2 - The Neural Control of Eye Movements in Acquired and Developmental Reading Disorders

Abstract

This chapter reviews the characteristics of the normal reader's eye movements during reading, abnormal eye movements and the relationship of different types of dyslexia to eye movement patterns. Developmental dyslexia is referred to as a specific reading disability in which the child has normal intelligence and is at least two years behind expected grade level in reading, has normal sensory acuity, and is without neurological damage and emotional problems. The chapter further discusses the types of eye movement disorders that result from lesions of the central nervous system and illustrates how these abnormalities affect reading. The chapter additionally presents eye movement data from two patients with eye movement disorders. It is shown that disordered efferent oculomotor control causes severe and insuperable problems. During reading, the eyes make a series of saccadic eye movements, generally in a left-to-right direction. The term “saccade” is used to distinguish this rapid, jerky type of eye movement, separated by fixational pauses, from pursuit or smooth tracking movements in which the eyes move slowly, maintaining fixation on a moving target or on a stationary point while the head moves. There are three types of saccadic eye movement disorders that are known to influence reading behavior: paralytic or slow saccades that result from lesions involving the basal ganglia, brain stem, and cerebral cortex; impaired saccadic initiation that results from acquired and congenital apraxias involving the left and right parietal lobe; and dysmetric saccades, which result from lesions involving the cerebellum and cerebellar pathways.