STIMULUS CONTEXT IN HEMINEGLECT

Rightward deviation on bisection of a horizontal line is well describe d 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), ad ditional models have been proposed to incorporate this finding into ex isting theories of hemineglect. We investigated a line-length effect i n six patients with left hemineglect, When presented with any set of l ines of uniform (reference) length, percentage rightward deviation on line bisection remained constant across different line lengths. When l ines of a second length were mixed into any uniform set of lines, bise ction performance on the reference lines changed. A rightward shift in the perceived midpoint of the reference line occurred if the added li nes were shorter than the reference lines; a leftward shift occurred i f the added lines were longer. Leftward shifts included shifts across the true midpoint, reproducing the crossover effect. Shifts in the per ceived midpoint occurred both on a manual line bisection task and on a line bisection discrimination task in which no manual response was re quired. We propose that the crossover effect may be part of a more gen eral 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 n ot hitherto been described in the neglect syndrome. A possible mechani sm for the effect is a generalization of length estimation produced by the combined influence of the focal stimulus and all stimuli that pre cede it.