Visual search performance in dyslexia

Abstract

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.

Stimulus effects on cancellation task performance in children with and without dyslexia

“…In comparison with children without dyslexia, children with developmental dyslexia may have prolonged attention-dwell time, difficulty shifting attention during visual or auditory processing, and less focused attention on visual tasks. Attention deficits as such are related to poor visual search performance on visual tasks (Casco & Prunetti, 1996;Facoetti, Lorusso, Paganoni, Umiltà, & Mascetti, 2003;Iles et al, 2000;Sireteanu, Goebel, Goertz, & Wandert, 2006). Monitoring children's directed attention would help to identify the visual processing strategies that they use when comprehending their surroundings and reading materials.…”

“…Monitoring children's directed attention would help to identify the visual processing strategies that they use when comprehending their surroundings and reading materials. Various studies on visual attention have compared children with and without dyslexia by using different methods, such as visual scanning, cancellation tasks, and continuous performance tests (Hawelka & Wimmer, 2005;Iles et al, 2000;Taroyan, Nicolson, & Fawcett, 2007). Results have suggested that children with developmental dyslexia or attention deficits or both showed impaired performance in both their speed of and capacity for information processing.…”

“…Dyslexic children usually perform less accurately on visual attention tasks, and their reaction times are affected by size effects in conjunction search with serial processing (Casco & Prunetti, 1996;Casco et al, 1998;Iles et al, 2000;Shalev & Tsal, 2003). Iles et al (2000) used tasks with different levels of similarity between targets and distractors, and different numbers of distractors in their experiments.…”

“…Iles et al (2000) used tasks with different levels of similarity between targets and distractors, and different numbers of distractors in their experiments. They concluded that dyslexic children with early visual processing deficits also have visual attention problems related to parietal cortex dysfunction.…”

“…For example, it is more challenging for a patient with the perceptual deficit of neglect to detect letters than lines, and complex shapes than letters, as well as to search targets embedded in random arrays rather than in structured arrays. The challenge of perceptual discrimination is a major determinant of the level of task difficulty, one that also increases the need for attentional control (Geldmacher, 1996;Iles, Walsh, & Richardson, 2000;Mesulam, 2000).…”