Horizontal space misrepresentation in unilateral brain damage. I. Visual and proprioceptive-motor influences in left unilateral neglect

Disconnection somewhere down the line: Multivariate lesion-symptom mapping of the line bisection error

Line Bisection is a simple task frequently used in stroke patients to diagnose disorders of spatial perception characterized by a directional bisection bias to the ipsilesional side. However, previous anatomical and behavioural findings are contradictory, and the diagnostic validity of the line bisection task has been challenged. We hereby aimed to re-analyse the anatomical basis of pathological line bisection by using multivariate lesion-symptom mapping and disconnection-symptom mapping based on support vector regression in a sample of 163 right hemispheric acute stroke patients. In line with some previous studies, we observed that pathological line bisection was related to more than a single focal lesion location. Cortical damage primarily to right parietal areas, particularly the inferior parietal lobe, including the angular gyrus, as well as damage to the right basal ganglia contributed to the pathology. In contrast to some previous studies, an involvement of frontal cortical brain areas in the line bisection task was not observed. Subcortically, damage to the right superior longitudinal fasciculus (I, II and III) and arcuate fasciculus as well as the internal capsule was associated with line bisection errors. Moreover, white matter damage of interhemispheric fibre bundles, such as the anterior commissure and posterior parts of the corpus callosum projecting into the left hemisphere, was predictive of pathological deviation in the line bisection task.

Prevalence and association of visual functional deficits with lesion characteristics and functional neurological deficits in patients with stroke

BACKGROUND

Stroke frequently induces visual problems, which impair activities of daily living, lead to falls, and require rehabilitation. However, visual dysfunction has not been well characterized in stroke.

OBJECTIVE

The purpose of this study was to characterize visual function in patients with stroke and the association of these characteristics with neurological dysfunction and lesion hemisphere.

METHODS

In 40 patients with stroke and 321 control subjects, we carried out an assessment of a broad panel of visual and neurological functional metrics to identify risk factors for specific visual impairments in stroke.

RESULTS

Patients with stroke exhibited a significantly higher rate of occurrence for impairments in all visual metrics assessed, when compared to healthy controls. Risk for particular visual deficits varied according to lesion side (right versus left hemisphere) and specific types of neurological dysfunction.

CONCLUSIONS

Detailed assessment of visual function in patients with stroke can help to clarify the risk of various types of visual impairment. Moreover, as visual function assessment in patients with stroke is difficult, knowledge of the correlation of visual impairments with different neurological dysfunctions observed in stroke and lesion side will help predict vision problems and inform optimal corrective measures in treating patients with stroke.

Bisecting or not bisecting: this is the neglect question. Line bisection performance in the diagnosis of neglect in right brain-damaged patients

In the present study we analysed the bisecting behaviour of 287 chronic right brain-damaged patients by taking into account the presence and severity of extrapersonal and/or personal neglect diagnosed with the hemineglect battery. We also analysed right brain-damaged patients who had (or did not have) neglect according to their line bisection performance. Our results showed that performance of the line bisection task correlates with performance of cancellation tasks, reading and perceptual tasks, but not with the presence of personal neglect. Personal neglect seems to be unrelated to line bisection behaviour. Indeed, patients affected by extrapersonal and personal neglect do not show more severe neglect in line bisection than patients with only extrapersonal neglect. Furthermore, we observed that 20.56% of the patients were considered affected or not by neglect on the line bisection task compared with the other spatial tasks of the hemineglect battery. We conclude that using a battery with multiple tests is the only way to guarantee a reliable diagnosis and effectively plan for rehabilitative training.

Long-lasting amelioration of walking trajectory in neglect after prismatic adaptation

In the present study we explored the effect of prismatic adaptation (PA) applied to the upper right limb on the walking trajectory of a neglect patient with more severe neglect in far than in near space. The patient was asked to bisect a line fixed to the floor by walking across it before and after four sessions of PA distributed over a time frame of 67 days. Gait path was analyzed by means of an optoelectronic motion analysis system. The walking trajectory improved following PA and the result was maintained at follow-up, 15 months after treatment. The improvement was greater for the predicted bisection error (estimated on the basis of the trajectory extrapolated from the first walking step) than for the observed bisection error (measured at line bisection). These results show that PA may act on high level spatial representation of gait trajectory rather than on lower level sensory-motor gait components and suggest that PA may have a long-lasting rehabilitative effect on neglect patients showing a deviated walking trajectory.

Neglect: a multisensory deficit?

Neglect is a neurological syndrome characterised by a lack of conscious perception of events localised in the contralesional side of space. Here, we consider the possible multisensory nature of this disorder, critically reviewing the literature devoted to multisensory manifestations and processing in neglect. Although its most striking manifestations have been observed in the visual domain, a number of studies demonstrate that neglect can affect virtually any sensory modality, in particular touch and audition. Furthermore, a few recent studies have reported a correlation in severity between visual and non-visual neglect-related deficits evaluated in the same patients, providing some preliminary support for a multisensory conception of neglect. Sensory stimulation and sensorimotor adaptation techniques, aimed at alleviating neglect, have also been shown to affect several sensory modalities, including some that were not directly affected by the intervention. Finally, in some cases neglect can bias multisensory interactions known to occur in healthy individuals, leading to abnormal behaviour or uncovering multisensory compensation mechanisms. This evidence, together with neurophysiological and neuroimaging data revealing the multisensory role played by the areas that are most commonly damaged in neglect patients, seems to speak in favour of neglect as a multisensory disorder. However, since most previous studies were not conducted with the specific purpose of systematically investigating the multisensory nature of neglect, we conclude that more research is needed to appropriately assess this question, and suggest some methodological guidelines that we hope will help clarify this issue. At present, the conception of neglect as a multisensory disorder remains a promising working hypothesis that may help define the pathophysiology of this syndrome.

Paradoxical extension into the contralesional hemispace in spatial neglect

To explore the idea of a perceptual distortion of space in spatial neglect, neglect patients, age-matched healthy controls and right hemisphere control patients judged the vanishing point of horizontally and vertically-moving stimuli. Hemifield of presentation and movement direction of the stimulus presentation was manipulated. The results suggest that neglect patients show a stronger response bias in the direction of the moving stimuli ("representational momentum") than healthy and right hemisphere controls. Furthermore, neglect patients, but not the control groups, showed a direction-specific response whereby the presence of neglect was associated with a larger representational momentum for leftward-moving stimuli. The one left-hemisphere patient with right-sided neglect showed the opposite effect. Thus, neglect patients showed a relative overextension into their neglected side of space. While these findings are in line with the idea of an extension in the representation of contralesional space, other explanations such as deficient spatial remapping, impairments in smooth pursuit and distortions in memorized visuo-motor movements are considered.

Monocular patching may induce ipsilateral "where" spatial bias

Spatial bias is an asymmetry of perception and/or representation of spatial information - "where" bias -, or of spatially directed actions - "aiming" bias. A monocular patch may induce contralateral "where" spatial bias (the Sprague effect [Sprague, J. M. (1966). Interaction of cortex and superior colliculus in mediation of visually guided behavior in cat. Science, 153(3743), 1544-1547]). However, an ipsilateral patch-induced spatial bias may be observed if visual occlusion results in top-down, compensatory re-allocation of spatial perceptual or representational resources toward the region of visual deprivation. Tactile distraction from a monocular patch may also contribute to an ipsilateral bias. To examine these hypotheses, neurologically normal adults bisected horizontal lines at baseline without a patch, while wearing a monocular patch, and while wearing tactile-only and visual-only monocular occlusion. We fractionated "where" and "aiming" spatial bias components using a video apparatus to reverse visual feedback for half of the test trials. The results support monocular patch-induced ipsilateral "where" spatial errors, which are not consistent with the Sprague effect. Further, the present findings suggested that the induced ipsilateral bias may be primarily induced by visual deprivation, consistent with compensatory "where" resource re-allocation.

Convergence between lesion-symptom mapping and functional magnetic resonance imaging of spatially selective attention in the intact brain

The parietal regions implicated in spatially selective attention differ between patient lesion studies and functional imaging of the intact brain. We aimed to resolve this discordance. In a voxel-based lesion-symptom mapping study in 20 ischemic stroke patients, we applied the same cognitive subtraction approach as in 23 healthy volunteers who underwent functional magnetic resonance imaging (fMRI) using identical tasks and stimuli. An instructive central cue directed attention to one visual quadrant. After a brief delay, a grating appeared in that quadrant together with an irrelevant grating in an uncued quadrant. Subjects had to discriminate the orientation of the grating in the cued quadrant. Patients with a right inferior parietal lesion were significantly more impaired during contralesional versus ipsilesional orienting when stimuli were bilateral and symmetrical than when stimuli occupied diagonally opposite quadrants or two quadrants within the same hemifield. In one area, the lesion-volume map overlapped with the activity map obtained in healthy volunteers: the lower bank of the middle third of the right intraparietal sulcus (IPS). In an additional 37 healthy fMRI subjects, we disentangled the effects of symmetry, bilaterality, and spatial configuration between stimuli on activity in the volume of overlap. Only the axis of configuration between stimuli had a significant effect, with highest activity when the configuration axis was horizontal. This constitutes converging evidence from patients and cognitively intact subjects that the lower bank of the middle third of the right IPS critically contributes to attentive selection between competing stimuli in a spatially anisotropic manner.

Assessment of sensory neglect: a study using moving images

A novel modified passive line-bisection test was performed using a computer display with a moving image. In the test, the subjects were required to give verbal responses instead of limb movements. The test, consisting of two subtasks, left-to-right and right-to-left tasks, was applied to patients with unilateral neglect and controls. The patient group showed an obvious mean deviation in both tasks, and showed greater rightward deviation in the right-to-left task than in the left-to-right task. The control group showed little differences between the two tasks. This paper discusses the results obtained from the two different conditions and their correlations. This method provides useful data for the assessment of unilateral neglect.

Unilateral neglect: A theory of proprioceptive space of a stimulus as determined by the cerebellar component of motor efference copy (and is autism a special case of neglect)

Unilateral neglect is a devastating condition, which manifests as a loss of a person's spatial awareness opposite the damaged side of the brain. It challenges our conception of the seat of the soul and its explanation is at the heart of the mind-body problem. A heuristic definition of the dorsal stream of a modality is here based on the categorization of parietal networks by the cerebellar component of motor efference copy. Taking this premise, the proprioceptive space of a stimulus is established as a concept in this paper. It is proposed that unilateral neglect is typically a dysfunction of proprioceptive space of a stimulus associated with lesions of the dorsal stream. Furthermore, most experimental findings of unilateral visual neglect (and by extrapolation, other sensory modalities), can be explained by two developmental mechanisms by which the proprioceptive space of a stimulus is encoded in the parietal cortex. Its right and left hemisphere representation can be dissociated from the hemifield of presentation of perceptual information, such that the left hemifield can have a left hemisphere representation through callosal connections and likewise, the right hemifield can have a right hemifield representation. The processing of a sensory stimulus in either parietal hemisphere is dynamically determined as shown by experimental modulation of performance. A theory of historical precedence will provide a developmental background to the organization of proprioceptive space and will invoke separate models according to specified terms of engagement. A model based on the expansion and contraction of the proprioceptive space of a stimulus as a gradient across both hemispheres and modulated by concurrent proprioceptive state will be differentiated from a model that is non-graduating but competitive and lacks such modulation. In other words, the dorsal representation of a sensory stimulus in the former case is shared to a varying degree by the two parietal hemispheres, whereas in the latter case the representation of left and right aspects of the same object stimulus is strictly divided between the two hemispheres. A further hypothesis of a dorsoventral gradient of the peripheral and foveal components of proprioceptive space characterizing dorsal stream networks will predict the double dissociation revealed by experimental paradigms. It will explain why some patients show neglect only in foveal while others only in peripheral vision. The paper proposes to unify neglect, extinction and optic ataxia on the one hand, and spatial and object-based neglect on the other hand, under a singularly proficient paradigmatic structure. A binding model as described is a component theory that acknowledges how the involved pathways or transitional zones in a pathway may contribute to a differential clinical picture as one progresses from posterior to anterior parietal cortex. Finally, a brief discussion is given on how autistic subjects neglect spatial cues and the inability of a spatial cognitive transformation underlies the impairments postulated for 'a theory of mind'.

Weights and measures: A new look at bisection behaviour in neglect

Horizontal line bisection is a ubiquitous task in the investigation of visual neglect. Patients with left neglect typically make rightward errors that increase with line length and for lines at more leftward positions. For short lines, or for lines presented in right space, these errors may 'cross over' to become leftward. We have taken a new approach to these phenomena by employing a different set of dependent and independent variables for their description. Rather than recording bisection error, we record the lateral position of the response within the workspace. We have studied how this varies when the locations of the left and right endpoints are manipulated independently. Across 30 patients with left neglect, we have observed a characteristic asymmetry between the 'weightings' accorded to the two endpoints, such that responses are less affected by changes in the location of the left endpoint than by changes in the location of the right. We show that a simple endpoint weightings analysis accounts readily for the effects of line length and spatial position, including cross-over effects, and leads to an index of neglect that is more sensitive than the standard measure. We argue that this novel approach is more parsimonious than the standard model and yields fresh insights into the nature of neglect impairment.

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

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.

Misprojection of landmarks onto the spatial map

It has been suggested that neglect patients misrepresent the metric spatial relations along the horizontal axis (anisometry). The "fabric" of their internal spatial medium would be distorted in such a way that physically equal distances appear relatively shorter on the contralesional side (canonical anisometry). The case of GL, a 76-year-old lady with left neglect on visual search tasks, is presented. GL showed severe relative overestimation on the left (contralesional) side on two independent tasks evaluating the metrics of her internal representation. A qualitatively similar pattern was found in two out of 10 other neglect patients who performed the second task. This behavior cannot be accounted for by the canonical anisometry hypothesis. Nevertheless, GL produced a relative left overextension (underestimation) when trying to set the endpoints of a virtual line given its midpoint (Endpoints Task). An interpretation of these results is offered in terms of a misprojection of relevant landmarks onto the internal representation without assuming distortion of its "fabric."

Underestimation of contralateral space in neglect: a deficit in the “where” task

Space perception was investigated in right brain damaged patients with ( N=13) and without neglect ( N=5; control group). Patients were requested to localise a target tachistoscopically flashed at various eccentricities along the horizontal meridian. All patients had an intact visual field and spared ability to manually point to a target. To segregate magno- and parvo-pathway activity, stimuli modulated in either luminance or chromatic contrast were used. Patients were required to verbally report the stimulus position (verbal task) or to manually point to the stimulus (pointing task). Neglect patients reported the stimuli in the left visual field closer to the centre than they actually were. In the verbal test, underestimation was about 7 deg at the most eccentric position tested (20 deg), and decreased linearly for smaller eccentricities. The effect was similar but less marked in the pointing task. No difference was found for stimuli with luminance or chromatic contrast. Space underestimation was confined to the contralesional space; no evidence of misperception was detected in the ipsilesional hemifield. The present findings are consistent with the view that contralesional space representation is compressed in neglect patients.

The anatomy of neglect without hemianopia: a key role for parietal-frontal disconnection?

We investigated the anatomical correlates of chronic unilateral neglect in right brain damaged patients with no visual field defects. Independently from basal ganglia or thalamus involvement, neglect patients had a subcortical area of maximal lesion overlap (maxov) in the superior longitudinal fasciculus (SLF). Neglect patients without lesion of subcortical nuclei had an additional maxov area in the rostralmost limit of the supramarginal gyrus (SmG). Patients without neglect showed a maxov area in the corticospinal tract with no involvement of the SLF or SmG. These findings call attention to the role of parietal-frontal disconnection in the pathogenesis of neglect. This disconnection can make neglect generalised and enduring also in patients suffering only partial damage of the parietal-temporal cortex, and who would otherwise show more selective attentional impairments.

Anisometry of space representation in neglect dyslexia

Patients with unilateral neglect show disorders in horizontal space perception. It has been argued that these disorders may depend on a left-right relaxation of the representational medium that becomes progressively "relaxed" toward the contralesional space and progressively "compressed" toward the ipsilesional space. We tested this hypothesis in 31 right-brain-damaged patients, 17 with neglect and 14 without neglect in two different experiments. Patients were asked to read words in canonical and anisometric letter spacing. Only in neglect patients, the manipulation of letters spacing may ameliorate neglect dyslexia. These results support the idea that the abnormalities observed in typical neglect tests are due to a distorted internal representation of the outside world. In addition, the space distortion seems to depend on the degree of horizontal relaxation of the representational medium and it is unrelated to hemianopia.

Line bisection following hemispherectomy

Two left- and right-hemispherectomized patients with contralateral hemianopia and 20 normal controls were administered a line bisection task. All hemispherectomized patients showed a strong bisection bias towards their blind visual field. This contralateral bias persisted when patients were forced to start scanning within their blind hemifield, supporting the idea of a strategic adaptation of attention towards the blind visual field. In all patients the hemispherectomy was performed as a result of cortical abnormality (congenital or acquired) and therefore early changes in functional cerebral organization may have occurred in these patients. The absence of a neglect-like ipsilateral bias and the presence of a hemianopic-like contralateral bias in line may represent a functional deficit or suggest that plastic changes following hemispherectomy induced an adaptive functional re-organization of spatial attention in both left- and right-hemispherectomized patients.

Horizontal space misrepresentation in unilateral brain damage. II. Eye-head centered modulation of visual misrepresentation in hemianopia without neglect

We used a visual distance reproduction task (endpoint task) to evaluate horizontal space representation in two left brain damaged (LBD) and three right brain damaged (RBD) patients with contralateral homonymous hemianopia and no neglect. All patients were examined in the chronic phase of the stroke and were aware of their visual field defect. Along with contralesional deviation in the line bisection task, all patients estimated size (Landmark task) and distances in the contralesional space as being longer than equivalent size and distances located in the ipsilesional space. Misreproduction of distances was abolished or reduced when the task was performed in the ipsilesional head-centred space with the head turned contralesionally. This finding points out that misrepresentation of horizontal space linked to hemianopia can be modulated by combined proprioceptive input from eye and neck muscles. The pattern of misrepresentation found in chronic hemianopic patients is opposite to the one described in chronic neglect patients with concomitant hemianopia. These different patterns of space misrepresentation are the likely consequence of the presence, in hemianopics, and the absence, in neglect patients with hemianopia, of compensatory strategies based on the non-retinotopic and multimodal coding of spatial positions falling in the retinotopically organised blind field.