Stimulus context in hemineglect

Abstract

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.

Biases in Attentional Orientation and Magnitude Estimation Explain Crossover: Neglect is a Disorder of Both

“…Poulton (1979Poulton ( , 1968 identified six types of "response bias" in magnitude estimation that can lower the size of the power function exponent. So far, contextual effects are the only form of response bias that has been examined in neglect (Ishiai, Koyama, et al, 2004;Ricci & Chatterjee, 2001;Marshall et al, 1998). Both Ishiai, Koyama, et al (2004) and Marshall et al (1998) explain crossover as due to exaggerated stimulus context effects in neglect.…”

“…So far, contextual effects are the only form of response bias that has been examined in neglect (Ishiai, Koyama, et al, 2004;Ricci & Chatterjee, 2001;Marshall et al, 1998). Both Ishiai, Koyama, et al (2004) and Marshall et al (1998) explain crossover as due to exaggerated stimulus context effects in neglect. In contrast, Ricci and Chatterjee (2001) found that absolute line length influences line bisection performance more than stimulus context in neglect.…”

“…Although confabulation is also assumed to occur on long lines, it is masked by a restricted window of attention located contralateral to brain injury. Finally, Marshall, Lazar, Krakauer, and Sharma (1998) explained misperceiving line length in terms of stimulus context effects that cause short lines to be perceived as longer and long lines to be perceived as shorter than their objective length (Cross, 1973;Hollingworth, 1909). Recently, Ishiai, Koyama, et al (2004) proposed that mental representations of short lines are exaggerated in neglect due to stimulus context.…”

Pseudoneglect: a review and meta-analysis of performance factors in line bisection tasks

“…McCourt and Jewell [82] found rightward errors with short lines and leftward errors with medium and long length lines. Finally, Marshall et al [78] report that the crossover eect in neglect patients occurs only when lines of heterogeneous lengths are presented within single blocks of trials, and is not found when a homogeneous block of short lines is presented. If similar context-dependent eects of line length are shown to occur with respect to pseudoneglect, this may explain some portion of the between-study variability.…”

Reemergence of Stroke Deficits With Midazolam Challenge

“…The perceived midpoint was calculated by averaging the percent deviations from the true midpoint on a scale from Ϫ100% (left end of the line) to ϩ100% (right end of the line). 10 Letter cancellation consisted of 72 black letters in nonlinear array with 12 A's in the left and right visual fields, respectively, and 24 distractive elements in each field. The patient was instructed to touch each letter A.…”