Orientation bias in unilateral neglect: representational contributions

Left-neglect patients bisect horizontal lines to the right of true center. Longer lines are bisected further to the right than shorter lines. This line-length effect might be explained by an increase in the rightward bias of attention because longer lines extend further ipsilesionally. Alternatively, neglect patients might be limited in their abilities to internally represent horizontal magnitudes. Patients might orient further rightward with longer lines because these lines have longer representations. If the line-length effect occurs on lines of identical objective length but they are represented differently, then central mechanisms must contribute to the orientation bias. We constructed two types of lines that were perceived by normal subjects as having different lengths, but were of identical extents. Three neglect patients bisected lines perceived as longer, further to the right than lines perceived as shorter. These results demonstrate that relative magnitudes of internal representations contribute to the degree of bias in neglect patients.     

Space representation in unilateral spatial neglect.

Patients with unilateral brain lesions were given a task requiring exploration of space with the hand in order to assess the visual dependency of unilateral spatial neglect. The task was carried out both without visual control and under visual control. Performances were compared with that of normal subjects. Results were :(1) patients with right brain damage with no visual field defect demonstrated left-sided neglect only when the exploration was not controlled visually; (2) patients with left and right brain damage with visual field defect demonstrated contralateral neglect only when the exploration was under visual guidance. The performance of the patients with right brain damage without visual field defect in not clearly understood. The other results suggest that inner spatial representation remains intact in most cases of spatial neglect. The role of parietal lobe damage in the development of this visually induced phenomenon is hypothesised. The dominant position of vision among the senses is indicated.     

The mechanism of unilateral spatial neglect

We investigated a test for unilateral spatial neglect and discussed the mechanism of this symptom. Seventy eight patients with lesions on the right cerebral hemisphere were examined on line bisection, line cancellation, figure copying, clock drawing and location on the map. The statistical analysis revealed (1) there were no correlations between line bisection and clock drawing or between line bisection and location; (2) those with abnormal results on line bisection were apt to indicate normal results on line cancellation and figure copying, while in line cancellation, figure copying, clock drawing and location, normal results on one task meant normal results on the other. These results indicated clock drawing and location might include different factors from other tasks, line bisection was the most sensitive task for unilateral spatial neglect, and abnormality on line cancellation might be similar to that figure copying. Unilateral spatial neglect may be based on several disorders, and it may be assumed to have several aspects. One of these disorders may be defective searching. Unilateral spatial neglect may be more severe when there is sensory disturbance (hemianopsia) with a searching deficit. Other disorders, such as cognitive disturbance, memory disturbance and so on, may also be associated with unilateral spatial neglect, and it is necessary to consider the versatility of this symptom.     

Picturing unilateral spatial neglect: viewer versus object centred reference frames.

Vision theorists postulate that knowledge of objects in space is shaped by different spatial coordinate systems. An object may be represented by its location in relation to the viewer, or it may be represented by its own intrinsic spatial properties. When patients with left sided neglect fail to respond to stimuli on the left, it is not clear whether "left" refers to a viewer or an object centred reference frame. To uncouple these two reference frames, eight patients with neglect were asked to centre lines and objects in photographs. Viewer centred neglect would result in images appearing on the right side of photographs and object centred neglect would result in images appearing on the left. Four patients demonstrated viewer centred neglect and three demonstrated object centred neglect. One patient had variable performance, perhaps resulting from competing effects of both viewer and object centred neglect. Stimuli characteristics did not affect the spatial coordinate system in which neglect occurred. These results suggest that viewer centred and object centred reference frames are functionally dissociable, and that patients may have spatial neglect predominantly in either coordinate system.     

Experimental remission of unilateral spatial neglect

Over the past several decades a growing amount of research has focused on the possibility of transiently reducing left neglect signs in right brain-damaged patients by using vestibular and/or visuo-proprioceptive stimulations. Here we review seminal papers dealing with these visuo-vestibulo-proprioceptive stimulations in normal controls, right brain-damaged (RBD) patients, and animals. We discuss these data in terms of clinical implications but also with regards to theoretical frameworks commonly used to explain the unilateral neglect syndrome. We undermine the effect of these stimulations on the position of the egocentric reference and extend the notion that the positive effects of these stimulation techniques may stem from a reorientation of attention towards the neglected side of space or from a recalibration of sensori-motor correlations. We conclude this review with discussing the possible interaction between experimental rehabilitation, models of neglect and basic spatial cognition research.     

Mirror images and unilateral spatial neglect

Responding correctly to a mirror image requires the creation of a rather peculiar form of dual representation. Mirror agnosia and mirror ataxia, i.e. a deficit in reaching an object reflected in a mirror, have been reported to be associated with parietal lobe lesions. This prospective study was conducted to investigate the capacity of subjects with neglect to identify the mirror image nature of visual information. Four consecutive brain-damaged patients with neglect, selected on the basis of specific criteria, and four control subjects performed grasping and object displacement tests under two response conditions (normal mirror and inverted mirror). Video recordings of the tests were analyzed to assess performance using the following criteria: (i) direction of the arm movement during the initial phase of movement, (ii) number of corrections of the hand position before grasping. The control subjects successfully grasped the objects in both experimental conditions. The patients (1) neglected the contralesional space, grasping objects correctly in the ipsilesional space (normal mirror condition) and (2) neglected the ipsilesional space, grasping correctly objects in the contralesional space (inverted mirror). Controls used real object-centered correction clues to modify the position and direction of their hand movement. The patients only produced horizontal displacements of the upper limb in the ‘healthy’ and neglected space. These results suggest that patients with neglect do not use the same clues and do not modify their procedures as they cannot recalibrate their spatial representations. These differences concerned non-mirror-image clues and directional and positional as well as attentional vectors. Theoretical and rehabilitative implications are discussed.     

Neurobiology of unilateral spatial neglect.

Hemispatial neglect is a common and disabling consequence of stroke. Earlier studies aimed to identify a single area of the brain where damage caused neglect and sought a single disrupted process that could account for the symptoms. Recent studies have shifted toward identifying component processes and representations underlying spatial attention required for various tasks and identifying areas of the brain responsible for each component that together constitute the network of regions responsible for neglect. This review focuses on recent insights into the mechanisms of neglect, regions of neural dysfunction that cause disruption of particular components or forms of neglect, and potential means of ameliorating neglect. Converging evidence supporting these insights comes from new imaging modalities in acute stroke, functional imaging, transcranial magnetic stimulation, electrophysiological studies in humans, and single-cell recording studies in nonhuman primates.     

Time and neglect: abnormal temporal dynamics in unilateral spatial neglect

Temporality enters our immediate experience as passage and becoming: the role time plays in the construction of a world of enduring entities tends to go unnoticed. This paper examines the relation between time and ontology in the context of unilateral neglect, a neuropsychological syndrome in which patients fail to perceive or respond to stimuli in the contralateral hemifield, behaving as if that half of space does not exist. Traditional models characterize neglect exclusively in spatial terms. Based on recent investigations suggesting abnormal temporal dynamics, here we highlight the impact of time factors on the presentation of the disorder. Neglect patients do not simply miss the presence of stimuli on the left: they also ignore the past as well as the future of neglected stimuli. We claim that, if this occurs, it is because time, and not only space, is impaired.     

Fatigue versus disengagement in unilateral neglect.

A letter cancellation task revealed neglect of the lower left quadrant when cancellation began at the top, but of the upper left quadrant when it began in the bottom. When the cancellation was done line by line through a "window" which covered all but the line currently worked on, the quadrant effects disappeared, so that fatigue could not explain these effects. An alternative hypothesis is that as the cancellation progressed, the subject's attention was attracted by progressively more stimuli on the right, and that disengagement difficulty and hence neglect increased proportionately to the number of stimuli.     

Leftward search in left unilateral spatial neglect.

The authors' previous study with an eye camera revealed that when asked to mark the centre of a line patients with left unilateral spatial neglect persist in fixating a point on its right part and place the subjective midpoint there without searching leftwards. The present study examined the patterns of leftward search of nine patients with left unilateral spatial neglect when they were required to search for the left endpoint of the line after the bisection. The patients could search leftwards beyond the subjective midpoint to place the mark at the subjective left endpoint. The initial fixation in this search always fell near the point located to the left of the subjective midpoint by the distance between the subjective midpoint and the right endpoint of the line. In patients with severe neglect the search further to the left of this point was laborious and fell short of the true left endpoint in about 80% of the trials. Our results suggest that when asked to bisect a line patients with left unilateral spatial neglect subjectively see the line as it extends equally to either side of the point where they are going to mark the subjective midpoint.     

Improvement of unilateral spatial neglect with numbering

We examined 8 patients with moderate to severe left unilateral spatial neglect by means of a series of line cancellation tasks. We asked the patients to cross out lines in the 1st trial, to number lines in the 2nd trial, and again to cross out lines in the 3rd trial. In the 2nd trial with numbering, all the patients showed improvement of left unilateral spatial neglect. Numbering involved the successive use of increasing numbers. This process seemed to motivate the patients to continue searching for another line, even at the point when they would be satisfied that they had completed the task if they used simple crossing-out. Insufficient motivation for visuospatial searching may play an important role in unilateral spatial neglect observed in cancellation tasks.     

Eye-fixation patterns in homonymous hemianopia and unilateral spatial neglect

Eye-fixation patterns while viewing simple patterns were quantitatively analysed in homonymous hemianopic subjects with or without unilateral spatial neglect (USN), using an eye camera. Hemianopic subjects without USN used the strategy to look at the hemianopic side of the patterns longer in order to compensate for their visual field defect. However, those with USN lacked this compensatory eye-fixation pattern.     

Darkness improves line bisection in unilateral spatial neglect.

This study was performed to determine the influence of background illumination on the performance of patients with left visual neglect. We examined six RBD patients with unilateral visual neglect, and as controls six RBD patients and six LBD patients without neglect. A luminous line consisting of LEDs was used in a line bisection task under two conditions, normal illumination and darkness. As expected, neglect patients made large rightward errors under normal illumination. Their performance improved by about 43% in darkness. Our results are discussed with reference to earlier studies of space exploration without visual guidance.     

Is unilateral spatial neglect a single phenomenon?

The aim of this study is to report the preliminary findings of a traditional battery of tests and our original battery capable of assessing the presence of components and extent of lesions in patients with unilateral spatial neglect. Thirty patients who had unilateral spatial neglect with a stroke in the right hemisphere were assessed for unilateral spatial neglect on exploratory-motor (E-M) tasks, visual-counting (V-C) tasks, and traditional neglect batteries at least 4 weeks after the onset. Other neuropsychological tests and computed tomography were also performed to investigate the relationship with neglect. A factor analysis showed that our tasks loaded significantly on three factors. E-M neglect was found in 16 patients, and V-C neglect in 22 patients with unilateral spatial neglect. There were high correlations between E-M neglect and motor paralysis, word fluency, backward digit span and motor impersistence. There were high correlations between V-C neglect and visual-field defect, line bisection, line cancellation and figure copying. Lesions in the frontal lobe, caudate, insula, and anterior portion of the paraventricular white matter were commonly associated with E-M neglect. Lesions in the occipital lobe were also associated with V-C neglect. We suggest that unilateral neglect is not a single phenomenon, but rather involves several different components. We propose that E-M and V-C tasks are useful methods for evaluating the extent of lesions in patients with unilateral spatial neglect. Received: 18 November 1996 Received in revised form: 14 March 1997 Accepted: 6 April 1997     

Preserved visuo-spatial transformations in representational neglect

Patients with representational unilateral neglect were impaired in immediate recall of novel material on the neglected side as presented (viewed and removed or verbally described) or following mental rotation. Transforming material from the neglected (left) side to the non-neglected (right) side resulted in no additional loss; patients were unimpaired in directing attention to the neglected side of their representation in order to perform the mental rotation. These findings cannot be explained by the widely adopted attention deficit hypothesis for representational neglect. It is suggested that the disorder arises from damage to temporary storage functions of visuo-spatial working memory.     

Prism adaptation improves representational neglect

Previous work has shown that various symptoms of unilateral neglect, including the pathological shift of the subjective midline to the right, may be improved by a short adaptation period to a prismatic shift of the visual field to the right. We report here, in two neglect patients the improvement of imagery neglect after prism exposure. Despite a strong neglect observed for mental images, as well as for conventional tests, the mental evocation of left-sided information from an internal image of the map of France was fully recovered following prism adaptation to the right. This improvement could not be explained by the alteration of visuomotor responses induced by the prism adaptation. Therefore, prism adaptation may act not only on sensory-motor levels but also on a higher cognitive level of mental space representation and/or exploration.     

Differential processing of word and color in unilateral spatial neglect

The aim of the present study was to investigate mechanisms underlying processing of contralesional visual stimuli in brain-damaged patients with unilateral spatial neglect (USN). Nine right-hemisphere-damaged stroke patients with left-sided neglect and nine controls performed a reaction-time task involving manual response to a central color patch (target stimulus) flanked to the left or right by a Stroop stimulus they had to ignore. While the word dimension of the flanker affected patients' responses considerably and equally when presented to either side, the color dimension of the flanker had no effect when presented to the left, but had a large effect when presented to the right. Four of the patients performed a control task requiring same/different judgments between either of the two flanker dimensions (color and word) and the central target. Their performance indicated that they were able to process color information from the contralesional field, despite their results in the first experiment. These findings demonstrate a dissociation between how the patients processed different dimensions of the same stimuli and imply that the extent of processing in the contralesional hemifield depends both on task requirements and on the exact features of the stimuli. The implications of these results on normal attentional mechanisms is also discussed.     

Effects of cueing on visuospatial processing in unilateral spatial neglect

Patients with typical left unilateral spatial neglect bisected lines after cueing to the left end-point, the fixation point being monitored with an eye camera. They persisted with the point of initial fixation made after cueing and placed the mark there without searching leftwards again. The rightward shift of fixation to the initial point of fixation thus determined the location of the subjective midpoint. We consider that rightward attentional bias increased the amplitude of this shift that was planned on the basis of the perception of the whole line while cueing. This hypothesis may explain smaller but obvious rightward bisection errors found in the cueing condition.     

Disappearance of unilateral spatial neglect following a simple instruction

OBJECTIVES—To clarify the reason why patientswith left unilateral spatial neglect fail to copy the left side of adaisy like flower, not continuing to draw petals all around.
METHODS—A flower was simplified and a figure wasmade that consisted of a large central circle and small circlessurrounding it. Four patients with typical left unilateral spatialneglect performed copying and arrangement tasks to make this figure. Inthe arrangement task, they were instructed to arrange small circles allaround the printed central circle.
RESULTS—The patients' identification of thecomposition seemed flawless. In the copying task, they showed neglect,leaving a space on the left side. They seemed to adhere to their planto place the same number of small circles as those of the model figure. By contrast, neglect disappeared in the arrangement task.
CONCLUSION—Patients with neglect can draw thefigure satisfactorily if they use a spatial strategy to arrange smallcircles all around. This strategy seems to improve motivation fordrawing and awareness for the left space. It is considered that in thecopying of figures such as a daisy, failure to use a spatial strategyplays an important part in the appearance of left unilateral spatial neglect.