The novelty effect in recovered hemineglect

Uncovering the (un)attended: Pupil light responses index persistent biases of spatial attention in neglect

Visuospatial neglect is a frequent and disabling disorder, mostly after stroke, that presents in impaired awareness to stimuli on one side of space. Neglect causes disability and functional dependence, even long after the injury. Improving measurements of the core attentional deficit might hold the key for better understanding of the condition and development of treatment. We present a rapid, pupillometry-based method that assesses automatic biases in (covert) attention, without requiring behavioral responses. We exploit the phenomenon that pupil light responses scale with the degree of covert attention to stimuli, and thereby reveal what draws (no) attention. Participants with left-sided neglect after right-sided lesions following stroke (n = 5), participants with hemianopia/quadrantanopia following stroke (n = 11), and controls (n = 22) were presented with two vertical bars, one of which was white and one of which was black, while fixating the center. We varied which brightness was left and right, respectively across trials. In line with the hypotheses, participants with neglect demonstrated biased pupil light responses to the brightness on the right side. Participants with hemianopia showed similar biases to intact parts of the visual field, whilst controls exhibited no bias. Together, this demonstrates that the pupil light response can reveal not only visual, but also attentional deficits. Strikingly, our pupillometry-based bias estimates were not in agreement with neuropsychological paper-and-pencil assessments conducted on the same day, but were with those administered in an earlier phase post-stroke. Potentially, we pick up on persistent biases in the covert attentional system that participants increasingly compensate for in classical neuropsychological tasks and everyday life. The here proposed method may not only find clinical application, but also advance theory and aid the development of successful restoration therapies by introducing a precise, longitudinally valid, and objective measurement that might not be affected by compensation.

Machine learning algorithms on eye tracking trajectories to classify patients with spatial neglect

BACKGROUND AND OBJECTIVE

Eye-movement trajectories are rich behavioral data, providing a window on how the brain processes information. We address the challenge of characterizing signs of visuo-spatial neglect from saccadic eye trajectories recorded in brain-damaged patients with spatial neglect as well as in healthy controls during a visual search task.

METHODS

We establish a standardized pre-processing pipeline adaptable to other task-based eye-tracker measurements. We use traditional machine learning algorithms together with deep convolutional networks (both 1D and 2D) to automatically analyze eye trajectories.

RESULTS

Our top-performing machine learning models classified neglect patients vs. healthy individuals with an Area Under the ROC curve (AUC) ranging from 0.83 to 0.86. Moreover, the 1D convolutional neural network scores correlated with the degree of severity of neglect behavior as estimated with standardized paper-and-pencil tests and with the integrity of white matter tracts measured from Diffusion Tensor Imaging (DTI). Interestingly, the latter showed a clear correlation with the third branch of the superior longitudinal fasciculus (SLF), especially damaged in neglect.

CONCLUSIONS

The study introduces new methods for both the pre-processing and the classification of eye-movement trajectories in patients with neglect syndrome. The proposed methods can likely be applied to other types of neurological diseases opening the possibility of new computer-aided, precise, sensitive and non-invasive diagnostic tools.

From competition to cooperation: Visual neglect across the hemispheres

Visuospatial neglect is a frequent and disabling consequence of injuries to the right hemisphere. Patients with neglect show signs of impaired attention for left-sided events, which depends on dysfunction of fronto-parietal networks. After unilateral injury, such as stroke, these networks and their contralateral homologs can reorganize following multiple potential trajectories, which can be either adaptive or maladaptive. This article presents possible factors influencing the profile of evolution of neglect towards recovery or chronicity, and highlights potential mechanisms that may constrain these processes in time and space. The integrity of white matter pathways within and between the hemisphere appears to pose crucial connectivity constraints for compensatory brain plasticity from remote brain regions. Specifically, the availability of a sufficient degree of inter-hemispheric connectivity might be critical to shift the role of the undamaged left hemisphere in spatial neglect, from exerting maladaptive effects, to promoting compensatory activity.

E-TAN, a technology-enhanced platform with tangible objects for the assessment of visual neglect: A multiple single-case study

Visual neglect is a frequent and disabling consequence of right brain damage. Traditional paper-and pencil tests of neglect have limited sensitivity and ecological validity. The Baking Tray Task (BTT), instead, approaches real-life situations, because it requires participants to place 16 physical objects on a board. The number of objects placed on the left and right portions of the board provides a clinical index of visual neglect. Here we present E-TAN, a technology-enhanced platform for BTT (E-BTT). E-BTT automatically determines the object locations on the board, and also records the sequence and timing of their placement. We used E-BTT to test 9 patients with right hemisphere damage and compared their performance with that obtained by 115 healthy participants. To this end, we developed a new method of analysis of participants' performance, based on the use of the convex hull described by the objects on the board. This measure provides an estimate of the portion of space processed by each participant and can effectively discriminate neglect patients from patients without neglect. E-TAN allows clinicians to assess visuospatial performance by using a convenient, fast, and relatively automatized procedure, that patients can even perform at home to follow-up the effects of rehabilitation.

Investigating the Characteristics of Covert Unilateral Spatial Neglect Using the Modified Posner Task: A Single-subject Design Study

Objectives:

Patients identified as asymptomatic for unilateral spatial neglect (USN) based on paper-and-pen tests nonetheless often collide with objects to their left while walking. This study aimed to investigate chronic USN in subjects who experienced collisions while walking.

Methods:

Two patients with chronic USN who experienced collisions while walking were evaluated using the Behavioral Inattention Test-conventional (BIT-c). Additionally, the modified Posner task (MPT) was used to evaluate the left and right reaction times. MPT targets randomly appeared either on the side indicated by the cue (valid condition) or on the opposite side (invalid condition). This study used an alternating treatments single-case design. The valid and invalid conditions of the MPT alternated rapidly and randomly to determine differences in reaction time. Statistical analysis compared left and right reaction times using a one-tailed randomization test to study valid and invalid conditions.

Results:

The total BIT-c score was in the normal range for both subjects, whereas MPT reaction times were higher on the left side than on the right side for the invalid condition. However, for the valid condition, only Case B had increased reaction times on the left side.

Conclusions:

The MPT valid condition evaluates voluntary attention, whereas the invalid condition evaluates the reorientation of attention. Consequently, for Case A, a left reorientation of attention deficit was observed, whereas, for Case B, left voluntary attention and left reorientation of attention deficits were observed. The MPT results revealed the characteristics of covert neglect signs. USN evaluation would benefit from additional research using MPT.

Anterior insula and inferior frontal gyrus: where ventral and dorsal visual attention systems meet

The clinical link between spatial and non-spatial attentional aspects in patients with hemispatial neglect is well known; in particular, an increase in alerting can transitorily help to allocate attention towards the contralesional side. In models of attention, this phenomenon is postulated to rely on an interaction between ventral and dorsal cortical networks, subtending non-spatial and spatial attentional aspects, respectively. However, the exact neural underpinnings of the interaction between these two networks are still poorly understood. In the present study, we included 80 right-hemispheric patients with subacute stroke (50% women; age range: 24-96), 33 with and 47 without neglect, as assessed by paper-pencil cancellation tests. The patients performed a computerized task in which they were asked to respond as quickly as possible by button-press to central targets, which were either preceded or not preceded by non-spatial, auditory warning tones. Reaction times in the two different conditions were measured. In neglect patients, a warning tone, enhancing activity within the ventral attentional 'alerting' network, could boost the reaction (in terms of shorter reaction times) of the dorsal attentional network to a visual stimulus up to the level of patients without neglect. Critically, using voxel-based lesion-symptom mapping analyses, we show that this effect significantly depends on the integrity of the right anterior insula and adjacent inferior frontal gyrus, i.e., right-hemispheric patients with lesions involving these areas were significantly less likely to show shorter reaction times when a warning tone was presented prior to visual target appearance. We propose that the right anterior insula and inferior frontal gyrus are a critical hub through which the ventral attentional network can 'alert' and increase the efficiency of the activity of the dorsal attentional network.

How does the number of targets affect visual search performance in visuospatial neglect?

INTRODUCTION

Impairments in visual search are a common symptom in visuospatial neglect (VSN). The severity of the lateralized attention bias in visual search tasks can vary depending on the number of distractors: the more distractors, the more targets are missed. However, little is known about how the number of targets affect search performance in VSN. The aim of the current study was to examine the effect of the number of targets on hit rate in VSN.

METHODS

We included 23 stroke patients with right-brain damage and VSN, 55 with right-brain damage without VSN, and 49 with left-brain damage without VSN, all admitted for inpatient rehabilitation. In a visual search task, patients had to find and tap targets, presented along with non-targets. The location and number of targets varied from trial to trial, allowing the evaluation of the effects of number and location of targets on hit rate.

RESULTS

VSN patients detected a lower percentage of targets when more targets were present. For patients with right-brain damage without VSN, adding targets only reduced the hit rate of the most contralesional target. No effect of number of targets on hit rate was seen in patients with left-brain damage. Additionally, VSN patients found less contralesional targets than ipsilesional targets, made more delayed revisits, and had an initial rightward bias when compared to the other groups. There were no differences in search time, search consistency, or immediate revisits between groups. There was a moderate positive relation between the hit rate asymmetry score in our search task and conventional paper-and-pencil VSN tasks, and neglect behavior in daily life.

CONCLUSIONS

In VSN patients, a higher number of targets reduces the hit rate. The reduced hit rate in visual search evoked by additional targets should be taken into account when assessing visual search in VSN.

Occurrence and Recovery of Different Neglect-Related Symptoms in Right Hemisphere Infarct Patients during a 1-Year Follow-Up

OBJECTIVES

To examine the occurrence of and recovery from visual neglect-related symptoms with the focus on neglect laterality, ipsilateral orienting bias, and slowed processing speed in right hemisphere (RH) infarct patients during a 1-year follow-up. Furthermore, to propose guidelines for assessing processing speed alongside the Behavioural Inattention Test (BIT).

METHODS

We studied three RH patient groups: neglect (N+), mild left inattention (MLI+), and non-neglect (N-) patients, and healthy controls. The BIT with some additional analyses was conducted at the acute phase and at 6 and 12 months.

RESULTS

The N+ group's BIT score increased and originally lateralized omissions became more evenly distributed during the follow-up. The N+ and MLI+ groups' starting points were more rightward located than the healthy group's at the acute phase and at 6, and partly at 12 months. Patient groups were slower than the controls in performing cancellation tests at the acute phase. The N+ and MLI+ groups remained slower than the controls throughout the follow-up.

CONCLUSIONS

During the first year after RH infarct, originally left-sided manifestation of neglect shifted toward milder non-lateralized attentional deficit. Ipsilateral orienting bias and slowed processing speed appeared to be rather persistent neglect-related symptoms both in neglect patients and patients with initially milder inattention. We propose some effortless, tentative ways of examining processing speed and ipsilateral orienting bias alongside the BIT to better recognize these neglect-related symptoms, and highlight the need to assess and treat patients with initially milder inattention, who have been under-recognized and under-treated in clinical work. (JINS, 2018, 24, 617-628).

When brain damage "improves" perception: neglect patients can localize motion-shifted probes better than controls

When we look at bars flashed against a moving background, we see them displaced in the direction of the upcoming motion (flash-grab illusion). It is still debated whether these motion-induced position shifts are low-level, reflexive consequences of stimulus motion or high-level compensation engaged only when the stimulus is tracked with attention. To investigate whether attention is a causal factor for this striking illusory position shift, we evaluated the flash-grab illusion in six patients with damaged attentional networks in the right hemisphere and signs of left visual neglect and six age-matched controls. With stimuli in the top, right, and bottom visual fields, neglect patients experienced the same amount of illusion as controls. However, patients showed no significant shift when the test was presented in their left hemifield, despite having equally precise judgments. Thus, paradoxically, neglect patients perceived the position of the flash more veridically in their neglected hemifield. These results suggest that impaired attentional processes can reduce the interaction between a moving background and a superimposed stationary flash, and indicate that attention is a critical factor in generating the illusory motion-induced shifts of location.

Unveiling residual, spontaneous recovery from subtle hemispatial neglect three years after stroke

A common and disabling consequence of stroke is the difficulty in processing contralesional space (i.e., hemispatial neglect). According to paper-and-pencil tests, neglect remits or stabilizes in severity within a few months after a brain injury. This arbitrary temporal limit, however, is at odds with neglect’s well-known dependency on task-sensitivity. The present study tested the hypothesis that the putative early resolution of neglect might be due to the insensitivity of testing methods rather than to the lack of spontaneous recovery at later stages. A right hemisphere stroke patient was studied longitudinally for 3 years. According to paper-and-pencil tests the patient showed no symptom of hemispatial neglect 1 month post stroke. Awareness of spatially lateralized visual targets was then assessed by means of computer-based single- and dual-tasks requiring an additional top-down deployment of attention for the parallel processing of visual or auditory stimuli. Errorless performance at computer-based tasks was reached at month 12 and maintained until month 29 after stroke. A bottom-up manipulation was then implemented by reducing target diameter. Following this change, more than 50% of contralesional targets were omitted, mostly under dual-tasking. At months 40 and 41 the same task revealed a significant (but not complete) reduction in the number of contralesional omissions. Ipsilesional targets were, in contrast, still errorless detected. The coupling of a bottom-up (target change) and a top-down (dual-tasking) manipulation revealed the presence of a long-lasting spontaneous recovery from contralesional spatial awareness deficits. In contrast, neither manipulation was effective when implemented separately. After having excluded the potential confound of practice effects, it was concluded that not only the presence but also the time course of hemispatial neglect strongly depends on the degree of attentional engagement required by the task.

Distance-mediated spatial neglect

Unilateral spatial neglect (USN) is usually assessed by means of individual stimuli or single arrays of stimuli. Seldom are stimuli presented as multiple objects or in spatially separated blocks, except in some tests for object-based neglect. The distance between individual objects or blocks of stimuli in such stimuli is implicitly considered irrelevant. We report on the case of a patient, EC, who showed severe USN in his everyday behavior, yet performed normally on standard tests for USN. Presented with stimuli in separate blocks, he performed flawlessly with 4 cm gaps between blocks, yet ignored all leftward blocks of stimuli when the gap was larger than this. EC's dissociation between good performance on standard tasks and severe neglect with separate groups of stimuli, and the distance-mediated nature of his USN are novel observations with relevant theoretical and clinical implications.

Neglect and extinction depend greatly on task demands: a review

This review illustrates how, after unilateral brain damage, the presence and severity of spatial awareness deficits for the contralesional hemispace depend greatly on the quantity of attentional resources available for performance. After a brief description of neglect and extinction, different frameworks accounting for spatial and non-spatial attentional processes will be outlined. The central part of the review describes how the performance of brain-damaged patients is negatively affected by increased task demands, which can result in the emergence of severe awareness deficits for contralesional space even in patients who perform normally on paper-and-pencil tests. Throughout the review neglect is described as a spatial syndrome that can be exacerbated in the presence and severity by both spatial and non-spatial tasks. The take-home message is that the presence and degree of contralesional neglect and extinction can be dramatically overlooked based on standard clinical (paper-and-pencil) testing, where patients can easily compensate for their deficits. Only tasks where compensation is made impossible represent an appropriate approach to detect these disabling contralesional deficits of awareness when they become subtle in post-acute stroke phases.

Brain networks of visuospatial attention and their disruption in visual neglect

Visual neglect is a multi-component syndrome including prominent attentional disorders. Research on the functional mechanisms of neglect is now moving from the description of dissociations in patients' performance to the identification of the possible component deficits and of their interaction with compensatory strategies. In recent years, the dissection of attentional deficits in neglect has progressed in parallel with increasing comprehension of the anatomy and function of large-scale brain networks implicated in attentional processes. This review focuses on the anatomy and putative functions of attentional circuits in the brain, mainly subserved by fronto-parietal networks, with a peculiar although not yet completely elucidated role for the right hemisphere. Recent results are discussed concerning the influence of a non-spatial attentional function, phasic alertness, on conscious perception in normal participants and on conflict resolution in neglect patients. The rapid rate of expansion of our knowledge of these systems raises hopes for the development of effective strategies to improve the functioning of the attentional networks in brain-damaged patients.

Rightward bias in right hemisphere infarct patients with or without thrombolytic treatment and in healthy controls

Right hemisphere (RH) infarct patients have a tendency to begin visual scanning from the right side of a given stimulus. Our aim was to find out whether RH patients with (T+) or without (T-) thrombolytic treatment and healthy controls differ in their starting points in three cancellation tasks. Our sample comprised of 77 patients and 62 controls. Thirty-four patients received thrombolysis. Rightward orientation bias was more evident in the T- group than in the T+ group. The T+ group showed a robust tendency to start all cancellation tasks more often on the right side than the controls. Regardless of whether they had visual neglect, patients in the T+ group showed still defective rightward orienting, possibly indicating residual attentional problems. The analyses of starting points in visual cancellation tasks provide additional information on residual symptoms of attention difficulties after stroke.

Visual neglect

PURPOSE OF REVIEW

Left visual neglect is a frequent and dramatic consequence of right hemisphere lesions. Diagnosis is important because behavioural and pharmacological treatments are available. Furthermore, neglect raises important issues concerning the brain mechanisms of consciousness, perception and attention.

RECENT FINDINGS

Recent behavioural findings and new techniques, such as transcranial magnetic stimulation, direct cortical and subcortical stimulation during brain surgery, and diffusion tensor imaging tractography, have provided evidence relevant to the debate concerning the functional mechanisms and the anatomical bases of neglect.

SUMMARY

Several component deficits appear to interact in producing different forms of neglect. Rather than lesions at single cortical levels, dysfunction of large-scale brain networks, often induced by white matter disconnection, may constitute the crucial antecedent of neglect signs.

A left attentional bias in chronic neglect: a case study using temporal order judgments

Previous studies of left visuospatial neglect using temporal order judgments (TOJs) have reported a temporal advantage for a stimulus presented on the right of fixation. The present case study examines an individual who shows a left temporal advantage on TOJ tasks, despite classic left-sided neglect on other tasks and in self-report. Experiment 1 found a continued left advantage on TOJs when employing a novel red/blue TOJ task to reduce potential response bias. Phasic alerting tones presented prior to random trials in Experiment 2 did not improve the abnormal attentional bias, as has been reported in previous studies of neglect. The addition of unilateral trials mixed within bilateral trials in Experiment 3 reduced the observed left advantage, suggesting a flexible attentional focus and implicating a role for strategic endogenous attentional strategies in this individual. Some implications for our understanding of endogenous orienting and relevance to rehabilitation therapy are discussed.

Time to imagine space: a chronometric exploration of representational neglect

When describing known places from memory, patients with left spatial neglect may mention more right- than left-sided items, thus showing representational, or imaginal, neglect. This suggests that these patients cannot either build or explore left locations in visual mental imagery. However, in place description there is no guarantee that patients are really employing visual mental imagery abilities, rather than verbal-propositional knowledge. Thus, patients providing symmetrical descriptions might be using other strategies than visual mental imagery. To address this issue, we devised a new test which strongly encourages the use of visual mental imagery. Twelve participants without brain damage and 12 right brain-damaged patients, of whom 7 had visual neglect, were invited to conjure up a visual mental image of the map of France. They subsequently had to state by pressing a left- or a right-sided key whether auditorily presented towns or regions were situated to the left or right of Paris on the imagined map. This provided measures of response time and accuracy for imagined locations. A further task, devised to assess response bias, used the words "left" or "right" as stimuli and the same keypress responses. Controls and non-neglect patients performed symmetrically. Neglect patients were slower for left than for right imagined locations. On single-case analysis, two patients with visual neglect had a greater response time asymmetry on the geographical task than predicted by the response bias task, but with symmetrical accuracy. The dissociation between response times and accuracy suggests that, in these patients, the left side of the mental map of space was not lost, but only "explored" less efficiently.

Ocular scanning and perceptual size distortion in hemispatial neglect: effects of prism adaptation and sequential stimulus presentation

When asked to compare two lateralized shapes for horizontal size, neglect patients often indicate the left stimulus to be smaller. Gainotti and Tiacci (1971) hypothesized that this phenomenon might be related to a rightward bias in the patients' gaze. This study aimed to assess the relation between this size underestimation and oculomotor asymmetries. Eye movements were recorded while three neglect patients judged the horizontal extent of two rectangles. Two experimental manipulations were performed to increase the likelihood of symmetrical scanning of the stimulus display. The first manipulation entailed a sequential, rather than simultaneous presentation of the two rectangles. The second required adaptation to rightward displacing prisms, which is known to reduce many manifestations of neglect. All patients consistently underestimated the left rectangle, but the pattern of verbal responses and eye movements suggested different underlying causes. These include a distortion of space perception without ocular asymmetry, a failure to view the full leftward extent of the left stimulus, and a high-level response bias. Sequential presentation of the rectangles and prism adaptation reduced ocular asymmetries without affecting size underestimation. Overall, the results suggest that leftward size underestimation in neglect can arise for a number of different reasons. Incomplete leftward scanning may perhaps be sufficient to induce perceptual size distortion, but it is not a necessary prerequisite.

Sensitivity of clinical and behavioural tests of spatial neglect after right hemisphere stroke

OBJECTIVES

The lack of agreement regarding assessment methods is responsible for the variability in the reported rate of occurrence of spatial neglect after stroke. The aim of this study was to assess the sensitivity of different tests of neglect after right hemisphere stroke.

METHODS

Two hundred and six subacute right hemisphere stroke patients were given a test battery including a preliminary assessment of anosognosia and of visual extinction, a clinical assessment of gaze orientation and of personal neglect, and paper and pencil tests of spatial neglect in the peripersonal space. Patients were compared with a previously reported control group. A subgroup of patients (n=69) received a behavioural assessment of neglect in daily life situations.

RESULTS

The most sensitive paper and pencil measure was the starting point in the cancellation task. The whole battery was more sensitive than any single test alone. About 85% of patients presented some degree of neglect on at least one measure. An important finding was that behavioural assessment of neglect in daily life was more sensitive than any other single measure of neglect. Behavioural neglect was considered as moderate to severe in 36% of cases. A factorial analysis revealed that paper and pencil tests were related to two underlying factors. Dissociations were found between extrapersonal neglect, personal neglect, anosognosia, and extinction. Anatomical analyses showed that neglect was more common and severe when the posterior association cortex was damaged.

CONCLUSIONS

The automatic rightward orientation bias is the most sensitive clinical measure of neglect. Behavioural assessment is more sensitive than any single paper and pencil test. The results also support the assumption that neglect is a heterogeneous disorder.

Orienting of attention in left unilateral neglect

After right posterior brain damage, patients may ignore events occurring on their left, a condition known as unilateral neglect. Although deficits at different levels of impairment may be at work in different patients, the frequency and severity of attentional problems in neglect patients have been repeatedly underlined. Recent advances in the knowledge of the mechanisms of spatial attention in normals may help characterizing these deficits. The present review focuses on studies exploring several aspect of attentional processing in unilateral neglect, with particular reference to the dichotomy between 'exogenous', or stimulus-related, and 'endogenous', or strategy-driven, orienting of attention. A large amount of neuropsychological evidence suggests that a basic mechanism leading to left neglect behavior is an impaired exogenous orienting toward left-sided targets. In contrast, endogenous processes seem to be relatively preserved, if slowed, in left unilateral neglect. Other component deficits, such as a general slowing of the operations of spatial attention, might contribute to neglect behavior. These results are presented and discussed, and their implications for hemispheric specialization in attentional orienting and for the mechanisms of visual consciousness are explored.

The traffic light paradigm: a reaction time task to study laterally directed arm movements

Patients with unilateral brain damage may show slowed or hypometric arm movements toward the contralesional space, as compared to movements directed towards the side of the brain lesion. The present article describes a reaction time paradigm devised to study accuracy and latency of directional arm movements in normal human subjects and brain-damaged patients. Experimental paradigms hitherto used to explore directional motor disorders often do not reliably disentangle between perceptual and motor factors, because they employ lateralized perceptual stimuli. The traffic light paradigm, instead, consists of visual stimuli presented on the vertical midline (like a traffic light) and hand responses to be produced in either hemispace. Thus, participants have to produce lateralized arm responses to central visual stimuli. Performance on this 'motor' paradigm can be contrasted with performance on a 'perceptual' reaction time task, consisting of similar, but lateralized visual stimuli and central motor responses. Results obtained with these paradigms on normal participants and brain-damaged patients are presented and discussed. These results give empirical support to the claim that the traffic light paradigm is suitable to study directional motor disorders in relative isolation from perceptual biases.

Laterally directed arm movements and right unilateral neglect after left hemisphere damage

Signs of unilateral neglect for events occurring in one hemispace most often result from right hemisphere lesions. Right unilateral neglect after left hemisphere damage is much rarer, and has received less attention. The present study explores the relationships between right unilateral neglect and asymmetries in producing laterally directed arm movements in the horizontal plane in left brain-damaged (LBD) patients. Participants produced right- or left-directed arm movements with their left arm in response to centrally located visual stimuli. Results showed that LBD patients with signs of right unilateral neglect were consistently slowed when producing arm movements toward the right (neglected) side, as compared to left-directed movements. Taking into account patients with and without signs of neglect, this directional asymmetry positively correlated with a reaction-time measure of perceptual spatial bias. These findings stand in contrast with previous results obtained with the same experimental paradigm in right brain-damaged patients, in whom a consistent slowing of leftward-directed movements was rare and apparently unrelated to the presence and severity of left neglect. These conflicting results are discussed with respect to the hypothesis that different mechanisms may underlie left and right unilateral neglect.

Unilateral neglect: the effect of competing stimuli on estimated line length

Normal subjects and patients with right hemisphere lesions with or without signs of left unilateral neglect judged the length of a horizontal line presented on the left or on the right side of space. In half of the trials, the line was presented with a centrally located square or diamond, and subjects had to identify the central stimulus before performing the judgment of length. The presence of the central stimulus improved accuracy of performance in controls and in patients without neglect; neglect patients, however, produced more overestimations of left-sided lines when these was presented with a central stimulus than when the lines occurred in isolation. This finding underlines the importance of attentional factors in length estimates performed by neglect patients in their neglected space.

Variability of response times as a marker of diverted attention

Anderson et al. (Variability not ability: another basis for performance decrements in neglect. Neuropsychologia 2000;38:785-796) have recently reported that variability of response times (RTs) progressively increases from the right to the left side in left neglect patients. Anderson et al. propose that this lack of consistency is an important determinant of patients' behaviour, and may result from a deficit independent of other mechanisms causing neglect. Here we suggest that an increase of variability, and not only of RTs, is to be expected when attention is exogenously biased away from the probed location. Consequently, space-based variability can be interpreted in the framework of existing models of unilateral neglect. According to one such model, a basic impairment in left neglect is a bias toward rightward exogenous orienting of attention. As a result, left targets often fail to rapidly capture patients' attention, thus yielding slow RTs. However, since the probability for a left target attracting attention is low but not null, relatively fast RTs can occur on those rare occasions in which a left target does capture patients' attention. The coexistence of these relatively fast with slow RTs could be at the basis of space-based variability in neglect. Empirical support for our hypothesis comes from the results of a re-analysis for variability of cued RTs obtained in 18 normal individuals and six left neglect patients. Cues were peripheral and non-informative, thus eliciting an exogenous attentional shift. For normal individuals, invalid trials yielded less consistent response times than valid trials at short (150 ms) cue-target interval; for neglect patients, a similar phenomenon occurred for left invalidly-cued targets, thus paralleling the disproportionate cost in RTs typically evoked by this condition in unilateral neglect. We conclude by discussing some possible determinants of gradient-shaped effects and by outlining the implications of space-based variability for current models of unilateral neglect.

Variability not ability: another basis for performance decrements in neglect

OBJECTIVE

To determine reaction time (RT) and its variability, as a function of horizontal spatial position, in subjects with neglect.

BACKGROUND

In neglect, performance is frequently reported a as mean and a decreased ability to perform the task inferred by comparison to control groups. Few studies have examined how consistency and optimal performance relate to spatial neglect.

METHODS

Ten subjects with brain damage, five with and five without spatial neglect, were assessed on a RT task. Subjects responded by pushing a computer key to the onset of a white square appearing on a black screen. The locations of stimuli were randomly varied along the horizontal meridian.

RESULTS

For three of five neglect subjects, optimal RT showed no or little relation to horizontal location. Four of five neglect subjects demonstrated an increased variability in RT that correlated with spatial position and which was not present in our brain damaged subjects without neglect. The relationship was not an artifact of left sided stimuli, in general, being processed differently. For the two neglect subjects with the most trials, a significant correlation between RT variability and spatial position existed for left-sided trials alone. Increased variability was not a consequence of simply looking left proportionately less often, nor could a model of multiple compensatory systems operating in parallel explain the enhanced variability. Neither hemianopsia alone nor brain damage per se could account for the spatial modulation of RT variability.

CONCLUSIONS

That neglect subjects perform the RT task normally on some trials, even in their 'neglected field', challenges the notion that neglect must reflect an irreparably damaged cognitive system. Performance decrements in neglect can reflect an inability to consistently detect and respond. Evaluating optimal performance and variability of performance can indicate if a capacity has been lost absolutely or merely degraded such that normal performance cannot be sustained.

Facilitation instead of inhibition for repeated right-sided events in left neglect

When two visual events appear consecutively in the same spatial location, our response to the second event is slower than that to the first. This inhibition for repeated events may reflect a bias toward sampling novel locations, a bias useful for exploring the visual space. Patients with right hemisphere damage and left neglect explore asymmetrically a visual scene. They are initially attracted by right-sided items and become stuck to them, being unable to reorient their attention toward the left. Here we show that neglect patients show facilitation instead of inhibition for repeated events on the right, non-neglected side. Patients without neglect showed normal inhibition. Our observation may explain why neglect patients' exploration of space cannot extend beyond a few right-sided objects.

Perception and action in hemispatial neglect

Hemispatial neglect is a neurological disorder which entails a spatial bias that penalizes events occurring in the hemispace contralateral to a brain lesion. Mechanisms operating upon various stages ranging from perception to action have been invoked to explain neglect. The present study explores the contribution of a defective programming of arm movements towards the neglected hemispace to neglect behaviour. Two reaction time tasks -- a "perceptual" task and a "motor" task -- were performed by right brain-damaged (RBD) patients with left hemispatial neglect, RBD patients without signs of neglect and control subjects. The perceptual task consisted of lateralized visual stimuli and central motor responses, whereas the motor task consisted of visual stimuli presented on the vertical midline and hand responses to be produced in either hemispace. Neglect patients showed a rightward bias on the perceptual task, but only two RBD patients (showing no signs of severe neglect) were consistently slowed in producing leftward motor responses. Different reference frames may thus be used in perceptual tasks and tasks involving arm movements. We conclude that hemispatial neglect commonly results from attentional impairments operating upon a visual perceptual frame of reference; additional deficits appear to be necessary to produce a directional motor disorder.

Patterns of dissociation between left hemineglect and deviation of the egocentric reference

Sixteen control subjects and six right brain-damaged patients with left hemiparesis (three showing signs of left unilateral neglect, three with no signs of neglect) performed a straight-ahead pointing task with their right hand while blindfolded. The aim was to test the hypothesis that the egocentric reference shows significant ipsilesional deviation in left neglect patients. We found no correlation between the position of the egocentric reference and the presence of neglect signs. Neglect patients, like non-neglect patients, showed leftward, rightward or no significant deviation when pointing straight ahead. Results are discussed with reference to egocentric hypotheses of neglect and experimental remission of neglect.