While considerable progress has been made in the study of auditory and linguistic prerequisites for reading, we still know very little about which visual physical attributes of text are most critical for letter and word identification in young readers and what visual perceptual skill are necessary to develop the rapid processing needed for fluent reading. Reading begins as a visual task (for all but the blind). If the quality of visual information has been compromised by processing impairments in the visual system, subsequent linguistic analysis may be affected by the poverty of the cognitive visual representation. Linguistic impairments have been well documented for developmental dyslexic readers (Liberman, 1983), but many dyslexic studies also have reported visual perceptual deficits (Lovegrove, Martin, & Slaghuis, 1986; Williams & Lecluyse, 1990) as well as visual neuroantomical abnormalities (Livingstone, Rosen, Drislane, & Calaburda, 1991). This project proposes: 1) to learn how physical attributes, such as processing speed in the visual system or spatial frequency components, affect normal reading function; 2) to study how children's abilities to process these attributes change with age and reading ability; 3) to compare normal and dyslexic readers in the visual perceptual skills to determine how visual processes affect dyslexic reading performance; and 4) to build a computationally sophisticated and more biologically plausible model of letter and word recognition. These ideas will be explored over a five years through a series of cross- sectional studies. This project has stringent subject selection criteria, sophisticated design features, and well designed experiments to test specific hypotheses about the relationship of visual processes and reading development. Five groups of subjects will be studied: reading impaired (decompensated) adult dyslexics, compensated adult dyslexics, normal adult readers, normal child readers, and dyslexic children. Particular care has been taken to insure that the developmental groups of dyslexic and normal readers who are selected for study are representative of the larger school population from which they were drawn. The results of this project will further our understanding of the relationship between neurophysiological processes and cognitive functions with regard to reading development. In addition it may help to explain why some dyslexic adults have been able to compensate for their reading problems and others have not.