The end of the line for a brain-damaged model of unilateral neglect
For over a century, it has been known that damage to the right hemisphere of
the brain can cause patients to be unaware of the contralesional side of
space. This condition, known as unilateral neglect, represents a
collection of clinically related spatial disorders characterized by the
failure in free vision to respond, explore, or orient to stimuli predominantly
located on the side of space opposite the damaged hemisphere. Recent studies
using the simple task of line bisection, a conventional diagnostic test,
have proved surprisingly revealing with respect to the spatial and attentional
impairments involved in neglect. In line bisection, the patient is asked
to mark the midpoint of a thin horizontal line on a sheet of paper. Neglect
patients generally transect far to the right of the center. Extensive studies
of line bisection have been conducted, manipulating--among other
factors--line length, orientation, and
position. We have simulated the pattern of results using an existing
computational model of visual perception and selective attention called
MORSEL (Mozer, 1991). MORSEL has already been used to model data in a
related disorder, neglect dyslexia (Mozer & Behrmann, 1990). In this earlier
work, MORSEL was "lesioned" in accordance with the damage we suppose to
have occurred in the brains of neglect patients. The same model and lesion
can simulate the detailed pattern of performance on line bisection, including
the following observations: (1) no consistent across-subject bias is found in
normals; (2) transection displacements are proportional to line length in
neglect patients; (3) variability of displacements are proportional to line
length, in both normals and patients; (4) position of the lines with respect
to the body or the page on which they are drawn has little effect; (5) for
lines drawn at different orientations, displacements are proportional to the
cosine of the orientation angle. MORSEL fails to account for one observation:
across patients, the variability of displacements for a particular line length
is roughly proportional to mean displacement. Nonetheless, the overall fit of
the model is sufficiently good that we believe MORSEL can be used as a
diagnostic tool to characterize the specific nature of a patient's deficit,
and thereby has potential down the line in therapy.
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