Neuroanatomical Dissections of Unilateral Visual Neglect Symptoms: ALE Meta-Analysis of Lesion-Symptom Mapping

Right Hemisphere Cognitive Functions: From Clinical and Anatomic Bases to Brain Mapping During Awake Craniotomy Part I: Clinical and Functional Anatomy

The nondominant hemisphere (usually the right) is responsible for primary cognitive functions such as visuospatial and social cognition. Awake surgery using direct electric stimulation for right cerebral tumor removal remains challenging because of the complexity of the functional anatomy and difficulties in adapting standard bedside tasks to awake surgery conditions. An understanding of semiology and anatomic bases, along with an analysis of the available cognitive tasks for visuospatial and social cognition per operative mapping allow neurosurgeons to better appreciate the functional anatomy of the right hemisphere and its relevance to tumor surgery. In this article, the first of a 2-part review, we discuss the anatomic and functional basis of right hemisphere function. Whereas part II of the review focuses primarily on semiology and surgical management of right-sided tumors under awake conditions, this article provides a comprehensive review of knowledge underpinning awake surgery on the right hemisphere.

Unilateral spatial neglect after posterior parietal damage

Unilateral spatial neglect is a disabling neurologic deficit, most frequent and severe after right-hemispheric lesions. In most patients neglect involves the left side of space, contralateral to a right-hemispheric lesion. About 50% of stroke patients exhibit neglect in the acute phase. Patients fail to orient, respond to, and report sensory events occurring in the contralateral sides of space and of the body, to explore these portions of space through movements by action effectors (eye, limbs), and to move the contralateral limbs. Neglect is a multicomponent higher-level disorder of spatial awareness, cognition, and attention. Spatial neglect may occur independently of elementary sensory and motor neurologic deficits, but it can mimic and make them more severe. Diagnostic tests include: motor exploratory target cancellation; setting the midpoint of a horizontal line (bisection), that requires the estimation of lateral extent; drawing by copy and from memory; reading, assessing neglect dyslexia; and exploring the side of the body contralateral to the lesion. Activities of daily living scales are also used. Patients are typically not aware of neglect, although they may exhibit varying degrees of awareness toward different components of the deficit. The neural correlates include lesions to the inferior parietal lobule of the posterior parietal cortex, which was long considered the unique neuropathologic correlate of neglect, to the premotor and to the dorsolateral prefrontal cortices, to the posterior superior temporal gyrus, at the temporoparietal junction, to subcortical gray nuclei (thalamus, basal ganglia), and to parietofrontal white-matter fiber tracts, such as the superior longitudinal fascicle. Damage to the inferior parietal lobule of the posterior parietal cortex is specifically associated with the mainly egocentric, perceptual, and exploratory extrapersonal, and with the personal, bodily components of neglect. Productive manifestations, such as perseveration, are not a correlate of posterior parietal cortex damage.

The influences and neural correlates of past and present during gambling in humans

During financial decision-making tasks, humans often make "rational" decisions, where they maximize expected reward. However, this rationality may compete with a bias that reflects past outcomes. That is, if one just lost money or won money, this may impact future decisions. It is unclear how past outcomes influence future decisions in humans, and how neural circuits encode present and past information. In this study, six human subjects performed a financial decision-making task while we recorded local field potentials from multiple brain structures. We constructed a model for each subject characterizing bets on each trial as a function of present and past information. The models suggest that some patients are more influenced by previous trial outcomes (i.e., previous return and risk) than others who stick to more fixed decision strategies. In addition, past return and present risk modulated with the activity in the cuneus; while present return and past risk modulated with the activity in the superior temporal gyrus and the angular gyrus, respectively. Our findings suggest that these structures play a role in decision-making beyond their classical functions by incorporating predictions and risks in humans' decision strategy, and provide new insight into how humans link their internal biases to decisions.

Development of Visuospatial Attention in Typically Developing Children

The aim of the present study is to investigate the development of visuospatial attention in typically developing children and to propose reference values for children for the following six visuospatial attention tests: star cancellation, Ogden figure, reading test, line bisection, proprioceptive pointing and visuo-proprioceptive pointing. Data of 159 children attending primary or secondary school in the Fédération Wallonie Bruxelles (Belgium) were analyzed. Results showed that the children's performance on star cancellation, Ogden figure and reading test improved until the age of 13 years, whereas their performance on proprioceptive pointing, visuo-proprioceptive pointing and line bisection was stable with increasing age. These results suggest that the execution of different types of visuospatial attention tasks are not following the same developmental trajectories. This dissociation is strengthened by the lack of correlation observed between tests assessing egocentric and allocentric visuospatial attention, except for the star cancellation test (egocentric) and the Ogden figure copy (ego- and allocentric). Reference values are proposed that may be useful to examine children with clinical disorders of visuospatial attention.

Superior colliculus resting state networks in post-traumatic stress disorder and its dissociative subtype

OBJECTIVES

The innate alarm system (IAS) models the neurocircuitry involved in threat processing in posttraumatic stress disorder (PTSD). Here, we investigate a primary subcortical structure of the IAS model, the superior colliculus (SC), where the SC is thought to contribute to the mechanisms underlying threat-detection in PTSD. Critically, the functional connectivity between the SC and other nodes of the IAS remains unexplored.

EXPERIMENTAL DESIGN

We conducted a resting-state fMRI study to investigate the functional architecture of the IAS, focusing on connectivity of the SC in PTSD (n = 67), its dissociative subtype (n = 41), and healthy controls (n = 50) using region-of-interest seed-based analysis.

PRINCIPAL OBSERVATIONS

We observed group-specific resting state functional connectivity between the SC for both PTSD and its dissociative subtype, indicative of dedicated IAS collicular pathways in each group of patients. When comparing PTSD to its dissociative subtype, we observed increased resting state functional connectivity between the left SC and the right dorsolateral prefrontal cortex (DLPFC) in PTSD. The DLPFC is involved in modulation of emotional processes associated with active defensive responses characterising PTSD. Moreover, when comparing PTSD to its dissociative subtype, increased resting state functional connectivity was observed between the right SC and the right temporoparietal junction in the dissociative subtype. The temporoparietal junction is involved in depersonalization responses associated with passive defensive responses typical of the dissociative subtype.

CONCLUSIONS

Our findings suggest that unique resting state functional connectivity of the SC parallels the unique symptom profile and defensive responses observed in PTSD and its dissociative subtype. Hum Brain Mapp 39:563-574, 2018. © 2017 Wiley Periodicals, Inc.

Neural Basis of Acquired Amusia and Its Recovery after Stroke

Although acquired amusia is a relatively common disorder after stroke, its precise neuroanatomical basis is still unknown. To evaluate which brain regions form the neural substrate for acquired amusia and its recovery, we performed a voxel-based lesion-symptom mapping (VLSM) and morphometry (VBM) study with 77 human stroke subjects. Structural MRIs were acquired at acute and 6 month poststroke stages. Amusia and aphasia were behaviorally assessed at acute and 3 month poststroke stages using the Scale and Rhythm subtests of the Montreal Battery of Evaluation of Amusia (MBEA) and language tests. VLSM analyses indicated that amusia was associated with a lesion area comprising the superior temporal gyrus, Heschl's gyrus, insula, and striatum in the right hemisphere, clearly different from the lesion pattern associated with aphasia. Parametric analyses of MBEA Pitch and Rhythm scores showed extensive lesion overlap in the right striatum, as well as in the right Heschl's gyrus and superior temporal gyrus. Lesions associated with Rhythm scores extended more superiorly and posterolaterally. VBM analysis of volume changes from the acute to the 6 month stage showed a clear decrease in gray matter volume in the right superior and middle temporal gyri in nonrecovered amusic patients compared with nonamusic patients. This increased atrophy was more evident in anterior temporal areas in rhythm amusia and in posterior temporal and temporoparietal areas in pitch amusia. Overall, the results implicate right temporal and subcortical regions as the crucial neural substrate for acquired amusia and highlight the importance of different temporal lobe regions for the recovery of amusia after stroke.

SIGNIFICANCE STATEMENT

Lesion studies are essential in uncovering the brain regions causally linked to a given behavior or skill. For music perception ability, previous lesion studies of amusia have been methodologically limited in both spatial accuracy and time domain as well as by small sample sizes, providing coarse and equivocal information about which brain areas underlie amusia. By using longitudinal MRI and behavioral data from a large sample of stroke patients coupled with modern voxel-based analyses methods, we were able provide the first systematic evidence for the causal role of right temporal and striatal areas in music perception. Clinically, these results have important implications for the diagnosis and prognosis of amusia after stroke and for rehabilitation planning.

"Nonspatial" Attentional Deficits Interact with Spatial Position in Neglect

A widely debated question concerns whether or not spatial and nonspatial components of visual attention interact in attentional performance. Spatial neglect is a common consequence of brain injury where individuals fail to respond to stimuli presented on their contralesional side. It has been argued that, beyond the spatial bias, these individuals also tend to exhibit nonspatial perceptual deficits. Here we demonstrate that the "nonspatial" deficits affecting the temporal dynamics of attentional deployment are in fact modulated by spatial position. Specifically, we observed that the pathological attentional blink of chronic neglect is enhanced when stimuli are presented on the contralesional side of the trunk while keeping retinal and head-centered coordinates constant. We did not find this pattern in right brain-damaged patients without neglect or in patients who had recovered from neglect. Our work suggests that the nonspatial attentional deficits observed in neglect are heavily modulated by egocentric spatial position. This provides strong evidence against models that suggest independent modules for spatial and nonspatial attentional functions while also providing strong evidence that trunk position plays an important role in neglect.

Individual differences in children's global motion sensitivity correlate with TBSS-based measures of the superior longitudinal fasciculus

Reduced global motion sensitivity, relative to global static form sensitivity, has been found in children with many neurodevelopmental disorders, leading to the "dorsal stream vulnerability" hypothesis (Braddick et al., 2003). Individual differences in typically developing children's global motion thresholds have been shown to be associated with variations in specific parietal cortical areas (Braddick et al., 2016). Here, in 125 children aged 5-12years, we relate individual differences in global motion and form coherence thresholds to fractional anisotropy (FA) in the superior longitudinal fasciculus (SLF), a major fibre tract communicating between parietal lobe and anterior cortical areas. We find a positive correlation between FA of the right SLF and individual children's sensitivity to global motion coherence, while FA of the left SLF shows a negative correlation. Further analysis of parietal cortical area data shows that this is also asymmetrical, showing a stronger association with global motion sensitivity in the left hemisphere. None of these associations hold for an analogous measure of global form sensitivity. We conclude that a complex pattern of structural asymmetry, including the parietal lobe and the superior longitudinal fasciculus, is specifically linked to the development of sensitivity to global visual motion. This pattern suggests that individual differences in motion sensitivity are primarily linked to parietal brain areas interacting with frontal systems in making decisions on integrated motion signals, rather than in the extra-striate visual areas that perform the initial integration. The basis of motion processing deficits in neurodevelopmental disorders may depend on these same structures.

The complexity of the relationship between neuropsychological deficits and impairment in everyday tasks after stroke

BACKGROUND AND PURPOSE

A large body of research reports that stroke patients are debilitated in terms of daily independence after dismissal from the hospital unit. Patients struggle with the use of daily objects or performing complex actions. Differences between individual deficits of patients are often associated with the site of the brain damage. However, clinical studies suggest that patients exhibit varied constellations of action-associated difficulties and neuropsychological deficits. There is a lack of conclusive evidence indicating how different neuropsychological symptoms link to the impaired ability to perform activities of daily living (ADL).

MATERIALS AND METHODS

To further address this matter, in this study we compared the behavior of patients with left brain damage (LBD) and right brain damage (RBD) following stroke in two naturalistic task scenarios (tea making and document filing), and compared the committed action errors to the neuropsychological screening results.

RESULTS

We observed mild to severe impairments in both the LBD and RBD groups amounting to 37-55% of failure rate in attainment of action goal. Interestingly, the performance on both tasks was not correlated to each other, suggesting that the tasks involved a different set of higher cognitive functions. Despite similar behavioral manifestations, in the LBD group poor task performance was related to deficits in praxis performance and unilateral tactile and visual extinction. The presence of aphasia did not correlate with task performance, except for a link between low scores in Aachen aphasia test scales and misestimation error in the tea making task. In the RBD group, difficulties with performance were primarily linked to deficit in praxis and unilateral visual extinction.

CONCLUSIONS

Despite similar behavior, the underlying mechanisms of the deficits after stroke might be different (in patients with LBD and RBD) and reveal complex interlinks of cognitive networks involved in the ability to carry on everyday tasks.

Intra- and Inter-Task Reliability of Spatial Attention Measures in Pseudoneglect

Healthy young adults display a leftward asymmetry of spatial attention ("pseudoneglect") that has been measured with a wide range of different tasks. Yet at present there is a lack of systematic evidence that the tasks commonly used in research today are i) stable measures over time and ii) provide similar measures of spatial bias. Fifty right-handed young adults were tested on five tasks (manual line bisection, landmark, greyscales, gratingscales and lateralised visual detection) on two different days. All five tasks were found to be stable measures of bias over the two testing sessions, indicating that each is a reliable measure in itself. Surprisingly, no strongly significant inter-task correlations were found. However, principal component analysis revealed left-right asymmetries to be subdivided in 4 main components, namely asymmetries in size judgements (manual line bisection and landmark), luminance judgements (greyscales), stimulus detection (lateralised visual detection) and judgements of global/local features (manual line bisection and grating scales). The results align with recent research on hemispatial neglect which conceptualises the condition as multi-component rather than a single pathological deficit of spatial attention. We conclude that spatial biases in judgment of visual stimulus features in healthy adults (e.g., pseudoneglect) is also a multi-component phenomenon that may be captured by variations in task demands which engage task-dependent patterns of activation within the attention network.

A Neural Decomposition of Visual Search Using Voxel-based Morphometry

The ability to search efficiently for visual targets among distractors can break down after a variety of brain lesions, but the specific processes affected by the lesions are unclear. We examined search over space (conjunction search) and over time plus space (preview search) in a consecutive series of patients with acquired brain lesions. We also assessed performance on standard neuropsychological measures of visuospatial short-term memory (Corsi Block), sustained attention and memory updating (the contrast between forward and backward digit span), and visual neglect. Voxel-based morphometry analyses revealed regions in the occipital (middle occipital gyrus), posterior parietal (angular gyrus), and temporal cortices (superior and middle temporal gyri extending to the insula), along with underlying white matter pathways, associated with poor search. Going beyond standard voxel-based morphometry analyses, we then report correlation measures of structural damage in these regions and the independent neuropsychological measures of other cognitive functions. We find distinct patterns of correlation in areas linked to poor search, suggesting that the areas play functionally different roles in search. We conclude that neuropsychological disorders of search can be linked to necessary and distinct cognitive functions, according to the site of lesion.

Specific locations within the white matter and cortex are involved in the cognitive impairments associated with periventricular white matter lesions (PWMLs)

The aim of this study was to test the hypothesis that both white matter disruption and the corresponding cortical dysfunction are involved in the cognitive impairments associated with periventricular white matter lesions (PWMLs). Twenty-two PWMLs subjects were divided into cognitively impaired (PWMLs-CI) and normal (PWMLs-CN) groups. Twelve subjects with normal magnetic resonance imaging (MRI) and cognition were recruited as controls. After cognitive evaluation, diffusion tension image (DTI) and resting-state functional MRI (rfMRI) scans, the fractional anisotropy (FA) values of DTI and the fractional amplitude of low-frequency fluctuation (fALFF) values of rfMRI were measured. Finally, correlations between the cognitive scores and MRI values were analyzed in PWMLs subjects. Our results demonstrated that compared with the other groups, the PWMLs-CI group demonstrated significantly decreased scores in Trail-Making Test (TMT), Symbol Digit Modalities Test (SDMT) and Logical Memory Test (LMT). Compared with the PWMLs-CN group, the PWMLs-CI group displayed decreased FA values in the right splenium of the corpus callosum and right posterior cingulum bundle; lower fALFF values in the left frontal middle gyrus, left precentral gyrus, right angular gyrus and right precuneus; and higher fALFF values in the right mid cingulum cortex, right hippocampus amygdala, right cerebellar hemisphere and left vermis. Meanwhile, the cognitive assessment scores were significantly correlated with the FA or fALFF values in some of the above-mentioned white matter or cortical regions. Conclusively, our results indicate that specific regions of WMLs and cortical dysfunction are involved in the cognitive impairments associated with PWMLs.

Lesion Sites Associated with Allocentric and Egocentric Visuospatial Neglect in Acute Stroke

Visuospatial neglect is a disorder that can often result from stroke and is characterized by an inability to attend to contralesional stimuli. Two common subtypes include allocentric (object-centered) neglect and egocentric (viewer-centered) neglect. In allocentric neglect, spatial inattention is localized to the contralesional side of an object regardless of its relative position to the observer. In egocentric neglect, spatial inattention is localized to the contralesional side of the individual's midline. The neuroanatomical correlates of each subtype are unknown. However, recent work has suggested that damage to temporal, inferior parietal, and occipital areas may result in allocentric neglect and that damage to frontoparietal areas may result in egocentric neglect. We used voxel-based lesion-symptom mapping (VLSM) to compare lesion location to behavioral performance on the conventional six subtests of the Behavioral Inattention Test (BIT) in 62 subjects with acute right hemisphere ischemic stroke. Results identified an anatomical dissociation in lesion location between subjects with neglect based on poor performance on allocentric tests (line bisection, copying, and drawing tasks) and on egocentric tests (star, letter, and line cancellation). VLSM analyses revealed that poor performance on the allocentric tests was associated with lesions to the superior and inferior parietal cortices, and the superior and middle temporal gyri. In contrast, poor performance on the egocentric tests was associated with lesions in the precentral gyrus, middle frontal gyrus, insula, and putamen. Interestingly, the letter cancellation test and average performance on egocentric tests were associated with frontal and parietal lesions. Some of these parietal lesion locations overlapped with lesion locations associated with allocentric neglect. These findings are consistent with suggestions that damage to temporal and parietal areas is more associated with allocentric neglect and damage to frontal lobe areas is more associated with egocentric neglect.

White matter lesional predictors of chronic visual neglect: a longitudinal study

Chronic visual neglect prevents brain-damaged patients from returning to an independent and active life. Detecting predictors of persistent neglect as early as possible after the stroke is therefore crucial to plan the relevant interventions. Neglect signs do not only depend on focal brain lesions, but also on dysfunction of large-scale brain networks connected by white matter bundles. We explored the relationship between markers of axonal degeneration occurring after the stroke and visual neglect chronicity. A group of 45 patients with unilateral strokes in the right hemisphere underwent cognitive testing for neglect twice, first at the subacute phase (<3 months after onset) and then at the chronic phase (>1 year). For each patient, magnetic resonance imaging including diffusion sequences was performed at least 4 months after the stroke. After masking each patient's lesion, we used tract-based spatial statistics to obtain a voxel-wise statistical analysis of the fractional anisotropy data. Twenty-seven patients had signs of visual neglect at initial testing. Only 10 of these patients had recovered from neglect at follow-up. When compared with patients without neglect, the group including all subacute neglect patients had decreased fractional anisotropy in the second (II) and third (III) branches of the right superior longitudinal fasciculus, as well as in the splenium of the corpus callosum. The subgroup of chronic patients showed reduced fractional anisotropy in a portion the splenium, the forceps major, which provides interhemispheric communication between regions of the occipital lobe and of the superior parietal lobules. The severity of neglect correlated with fractional anisotropy values in superior longitudinal fasciculus II/III for subacute patients and in its caudal portion for chronic patients. Our results confirm a key role of fronto-parietal disconnection in the emergence and chronic persistence of neglect, and demonstrate an implication of caudal interhemispheric disconnection in chronic neglect. Splenial disconnection may prevent fronto-parietal networks in the left hemisphere from resolving the activity imbalance with their right hemisphere counterparts, thus leading to persistent neglect.

Dissociating the neural mechanisms of distance and spatial reference frames

This study investigated if the neural mechanisms involved in processing distance (near and far) and frame of reference (egocentric and allocentric) can be dissociated. 36 participants completed a conjunction visual search task using either an egocentric (deciding if the target was to their left or right) or an allocentric (deciding if the target was to the left or right of a reference object) frame. Both tasks were performed in near (57 cm) and far (171 cm) space conditions. Participants were separated into three groups, and each received transcranial magnetic stimulation (TMS) to a different site; right posterior parietal cortex (rPPC), right ventral occipital cortex (rVO), or right frontal eye field (rFEF) in addition to sham TMS. The results show that rFEF is critical in the processing of each search at each distance whereas, contrary to previous detection results, TMS over rVO did not affect performance for any condition. TMS over rPPC revealed that specialised egocentric processing in the parietal cortex does not generalise to far space, providing evidence of a separation of the reference frame/distance conflation in the literature.

Effects of contralesional robot-assisted hand training in patients with unilateral spatial neglect following stroke: a case series study

Background

A reduction of hemispatial neglect due to stroke has been associated with activation of the contralesional hand in the contralesional hemispace. Robot-assisted upper limb training was found to effectively improve paretic arm function in stroke patients. To date no proof of concept of robot-assisted hemispatial neglect therapy has been reported in literature. This study aimed to determine whether robot-assisted left (contralesional) hand activation alone could lead to an improvement in hemispatial neglect following stroke.

Methods

Three stroke patients with right brain injury underwent a 2-week training program of robotic left hand activation with the Gloreha® hand rehabilitation glove, which provides repetitive, passive mobilization of the fingers. Outcomes were assessed using the Line Crossing test, the Bells test, the Sentence Reading test, the Saccadic Training, the Sustained Attention to Response Task, and the Purdue Pegboard test.

Results

Changes were observed after treatment as follows. Line Crossing test: all patients showed improved performance (6.7%, 89.5% and 80% increase in lines crossed) with two patients reaching normal performance levels. Bells test: one patient improved performance (50% increase), while one patient showed no change and one patient declined (−10.3% change); no patient reached normal performance levels. Sentence Reading test: all patients showed improved performance (800%, 57.1% and 42.9% increase in number of sentences read) with no patient reaching normal performance level. Saccadic Training: all patients showed improved performance (−62.8%, −15.5% and −9.7% change of the left hemifield reaction time). Sustained Attention to Response Task: all patients showed improved performance (−20.5%, −5.8% and −10% change of the reaction time) with two patients reducing incorrect responses (−42.9% and −73.3%) and one patient increasing them (9.1%). Purdue Pegboard test: all patients showed improved performance (100%, 27.3% and 75% change in the left + right + both hands sub-item score).

Conclusions

Some caution is warranted when interpreting our results, as the responses to the intervention were variable and might have been due to a placebo effect or fluctuating clinical conditions. However, robot-assisted hemispatial neglect therapy might be useful in stroke patients. Larger-scale investigations are needed to confirm our preliminary findings.

Electronic supplementary material

The online version of this article (doi:10.1186/1743-0003-11-160) contains supplementary material, which is available to authorized users.

The angular gyrus and visuospatial attention in decision-making under risk

Recent neuroimaging studies on decision-making under risk indicate that the angular gyrus (AG) is sensitive to the probability and variance of outcomes during choice. A separate body of research has established the AG as a key area in visual attention. The current study used repetitive transcranial magnetic stimulation (rTMS) in healthy volunteers to test whether the causal contribution of the AG to decision-making is independent of or linked to the guidance of visuospatial attention. A within-subject design compared decision making on a laboratory gambling task under three conditions: following rTMS to the AG, following rTMS to the premotor cortex (PMC, as an active control condition) and without TMS. The task presented two different trial types, 'visual' and 'auditory' trials, which entailed a high versus minimal demand for visuospatial attention, respectively. Our results showed a systematic effect of rTMS to the AG upon decision-making behavior in visual trials. Without TMS and following rTMS to the control region, decision latencies reflected the odds of winning; this relationship was disrupted by rTMS to the AG. In contrast, no significant effects of rTMS to the AG (or to the PMC) upon choice behavior in auditory trials were found. Thus, rTMS to the AG affected decision-making only in the task condition requiring visuospatial attention. The current findings suggest that the AG contributes to decision-making by guiding attention to relevant information about reward and punishment in the visual environment.

Effector-dependent neglect and splenial disconnection: a spherical deconvolution tractography study

We present the case of a patient with left homonymous hemianopia and chronic left neglect consequent to a stroke in the occipito-temporal regions of the right hemisphere. When the patient performed cancellation tasks with her right (dominant) hand, she had severe and persistent left neglect at retest 7 and 8 years after onset. However, her performance on line bisection was invariably within normal limits. Strikingly, performance on cancellation tests reverted to normal when the patient used her left hand. White matter tractography using spherical deconvolution demonstrated damage to the splenium of the corpus callosum, as well as a relative preservation of the right fronto-parietal network. Effector-dependent neglect may occur because splenial disconnection deprives the right fronto-parietal network from visual information processed by the left hemisphere. Consequently, spatial exploration reverts to normal when the patient uses her left hand, thus involving more directly the fronto-parietal attentional networks in the right-hemisphere.

Egocentric representations of space co-exist with allocentric representations: evidence from spatial neglect

The brain transforms sensory input to motor coordinates to accommodate for changes of posture and gaze direction. Neurophysiological and neuropsychological evidence supports the existence of multiple representations of space. A debated issue regards whether objects that we see are encoded in egocentric frames only or also maintain an object-centered frame of reference. Previous clinical studies were unable to discriminate between these models as the stimuli used to determine object-based (allocentric) biases are contaminated by body-centered (egocentric) effects. To address this, we used stimuli where allocentric side was elicited by gestalt configuration rather than egocentric location. We then displayed these stimuli at different egocentric positions allowing us to independently measure the effects of allocentric position, egocentric position as well as their interaction. In a group of stroke patients with neglect we demonstrate that allocentric biases are modulated as a function of egocentric position. These findings help adjudicate between the different models of space representation, demonstrating that specific allocentric deficits not only exist but also often co-exist with egocentric biases.

Neuroanatomical substrates of action perception and understanding: an anatomic likelihood estimation meta-analysis of lesion-symptom mapping studies in brain injured patients

Several neurophysiologic and neuroimaging studies suggested that motor and perceptual systems are tightly linked along a continuum rather than providing segregated mechanisms supporting different functions. Using correlational approaches, these studies demonstrated that action observation activates not only visual but also motor brain regions. On the other hand, brain stimulation and brain lesion evidence allows tackling the critical question of whether our action representations are necessary to perceive and understand others’ actions. In particular, recent neuropsychological studies have shown that patients with temporal, parietal, and frontal lesions exhibit a number of possible deficits in the visual perception and the understanding of others’ actions. The specific anatomical substrates of such neuropsychological deficits however, are still a matter of debate. Here we review the existing literature on this issue and perform an anatomic likelihood estimation meta-analysis of studies using lesion-symptom mapping methods on the causal relation between brain lesions and non-linguistic action perception and understanding deficits. The meta-analysis encompassed data from 361 patients tested in 11 studies and identified regions in the inferior frontal cortex, the inferior parietal cortex and the middle/superior temporal cortex, whose damage is consistently associated with poor performance in action perception and understanding tasks across studies. Interestingly, these areas correspond to the three nodes of the action observation network that are strongly activated in response to visual action perception in neuroimaging research and that have been targeted in previous brain stimulation studies. Thus, brain lesion mapping research provides converging causal evidence that premotor, parietal and temporal regions play a crucial role in action recognition and understanding.

Hierarchical processing in Balint's syndrome: a failure of flexible top-down attention

Patients with Balint’ s syndrome are typically impaired at perceiving multiple objects simultaneously, and at evaluating the relationship between multiple objects in a scene (simultanagnosia). These deficits may not only be observed in complex scenes, but also when local elements of individual objects must be integrated into a perceptual global whole. Thus, unlike normal observers, patients with simultanagnosia typically show a bias towards the local forms, even to the extent that they cannot identify the global stimuli. However, we have previously shown that global processing is still attainable in Balint patients in certain scenarios (e.g., when local elements are unfamiliar). This suggests that in addition to a possible perceptual deficit that favors the local elements in these patients, impaired attentional control may be at the core of their unique performance. To test this hypothesis we manipulated the perceptual saliency of the local and global elements in a compound letter task so that it included global-more-salient or local-more-salient displays. We show that a Balint patient was able to accurately identify both global and local targets as long as they were the salient aspect of the compound letter. However, substantial impairment was evident when either the global or local elements were the less salient aspect of the compound letter. We conclude that in Balint’ s syndrome there is a failure of flexible top-down attention both in biasing attention away from salient irrelevant aspects of the display (salience-based-selection) and in impaired disengagement from irrelevant but salient items once they have been selected.

The frequency and severity of extinction after stroke affecting different vascular territories

We examined the frequency and severity of visual versus tactile extinction based on data from a large group of sub-acute patients (n=454) with strokes affecting different vascular territories. After right hemisphere damage visual and tactile extinction were equally common. However, after left hemisphere damage tactile extinction was more common than visual. The frequency of extinction was significantly higher in patients with right compared to left hemisphere damage in both visual and tactile modalities but this held only for strokes affecting the MCA and PCA territories and not for strokes affecting other vascular territories. Furthermore, the severity of extinction did not differ as a function of either the stimulus modality (visual versus tactile), the affected hemisphere (left versus right) or the stroke territory (MCA, PCA or other vascular territories). We conclude that the frequency but not severity of extinction in both modalities relates to the side of damage (i.e. left versus right hemisphere) and the vascular territories affected by the stroke, and that left hemisphere dominance for motor control may link to the greater incidence of tactile than visual extinction after left hemisphere stroke. We discuss the implications of our findings for understanding hemispheric lateralization within visuospatial attention networks.

Effect of eye patching in rehabilitation of hemispatial neglect

Eye patching (EP; monocular or right hemifield) has been proposed to improve visuospatial attention to the ignored field in patients with hemispatial neglect. The aim of this paper is to review the literature on the effects of EP in hemispatial neglect after stroke in order to convey evidence-based recommendations to clinicians in stroke rehabilitation. Thirteen intervention studies were selected from the Medline, EMBASE, Scopus, Cochrane Library, CINAHL, PsychINFO, EBRSR, and Health Star databases. Methodological quality was defined according to the Physiotherapy Evidence Database. Overall, seven studies used monocular EP, five used right hemifield patching, and one compared right monocular with right hemifield patching. Seven studies compared normal viewing to monocular or hemifield patching conditions. Six studies included a period of treatment. As to the monocular EP, four studies reported positive effects of right monocular patching. One study showed an improvement in hemispatial neglect with left monocular patching. Two studies found no superiority of right vs. left monocular patching. One study found no effects of right monocular patching. As to the right hemifield EP, one study showed improvements in neglect after right hemifield patching. Three studies found that right hemifield patching combined with another rehabilitation technique was more effective than that treatment alone. One study found no differences between right hemifield patching combined with another treatment and that treatment alone. One study found the same effect between right hemifield patching alone and another rehabilitation technique. Our results globally tend to support the usefulness of right hemifield EP in clinical practice. In order to define a level of evidence with the standard rehabilitation evidence rating tools, further properly powered randomized controlled trials or meta-analysis are needed.

Beyond the lesion: neuroimaging foundations for post-stroke recovery

A shift is emerging in the way in which we view post-stroke recovery. This shift, supported by evidence from neuroimaging studies, encourages us to look beyond the lesion and to identify viable brain networks with capacity for plasticity. In this article, the authors review current advances in neuroimaging techniques and the new insights that they have contributed. The ability to quantify salvageable tissue, evidence of changes in remote networks, changes of functional and structural connectivity, and alterations in cortical thickness are reviewed in the context of their impact on post-stroke recovery. The value of monitoring spared structural connections and functional connectivity of brain networks within and across hemispheres is highlighted.

Mapping the functional neuroanatomy of spatial neglect and human parietal lobe functions: progress and challenges

Spatial neglect is generally defined by various deficits in processing information from one (e.g., left) side of space contralateral to focal (e.g., right) hemisphere damage. Although classically associated with parietal lobe functions, there is now compelling evidence that neglect can follow lesions in many different cortical and subcortical sites, suggesting a dysfunction in distributed brain networks. In addition, neglect is likely to result from a combination of distinct deficits that co-occur due to concomitant damage affecting juxtaposed brain areas and their connections, but the exact nature of core deficits and their neural substrates still remains unclear. The present review describes recent progress in identifying functional components of the neglect syndrome and relating them to distinct subregions of parietal cortex. A comprehensive understanding of spatial neglect will require a more precise definition of cognitive processes implicated in different behavioral manifestations, as well as meticulous mapping of these processes onto specific brain circuits, while taking into account functional changes in activity that may arise in structurally intact areas subsequent to damage in distant portions of the relevant networks.

The parietal cortex and saccade planning: lessons from human lesion studies

The parietal cortex is a critical interface for attention and integration of multiple sensory signals that can be used for the implementation of motor plans. Many neurons in this region exhibit strong attention-, reach-, grasp- or saccade-related activity. Here, we review human lesion studies supporting the critical role of the parietal cortex in saccade planning. Studies of patients with unilateral parietal damage and spatial neglect reveal characteristic spatially lateralized deficits of saccade programming when multiple stimuli compete for attention. However, these patients also show bilateral impairments of saccade initiation and control that are difficult to explain in the context of their lateralized deficits of visual attention. These findings are reminiscent of the deficits of oculomotor control observed in patients with Bálint's syndrome consecutive to bilateral parietal damage. We propose that some oculomotor deficits following parietal damage are compatible with a decisive role of the parietal cortex in saccade planning under conditions of sensory competition, while other deficits reflect disinhibition of low-level structures of the oculomotor network in the absence of top-down parietal modulation.

A world unglued: simultanagnosia as a spatial restriction of attention

Simultanagnosia is a disorder of visual attention that leaves a patient's world unglued: scenes and objects are perceived in a piecemeal manner. It is generally agreed that simultanagnosia is related to an impairment of attention, but it is unclear whether this impairment is object- or space-based in nature. We first consider the findings that support a concept of simultanagnosia as deficit of object-based attention. We then examine the evidence suggesting that simultanagnosia results from damage to a space-based attentional system, and in particular a model of simultanagnosia as a narrowed spatial window of attention. We ask whether seemingly object-based deficits can be explained by space-based mechanisms, and consider the evidence that object processing influences spatial deficits in this condition. Finally, we discuss limitations of a space-based attentional explanation.

Prism Adaptation Improves Ego-Centered but Not Allocentric Neglect in Early Rehabilitation Patients

Background and Objective. Unilateral neglect due to parieto-temporo-frontal lesions has a negative impact on the success of rehabilitation, and prism adaptation (PA) enhances recovery from neglect. However, it is unclear if this effect holds also in severely impaired patients and/or in the postacute phase of rehabilitation. Moreover, it is not known whether PA affects all aspects of neglect recovery or ego-centered spatial orientation only. Methods. Sixteen patients in a postacute stage (on average 36 days after a large right cerebrovascular stroke) were entered into a series of single case design studies with 4 measurements: 2 before and 2 after 1 week of PA treatment. All patients had severe neglect (showing trunk, head, and eye deviation; canceling less than 20% of targets in a visual cancellation test). Lesions were transferred to a standard brain to analyze size and location. Results. Patients improved in cued body orientation and in the cancellation task, that is, in ego-centered neglect. However, none of the measures used to evaluate neglect of left side of objects irrespective of their position on the right or left side of the patient (allocentric neglect) showed an improvement. Treatment effects were not influenced by total lesion size, but lesions including the postcentral cortex were related to smaller recovery gains. Conclusion. PA is helpful in treating severely impaired patients in the postacute phase, but the effect is restricted to ego-centered neglect. Lesions in the postcentral cortex (middle occipito-temporal, middle temporal, and posterior parietal areas) seem to limit the effect of PA.

The prognosis of allocentric and egocentric neglect: evidence from clinical scans

We contrasted the neuroanatomical substrates of sub-acute and chronic visuospatial deficits associated with different aspects of unilateral neglect using computed tomography scans acquired as part of routine clinical diagnosis. Voxel-wise statistical analyses were conducted on a group of 160 stroke patients scanned at a sub-acute stage. Lesion-deficit relationships were assessed across the whole brain, separately for grey and white matter. We assessed lesions that were associated with behavioural performance (i) at a sub-acute stage (within 3 months of the stroke) and (ii) at a chronic stage (after 9 months post stroke). Allocentric and egocentric neglect symptoms at the sub-acute stage were associated with lesions to dissociated regions within the frontal lobe, amongst other regions. However the frontal lesions were not associated with neglect at the chronic stage. On the other hand, lesions in the angular gyrus were associated with persistent allocentric neglect. In contrast, lesions within the superior temporal gyrus extending into the supramarginal gyrus, as well as lesions within the basal ganglia and insula, were associated with persistent egocentric neglect. Damage within the temporo-parietal junction was associated with both types of neglect at the sub-acute stage and 9 months later. Furthermore, white matter disconnections resulting from damage along the superior longitudinal fasciculus were associated with both types of neglect and critically related to both sub-acute and chronic deficits. Finally, there was a significant difference in the lesion volume between patients who recovered from neglect and patients with chronic deficits. The findings presented provide evidence that (i) the lesion location and lesion size can be used to successfully predict the outcome of neglect based on clinical CT scans, (ii) lesion location alone can serve as a critical predictor for persistent neglect symptoms, (iii) wide spread lesions are associated with neglect symptoms at the sub-acute stage but only some of these are critical for predicting whether neglect will become a chronic disorder and (iv) the severity of behavioural symptoms can be a useful predictor of recovery in the absence of neuroimaging findings on clinical scans. We discuss the implications for understanding the symptoms of the neglect syndrome, the recovery of function and the use of clinical scans to predict outcome.