Visuospatial attention in line bisection: stimulusmodulation of pseudoneglect
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.
Section snippets
Subjects
A total of 217 strongly right-handed male subjects participated in one or more of theexperiments reported here. Subject laterality was assessed using a standard instrument [56] onwhich a composite score of −100 denotes exclusive left-handedness, and +100 denotes exclusiveright-handedness. The mean laterality score of all subjects participating in the experiments was +87.3(S.E. = 2.1).
Instrumentation and calibration
Subject responses were sensed and collected, and stimuli were presented usingIBM-compatible microcomputers with
Experiment 1: azimuthal position and positional uncertainty
Numerous studies of neglect have manipulated the azimuthal spatial position in which linestimuli are presented 18, 28, 49, 50, 51, 55, 64. The pattern ofbisection performance across these studies is quite consistent. Rightward errors are most extremefor lines presented in left hemispace, are intermediate for lines presented on or near the midsagittalplane, and bisections are most veridical for lines presented in right hemispace. Most linebisection studies present line stimuli drawn on paper
Experiment 2: elevation and aspect ratio
The rightward bisection errors of neglect patients are reduced when the stimuli being bisectedare not lines, but are instead objects with aspect ratios nearer unity such as circles or squares 26, 27, 72. In addition, bisection and cancellation errors vary systematically across visualfield quadrants, being most pronounced in the left inferior quadrant of the visual field [45]. Previc [59] has recently reviewed the physiological, anatomical and neuropsychologicalliterature regarding the
Experiment 3: line length
It is well established that bisection errors in neglect patients are significantly modulated byline length. Specifically, whereas patients misbisect long lines far to the right of center thesesame patients will, as the line segments shorten, make progressively smaller rightward errors.Patients frequently cross over to misbisect left of center for very short lines 2, 5, 15, 16, 25, 46, 55, 64. A recent meta-analysis of thepseudoneglect literature [36] examined the possible effect of line length
Experiment 4: stimulus contrast and backward masking
According to our recent meta-analysis [36] one of the most significant variablesmodulating the magnitude of pseudoneglect in line bisection tasks is motor (eye or limb) scanning,where left-to-right patterns exacerbate leftward error, and right-to-left patterns actually producerightward error. The tachistoscopic method of presenting line stimuli used here 48, 62is an effective control for overt scanning eye movements. Likewise, utilizing the method of constantstimuli in combination with delayed
Conclusions
Measured using a sensitive and well-controlled methodology, pseudoneglect is not artifactual,but is a reliable perceptual phenomenon of moderate effect size. As such, some attention mightusefully be devoted to incorporating this normal attentional asymmetry into computational and⧸orconnectionist models of spatial attention. Stimulus positional uncertainty does not abolishpseudoneglect, indicating that observers normally make bisection judgements within anobject-centered frame of reference.
Acknowledgements
This research was supported by a grants (to MEM) from the National Eye Institute(EY12267-01) and North Dakota EPSCoR. The authors are grateful to Dr Patricia Reuter-Lorenzfor helpful and detailed comments on the penultimate draft of this paper. The authors also thankPhillip Gunderson, Heidi Hoistad, Matt Garlinghouse, Christi Jarland and Jessica Slater for help withdata collection and analysis.
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