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Abstract:
Patients with unilateral brain damage leading to neglect tend
to bisect horizontal lines incorrectly, such that their mark is
displaced towards the ipsilesional side. It has recently been shown
(Halligan and Marshall, Neuropsychologia, 1994) that adding a
vertical line on the ipsilesional end of the horizontal line may
reduce these bisection errors. This appears surprising, since one
would expect that salient information on the ipsilesional side
might bias attention in an ipsilesional direction, and even
increase the bisection errors. One possible explanation is that the
vertical line provides an estimate of the length of the horizontal
line. In order to test this, we asked patients to bisect line that
could have vertical lines added to either the ipsilesional or the
contralesional side. When the ipsilesional vertical line was always
the same length as the horizontal line that had to be bisected,
bisection errors were greatly reduced. However, this improvement
was much less when the vertical lines had a variable and
unpredictable relation to the horizontal lines. These results
suggest that line bisection errors may be reduced by providing
patients with a reliable estimate of the line length by a vertical
line that is always the same length.
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