Posterior Parietal Visual Dysfunction: An Explanatory Review

Abstract

Dysfunction or injury of the posterior parietal lobes impairs 3D mapping of the visual scene and can… (1) Make body movement guided by vision inaccurate (optic ataxia). (2) Limit the number of items seen at a glance, due to simultanagnostic visual dysfunction. (3) Cause inability to move the eyes to a specified target (apraxia of gaze), despite intact pursuit and fast (saccadic) eye movements. (Perhaps because the target cannot be located.) When severe, these features comprise Balint syndrome, but when less marked, the term dorsal stream dysfunction has been used. Associated lower visual field impairment due to the superior optic radiations passing through the parietal lobes being affected as well, can cause the feet not to be seen during locomotion. While accompanying inability to see fast moving targets, or dyskinetopsia is not uncommon. In our experience, a wider inflight gap between the fingers and thumb of a hand reaching in the lower visual field than when reaching in the upper visual field, and disability locating where sounds are coming from, may also be evident. Central and peripheral visual functions can be diminished if the occipital lobes are affected as well, while recognition can be impaired by this and / or associated temporal lobe dysfunction. Those with accompanying intellectual impairment and four-limb cerebral palsy may ‘wake up’ and start to look around, even reaching out for a single object, when surrounded by a monochromatic tent that excludes extraneous visual and auditory distraction. In the authors’ experience, dorsal stream dysfunction is the commonest pattern of cerebral visual impairment seen in children and in many affected (often previously undiagnosed) adults. A series of case history data is presented to illustrate the origin, nature and heterogeneity of this condition, as well as its potential management.