Causes of cross-over in unilateral neglect: between-group comparisons, within-patient dissociations and eye movements

Abstract

Patients with left unilateral neglect bisect long horizontal lines to the right of the true centre. However, when given short lines, many of the same patients mark the midpoint to the left of the true centre, towards the otherwise neglected space. This paradoxical phenomenon has been termed 'cross-over' and is difficult to explain based on current accounts of the neglect syndrome. To explore the causes of cross-over, in a first study we evaluated bisection of 20, 100 and 200 mm horizontal lines in groups of unilateral brain-damaged patients with neglect and hemianopia, with neglect and no hemianopia, with hemianopia and no neglect and without neglect or hemianopia. Cross-over of 20 mm lines was found only in neglect patients with hemianopia. To ascertain further the influence of visual field defects on cross-over, in a second study we compared the performance of two right-brain-damaged patients with contralesional neglect and inferior quadrantanopia with that of a patient with inferior quadrantanopia and no neglect. Patients bisected lines oriented so as to cross or uncross the blind quadrant of the visual field. When short 20 mm lines crossed the blind quadrant, neglect patients showed cross-over; when the same lines crossed the seeing quadrants cross-over was absent. These findings were confirmed by the examination of a neglect patient with sparing of the central 5 degrees of the contralesional left visual hemifield in the right eye and no sparing in the left eye. In monocular viewing, cross-over was present when 20 mm lines were bisected with the left eye and absent when bisected with the right eye. Recording of eye movements showed that at the moment of bisection left eye fixations shifted towards the contralesional line endpoint whereas right eye fixations remained anchored to the centre of the line. With long lines, both eyes deviated ipsilesionally. These results show that in neglect patients ipsilesional deviation in the bisection of long lines turns into apparently paradoxical contralesional bisection of short ones only when these cross a retinotopically blind sector of the neglected space. Cross-over seems to depend on the small spatial effects produced by reflexive contralesional gaze shifts allowing eccentric fixations with the seeing hemifield. During the bisection of long lines, these effects are cancelled out by the strong attentional deviation induced by the marked extension of the ipsilesional line segment. This explanation establishes coherence between cross-over and current accounts of the neglect syndrome.

Line bisection in left neglect: The importance of starting right

“…As a consequence, the left extremity of the right segment, which corresponds to the centre of the line, may be displaced rightward, thus giving rise to the bisection error. Left hemianopia may further decrease the saliency of the left segment, thus aggravating the rightward bias (Bartolomeo, 1987;D'Erme et al, 1987;Doricchi et al, 2005). Conversely, a reduction or absence of right-sided stimuli, as in the present imagery conditions, may reduce or abolish the rightward bias.…”

“…A large body of evidence indicates that this pattern of performance can not be completely explained by elementary sensory deficits (Fuchs, 1920;D'Erme et al, 1987;Barton and Black, 1998), or by deficits in programming hand movements towards the left side (Milner et al, 1993), although such problems may well contribute to the patients' final performance (D'Erme et al, 1987;Bisiach et al, 1990;Doricchi and Angelelli, 1999;Doricchi et al, 2005).…”

Comparison of the Behavioural Inattention Test and the Catherine Bergego Scale in assessment of hemispatial neglect

“…However, patients with chronic neglect often have additional visual field deficits which they do not learn to compensate with eye movements as is the case in patients with only hemianopia (Müller-Oehring et al, 2003). Visual fields are easily assessed by confrontation with fixation point and this clinical testing has been shown to be nearly as reliable as Goldman kinetic perimetry or visual evoked potentials to contralateral stimuli (Doricchi et al, 1999(Doricchi et al, , 2005.…”

Stimulus- and state-dependence of systematic bias in spatial attention: Additive effects of stimulus-size and time-on-task

“…A second candidate mechanism is bilateral attentional competition, which should increase with increasing line length and therefore bilateral peripheral stimulation. In neglect patients, the level of inter-hemispheric competition exacerbates attentional imbalance (Doricchi et al, 2005;Urbanski and Bartolomeo, 2008;Bonato et al, 2010;Charras et al, 2010) with right-sided stimuli having a negative impact on the processing of left-sided stimuli (Posner et al, 1984;Baylis et al, 1993;Urbanski and Bartolomeo, 2008;Charras et al, 2010). If this extinction-like behaviour flips side in healthy participants (as does pseudoneglect), then extinction would occur more for right parts of the visual field during bisection of long lines leading to a more consistent leftward bias.…”