Visuospatial Processes of Line Bisection and the Mechanisms Underlying Unilateral Spatial Neglect

Abstract

Patients with unilateral spatial neglect fail to report or respond to stimuli contralateral to the lesion which usually involves the right parietal lobe. When asked to mark the centre of a horizontal line, these patients place the mark to the right of the true midpoint. It has been considered that they neglect the left part of the line and bisect the perceived line segment. We investigated the eye-fixation patterns of hemianopic patients with or without unilateral spatial neglect during the bisection of lines, using an eye camera. Hemianopic patients without unilateral spatial neglect saw the whole lines, searching to the endpoint on the hemianopic side, and bisected it correctly. In contrast, left hemianopics with unilateral spatial neglect never searched to the left hemianopic side. Once they fixated a certain point on the right part of the line, they persisted with this point and marked the subjective midpoint there. Taking left homonymous hemianopia into account, the subjective midpoint appeared to be marked, not at the centre of the line segment perceived in the seeing right visual field, but at the leftmost point of it. However, they could appreciate the deviation of the subjective midpoint in the right visual field when forced to fixate the left endpoint of the line. These findings suggest that the left hemisphere has the ability to estimate the midpoint of the line through the right visual field and that visuospatial disorder in the line bisection test is attributable to the pathological change in the right hemisphere. The results are interpreted to mean that left hemianopic patients with unilateral spatial neglect see a totalized image of a line extending equally to either side of the point where they are going to mark the subjective midpoint. We considered that the right hemisphere completes the line, using the visual input relating to the right part of the line perceived by the left hemisphere.

Fast responses to neglected targets in visual search reflect pre-attentive processes: an exploration of response times in visual neglect

“…The outcome would then necessarily be a premature conclusion about the absence of the target. This account seems to us highly consistent with well-known aspects of the phenomenology of neglect, like the presence of ineffective attentional and ocular shifts or "hypodirectionality" of movement [8,12,[40][41][42][43]71] or the inability to "disengage" from a previous stimulus that is right-sided in relation to a left-sided target [65,66]. In the former case, searches may be based on incorrect information or in the latter cases simply aborted.…”

“…The attention window may not move away from a currently attended location [53] or a shift could be quickly aborted or be guided by incomplete, weakened, or biased spatial information so as to undershoot the target's position (cf. [8,40,41]). Because we assume that a present target is acknowledged as such only when the attributes stored in the target template's representation and the content of the attention window match, all of the above forms of impairment would result in AE giving fast responses to present targets while at the same time not acknowledging their presence.…”

Line bisection in left neglect: The importance of starting right

“…The remarkable work of the Ishiai group on eye movements in line bisection (Ishiai et al, 1989;1996; the case where the patients marked not only the midpoint of the virtual line, but also its endpoints, and address more directly the issue of mental imagery, because our participants did not see a physical line before each imaginal bisection; consequently, they could not make a bisection judgment on the physically presented line before it disappeared. Overall, 4 At variance with the Bisiach et al's (1996) results, but similar to the performance of our patients JEC and MAL, the subjective distance from the centre was always shorter than the real distance.…”

“…Conversely, a reduction or absence of right-sided stimuli, as in the present imagery conditions, may reduce or abolish the rightward bias. The importance of starting from the right endpoint in neglect patients' line bisection performance is also underlined by eye movements studies, showing that patients' fixations often land on the right part of the line (Ishiai et al, 1989). This preferential scanning direction is nicely illustrated by the funny drawings which a distinguished artist with neglect could not help adding to his bisections (Cantagallo and Della Sala, 1998) (Fig.…”

“…Eye-movement recordings confirmed that neglect patients adopt a right-to-left scanning strategy, and often fail to reach the leftmost extremity (Ishiai et al, 1992;Ishiai et al, 1989).…”