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A Computational Model of Spatial Representations That Explains Object-Centered Neglect in Parietal Patients |
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@Article{Rao:CNS96,
author = {Rajesh P.N. Rao and Dana H. Ballard},
title = {A Computational Model of Spatial Representations That Explains Object-Centered Neglect in Parietal Patients},
journal = {Computational Neuroscience},
year = {1996},
introduction = {Patients with parietal cortex lesions exhibit unusual visual deficits, typically involving the neglect of a part of their
visual space. Without being consciously aware of it, they act as if that part of space is not visible. At first, it
was thought that this neglected space coincided with the visual hemifield contralateral to the lesioned hemisphere,
but more recently, an increasing number of experiments [1, 2, 5, 6, 7, 8, 11] have shown that in many cases, the
neglected part of space is related to a reference object of immediate interest. For example, in a recent experiment
by Behrmann et al. [3], a patient with a right parietal lobe lesion was asked to count the number of instances of
the letter ``A'' in a field of letters on a TV screen (see Figure 1). The eye movements recorded from the subjects
showed that they typically neglected to look at most of the ``A''s in the left side of the TV screen. Note that the
neglect cannot be explained as a visual hemifield neglect because, as the patient makes eye movements, the letters
that appear in each hemifield change.
We know that the observed behavior pertains to the object of immediate interest owing to another experiment
by Behrmann et al. [2]. In this case, the patient gazes at a dumbbellshaped object consisting of two circles (one
red and one blue) joined by a horizontal bar. The patient is asked to press a button when a target (a small white
circle) appears within either the left circle or the right. As expected, the response to targets in the contralesional
circle (opposite side of the lesion) is typically much slower. Now the patient sees the dumbbell slowly rotate 180
degrees clockwise about its midpoint so that the colored circles have now exchanged positions. When the same
test is repeated using the rotated dumbbell, the patient is now much slower with respect the ipsilesional circle
(same side as the lesion). This result is explained if the patient assigns an object centered frame to the dumbbell
initially and maintains that frame throughout the experiment. The neglect is thus consistently in objectcentered
coordinates. Other experiments [6, 7] have demonstrated similar results.
While neglect in objectcentered coordinates is easy to understand abstractly, it is much more difficult to
explain how the brain's object recognition machinery could be organized to explain such results. The purpose
of this paper is to describe a systemslevel model of spatial representations in the parietal cortex, and show by
simulations that it can explain the above experimental data in a concise manner.}
}