A study of visuomotor behaviour in normal and brain lesioned human subjects, with special reference to line bisection performance in patients with hemispatial neglect

In Experiments 1 to 8 an attempt was made to examine the nature of the displacements found in the traditional line bisection test when applied to normal (right-handed), as well as brain lesioned subjects. The problem with this test is that it invariably confounds perceptual and motor components which might both contribute to the observed errors. However, use of the 'landmark task' enables an examination of perceptual effects in isolation. It was found that five out of six neglect patients judged the left half-line of a centrally bisected line as shorter than the right half-line. Moreover, it was consistently shown that cueing strongly influenced judgements in normal and left and right hemisphere lesioned subjects (without neglect) in that it caused them to overestimate the cued part of the line. It was argued that the perception of relative size is subject to systematic distortion as a function of this selective attention within the visual field. Neglect patients may present an abnormal example of this attentionally- induced illusion in that their attentional resources may be abnormally biased towards the ipsilesional space. The result of this imbalance may be to cause, quite directly, a gross abnormality of size perception. Nonetheless one of the neglect patients did not show spatial misperception but spatially misdirected actions, in line with what has been described as directional hypokinesia. Experiments 9 to 12 were designed to demonstrate any possible contribution the right hemisphere might make to visuomotor control, but the data on normal subjects gave little indication of a specific right hemisphere involvement in such tasks. Neither use of a spatial bisection task, nor absence of visual feedback of the moving hand or arm seemed to produce left hand advantages on the dependent measures. On the other hand, RCVA patients proved to be impaired in their reaching behaviour in that they erred systematically to the right of the true target over all three spatial positions, in the absence of visual feedback. The bias was interpreted as a pure example of directional hypokinesia