The magnocellular theory is a prominent, albeit controversial view asserting that many reading disabled (RD) individuals suffer from a specific impairment within the visual magnocellular pathway. In order to assess the validity of this theory we tested its two basic predictions. The first is that a subpopulation of RD subjects will show impaired performance across a broad range of psychophysical tasks relying on magnocellular functions. The second is that this subpopulation will not be consistently impaired across tasks that do not rely on magnocellular functions. We defined a behavioural criterion for magnocellular function, which incorporates performance in flicker detection, detection of drifting gratings (at low spatial frequencies), speed discrimination and detection of coherent dot motion. We found that some RD subjects (six out of 30) had impaired magnocellular function. Nevertheless, these RD subjects were also consistently impaired on a broad range of other perceptual tasks. The performance of the other subgroup of RD subjects on magnocellular tasks did not differ from that of controls. However, they did show impaired performance in both visual and auditory non-magnocellular tasks requiring fine frequency discriminations. The stimuli used in these tasks were neither modulated in time nor briefly presented. We conclude that some RD subjects have generally impaired perceptual skills. Many RD subjects have more specific perceptual deficits; however, the "magnocellular" level of description did not capture the nature of the perceptual difficulties in any of the RD individuals assessed by us.
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Neuropsychological findings together with recent advances in neuroanatomical and neuroimaging techniques have spurred the investigation of cerebellar contributions to cognition. One cognitive process that has been the focus of much research is working memory, in particular its verbal component. Influenced by Baddeley's cognitive theory of working memory, cerebellar activation during verbal working memory tasks has been predominantly attributed to the cerebellum's involvement in an articulatory rehearsal network. Recent neuroimaging and neuropsychological findings are inconsistent with a simple motor view of the cerebellum's function in verbal working memory. The present article examines these findings and their implications for an articulatory rehearsal proposal of cerebellar function. Moving beyond cognitive theory, we propose two alternative explanations for cerebellar involvement in verbal working memory: Error-driven adjustment and internal timing. These general theories of cerebellar function have been successfully adapted from the motor literature to explain cognitive functions of the cerebellum. We argue that these theories may also provide a useful framework to understand the non-motor contributions of the cerebellum to verbal working memory.
Developmental dyslexia is a specific reading disability that affects 5-10% of the population. Recent studies have suggested that dyslexics may experience a deficit in the visual magnocellular pathway. The most extensively studied prediction deriving from this hypothesis is impaired contrast sensitivity to transient, low-luminance stimuli at low spatial frequencies. However, the findings are inconsistent across studies and even seemingly contradictory. In the present study, we administered several different paradigms for assessing temporal contrast sensitivity, and found both impaired and normal contrast sensitivity within the same group of dyslexic participants. Under sequential presentation, in a temporal forced choice paradigm, dyslexics showed impaired sensitivity to both drifting and flickering gratings. However, under simultaneous presentation, with a spatial forced choice paradigm, dyslexics' sensitivity did not differ from that of the controls. Within each paradigm, dyslexics' sensitivity was poorer at higher temporal frequencies, consistent with the magnocellular hypothesis. These results suggest that a basic perceptual impairment in dyslexics may be their limited ability to retain-and-compare perceptual traces across brief intervals.
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