The aim of the first of two experiments was to investigate the effect of practice on sensitivity to global motion and global form in a group of adults with dyslexia, a group of normal readers with visual discomfort, a group with dyslexia and visual discomfort, and a control group. In comparison to the control group, and regardless of the effect of practice, the group with dyslexia was significantly less sensitive to global motion. No differences in global motion sensitivity were found when individuals with dyslexia, with or without visual discomfort, were compared. The normal reading group with visual discomfort was less sensitive to global motion than the control group at baseline, but not when a second estimate of motion sensitivity was obtained. About 30% of the group with dyslexia had a global motion deficit on each threshold estimate. In contrast, there were no significant effects of practice or group on sensitivity to global form. In Experiment 2, performance on a number of cognitive and visual processing tasks was measured in four groups: two with dyslexia; one with and one without a global motion deficit, a normal reading group with visual discomfort, and a control group. The group with visual discomfort had reduced visual processing speed only. Regardless of whether a global motion processing deficit was present or absent in individuals with dyslexia, reduced accuracy was found on the language and visual processing measures, and on a rapid temporal sequencing task. Individuals with dyslexia and a global motion deficit had poorer accuracy than individuals with dyslexia and no motion deficit on the phonological processing and verbal short term memory tasks. We concluded that some adults with dyslexia have a persistent deficit when processing global motion but not global form. This is consistent with reports of a magnocellular pathway deficit in this group. Individuals with visual discomfort do not have a magnocellular processing deficit, but have perceptual difficulties when performing complex visual processing tasks. (Contains 5 figures and 4 tables.)