To see or not to see: the effects of visible and invisible cues on line bisection judgements in unilateral neglect

Neglect and Motion Stimuli--Insights from a Touchscreen-Based Cancellation Task

Background and Purpose

In stroke patients, neglect diagnostic is often performed by means of paper-pencil cancellation tasks. These tasks entail static stimuli, and provide no information concerning possible changes in the severity of neglect symptoms when patients are confronted with motion. We therefore aimed to directly contrast the cancellation behaviour of neglect patients under static and dynamic conditions. Since visual field deficits often occur in neglect patients, we analysed whether the integrity of the optic radiation would influence cancellation behaviour.

Methods

Twenty-five patients with left spatial neglect after right-hemispheric stroke were tested with a touchscreen cancellation task, once when the evenly distributed targets were stationary, and once when the identic targets moved with constant speed on a random path. The integrity of the right optic radiation was analysed by means of a hodologic probabilistic approach.

Results

Motion influenced the cancellation behaviour of neglect patients, and the direction of this influence (i.e., an increase or decrease of neglect severity) was modulated by the integrity of the right optic radiation. In patients with an intact optic radiation, the severity of neglect significantly decreased in the dynamic condition. Conversely, in patients with damage to the optic radiation, the severity of neglect significantly increased in the dynamic condition.

Conclusion

Motion may influence neglect in stroke patients. The integrity of the optic radiation may be a predictor of whether motion increases or decreases the severity of neglect symptoms.

Flanker interference effects in a line bisection task

Previous studies have shown that flanking distractors influence line bisection. In the present study, we examined if reaching the flanker after bisecting the line resulted in a variation of flanker interference on line bisection. Right- and left-handed participants were asked to bisect a horizontal line flanked by a dot (bisection task, B-task) or to bisect the line and then to reach the dot (bisection plus reaching task, BR-task). The dot was placed laterally to, and above or below, the line edge. The results showed that in both tasks the subjective midpoint was shifted away from the position of the dot. However, this effect was greater in the BR-task than in the B-task. We suggest that the requirement to perform an action to the flanker in the BR-task induced participants to pay more attention to the dot, enhancing its salience and distorting effects on line bisection.

Representational pseudoneglect: a review

Pseudoneglect, the tendency to be biased towards the left-hand side of space, is a robust and consistent behavioural observation best demonstrated on the task of visuospatial line bisection, where participants are asked to centrally bisect visually presented horizontal lines at the perceived centre. A number of studies have revealed that a representational form of pseudoneglect exists, occurring when participants are asked to either mentally represent a stimulus or explore a stimulus using touch in the complete absence of direct visuospatial processing. Despite the growing number of studies that have demonstrated representational pseudoneglect there exists no current and comprehensive review of these findings and no discussion of a theoretical framework into which these findings may fall. An important gap in the current representational pseudoneglect literature is a discussion of the developmental trajectory of the bias. The focus of the current review is to outline studies that have observed representational pseudoneglect in healthy participants, consider a theoretical framework for these observations, and address the impact of lifespan factors such as cognitive ageing on the phenomenon.

Effect of limb movements on orienting of attention in right-hemisphere stroke

A deficit disengaging attention from the ipsilesional space in order to re-orient toward the contralesional space has been reported after right-hemisphere stroke (disengage deficit) and has been related to the severity of visuospatial neglect. Neglect rehabilitation studies have shown that left limb movements improve leftward orienting; the effect, however, is variable, and the mechanism of improvement is uncertain. Thus, this study examined whether limb movements specifically reduce the underlying disengage deficit of attention after right-hemisphere stroke. The effects of active and passive limb movements (vs. no limb movement) on orienting were examined using a covert exogenously cued orienting task in groups of right-hemisphere stroke patients with and without a significant disengage deficit (DD+, DD-) and healthy older adults. As previously seen, disengage deficit scores of stroke patients were positively correlated with the severity of neglect. The leftward disengage deficit was not affected by either active or passive limb movements, however, although movements did have both alerting and distracting effects on other aspects of orienting. Thus, our results suggest that the benefits of limb movements may not be related to changes in the underlying disengage deficit, but may impact other processes that underlie left-sided orienting (e.g., arousal and voluntary strategies).

The role of prestimulus activity in visual extinction

Patients with visual extinction following right-hemisphere damage sometimes see and sometimes miss stimuli in the left visual field, particularly when stimuli are presented simultaneously to both visual fields. Awareness of left visual field stimuli is associated with increased activity in bilateral parietal and frontal cortex. However, it is unknown why patients see or miss these stimuli. Previous neuroimaging studies in healthy adults show that prestimulus activity biases perceptual decisions, and biases in visual perception can be attributed to fluctuations in prestimulus activity in task relevant brain regions. Here, we used functional MRI to investigate whether prestimulus activity affected perception in the context of visual extinction following stroke. We measured prestimulus activity in stimulus-responsive cortical areas during an extinction paradigm in a patient with unilateral right parietal damage and visual extinction. This allowed us to compare prestimulus activity on physically identical bilateral trials that either did or did not lead to visual extinction. We found significantly increased activity prior to stimulus presentation in two areas that were also activated by visual stimulation: the left calcarine sulcus and right occipital inferior cortex. Using dynamic causal modelling (DCM) we found that both these differences in prestimulus activity and stimulus evoked responses could be explained by enhanced effective connectivity within and between visual areas, prior to stimulus presentation. Thus, we provide evidence for the idea that differences in ongoing neural activity in visually responsive areas prior to stimulus onset affect awareness in visual extinction, and that these differences are mediated by fluctuations in extrinsic and intrinsic connectivity.

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Prestimulus activity in visual extinction affects perception.

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The BOLD signal in two visual areas is indicative of perception in bilateral trials.

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Intrinsic and extrinsic connectivity is elevated prior to perceived trials.

Hemifield asymmetry in the potency of exogenous auditory and visual cues

Neurologically normal subjects misperceive the midpoints of lines (PSE) as reliably leftward of veridical center, a phenomenon known as pseudoneglect. This leftward bias reflects the dominance of the right cerebral hemisphere in deploying spatial attention. Transient visual cues, delivered to either the left or right endpoints of lines, modulate PSE such that leftward biases are increased by leftward cues, and are decreased by rightward cues, relative to a no-cue control condition. We ask whether lateralized auditory cues can similarly influence PSE in a tachistoscopic visual line bisection task, and describe how visual and auditory cues, in spatially synergistic or antagonistic combinations, jointly influence PSE. Our results demonstrate that whereas auditory and visual cues both modulate PSE, visual cues are overall more potent than auditory cues. Visual and auditory cues are weighted such that visual cues are significantly more potent than auditory cues when visual cues are delivered to left hemispace. Visual and auditory cues are equipotent when visual cues are delivered to right hemispace. These results are consistent with the existence of independent lateralized networks governing the deployment of visuospatial and audiospatial attention. An analysis of the weighting of unisensory visual and auditory cues which optimally predicts PSE in multisensory cue conditions shows that cues combine additively. There was no evidence for a superadditive multisensory cue combination.

Directional collisions during a route-following task

Neurologically normal people tend to collide with objects on the right side more frequently than with objects located on the left side of space. This phenomenon could be attributable to pseudoneglect wherein individuals selectively attend to the left field. The current study investigated this effect using a virtual route-following task that was presented centrally, in the lower field, and in the upper field. Handedness was also examined. Fifty-two participants (four left handed) completed this task, and when presented in the lower field, more left-side collisions emerged. In the upper condition, this bias reversed direction to the expected rightward bias. In the central condition, there was no significant directional bias in collision behavior. An interaction between handedness and presentation condition indicated that left-handed participants experienced more right-side collisions in the central condition. Collectively, these results suggest that directional biases (i.e., left vs. right) in collision behavior are modulated by both location in the visual field (central, upper, or lower) and handedness.

Line bisection in left neglect: the importance of starting right

When marking the subjective midpoint of a horizontal line, patients with left unilateral neglect typically deviate rightward. Different accounts of this pattern of performance refer either to a biased competition between the two hemi-segments of the line, with the right part being subjectively perceived as longer than the left part, or to a distortion of a cognitive representation of space, with spatial coordinates progressively relaxing from the right to the left. These accounts make different predictions about the role of the right part of the line, which is crucial in the biased competition account, but less important in the distortion account. To test these predictions, we asked participants to set the endpoints and the centre of perceived and imaginary lines. Contrary to previous studies, we controlled for the direction of performance of the endpoint task, with left-to-right trials and right-to-left trials being performed in separate blocks. Five patients with right-hemisphere lesions and left neglect demonstrated the typical asymmetries when a right-sided stimulus (segment or endpoint) was present, but showed either normal performance or a reversed (leftward) bias while setting the endpoints and the centre of an imaginary line starting from the left side, when no right-sided visual stimulus was present until completion of each trial. We concluded that the right-sided portion of the line has a crucial importance in determining patients' rightward deviations in line bisection, consistent with the biased competition hypothesis and with neurocognitive models of attentional orienting.

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.

Pseudoneglect in schizophrenia: a line bisection study with cueing

INTRODUCTION

Numerous authors have reported the existence of lateralised abnormalities towards the right side in patients with schizophrenia.

METHODS

In the present study, a manual line bisection task was used to assess the existence of a visuospatial bias in patients with schizophrenia as compared to healthy subjects and left unilateral neglect patients. In addition, we used a local cueing paradigm (consisting of a number placed on the right, on the left, or at both ends of the line).

RESULTS

Healthy subjects showed a leftwards trend in the "no cue" condition (known as pseudoneglect) and neglect patients showed a right bias in all cue conditions. In contrast, patients with schizophrenia placed their manual estimation of the centre further to the left than healthy subjects in all cue conditions, reflecting neglect of the right side of the line. Moreover, like healthy subjects and neglect patients, patients with schizophrenia were affected by the local cueing.

CONCLUSION

Hence, patients with schizophrenia show a bias in their spatial representation, which does not interfere with local context processing.

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'.

Pseudoneglect: Evidence for both perceptual and attentional factors

When neurologically normal individuals bisect a horizontal line as accurately as possible, they reliably show a slight leftward error. This leftward inaccuracy is called pseudoneglect because errors made by neurologically normal individuals are directionally opposite to those made by persons with visuospatial neglect (Jewell & McCourt, 2000). In the current study, normal right-handed observers bisected horizontal lines that were altered to bias line length judgments either toward the right or the left side of the line. Non-target dots were placed on or near the line stimuli using principles derived from a theory of visual illusions of length called centroid extraction (Morgan, Hole, & Glennerster, 1990). This theory argues that the position of a visual target is calculated as the mean position of all stimuli in close proximity to the target stimulus. We predicted that perceptual alterations that shifted the direction of centroid extraction would also shift the direction of line bisection errors. Our findings confirmed this prediction and support the idea that both perceptual and attentional factors contribute to the pseudoneglect effect.

Putting attention on the line: investigating the activation-orientation hypothesis of pseudoneglect

Neurologically healthy participants systematically misbisect horizontal lines to the left of centre, a phenomenon termed 'pseudoneglect'. According to the activation-orientation hypothesis, the distribution of attention is biased in the direction opposite to the more activated hemisphere. Since visuospatial tasks involve activation of the right hemisphere, the hypothesis suggests that a leftward line-bisection bias might be explained by the uneven distribution of attention to the left and right line segments. A crucial assumption of this explanation is that the more attended half of the line will be perceived as longer than the less attended half. This study uses a tachistoscopic Landmark test and an attention cueing paradigm to explore this assumption. Three conditions were met to demonstrate the relative elongation of the more attended half of the line: (1) attention was biased to the cued end of the line, (2) subjective line midpoint was shifted towards the cued end, and (3) alternative biasing factors were ruled out. The results also demonstrate that increased hemispheric activation, resulting from presentation of stimuli in one or the other visual field, leads to subjective midpoints that are biased away from the more activated hemisphere.

Understanding the effects of moving visual stimuli on unilateral neglect following stroke

Moving visual stimuli have been shown to reduce unilateral neglect (ULN), however, the mechanisms underlying these effects remain poorly understood. This study compared lateralised and non-lateralised moving visual stimuli to investigate whether the spatial characteristics or general alerting properties of moving visual stimuli are responsible for reducing neglect. Post-stroke left neglect patients as well as healthy and patient control subjects were tested on a computerised line bisection task under six visual stimulus conditions. The key finding was that, relative to the no stimulus condition, leftward moving and left-sided moving visual stimuli shifted neglect patients' bisection errors leftward while the non-lateralised random moving visual stimuli did not reduce neglect patients' rightward bisection errors. The results provide evidence that spatial characteristics rather than general alerting properties of moving visual stimuli reduce rightward bisection errors in ULN. Moreover, the pattern of findings strongly supports the notion that moving visual stimuli reduce neglect by capturing attention and drawing it to a spatial location rather than by activating the attentional system via superior collicular neurons.

Effects of emotional face cueing on line bisection in neglect: a single case study

One patient with left neglect (FM) and four right brain-damaged controls were tested on a line bisection task with pictures of neutral and emotional faces as unilateral cues. We thus manipulated the attentional salience of the cues (higher for emotional and lower for neutral faces) while keeping constant their physical dimensions. Our findings showed that left emotional faces were more effective than left neutral faces in reducing bisection errors only in FM. These data indicate that in the neglected hemispace cues bias attention rather than simply altering the perceptual point of balance of the line in the horizontal plane.

"Mind the gap": the size-distance dissociation in visual neglect is a cueing effect

There is a growing body of evidence that the processes mediating the allocation of spatial attention within objects may be separable from those governing attentional distribution between objects. In the neglect literature, a related proposal has been made regarding the perception of (within-object) sizes and (between-object) distances. This proposal follows observations that, in size-matching and bisection tasks, neglect is more strongly expressed when patients are required to attend to the sizes of discrete objects than to the (unfilled) distances between objects. These findings are consistent with a partial dissociation between size and distance processing, but a simpler alternative must also be considered. Whilst a neglect patient may fail to explore the full extent of a solid stimulus, the estimation of an unfilled distance requires that both endpoints be inspected before the task can be attempted at all. The attentional cueing implicit in distance estimation tasks might thus account for their superior performance by neglect patients. We report two bisection studies that address this issue. The first confirmed, amongst patients with left visual neglect, a reliable reduction of rightward error for unfilled "gap" stimuli as compared with solid lines. The second study assessed the cause of this reduction, deconfounding the effects of stimulus type (lines vs. gaps) and attentional cueing, by applying an explicit cueing manipulation to line and gap bisection tasks. Under these matched cueing conditions, all patients performed similarly on line and gap bisection tasks, suggesting that the reduction of neglect typically observed for gap stimuli may be attributable entirely to cueing effects. We found no evidence that a spatial extent, once fully attended, is judged any differently according to whether it is filled or unfilled.

An investigation of the relationship between free-viewing perceptual asymmetries for vertical and horizontal stimuli

Two experiments examine the relationship between free-viewing vertical and horizontal perceptual biases. In Experiment 1, normal participants (n=24) made forced-choice luminance judgments on two mirror-reversed luminance gradients (the 'grayscales' task). The stimuli were presented in vertical, horizontal and oblique (+/-45 degrees ) orientations. Leftward and upward biases were observed in the horizontal and vertical conditions, respectively. In the oblique conditions, leftward and upward biases combined to produce a strong shift of attention away from the lower/right space toward the upper/left. Regression analyses revealed that the oblique biases were the combined product of the vertical and horizontal biases. A lack of correlation between the vertical and horizontal biases, however, suggests they reflect the operation of independent cognitive/neural mechanisms. In Experiment 2, the same stimuli were given to right-hemisphere-lesioned patients with spatial neglect (n=4). Rightward and upward biases were observed for horizontal and vertical stimuli, respectively. The biases combined to produce a strong shift of attention away from the lower/left space toward the upper/right. While our research demonstrates that vertical and horizontal attentional biases are additive, it also appears that they reflect the operation of independent cognitive/neural mechanisms. Potential applications of these findings to the remediation of spatial neglect are discussed.

The contribution of spatial remapping impairments to unilateral visual neglect

Left visual neglect following right hemisphere damage is a heterogeneous phenomenon, in which several underlying impairments have been identified. Despite recent advances in understanding the neural and cognitive bases of these impairments, current theories of neglect, particularly those that emphasise attentional deficits, do not explain a number of phenomena, including: 'Ipsilesional' neglect after left orienting. Positive or 'productive' manifestations. Spatial transposition errors. Mislocalisations. Revisiting behaviour during visual search. Lack of awareness for objects toward the contralesional side of space. We propose that these manifestations of neglect can be accounted for by an additional underlying disorder of spatial remapping due to parietal dysfunction. In primary visual areas, retinotopic maps are renewed and thus overwritten at each new ocular fixation. Remapping processes operating in higher-level oculocentric visual maps of the parietal cortex ensure visual integration of these successive retinal images over time and space, by creating a constantly updated representation of stimulus locations in terms of distance and direction from the fovea. They consist in the storage, refreshment and re-localization of the different components of the visual scene that are successively attended during its exploration, and provide spatial constancy of visual perception and a spatial buffer for working memory [Cereb Cortex 5 (1995) 470; Visual Cogn 7 (2000) 17]. We begin this article by reviewing theoretical and experimental arguments that have highlighted the importance of parietal remapping processes in maintaining an accurate representation of space across saccadic shifts. We then focus on findings from the double-step saccade task, [Ann Neurol 38 (1995) 739] as a basis for our model of the role of remapping impairments in many of the symptoms of neglect. From these results, remapping impairments would be demonstrated when a saccade has to be guided across the midline after having fixated an object in either the left or right visual field for patients with either left- or right-side parietal lesions. In addition, patients with right-side lesions will have remapping impairments within the left visual field following a saccade to a left-side target (see Fig. 5). In a large part of the article, we seek to build our hypothesis based on this basic model and more speculative assumptions supported with extensive evidence from the literature.

Right parietal lesions, unilateral spatial neglect, and the egocentric frame of reference

Recently, the hypothesis has been proposed that the crucial mechanism leading to neglect is the disturbed transformation of sensory input into a supramodal egocentric frame of reference (ER), which causes in turn a deviation of this reference frame toward the side ipsilateral to the brain lesion. This egocentric coordinate system is normally centered on the midsagittal plane, but a unilateral brain lesion may cause a deviation of the egocentric reference due to an imbalance between the differentially lateralized neural processes which build this representation. Although neglect signs are often defined in an egocentric frame, and an ER shift may be observed in some right-brain-damaged patients (RBD), I present here several data that rule out any causal link between the deviation of the egocentric reference and the presence and/or severity of left-neglect signs in RBD patients.

Preserved obstacle avoidance during reaching in patients with left visual neglect

We asked 12 patients with left visual neglect to bisect the gap between two cylinders or to reach rapidly between them to a more distal target zone. Both tasks demanded a motor response but these responses were quite different in nature. The bisection response was a communicative act whereby the patient indicated the perceived midpoint. The reaching task carried no imperative to bisect the gap, only to maintain a safe distance from either cylinder while steering to the target zone. Optimal performance on either task could only be achieved by reference to the location of both cylinders. Our analysis focused upon the relative influence of the left and right cylinders on the lateral location of the response. In the bisection task, all neglect patients showed qualitatively the same asymmetry, with the left cylinder exerting less influence than the right. In the reaching task, the neglect group behaved like normal subjects, being influenced approximately equally by the two cylinders. This was true for all bar two of the patients, who showed clear neglect in both tasks. We conclude that the visuomotor processing underlying obstacle avoidance during reaching is preserved in most patients with left visual neglect.

The greyscales task: a perceptual measure of attentional bias following unilateral hemispheric damage

The two cerebral hemispheres in humans have been suggested to control contralaterally opposed attentional biases. These biases may be revealed by unilateral hemispheric damage, which often causes contralesional spatial neglect, particularly when the right hemisphere (RH) is affected. Subtle attentional biases have also been observed in normal observers in tasks requiring judgements of horizontal spatial extent, brightness, numerosity and size. Here, we examined attentional biases for judging the darker of two left-right mirror-reversed brightness gradients under conditions of free viewing (the greyscales task). We compared performances of patients with damage to the RH (n=78) and left hemisphere (LH; n=20) with those of normal controls (n=20). Controls showed a small but significant leftward bias, implying a subtle asymmetry favouring the RH. In contrast, RH and LH patients showed extreme rightward and leftward biases, respectively, both of which differed significantly from that of controls. For the patient groups, performance on clinical tests of neglect (cancellation and line bisection) did not predict their greyscales scores. Pathological biases were present in patients without clinical neglect or visual field defects, suggesting that the attentional bias measured by the greyscales task can be dissociated from clinical neglect and visual sensory loss. The greyscales task offers an efficient means of quantifying pathological attentional biases in unilateral lesion patients; it is easy to administer and score, and may be particularly useful for clinical trials of recovery and rehabilitation following stroke.

Visual, proprioceptive and tactile performance in left neglect patients

Patients with unilateral spatial neglect due to right-hemisphere lesions typically fail to attend to and explore left-sided stimulus objects. It has been postulated that in right-brain damaged (RBD) patients an ipsilesional displacement of the egocentric frame of reference (ER), whether visual or tactile, may be responsible for a contralesional supramodal spatial bias causing their left neglect behavior. However, this hypothesis had been proposed without testing, in the same patients, the position of the ER or their performance in the visual and tactile modalities. Thus, the aim of the present study was to test the hypothesis that an ipsilateral shift of the ER is responsible for a supramodal spatial bias in neglect. For this purpose, a within-subject design is required. Consequently, 12 left neglect patients and 12 control subjects were asked to perform a proprioceptive straight-ahead pointing task while blindfolded, as well as visual and tactile bisection tasks. In the left neglect patients, we found:no systematic deviation of the ER on the ipsilesional right side;a significant rightward bias in visual bisection, and normal performance in tactile bisection;no correlation among the three tasks;that only visual bisection correlated with the severity of neglect. These results are discussed with regard to the egocentric and attentional hypothesis of neglect.

Can free-viewing perceptual asymmetries be explained by scanning, pre-motor or attentional biases?

Judgments of relative magnitude between the left and right sides of a stimulus are generally weighted toward the features contained on the left side. This leftward perceptual bias could be the result of, (a) left-to-right scanning biases, (b) pre-motor activation of the right hemisphere, or (c) a left hemispatial attentional bias. The relative merits of these explanations of perceptual asymmetry were investigated. In Experiment 1, English and Hebrew readers made luminance judgements for two left/right mirror-reversed luminance gradients (greyscales task). Despite different reading/scanning habits, both groups exhibited a leftward perceptual bias. English and Hebrew readers also performed a line bisection task. Scanning biases were controlled by asking participants to follow a marker as it moved left-to-right or right-to-left and then stop it as it reached the midpoint of the line. Despite controlling scanning, a leftward bias was observed in both groups. In Experiment 2, peripheral spatial cues were presented prior to the greyscales stimuli. English readers showed a reduction in the leftward bias for right-sided cues as compared to left-sided and neutral cues. Right-side cues presumably overcame a pre-existing leftward attentional bias. In both experiments, pre-motor activation was controlled using bimanual responses. Despite this control, a leftward bias was observed throughout the study. The data support the attentional bias account of leftward perceptual biases over the scanning and pre-motor activation accounts. Whether or not unilateral hemispheric activation provides an adequate account of this attentional bias is discussed.

Effects of visible and invisible cueing on line bisection and Landmark performance in hemispatial neglect

A total of 12 patients with hemispatial neglect (and two control groups) were tested to examine the effects of lateralized cues on line bisection and Landmark judgements. The experiment was designed to investigate whether bisection and landmark biases induced by cueing are simply a result of a direct perceptual lengthening of the cued part of the line caused by the fact that the cue is visible, thus creating a composite 'line plus cue' or whether cueing indeed induces an attentional bias. Secondly, earlier work by Harvey et al. [Harvey M, Milner AD, Roberts RC. An investigation of hemispatial neglect using the landmark task, Brain and Cognition 1995; 27: 59-78] has shown that in neglect patients cues work by inducing orientational biases rather than via the alteration of subjective length perception. An attempt was made to replicate this finding and extend it to cues that are not physically present. The bisection data clearly showed that cues bias attention rather than work via a direct lengthening of the line: both visible and invisible cues biased bisection performance equally well. The Landmark data, however, revealed much less clear-cut results and we failed to repeat the earlier observation by Harvey et al. that cues induce orientational biases. Even when the neglect patients were categorised into premotor and perceptual categories a clear effect failed to emerge. It is hypothesised that the earlier reported effect may be linked to neglect severity rather than to perceptual type neglect.

Illusion processing in hemispatial neglect

Twelve patients with hemispatial neglect and two control groups were tested to examine the effects of the Müller-Lyer and Judd illusions on bisection behaviour. The studies were designed to investigate whether neglect patients were indeed unaware of the left sides of the illusory figures. In Experiment 1, participants were asked to describe the illusory figures prior to bisection, whereas in Experiment 2, they compared two illusions whose fins, in the critical condition, differed on the left and then performed the bisection. It was found that the illusions worked equally well in all three groups. Interestingly, apart from one exception, almost all neglect patients explicitly reported the left-sided fins in Experiment 1. Only five patients failed to do so but only on an average of 16% of trials. In Experiment 2, six patients made errors in the comparison task but four of these patients did not neglect any left-sided fins in Experiment 1 (with the exception of three overall trials for LC and EdR). This finding seems a good indication that the two tasks differ in their requirements. The comparison task may be perceived as harder as it requires discrimination rather than detection and thus lead to more neglect type errors than the bisection task. In one neglect patient, the illusions consistently failed to work. This patient presented with an occipito-temporal and basal ganglia lesion and the mechanisms responsible for the processing of simple visual features might have possibly been impaired in her case.

Effektivität experimenteller und rehabilitativer Interventionen bei visuellem Neglect - Eine Metaanalyse

Zusammenfassung: Präsentiert wird eine Metaanalyse zur Wirksamkeit neuropsychologischer Interventionen bei visuellem Hemineglect. Es wurden die publizierten Outcome-Ergebnisse Experimental- und Rehabilitationsstudien zur Modifikation und Modulation des Neglectsyndroms quantitativ integriert. 133 identifizierte Studien wurden in sechs Klassen eingeteilt. Studien-, Interventions- und Patientenmerkmale dienten zur Bestimmung moderierender Einflüsse, über Sensitivitätsanalysen wurde die Robustheit der Ergebnisse abgeschätzt. Die mittlere Gesamteffektivität der Interventionen liegt um 0.8 Standardeinheiten Symptomverbesserung gegenüber dem Prä-Niveau bei Patienten mit chronischem Neglect. Spontanremissionseffekte sind aufgrund der durchschnittlichen Erkrankungsdauer als gering zu veranschlagen und konnten empirisch nicht nachgewiesen werden. Explorationstrainings, personale Intervention und Hinweisreiz-Verfahren stellen sich als überdurchschnittlich effektiv und signifikant effektiver als bloße Reduktion der Umweltanforderungen heraus.

Pseudoneglect: a review and meta-analysis of performance factors in line bisection tasks

An exhaustive qualitative (vote-counting) review is conducted of the literature concerning visual and non-visual line bisection in neurologically normal subject populations. Although most of these studies report a leftward bisection error (i.e., pseudoneglect), considerable between-study variability and inconsistency characterize this literature. A meta-analysis of this same literature is performed in which the total quantitative data set, comprising 73 studies (or sub-studies) and 2191 subjects, is analyzed with respect to 26 performance factors. The meta-analytic results indicate a significant leftward bisection error in neurologically normal subjects, with an overall effect size of between -0.37 and -0.44 (depending on integration method), which is significantly modulated to varying degrees by a number of additional task or subject variables. For example, visual bisection tasks, midsagittal-pointing tasks and tactile bisection tasks all lead to leftward errors, while kinesthetic tasks result in rightward errors. Tachistoscopic forced-choice testing methods reveal much greater estimates of bisection error (effect size = -1.32) than do manual method-of-adjustment procedures (effect size= -0.40). Subject age significantly modulates line bisection performance such that older subjects err significantly rightward compared to younger subjects, and to veridical line midpoint. Male subjects make slightly larger leftward errors than do female subjects. Handedness has a small effect on bisection errors, with dextrals erring slightly further to the left than sinistral subjects. The hand used to perform manual bisection tasks modulated performance, where use of the left hand lead to greater leftward errors than those obtained using the right hand. One of the most significant factors modulating bisection error is the direction in which subjects initiate motor scanning (with either eye or hand), where a left-to-right scan pattern leads to large leftward errors while a right-to-left scan pattern leads to rightward errors.

Effects of visible and invisible cueing procedures on perceptual judgments in young and elderly subjects

Healthy subjects were tested in two experiments to examine the effects of lateralized cues on line bisection and landmark judgments. The studies were designed to investigate whether bisection and landmark biases induced by cueing are simply a result of a direct perceptual lengthening of the cued part of the line caused by the fact that the cue is visible, thus creating a composite 'line plus cue' or whether cueing induces an attentional bias operating on judgments of spatial extent by either reducing the magnitude of the parts of the stimulus receiving 'less' attention or magnifying those parts receiving 'more'. Lateralized cues were either visible letter cues or invisible marks drawn with a leadless pencil either by the subject him/herself (Experiment 1) or the experimenter (Experiment 2). Comparable to a previous study (Mattingley, Pierson, Bradshaw, Phillips and Bradshaw, 1993, Neuropsychologia, 31, 1201-1215), the first experiment showed that only visible cues affected line bisection and landmark judgments thus favouring the perceptual explanation. The second study, however, revealed bisection and judgment biases for invisible as well as visible cues with the attended part of the line appearing subjectively longer. These results indicate that attentional modulations can increase the salience of a line in a similar vein to physical changes. It seems likely that the requirement of a motor response in Experiment 1 cancelled out all attentional properties supposedly induced by placing that invisible cue.

Free-viewing perceptual asymmetries for the judgement of brightness, numerosity and size

Perceptual asymmetries under free-viewing conditions were investigated in 24 normal dextral adults. Three tasks were administered that required participants to chose between a pair of left/right reversed stimuli on the basis of their brightness, numerosity or size. These stimulus features were represented asymmetrically within the stimuli, so that each stimulus appeared darker, larger or more numerous on the left or right sides. Participants more often selected the stimulus with the relevant feature on the left-hand side for all three tasks. Response times for leftward responses were faster than rightward responses. Split-half reliabilities revealed a high level of consistency within the tasks. However, the correlation between tasks was low. These results suggest that the different tasks, while showing similar levels of perceptual asymmetry, engage distinct sets of lateralised processes.

Scanning direction and line bisection: a study of normal subjects and unilateral neglect patients with opposite reading habits

Sixty normal dextrals (30 left-to-right and 30 right-to-left readers) and two left unilateral neglect patient with opposite reading habits performed a passive line bisection task. In order to study the effect of scanning direction on performance, subjects had to stop a mark moving on the to-be-bisected line either from the left to the right or in the opposite direction. Results showed that the position of the subjective middle was dependent upon the scanning direction of the line for all the subjects. A leftward deviation appeared for left to right scanning, whereas a rightward shift occurred when the mark moved from the right to the left. These results emphasize the role of scanning direction in space organization and are discussed with respect to the explanatory hypotheses of unilateral neglect.

Diagonal spatial neglect

OBJECTIVE

To determine whether stroke patients with diagonal neglect on cancellation may show diagonal neglect on line bisection, and hence to indicate whether diagonal neglect may be related solely to the type of test used or whether instead it may reflect a fundamental spatial disorder.

METHODS

Nine patients with subacute right hemispheric stroke who neglected targets primarily in the near left direction on line cancellation bisected diagonal lines of two opposing orientations: near left to far right and far left to near right. The errors were assessed to determine whether line orientation significantly affected bisection error.

RESULTS

Eight patients had significant bisection errors. One of these showed no effect of line orientation on error, consistent with lateral neglect. The remaining seven patients had a line orientation effect, indicating a net diagonal spatial bias. For the group, cancellation errors were significantly correlated with the line orientation effect on bisection errors.

CONCLUSIONS

A significant diagonal bias on two tests of spatial attention may appear in stroke patients, although the directions of the biases may differ within individual patients. None the less, diagonal neglect may be a fundamental spatial attentional disturbance of right hemispheric stroke. Greater severity of stroke deficit as indicated by cancellation error score may be associated with a greater degree of diagonal neglect on line bisection.

The effects of gender, menstrual phase and practice on the perceived location of the midsagittal plane

Women show menstrual phase-related cognitive changes that suggest altered hemispheric activation for a particular task, such that they demonstrate the greatest lateral performance differences on prototypical left hemisphere tasks during the luteal phase and on prototypical right hemisphere tasks during menstruation. Additionally, menstrual phase may alter total cerebral responsiveness, such that response times and performance accuracy for many tasks are best during the luteal phase and most impaired during the menstrual phase. We evaluated the effect of menstrual phase on spatial bisection (a perceptuospatial task) to help further understand hormonally-mediated changes in interhemispheric dynamics. Healthy young adult women and men blindly pointed to their midsagittal plane with either hand. Women were repeatedly tested according to menstrual phase, and men were tested at similar intervals. The mean pointing error in the luteal phase differed significantly from that of all other phases and did not differ significantly from those of men, who pointed significantly to the left across test sessions. These findings suggest that, in space bisection tasks, women are more likely to have asymmetric hemispheric activation during the luteal phase than during the menstrual phase. Thus, space bisection did not resemble other prototypical right hemisphere behaviors. The luteal phase may have nonspecifically activated both hemispheres on this task instead of suppressing right hemisphere function, and a slight functional asymmetry favoring the right hemisphere may have been promoted. In addition, intermanual pointing discrepancies in both subject groups decreased over repeated sessions. This suggests that, while practice alters an internal kinesthetic reference, it does not influence an imaginal extrapersonal spatial reference.

Contributions of the left and right cerebral hemispheres to line bisection

Thirty-one right-hemisphere lesioned (RHL) patients, 11 left-hemisphere lesioned patients (LHL) and 10 normal controls (NC) bisected lines in three spatial location and four directional cuing conditions. The error direction and error size were analyzed as separate and combined variables. Seventy-seven percent of RHL patients and 45% of LHL patients made abnormally large errors in line bisection. Right-hemisphere lesioned patients were more sensitive to spatial location and directional cuing than NC subjects. In contrast, LHL patients were less sensitive to either condition than NCs. The error direction and error size emerged as dissociable components of line bisection. Right-hemisphere lesioned patients and NC subjects bisected lines consistently to one side of the true center. Left-hemisphere lesioned patients bisected lines equally often on both sides of the true center. Both RHL and LHL patients made larger absolute bisection errors than NC subjects, but the RHL patient's errors were larger than those of the LHL patients. We propose that the greater sensitivity of RHL patients to spatial location and directional cues and the directional consistency of their bisection errors represent contributions of the intact left cerebral hemisphere to line bisection. In contrast, the LHL patient's unrestrained ability to orient to both ends of the line reflects a contribution of the intact right cerebral hemisphere to line bisection. The failure of both groups to accurately bisect lines reflects a common visuospatial processing deficit that is more pronounced following RHLs than LHLs.

Bisection and perception of horizontal lines in normal children

Simple line bisection is a complex task in which both perceptual/attentional factors and motor factors are involved. Using a judgment task on prebisected lines it is possible to assess independently the role of perceptual/attentional factors. The judgment-bisection relationship was investigated in 37 preschool children aged 4-5 and 70 school children aged 10-12. At bisection, a clear shift of the right hand with age from right to left bisections was observed, more pronounced in right- than in left-handers, and probably related to learned patterns. At judgment, a slight but significant left side overestimation was observed. In right-handers, judgment was related to left-hand but not to right-hand bisection. It is concluded that motor (manual) factors strongly influence visuo-motor bisection in normal children, and it is suggested that the judgment-bisection relationship has to be systematically investigated when bisection is used as a task informing on space perception or on direction of attention.

Effects of left-sided movements on line bisection in unilateral neglect

Thirteen patients with left neglect performed line bisection under four conditions: no cue, visual cueing involving the report of a digit placed at the left end of the line, circling the left-end digit, and digit circling plus tracing of the line with the right index finger from its left end to its midpoint before bisection. Digit circling plus finger tracing was unequivocally more effective in reducing left neglect than digit circling alone, which was in turn more effective than visual cueing; indeed, digit circling with tracing completely abolished the rightward bisection bias. Thus continuously directing visuomotor control to the left side of the line (even with the right hand) until bisection is performed reduces neglect more than only requiring patients to attend to left-sided visual cues. The facilitatory effects of the cueing procedures may reflect their differential efficacy in constraining as well as attracting attention and action to the left part of the target line. These findings have implications for neglect rehabilitation.

Visual and motor components in simple line bisection: an investigation in normal adults

Visuomotor bisection and bisection judgement of centrally presented 10-cm horizontal lines were proposed to 43 normal adults, both on paper and on the vertical screen of a computer. Tasks on paper showed slight left-side of the line overestimation and weak link between actual bisection and judgement. Tasks on screen showed, on the contrary, right-side of the line overestimation and stronger link between actual bisection and judgement. Centrally pretransected lines were often judged wrongly. These results suggest an important role of motor factors on classical paper-and-pencil visuomotor line bisection, and the relevance of the distinction between peripersonal and extrapersonal space.

Is left always right? Directional deviations in visual line bisection as a function of hand and initial scanning direction

Directional deviations in visual line bisection were investigated using normal subjects. Significant main effects were found for hand and initial scan direction resulting from significantly greater deviations to the left by the left hand compared with the right hand and by a scan from the left compared with a scan from the right. These results suggest that the amelioration of neglect can only be inferred from the left hand deviations of neglect patients if they are significantly leftwards of the objective middle and that the degree of leftward deviation in normal subjects results from an interaction between right hemispheric activation and unilateral allocation of attention.

Reading habits and line bisection: a developmental approach

120 normal right-handed subjects, children and adults, with opposite reading habits (60 French, left-to-right readers, 60 Israeli, right-to-left readers) and 60 pre-school children (30 French and 30 Israeli), were submitted to a visuo-motor bisection task. Bisection is found to be dependent upon reading habits with a leftward deviation of the subjective middle for left-to-right readers, and a rightward bias for right-to-left readers. Even before formal reading learning, French and Israeli pre-school children differ significantly in bisecting a line. Results are discussed with respect to hemispheric activation theories and directional hypotheses.

The effects of unilateral visuospatial neglect on perception of Müller-Lyer illusory figures

Left visuospatial neglect after right hemisphere damage is a lateralised disorder of spatial perception and cognition. A study is reported of the extent to which patients with left visuospatial neglect are susceptible to the illusory effects normally elicited by Müller-Lyer figures, in which inducing fins are located on one end or on both ends of a horizontal line. Seven patients with left visuospatial neglect and seven normal controls were tested on a task of horizontal-line bisection in which stimuli consisted of plain lines of three lengths (100, 150, and 200 mm), randomly interleaved with lines with unilateral or bilateral fins. As a group, normals made accurate bisection judgments in the baseline (no-fins) condition, and exhibited significant illusory effects in both the unilateral-fin and the bilateral-fin conditions. In contrast, patients made substantial rightward errors in the baseline condition, consistent with their neglect of the left end of the line. However, as a group, patients still exhibited significant illusory effects with left-sided outward-projecting fins on 100 mm lines and with left-sided inward-projecting fins on 150 mm lines. Moreover, at least one patient exhibited consistent illusory effects both for inward-projecting and for outward-projecting left-sided fins at all line lengths. Normal illusory effects in patients were also obtained with stimuli containing unilateral right-sided and bilateral fins. The existence of such effects with inducing elements on the contralesional extremity of horizontal line stimuli suggests preservation of low-level, perhaps preattentive, perceptual mechanisms responsible for coding elementary visual features. The results may be relevant for an understanding of the influence of attentional factors on illusory perception.

Horizontal visual motion modulates focal attention in left unilateral spatial neglect

Patients with unilateral spatial neglect are impaired in directing focal attention toward the contralesional side of space. Provision of static spatial cues on the neglected side has previously been shown to help overcome this deficit. Common movement of visual stimuli may also guide the allocation of spatial attention, although such effects have not been examined in patients with unilateral spatial neglect. Eleven patients with right hemisphere damage and clinical evidence of left unilateral spatial neglect, and 11 matched, healthy controls were tested on a task of horizontal line bisection. Lines were presented on a computer display, with a neutral, static, or slowly drifting, random dot background. Under conditions of motion, background stimuli drifted either leftward or rightward, across the full width of the display, at speeds that did not elicit optokinetic nystagmus or perceptual aftereffects. Controls were accurate in all conditions, and showed minimal effects of background conditions. By contrast, patients with left unilateral spatial neglect were sensitive to leftward background motion, showing a significant leftward shift in bisection error, relative to neutral, static, and rightward moving backgrounds. There was no significant effect of rightward motion in comparison with the neutral and static conditions. The extent to which patients were susceptible to the effects of background motion was not related to severity of unilateral spatial neglect, as measured by clinical tests. The benefits of leftward motion may reflect activity of preserved motion processing mechanisms, which provide input to an otherwise dysfunctional attentional network. The use of visual motion to assist in contralesionally guiding focal attention may be useful in the rehabilitation of unilateral spatial neglect.

Impairments of movement execution in unilateral neglect: a kinematic analysis of directional bradykinesia

A kinematic analysis was performed on goal-directed movements made by 14 patients with right hemisphere damage and left unilateral neglect (seven mild, seven severe), and 14 healthy controls. Leftward and rightward pen strokes of varying extents were made to targets of varying size on a centrally located digitising tablet. While mild unilateral neglect patients performed like controls, patients with severe unilateral neglect were slower to initiate leftward than rightward strokes and were slow and inefficient in movement execution. Leftward strokes made by severe unilateral neglect patients were characterised by prolonged movement time, lower peak velocity and departed from optimal bell-shaped velocity profiles. Their leftward strokes also showed prolonged accelerative phases, implying difficulties in force production, while the high proportion of their total movement time spent in decelerating with rightward strokes suggested an abnormal emphasis on terminal visual guidance. Leftward strokes made by these patients also contained more submovements than rightward strokes, suggesting poor force control. An impaired internal representation of the location of left-sided targets and desired movement trajectories in severe unilateral neglect causes breakdown in the temporal control of goal-directed movements.

Can task specific perceptual bias be distinguished from unilateral neglect?

The present study examined visuoperceptual bias in 12 right hemisphere damaged patients, eight of whom showed left unilateral neglect on standard clinical tests, and in 30 normal controls. In the chimeric faces task, subjects were required to judge which of a pair of faces appeared happier. Stimuli comprising each pair were mirror images, with the smiling half on the left of one face and on the right of the other. In the grey scales task, subjects were required to indicate which of two shaded rectangles appeared to be darker overall. Again, stimuli were mirror images, with the darker end appearing either on the left or on the right. Patients exhibited a significant rightward bias on both experimental tasks, in contrast to the significant leftward bias exhibited by controls. There was no significant correlation between patients' performances on standard clinical tests and the extent of bias on the two experimental tasks, suggesting that such patients exhibit distinct impairments of spatial cognition which are differentially indexed by the two types of task. Moreover, for both patients and controls, scores obtained on the two perceptual bias tasks were unrelated, suggesting that they may engage stimulus-specific processes which have different underlying patterns of asymmetrical processing. These data provide further support for models which propose that the heterogeneity of disorders of spatial cognition arise from disruption of distinct neural mechanisms.