Brain, Vol 121, Issue 6 1117-1131, Copyright © 1998 by Oxford University Press
JJ Barton, M Behrmann and S Black
We examined ocular fixations during line bisection in five patients with
left hemianopia, two patients with right hemianopia, nine patients with
left hemi-neglect and nine normal control subjects. Compared with measures
in control subjects, the median fixation, and left- and rightmost fixations
were shifted contralaterally in patients with hemianopia alone and
ipsilaterally in patients with hemi-neglect. The fixation with the longest
duration and the bisection point were also shifted contralaterally with
hemianopia and ipsilaterally with hemi- neglect. However, the number of
fixations and the spatial range spanned by fixations did not differ between
the groups, showing that ocular exploration was not truncated in any group.
Only some patients showed a previously reported directional search bias.
Overall, there was no directional bias in saccadic number or amplitude. The
distribution of fixations was most dense at the centre of the line in
normal subjects, while hemianopic patients fixated most frequently at the
ends of lines in their contralateral (blind) hemispace and at a central
locus that was biased slightly contralaterally, as was their bisection
judgement. This contralateral bias may reflect either an adaptive
contralateral attentional gradient or a non-veridical spatial
representation within the remaining normal hemifield. Hemi-neglect patients
had a broad distribution of fixation peaks in the ipsilateral hemispace. Of
two hemi-neglect patients with many fixations, one clustered fixations at a
position right of centre, as if a normal fixation pattern was shifted
rightward, while the other had two fixation peaks: one to the far right and
the other near the centre of the line, reminiscent of the dual peaks of
activity seen in some recent hemi-neglect models. These data reveal a
heterogeneity in the routes by which right-biased judgements of spatial
centre are reached by hemi-neglect patients.
ARTICLES
Ocular search during line bisection. The effects of hemi-neglect and hemianopia
Department of Neurology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215, USA.
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