Elsevier

Brain and Cognition

Volume 68, Issue 1, October 2008, Pages 9-14
Brain and Cognition

Perceptual–attentional and motor-intentional bias in near and far space

https://doi.org/10.1016/j.bandc.2008.02.006Get rights and content

Abstract

Spatial bias demonstrated in tasks such as line-bisection may stem from perceptual–attentional (PA) “where” and motor-intentional (MI) “aiming” influences. We tested normal participants’ line bisection performance in the presence of an asymmetric visual distracter with a video apparatus designed to dissociate PA from MI bias. An experimenter stood as a distractor to the left or right of a video monitor positioned in either near or far space, where participants viewed lines and a laser point they directed under (1) natural and (2) mirror-reversed conditions. Each trial started with the pointer positioned at either the top left or top right corner of the screen, and alternated thereafter. Data analysis indicated that participants made primarily PA leftward errors in near space, but not in far space. Furthermore, PA, but not MI, bias increased bilaterally in the direction of distraction. In contrast, MI, but not PA, bias was shifted bilaterally in the direction of startside. Results support the conclusion that a primarily PA left sided bias in near space is consistent with right hemisphere spatial attentional dominance. A bottom–up visual distractor specifically affected PA “where” spatial bias while top–down motor cuing influenced MI “aiming” bias.

Introduction

Patients with unilateral neglect demonstrate asymmetric spatial deficits behaviorally as a failure to report, respond to, or orient toward stimuli in contralesional space (Heilman & Valenstein, 1979). The heterogeneity of the neglect syndrome suggests that their deficits may stem from perceptual–attentional (PA) or motor-intentional (MI) sources or both (Barrett et al., 2006, Heilman et al., 2003). PA deficits represent a lack of awareness of or attention to stimuli in the contralesional side of space that is not due to primary sensory deficits. MI deficits, alternatively, denote a failure to respond to or initiate action toward contralesional stimuli, even if they fall into conscious awareness, that cannot be attributed to primary motor deficits. Neglect patients tend to demonstrate spatial biases based predominantly on either PA or MI deficits when they are dissociated (Adair et al., 1998, Bisiach et al., 1990, Coslett et al., 1990, Na et al., 1998, Tegnér and Levander, 1991).

PA “where” and MI “aiming” biases have been dissociated experimentally by asking subjects to perform a visual-motor task in which the direction of action is dissociated from their viewed perception of their movement. For example, Bisiach and colleagues (1990) devised a pulley system with which subjects marked the center of a line by pulling a string. A marker on the line either moved in the same direction as the string they pulled, or in the opposite direction. When the marker moved in the opposite direction that the subjects moved the string, responses differed depending upon whether their deficit was in attending or in moving their hand leftward. Some investigators use a video apparatus designed along similar principles to horizontally dissociate perception and action (Adair et al., 1998, Barrett et al., 2001, Na et al., 1998, Schwartz et al., 1997). Nico (1996) utilized an epidiascope (overhead projector) to achieve mirror-reversed viewing conditions as a technique to detect directional hypokinesia. A verbal task has also been developed as a more flexible method for making this same type of dissociation (Chiba, Yamaguchi, & Eto, 2005).

The dissociation of PA and MI biases among neglect patients may reflect different underlying mechanisms for the disorder, as well as different neural systems involved in spatial perception (Heilman, 2004). If so, similar dissociations should be observable among neurally intact participants when they demonstrate a spatial bias. Normal subjects may exhibit such a bias when performing the line bisection test, erring to the left when attempting to mark the veridical center of the line (Jewell & McCourt, 2000). Because the performance of this task requires the coordination of visual perceptual and motor control skills, PA or MI biases may underlie the asymmetric performance. If the bias is related to a failure to attend to one side of the line, or an abnormal propensity to attend to the other side, then asymmetric perceptual–attentional (PA) awareness may be primarily responsible for errors. Alternatively, the bias might originate from asymmetries in premotor or motor-intentional (MI) functioning, with subjects demonstrating a preferential turning or aiming tendency.

Though PA “where” and MI “aiming” systems are intimately interrelated in spatial attention and action, leftward PA bias may primarily predominate in normal control groups (Barrett et al., 2002, Schwartz et al., 1997), suggesting right hemispheric attentional systems critically support task performance. The standard clinical administration of the line bisection task is in paper and pencil format, and therefore it must be carried out within arms’ reach in near peripersonal space. It is possible, therefore, that right hemispheric specialization may be especially robust in near space. To examine this possibility, Dellatolas, Vanluchene, and Coutin (1996) had normal subjects bisect lines in near space on paper and in far space on computer screens. They found that the slight leftward bias on paper was not present when subjects bisected lines in far space. Varnava, McCarthy, and Beaumont (2002) reported similar results when normal subjects bisected lines on a completely computerized task at four different distances ranging from near to far space. Subjects demonstrated leftward error in the nearest conditions that decreased as the lines were positioned further away, supporting the hypothesis that these differences are continuously related to viewing distance. The functional differences in performance between near and far space might be attributable to a greater contribution of attentional processing in far space (Previc, 1990). However, it is not known whether in normal subjects an asymmetric bias may fractionate differently into PA and MI bias components depending upon whether tasks are performed in near versus far space.

In near and far space, other environmental conditions may influence spatial bias and cause distraction. Barrett, Schwartz, Crucian, Kim, and Heilman (2000) studied a patient with a left medial thalamic infarction who had spatial bias selective to far extrapersonal space. The subject used a laser pointer to bisect lines in near and far space. Although she performed comparably to control subjects in near space, she erred significantly rightward of controls (and the veridical center) when bisecting in far space. Interestingly, the patient also reported a tendency while driving to veer in the direction of objects appearing on the right side of the road, and the researchers included conditions in which an experimenter (distractor) stood physically to one side or the other of the line. The patient responded significantly rightward when the distractor stood on the right compared with trials in which the distractor was on the left, suggesting a distractor effect in far space only.

The asymmetric bias could have been either primarily PA or MI, as the motoric aspect of her responses was not spatially dissociated from visual-perceptual space. As a visual distractor would presumably engage bottom–up visual systems, it might primarily influence PA “where” bias. Alternatively, top–down motor programming may be expected to have a greater influence on MI “aiming” bias.

In the current study we experimentally examined normal control subjects’ spatial bias on the line bisection test by using a video camera, mixer, and monitor to dissociate subjects’ action space from the task viewing space. We expected to observe PA leftward bias in near space, but wished to identify whether reduced errors of a different character might occur when this task is performed in far space. Additionally, we hypothesized that the presence of an asymmetric distractor on the right or left might primarily influence PA response bias through a bottom–up influence, while asymmetric motor programming (i.e., instructing subjects to start on the left or the right side of the line) may primarily affect MI bias through a top–down influence.

Section snippets

Participants

Twenty-two right-handed volunteers (11 male) aged 21–35 (mean 24.8, SD 3.26) participated in the study. Subjects had no history of neurological or psychiatric conditions, and had normal or corrected to normal vision. They had a mean 16.6 (SD 1.60) years of education.

Apparatus

Subjects bisected lines by directing a Laserlyte laser pointer to a self-standing, non-glare, transparent acrylic workscreen, positioned on the floor (see Fig. 1). The workscreen held a white sheet of paper with a black horizontal

Results

Analyses were undertaken to determine if subjects would err further to the left in near space than in far space. Mean responses in near space (M = −0.84 mm, SD = 1.52) were indeed reliably to the left of those in far space (M = −0.01, SD = 1.45; significant main effect of Distance, F(1,20) = 7.65, p = .012). Furthermore, subjects erred significantly to the left of center when responding in near space alone (t = 2.60, p = .017, one-sample t-test), but not in far space (t = 0.43, n.s.).

Differences in error

Discussion

In this study, normal control subjects bisected lines in near and far space. By dissociating perceptual and action space, we sought to fractionate the effects of perceptual–attentional (PA) and motor-intentional (MI) bias.

Previous investigators found that normal subjects made greater leftward errors when they bisected lines in near versus far space (Dellatolas et al., 1996, Varnava et al., 2002). Our results are consistent with these prior reports. Additionally, the near leftward errors made by

Acknowledgments

Study supported by the National Institutes of Health/National Institute of Neurological Disorders and Stroke (K08 NS002085 and K02 NS47099), the Departments of Medicine and Neurology, the Penn State College of Medicine, the Henry H. Kessler Foundation and the General Clinical Research Center of the Penn State University College of Medicine (NIH/NCRR C06 RR016499 and M01 RR010732). We thank Dr. Kenneth Heilman for comments and assistance in planning the work on which this study was based. We

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