EYE-MOVEMENT PATTERNS IN HEMIANOPIC DYSLEXIA

Homonymous parafoveal field loss impairs reading at the visual-sensory level. To elucidate the role of parafoveal visual field in reading, r eading eye movements were recorded, by means of an infra-red registrat ion technique, in 50 patients with homonymous hemianopia and visual fi eld sparing ranging from 1 degrees to 5 degrees; for comparison, a gro up of 25 normal subjects was studied. The degree of reading impairment in patients was found to depend on the extent of visual field sparing . Patients with right-sided loss of parafoveal visual field were more impaired than patients with left-sided loss. Eye movement reading patt erns paralleled this observation. Left-sided field loss mainly impairs return eye movements to the beginning of a line, while right-sided fi eld loss is characteristically associated with prolonged fixations tim es, reduced amplitudes of saccades to the right and many regressive sa ccades. The analysis of the durations of fixations, and the amplitudes of saccadic eye shifts to the right, and their mutual dependencies, s uggests that the perceptual window('reading span') is altered: its spa tial size is reduced, while its temporal extent is increased. The anal ysis of reading eye movements in 20 patients, who were treated for the ir hemianopic reading disorder revealed, in part, a normalization of t he eye movement pattern after treatment, indicating that the lost para foveal field region can be successfully substituted by oculomotor adap tation. Our observations underline the importance of the parafoveal vi sual field for reading and support the hypothesis of a serial interpla y between sensory-perceptual and cognitive factors in reading. Further more, reading eye movements appear to be guided primarily by parafovea l information processing, however; eye movement patterns show relative plasticity with respect to 'local' adaptation when the parafoveal fie ld region is lost. This adaptation can best be explained by a change o f the perceptual window which appears to be guided mainly by top-down influences. As to the brain lesion which may be responsible for the la ck of effective oculomotor compensation, damage to the occipital white matter appears the most crucial condition because it may disconnect v isual cortical areas, and interrupt subcortical-cortical connections w hich are part of a neural network subserving directed visual attention and associated saccadic eye shifts.