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Abstract:
Abstract: Patients with left neglect bisect horizontal lines
to the right of true center. Longer lines are transected further to
the right than shorter lines. The obvious explanation for this
line-length effect is that attention is drawn further rightward
with longer lines that extend further into ipsilesional space.
Alternatively, if patients are limited in their abilities to
internally represent horizontal extension, then they might orient
further rightward because long lines have longer representations
than short lines. If a phenomenon analogous to the line-length
effect occurs in lines with identical extensions in space but are
perceived as having different lengths, then central representations
must also influence the orientation bias. Using principles
underlying the Oppel-Kundt illusion, we constructed two types of
lines made of relatively long or relatively short segments. Lines
made of shorter segments appear longer than lines made of longer
segments as judged by normal subjects. Three neglect patients with
left neglect bisected lines perceived by normal subjects as longer
significantly further to the right than lines perceived as shorter.
This modulation of patients' orientation biases by varying the
internal representation of line length without varying total
objective lengths cannot be explained by an abnormal attentional
gradient to stimuli extending further into ipsilesional external
space. Rather, the magnitude of internal representations must also
modulate the orientation bias in patients with neglect.
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