FUNCTIONAL MAGNETIC-RESONANCE-IMAGING OF EARLY VISUAL PATHWAYS IN DYSLEXIA

We measured brain activity, perceptual thresholds, and reading perform ance in a group of dyslexic and normal readers to test the hypothesis that dyslexia is associated with an abnormality in the magnocellular ( M) pathway of the early visual system. Functional magnetic resonance i maging (fMRI) was used to measure brain activity in conditions designe d to preferentially stimulate the M pathway. Speed discrimination thre sholds, which measure the minimal increase in stimulus speed that is j ust noticeable, were acquired in a paradigm modeled after a previous s tudy of M pathway-lesioned monkeys. Dyslexics showed reduced brain act ivity compared with controls both in primary visual cortex (V1) and in several extrastriate areas, including area MT and adjacent motion-sen sitive areas (MT+) that are believed to receive a predominant M pathwa y input. There was a strong three-way correlation between brain activi ty, speed discrimination thresholds, and reading speed. Subjects with higher V1 and MT; responses had lower perceptual thresholds (better pe rformance) and were faster readers. These results support the hypothes is for an M pathway abnormality in dyslexia and imply strong relations hips between the integrity of the M pathway, visual motion perception, and reading ability.