classifications of sub-types of specific reading disability. Some discussion of how a transient system deficit may influence the reading process is offered. b.c.Requests for reprints should be sent to

Contrast thresholds for sine-wave gratings of spatial frequencies of 2, 4, 12, and 16 cycles per degree were determined for normal and disabled readers at a range of stimulus durations. Normal readers demonstrated monotonically decreasing sensitivity with increasing spatial frequency at exposure durations between 40 and 100 milliseconds. At exposure durations of 150 to 1000 milliseconds, they showed peak sensitivity at 4 cycles per degree. In comparison, disabled readers showed monotonically decreasing sensitivity with increasing spatial frequency at all stimulus durations. The difference in sensitivity pattern across spatial frequencies was greatest at stimulus durations approximately equal to fixation durations during reading.

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.

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.