Eye movement patterns in hemianopic dyslexia.

Abstract

Homonymous parafoveal field loss impairs reading at the visual-sensory level. To elucidate the role of parafoveal visual field in reading, reading eye movements were recorded, by means of an infra-red registration technique, in 50 patients with homonymous hemianopia and visual field sparing ranging from 1 degree to 5 degrees; for comparison, a group 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 patterns paralleled this observation. Left-sided field loss mainly impairs return eye movements to the beginning of a line, while right-sided field loss is characteristically associated with prolonged fixations times, reduced amplitudes of saccades to the right, and many regressive saccades. The analysis of the durations of fixations, and the amplitudes of saccadic eye shifts to the right, and their mutual dependencies, suggests that the perceptual window ('reading span') is altered: its spatial size is reduced, while its temporal extent is increased. The analysis of reading eye movements in 20 patients, who were treated for their hemianopic reading disorder, revealed, in part, a normalization of the eye movement pattern after treatment, indicating that the lost parafoveal field region can be successfully substituted by oculomotor adaptation. Our observations underline the importance of the parafoveal visual field for reading and support the hypothesis of a serial interplay between sensory-perceptual and cognitive factors in reading. Furthermore, reading eye movements appear to be guided primarily by parafoveal information processing; however, eye movement patterns show relative plasticity with respect to 'local' adaptation when the parafoveal field region is lost. The adaptation can best be explained by a change of the perceptual window which appears to be guided mainly by top-down influences. As to the brain lesion which may be responsible for the lack of effective oculomotor compensation, damage to the occipital white matter appears the most crucial condition because it may disconnect visual cortical areas, and interrupt subcortical-cortical connections which are part of a neural network subserving directed visual attention and associated saccadic eye shifts.