Brain, Vol 121, Issue 10 2003-2010, Copyright © 1998 by Oxford University Press
RS Marshall, RM Lazar, JW Krakauer and R Sharma
Rightward deviation on bisection of a horizontal line is well described in
patients with right brain injury and left hemineglect. Because of the
observation that hemineglect patients may bisect very short lines to the
left of the true midpoint (the so-called crossover effect), additional
models have been proposed to incorporate this finding into existing
theories of hemineglect. We investigated a line-length effect in six
patients with left hemineglect. When presented with any set of lines of
uniform (reference) length, percentage rightward deviation on line
bisection remained constant across different line lengths. When lines of a
second length were mixed into any uniform set of lines, bisection
performance on the reference lines changed. A rightward shift in the
perceived midpoint of the reference line occurred if the added lines were
shorter than the reference lines; a leftward shift occurred if the added
lines were longer. Leftward shifts included shifts across the true
midpoint, reproducing the crossover effect. Shifts in the perceived
midpoint occurred both on a manual line bisection task and on a line
bisection discrimination task in which no manual response was required. We
propose that the crossover effect may be part of a more general
stimulus-context effect in which the perceived midpoint of a line is
related not to absolute length, but to the line's length relative to other
lines with which it is presented. Such a context effect has not hitherto
been described in the neglect syndrome. A possible mechanism for the effect
is a generalization of length estimation produced by the combined influence
of the focal stimulus and all stimuli that precede it.
ARTICLES
Stimulus context in hemineglect
Department of Neurology, Neurological Institute of New York, Columbia- Presbyterian Medical Center, New York 10032, USA. rsm2@columbia.edu
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