Elsevier

NeuroImage

Volume 19, Issue 3, July 2003, Pages 710-726
NeuroImage

Regular article
Right hemisphere control of visuospatial attention: line-bisection judgments evaluated with high-density electrical mapping and source analysis☆

https://doi.org/10.1016/S1053-8119(03)00057-0Get rights and content

Abstract

The “line-bisection” task has proven an especially useful clinical tool for assessment of spatial neglect syndrome in neurological patients. Here, we investigated the neural processes involved in performing this task by recording high-density event-related potentials from 128 scalp electrodes in normal observers. We characterized a robust net negative potential from 170–400 ms poststimulus presentation that correlates with line-bisection judgments. Topographic mapping shows three distinct phases to this negativity. The first phase (∼170–190 ms) has a scalp distribution exclusively over the right parieto-occipital and lateral occipital scalp, consistent with generators in the region of the right temporo-parietal junction and right lateral occipital cortices. The second phase (∼190–240 ms) sees the emergence of a second negative focus over the right central parietal scalp, consistent with subsequent involvement of right superior parietal cortices. In the third phase (∼240–400 ms), the topography becomes dominated by this right central parietal negativity. Inverse source modeling confirmed that right hemisphere lateral occipital, inferior parietal, and superior parietal regions were the likeliest generators of the bulk of the activity associated with this effect. The line stimuli were also presented at three contrast levels (3, 25, and 100%) in order to manipulate both the latency of stimulus processing and the relative contributions from magnocellular and parvocellular inputs. Through this manipulation, we show that the line-bisection effect systematically tracks/follows the latency of the N1 component, which is considered a temporal marker for object processing in the ventral visual stream. This pattern of effects suggests that this task invokes an allocentric (object-based) form of visuospatial attention. Further, at 3% contrast, the line-bisection effect was equivalent to the effects seen at higher contrast levels, suggesting that parvocellular inputs are not necessary for successful performance of this task.

Introduction

Line-bisection tasks are a commonly used metric for the clinical assessment of visuospatial neglect syndrome, a condition that predominantly results from vascular lesions to the right inferior parietal or temporoparietal cortex (e.g., Vallar et al 1987, Cappa et al 1991, Mesulam 2000, Na et al 2000, Kerkhoff 2001; see Karnath, 2001 for a comprehensive treatment). Patients with visuospatial neglect syndrome generally bisect lines significantly to the right of veridical center (Robertson and Halligan, 1999), due either to a decrement in the ability to allocate attention to left visual space or to hyperattention to rightward space. Clinical findings from neglect patients allied to the scores of neuroimaging studies showing a predomination of right parietal activity during tasks requiring visuospatial attention (e.g., Corbetta et al 2000, Coull et al 2001 have led to the formulation that while both left and right parietal areas are involved in attention to the right visual field, right parietal areas alone may control attention to the left visual hemifield (e.g., Heilman and van den Abell 1980, Weintraub and Mesulam 1987. A measure of support for this conjecture is derived from the finding that neurologically normal subjects demonstrate a phenomenon known as “pseudoneglect” (Bowers and Heilman, 1980), in which a significant and systematic misbisection of lines (or space) occurs that is leftward of veridical center. This leftward tendency in normal observers has been theorized to result from a profound parietal asymmetry in attentional control, which gives rise to some degree of hyper-attentiveness to the left visual field (in space-based attention), or to the left-hand side of individual objects (in object-based attention) (see Jewell and McCourt 2000, McCourt 2001, McCourt and Jewell 1999, McCourt et al 2000.

Functional imaging (fMRI) studies have confirmed a central role for right parietal cortices in performance of both line-bisection tasks Weiss et al 2000, Fink et al 2000a, Fink et al 2000b, Fink et al 2001, Fink et al 2002, Galati et al 2000 and judgments of object location relative to the midsagittal plane Vallar et al 1999, Galati et al 2000. There have, to date, been no electrophysiological investigations of line bisection; consequently the timing of right parietal involvement relative to the timing of ongoing stimulus processing is as yet unknown. In the current study, we performed high-density electrical mapping (from 128 scalp electrodes) of the visual event-related potential (ERP) while subjects engaged in either a tachistoscopic forced-choice line-bisection task (McCourt and Olafson, 1997) or a control task in which they simply judged whether or not lines were transected. Our main objectives were to both confirm the involvement of and define the timecourse of right parietal activity in performance of the line-bisection task. In particular, we wished to assess the relationship of parietal attentional processes in the dorsal visual processing stream to the processing of the line stimuli by object recognition areas of the ventral visual stream. An open question is whether line midpoint judgments can only be made upon completion of object recognition processes for the object that is to be bisected. In the present study, we systematically varied the timing of object-recognition processes by varying the contrast level at which lines were displayed. We used the well-characterized N1 component of the visual evoked potential (VEP) as an index of the timing of object recognition processes, since this component has repeatedly been shown to be correlated with the processes involved in object recognition (e.g., Allison et al 1999, Bentin et al 1999, Doniger et al 2000, Doniger et al 2001. Systematically varying the latency of this component allowed us to assess whether bisection processes tracked this latency manipulation.

Section snippets

Subjects

Nine (4 male) neurologically normal, paid volunteers (ages 19–45 years, mean = 29.2 years) participated. All subjects provided written informed consent, and the Institutional Review Boards of the Nathan Kline Research Institute and North Dakota State University approved all procedures. All subjects possessed normal or corrected-to-normal vision and were right-handed, as measured using the Oldfield (1971) laterality inventory (mean score = 58.3, SD = 22.7).

Instrumentation and stimuli

Subject responses were sensed and

Behavioural results

Fig. 2 presents group-averaged psychometric functions obtained in the Line-Bisection condition at each level of line contrast. Open symbols plot the mean percentage “left” responses (± 1 SEM) against transector location (in degrees relative to veridical line midpoint). The data are well behaved, and the range of transector locations sampled is observed to encompass perceived line midpoint in this sample of subjects. Solid lines depict cumulative Gaussian distributions fitted to the data by

Discussion

The current findings explicate the brain mechanisms underlying performance of the visual line-bisection task, a perceptual version of the perceptual-motor task that is frequently employed in the clinic to disclose the presence and severity of visuospatial neglect syndrome. We define an electrophysiological correlate of line-bisection judgments, which manifests as a right parieto-occipital negative potential that is significantly earlier in latency and larger in magnitude than that over left

Acknowledgements

We express our sincere appreciation to Deirdre Foxe, Beth Higgins, and Dr. Micah Murray for their technical help with this study. We are most grateful to Dr. Gereon Fink and his colleagues for generously allowing us to reproduce their fMRI data in the current report. Dr. Antigona Martinez provided valuable comments on an earlier version of the manuscript for which we are indebted to her. Our thanks also go to two anonymous reviewers for their careful and constructive comments. This work was

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