Visuospatial attention in line bisection: stimulusmodulation of pseudoneglect

Abstract

Neglect and pseudoneglect are asymmetries of spatial attention which are oftenassumed to possess a fundamental theoretical and neurological relationship to each other,although this assumption has never been directly tested and there is as yet no unifyingquantitative theory. A total of 217 subjects participated in five experiments demonstrating thatboth the magnitude and direction of bisection errors in normal subjects (pseudoneglect) aremodulated by stimulus factors that similarly influence the magnitude and direction of neglect.Stimulus positional uncertainty did not abolish pseudoneglect, indicating that bisectionjudgements are made within an object-centered frame of reference. Backward masking linestimuli had no influence on the magnitude of pseudoneglect, signifying that pseudoneglect is nota byproduct of covert directional scanning of the line stimulus in iconic or short-term visualmemory. Finally, bisection errors are influenced by the direction of contrast gradients imposedon line stimuli, such that perceived line midpoint is drawn toward the lower-contrast lineend. The magnitude and direction of pseudoneglect are modulated by stimulus factors that alsoinfluence the magnitude and direction of neglect. Both phenomena are succinctly described asbiases in attention (i.e., neglect is a right-bias, whereas pseudoneglect is a left-bias). The twophenomena are modulated by stimulus factors as follows. Line length: there is anincreased bias with increasing line length for both phenomena, and a cross-over to an reversedbias for short lines. Azimuthal line position: an increasing bias accompanies increasingleftward placement for both phenomena. Line aspect ratio: there is a decreasing bias withincreasing line height for both phenomena. Line elevation: there is a decreasing bias withincreasing elevation for neglect, and an increasing bias with increasing elevation forpseudoneglect. The only case in which a factors influence on the two phenomena is discrepant isfor elevation, and this difference is explicable. Taken together these congruencies stronglysupport the notion that neglect and pseudoneglect are phenomena that are twin manifestations ofparameter changes in a unitary set of underlying hemispheric attentional asymmetries.

Introduction

Persons with visuospatial neglect syndrome have difficulty orienting towards stimuli incontralesional hemispace, or toward the contralesional side of stimuli themselves. The inattentioncannot be completely explained by either primary motor or sensory defects [33]. It is lefthemispace that is most frequently affected, and the site of damage is usually the right inferior parietalor temporoparietal neocortex, although neglect can also occur following lesions to the left frontalor cingulate cortex, or to a variety of subcortical structures 13, 19, 31, 32, 33, 34, 75, 79, 80. Line bisection is a task frequently employed tomeasure the severity of neglect. Patients typically bisect horizontal lines of moderate length (>10°visual angle) significantly to the right of veridical center (i.e., as if they either ignore the majorityof the left-hand side of the stimulus or are, alternatively, hyperattentive to the right-hand side). Neurologically normal subjects also systematically misbisect space during visual line-bisectionor similar tasks, generally erring to the left of veridical center when bisecting horizontal lines, aphenomenon known as pseudoneglect [7]. The magnitude of bisection errors in normal subjectsis smaller than in neglect patients and it has been reported to be somewhat variable acrossindividuals 44, 48. A recently conducted meta-analysis of the pseudoneglect literaturewhich reviewed and compiled data across over 70 published articles reveals, however, that visualpseudoneglect has an effect size between 0.40–1.30 (depending on testing method), and is a highlyreliable and significant phenomenon [36]. The twin phenomena of neglect andpseudoneglect each reflect an asymmetry in the deployment of spatial attention into right and lefthemispace, reflecting an underlying neurological asymmetry between the right and left hemispheres.Neglect and pseudoneglect are often assumed to possess a fundamental theoretical and neurologicalrelationship to each other, although this assumption has never been directly tested and there is as ofyet no unifying quantitative theory. The experiments described in this report begin to address therelationship between neglect and pseudoneglect by quantitatively assessing the magnitude ofpseudoneglect as a function of a variety of stimulus parameters known to influence the magnitudeof neglect.