Reperfusion of specific cortical areas is associated with improvement in distinct forms of hemispatial neglect

Relative Improvement in Language vs Motor Functions with Reperfusion Therapies for Acute Stroke due to Large Vessel Occlusion

Background

When weighing potential risks versus benefits of reperfusion therapy, the functions likely to recover if blood flow can be restored should be considered. Because deep and motor areas of the brain often infarct relatively early in acute stroke, we hypothesized that reperfusion therapies are more likely to improve language function and neglect (cortical functions) more than motor function.

Methods

In this retrospective review of a prospectively collected database, patients with acute stroke due to large vessel occlusion), we evaluated percent improvement (mean change in score/maximum score) for different items of the National Institutes of Health Score Scale with and without endovascular thrombectomy, and/or intravenous thrombolysis.

Results

In total, 290 patients (mean age 61.8; SD 14.0; 47.9% female) met the inclusion criteria. For all outcome measures (percent change in language, total language, motor, and neglect) there were significant effects of treatment group (p<0.0001 for all), with the greatest change in the EVT +tPA group, then EVT only group, followed by tPA only, followed by no intervention. Differences between EVT + tPA and EVT only were not significant (p=.30 to 0.79 across outcomes). For patients with aphasia and/or right sided weakness before treatment, the percent change in language was significantly greater than the percent change in weakness (29.8% vs. 12.7%; t(93)=5.3;p<0.0001). Greater percent improvement in language was observed in all treatment groups (p=0.0003 to 0.03 across treatment groups).

Conclusions

In patients with acute ischemic stroke due to LVO, improvements in all neurological functions occur with tPA, and even more with EVT (with and without IV tPA). However, gains in language are even greater than gains in motor function with both interventions. Few patients had neglect before treatment, but of those who did, the majority improved, and most (92.8%) improved with EVT.

Unilateral Spatial Neglect Recovery Poststroke

Unilateral spatial neglect (USN) is a common and disabling cognitive consequence of stroke wherein individuals demonstrate decreased response to contralesional information. Here, we provide an updated narrative review of studies that shed light on the neural mechanisms and predictors of recovery of USN. Additionally, we report a rapid review of randomized controlled trials focusing on USN intervention, both nonpharmacological and pharmacological, published in the last 5 years. Randomized controlled trials are reviewed within the context of systematic reviews and meta-analyses of USN interventions published within the same time frame. The quality of randomized controlled trials of treatment is higher compared to quality reported in previous reviews and meta-analyses. However, remaining weaknesses in participant demographic reporting, as well as small, heterogenous samples, render generalizability and cross-study interpretation a challenge. Nevertheless, evidence regarding neural mechanisms underlying USN recovery and regarding the effectiveness of targeted USN interventions is accumulating and strengthening, setting the foundation for future investigations into patient-specific factors that may influence treatment response. We identify gaps and provide suggestions for future USN intervention research.

Various tests of left neglect are associated with distinct territories of hypoperfusion in acute stroke

Hemispatial neglect is among the most disabling consequences of right hemisphere stroke. However, there is no consensus on the optimal assessments to identify neglect. We hypothesized that different tests for neglect given the same day (i) detect distinct aspects and types of neglect, (ii) are sensitive to different cognitive functions (beyond spatially specific processing) and (iii) are associated with distinct regions of hypoperfusion. We examined data from 135 participants with acute, right-hemispheric ischaemic stroke who received an MRI and neglect testing within 48 h of acute infarct in a cross-sectional study. The volume of infarct was calculated on diffusion-weighted imaging. We also scored severity and location of fluid-attenuated inversion recovery hyperintense vessels in six areas (anterior cerebral artery territory, posterior cerebral artery territory and four within the middle cerebral artery territory) to estimate the volume and location of hypoperfusion in acute stroke. Neglect tests included gap detection, scene copy, line bisection, line cancellation, oral reading and picture description. We found strong correlations between tests that evaluated viewer-centred processing, as well as strong correlations between tests that evaluated stimulus-centred processing. The error rate on different tests was associated with hypoperfusion in different vascular territories, even after controlling for the volume of an infarct. Our results confirm that it is essential to administer a battery of different tests of hemispatial neglect to capture various deficits in attention and spatially specific processing that underlies neglect. Our results also show the potential usefulness of hyperintense vessel ratings as an indication of dysfunction beyond the infarct, as the ratings (and not infarct volume) were highly associated with many clinical deficits. Finally, results underscore that diverse types of neglect are clinically important in acute stroke, as they reflect different areas of hypoperfused tissue, which may be salvageable in the absence of infarct in those areas. As such, neglect batteries may be useful for detecting patients with cortical hypoperfusion who are candidates for reperfusion therapies.

Disruptions of the Human Connectome Associated With Hemispatial Neglect

BACKGROUND AND OBJECTIVES

Hemispatial neglect is a heterogeneous and complex disorder that can be classified by frame of reference for "left" vs "right," including viewer-centered neglect (VCN, affecting the contralesional side of the view), stimulus-centered neglect (SCN, affecting the contralesional side of the stimulus, irrespective of its location with respect to the viewer), or both. We investigated the effect of acute stroke lesions on the connectivity of neural networks that underlie VCN or SCN.

METHODS

A total of 174 patients within 48 hours of acute right hemispheric infarct underwent a detailed hemispatial neglect assessment that included oral reading, scene copy, line cancellation, gap detection, horizontal line bisection tests, and MRI. Each patient's connectivity map was generated. We performed a linear association analysis between network connectivity strength and continuous measures of neglect to identify lesion-induced disconnections associated with the presence or severity of VCN and SCN. Results were corrected for multiple comparisons.

RESULTS

About 42% of the participants with right hemisphere stroke had at least one type of neglect. The presence of any type of neglect was associated with lesions to tracts connecting the right inferior parietal cortex, orbitofrontal cortex, and right thalamus to other right-hemispheric structures. VCN only was strongly associated with tracts connecting the right putamen to other brain regions and tracts connecting right frontal regions with other brain regions. The presence of both types of neglect was most strongly associated with tracts connecting the right inferior and superior parietal cortex to other brain regions and those connecting left or right mesial temporal cortex to other brain regions.

DISCUSSION

Our study provides new evidence for the specific white matter tracts where disruption can cause hemispatial neglect in a relatively large number of participants and homogeneous time after onset. We obtained MRI and behavioral testing acutely, before the opportunity for rehabilitation or substantial recovery.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that damage to specific white matter tracts identified on MRI are associated with the presence of neglect following right hemispheric stroke.

Spatial Neglect Subtypes, Definitions and Assessment Tools: A Scoping Review

Objective: The objective of this scoping review was to capture the reported definitions for the subtypes of neglect post stroke and map the range of assessment tools employed for each neglect subtype.

Methods: EMBASE, Emcare, Medline, and psychINFO were searched from database inception. Searching included all allied terms and mesh headings for stroke, spatial neglect, measurement, screening tools, psychometric properties. Two reviewers independently screened studies for inclusion. Primary studies with documented protocols of a spatial neglect tool for adults post stroke, with some aspect of validity or reliability were included. Two reviewers independently reviewed the documented protocols of each tool to determine the underlying subtypes and disagreements were resolved through discussion.

Results: There were 371 articles included with 292 tools used for the screening or diagnosis of neglect. The majority of studies (67%) included a tool that did not specify the neglect subtype being assessed, therefore an analysis of the underlying subtypes for each tool is presented.

Conclusions: There is no consistency with the terms used to refer to the syndrome of spatial neglect with over 200 different terms used within the included studies to refer to the syndrome as a whole or one of its subtypes. It is essential to unify the terminology and definition for each neglect subtype. There are hundreds of neglect tools available, however many are not able to differentiate presenting subtypes. It is important for clinicians and researchers to critically evaluate the neglect tools being used for the screening and diagnosis of neglect.

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.

The 3s Spreadsheet Test version 2 for assessing egocentric viewer-centered and allocentric stimulus-centered spatial neglect

The present study established the norms of the 3 s Spreadsheet Test version 2 (3S-v2 Test) with 186 healthy adults, compared performance of 23 individuals with spatial neglect after right brain stroke to the norms, and examined the extent that allocentric neglect is independent from egocentric neglect. The task required in the 3S-v2 Test is to cross out the target digit "3" in a spreadsheet that contained 10 columns and 14 rows of digit strings, including 120 target digits and 720 non-target digits. Each target is categorized with respect to its location on the page (egocentric viewer-centered) and its position within the digit string (allocentric stimulus-centered). Patients completed the 3S-v2 Test, the Apples Test, and Scene Copying Test (a five-object figure copying test). Based on the neglect classification criteria of these three tests, 18 patients (78.3%) were identified with both forms of neglect, three patients (13.0%) had isolated egocentric neglect, and two (8.7%) had isolated allocentric neglect. Among patients who were determined as having allocentric neglect by a given test, we found no significant correlation between severity of allocentric neglect and stimulus location in the egocentric reference frame. Based on the present findings, we suggest that including the 3S-v2 Test, a functionally relevant task and different from the currently available tests, may increase the comprehensiveness of neglect assessment. In addition, allocentric neglect symptoms are independent of egocentric locations.

Allocentric Versus Egocentric Neglect in Stroke Patients: A Pilot Study Investigating the Assessment of Neglect Subtypes and Their Impacts on Functional Outcome Using Eye Tracking

OBJECTIVE

Few studies have investigated the assessment and functional impact of egocentric and allocentric neglect among stroke patients. This pilot study aimed to determine (1) whether allocentric and egocentric neglect could be dissociated among a sample of stroke patients using eye tracking; (2) the specific patterns of attention associated with each subtype; and (3) the nature of the relationship between neglect subtype and functional outcome.

METHOD

Twenty acute stroke patients were administered neuropsychological assessment batteries, a pencil-and-paper Apples Test to measure neglect subtype, and an adaptation of the Apples Test with an eye tracking measure. To test clinical discriminability, twenty age- and education-matched control participants were administered the eye tracking measure of neglect.

RESULTS

The eye tracking measure identified a greater number of individuals as having egocentric and/or allocentric neglect than the pencil-and-paper Apples Test. Classification of neglect subtype based on eye tracking performance was a significant predictor of functional outcome beyond that accounted for by the neuropsychological test performance and Apples Test neglect classification. Preliminary evidence suggests that patients with no neglect symptoms had superior functional outcomes compared with patients with neglect. Patients with combined egocentric and allocentric neglect had poorer functional outcomes than those with either subtype. Functional outcomes of patients with either allocentric or egocentric neglect did not differ significantly. The applications of our findings, to improve neglect detection, are discussed.

CONCLUSION

Results highlight the potential clinical utility of eye tracking for the assessment and identification of neglect subtype among stroke patients to predict functional outcomes. (JINS, 2019, 25, 479-489).

The role of the right superior temporal gyrus in stimulus-centered spatial processing

Although emerging neuropsychological evidence supports the involvement of temporal areas, and in particular the right superior temporal gyrus (STG), in allocentric neglect deficits, the role of STG in healthy spatial processing remains elusive. While several functional brain imaging studies have demonstrated involvement of the STG in tasks involving explicit stimulus-centered judgments, prior rTMS studies targeting the right STG did not find the expected neglect-like rightward bias in size judgments using the conventional landmark task. The objective of the current study was to investigate whether disruption of the right STG using inhibitory repetitive transcranial magnetic stimulation (rTMS) could impact stimulus-centered, allocentric spatial processing in healthy individuals. A lateralized version of the landmark task was developed to accentuate the dissociation between viewer-centered and stimulus-centered reference frames. We predicted that inhibiting activity in the right STG would decrease accuracy because of induced rightward bias centered on the line stimulus irrespective of its viewer-centered or egocentric locations. Eleven healthy, right-handed adults underwent the lateralized landmark task. After viewing each stimulus, participants had to judge whether the line was bisected, or whether the left (left-long trials) or the right segment (right-long trials) of the line was longer. Participants repeated the task before (pre-rTMS) and after (post-rTMS) receiving 20 min of 1 Hz rTMS over the right STG, the right supramarginal gyrus (SMG), and the vertex (a control site) during three separate visits. Linear mixed models for binomial data were generated with either accuracy or judgment errors as dependent variables, to compare 1) performance across trial types (bisection, non-bisection), and 2) pre- vs. post-rTMS performance between the vertex and the STG and the vertex and the SMG. Line eccentricity (z = 4.31, p < 0.0001) and line bisection (z = 5.49, p < 0.0001) were significant predictors of accuracy. In the models comparing the effects of rTMS, a significant two-way interaction with STG (z = -3.09, p = 0.002) revealed a decrease in accuracy of 9.5% and an increase in errors of the right-long type by 10.7% on bisection trials, in both left and right viewer-centered locations. No significant changes in leftward errors were found. These findings suggested an induced stimulus-centered rightward bias in our participants after STG stimulation. Notably, accuracy or errors were not influenced by SMG stimulation compared to vertex. In line with our predictions, the findings provide compelling evidence for right STG's involvement in healthy stimulus-centered spatial processing.

Extrastriate visual cortex reorganizes despite sequential bilateral occipital stroke: implications for vision recovery

The extent of visual cortex reorganization following injury remains controversial. We report serial functional magnetic resonance imaging (fMRI) data from a patient with sequential posterior circulation strokes occurring 3 weeks apart, compared with data from an age-matched healthy control subject. At 8 days following a left occipital stroke, contralesional visual cortical activation was within expected striate and extrastriate sites, comparable to that seen in controls. Despite a further infarct in the right (previously unaffected hemisphere), there was evolution of visual cortical reorganization progressed. In this patient, there was evidence of utilization of peri-infarct sites (right-sided) and recruitment of new activation sites in extrastriate cortices, including in the lateral middle and inferior temporal lobes. The changes over time corresponded topographically with the patient's lesion site and its connections. Reorganization of the surviving visual cortex was demonstrated 8 days after the first stroke. Ongoing reorganization in extant cortex was demonstrated at the 6 month scan. We present a summary of mechanisms of recovery following stroke relevant to the visual system. We conclude that mature primary visual cortex displays considerable plasticity and capacity to reorganize, associated with evolution of visual field deficits. We discuss these findings and their implications for therapy within the context of current concepts in visual compensatory and restorative therapies.

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.

Interhemispheric cerebral blood flow balance during recovery of motor hand function after ischemic stroke--a longitudinal MRI study using arterial spin labeling perfusion

BACKGROUND

Unilateral ischemic stroke disrupts the well balanced interactions within bilateral cortical networks. Restitution of interhemispheric balance is thought to contribute to post-stroke recovery. Longitudinal measurements of cerebral blood flow (CBF) changes might act as surrogate marker for this process.

OBJECTIVE

To quantify longitudinal CBF changes using arterial spin labeling MRI (ASL) and interhemispheric balance within the cortical sensorimotor network and to assess their relationship with motor hand function recovery.

METHODS

Longitudinal CBF data were acquired in 23 patients at 3 and 9 months after cortical sensorimotor stroke and in 20 healthy controls using pulsed ASL. Recovery of grip force and manual dexterity was assessed with tasks requiring power and precision grips. Voxel-based analysis was performed to identify areas of significant CBF change. Region-of-interest analyses were used to quantify the interhemispheric balance across nodes of the cortical sensorimotor network.

RESULTS

Dexterity was more affected, and recovered at a slower pace than grip force. In patients with successful recovery of dexterous hand function, CBF decreased over time in the contralesional supplementary motor area, paralimbic anterior cingulate cortex and superior precuneus, and interhemispheric balance returned to healthy control levels. In contrast, patients with poor recovery presented with sustained hypoperfusion in the sensorimotor cortices encompassing the ischemic tissue, and CBF remained lateralized to the contralesional hemisphere.

CONCLUSIONS

Sustained perfusion imbalance within the cortical sensorimotor network, as measured with task-unrelated ASL, is associated with poor recovery of dexterous hand function after stroke. CBF at rest might be used to monitor recovery and gain prognostic information.

Attention-network specific alterations of structural connectivity in the undamaged white matter in acute neglect

Visual neglect results from dysfunction within the spatial attention network. The structural connectivity in undamaged brain tissue in neglect has barely been investigated until now. In the present study, we explored the microstructural white matter characteristics of the contralesional hemisphere in relation to neglect severity and recovery in acute stroke patients. We compared age-matched healthy subjects and three groups of acute stroke patients (9 ± 0.5 days after stroke): (i) patients with nonrecovered neglect (n = 12); (ii) patients with rapid recovery from initial neglect (within the first week post-stroke, n = 7), (iii) stroke patients without neglect (n = 17). We analyzed the differences between groups in grey and white matter density and fractional anisotropy (FA) and used fiber tracking to identify the affected fibers. Patients with nonrecovered neglect differed from those with rapid recovery by FA-reduction in the left inferior parietal lobe. Fibers passing through this region connect the left-hemispheric analogues of the ventral attention system. Compared with healthy subjects, neglect patients with persisting neglect had FA-reduction in the left superior parietal lobe, optic radiation, and left corpus callosum/cingulum. Fibers passing through these regions connect centers of the left dorsal attention system. FA-reduction in the identified regions correlated with neglect severity. The study shows for the first time white matter changes within the spatial attention system remote from the lesion and correlating with the extent and persistence of neglect. The data support the concept of neglect as disintegration within the whole attention system and illustrate the dynamics of structural-functional correlates in acute stroke.

Stroke Recovery: Surprising Influences and Residual Consequences

There is startling individual variability in the degree to which people recover from stroke, and the duration of time over which recovery of some symptoms occur. There are a variety of mechanisms of recovery from stroke, which take place at distinct time points after stroke and are influenced by different variables. We review recent studies from our laboratory that unveil some surprising findings, such as the role of education in chronic recovery. We also report data showing that the consequences that most plague survivors of stroke and their caregivers are not dependence in activities of daily living, but loss of more high level functions, such as empathy or written language. These results have implications for rehabilitation and management of stroke.

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.

Object-based neglect in number processing

Recent evidence suggests that neglect patients seem to have particular problems representing relatively smaller numbers corresponding to the left part of the mental number line. However, while this indicates space-based neglect for representational number space little is known about whether and - if so - how object-based neglect influences number processing.

To evaluate influences of object-based neglect in numerical cognition, a group of neglect patients and two control groups had to compare two-digit numbers to an internally represented standard. Conceptualizing two-digit numbers as objects of which the left part (i.e., the tens digit should be specifically neglected) we were able to evaluate object-based neglect for number magnitude processing.

Object-based neglect was indicated by a larger unit-decade compatibility effect actually reflecting impaired processing of the leftward tens digits. Additionally, faster processing of within- as compared to between-decade items provided further evidence suggesting particular difficulties in integrating tens and units into the place-value structure of the Arabic number system.

In summary, the present study indicates that, in addition to the spatial representation of number magnitude, also the processing of place-value information of multi-digit numbers seems specifically impaired in neglect patients.

Is 'object-centred neglect' a homogeneous entity?

The nature of object-centred (allocentric) neglect and the possibility of dissociating it from egocentric (subject-centred) forms of neglect are controversial. Originally, allocentric neglect was described by Gainotti, D'Erme, Monteleone & Silveri (1986) and Gainotti, Messerli, & Tissot (1972) in patients who reproduced all the elements of a multi-object scene, but left unfinished the left side of one or more of them. More recently, however, Karnath, Mandler, and Clavagnier (2011) have claimed that the severity of allocentric neglect worsens when a complex 'object' shifts from an ipsilesional to a contralesional egocentric position. On the basis of these and of other clinical data, showing that allocentric and egocentric neglect are strongly associated, they have questioned the possibility of dissociating these two forms of neglect, suggesting that egocentric and allocentric neglect constitute different manifestations of the same disturbed system. Since these statements were inconsistent with the clinical findings which had prompted the construct of object-centred neglect, we checked in a group of right brain-damaged patients, who had copied the original multi-object scene, if the degree of neglect for the left side of figures varied as a function of their position on the horizontal axis. Furthermore, we reviewed all papers where copies of other multi-object scenes had been reported. Results of both studies failed to confirm the assumption of a relationship between spatial location of the stimulus and severity of object-centred neglect. This discrepancy between our data and those obtained by Karnath et al. (2011) could be due to the characteristics of stimuli and of procedures used to evaluate 'object-centred' neglect. If the stimulus is complex and the task requires its thorough exploration, the spatial location of the stimulus will influence the severity of 'object-centred neglect'. If, on the contrary, the stimulus is simple and can be identified with few eye fixations, the spatial location of the stimulus should not influence the severity of 'object-centred neglect'. In any case, our data confirm the possibility of dissociating allocentric from egocentric neglect.

Damage to white matter pathways in subacute and chronic spatial neglect: a group study and 2 single-case studies with complete virtual "in vivo" tractography dissection

The exact anatomical localization of right hemisphere lesions that lead to left spatial neglect is still debated. The effect of confounding factors such as acute diaschisis and hypoperfusion, visual field defects, and lesion size may account for conflicting results that have been reported in the literature. Here, we present a comprehensive anatomical investigation of the gray- and white matter lesion correlates of left spatial neglect, which was run in a sample 58 patients with subacute or chronic vascular strokes in the territory of the right middle cerebral artery. Standard voxel-based correlates confirmed the role played by lesions in the posterior parietal cortex (supramarginal gyrus, angular gyrus, and temporal-parietal junction), in the frontal cortex (frontal eye field, middle and inferior frontal gyrus), and in the underlying parietal-frontal white matter. Using a new diffusion tensor imaging-based atlas of the human brain, we were able to run, for the first time, a detailed analysis of the lesion involvement of subcortical white matter pathways. The results of this analysis revealed that, among the different pathways linking parietal with frontal areas, damage to the second branch of the superior longitudinal fasciculus (SLF II) was the best predictor of left spatial neglect. The group study also revealed a subsample of patients with neglect due to focal lesion in the lateral-dorsal portion of the thalamus, which connects the premotor cortex with the inferior parietal lobule. The relevance of fronto-parietal disconnection was further supported by complete in vivo tractography dissection of white matter pathways in 2 patients, one with and the other without signs of neglect. These 2 patients were studied both in the acute phase and 1 year after stroke and were perfectly matched for age, handedness, stroke onset, lesion size, and for cortical lesion involvement. Taken together, the results of the present study support the hypothesis that anatomical disconnections leading to a functional breakdown of parietal-frontal networks are an important pathophysiological factor leading to chronic left spatial neglect. Here, we propose that different loci of SLF disconnection on the rostro-caudal axis can also be associated with disconnection of short-range white matter pathways within the frontal or parietal areas. Such different local disconnection patterns can play a role in the important clinical variability of the neglect syndrome.

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.

No inherent left and right side in human 'mental number line': evidence from right brain damage

Spatial reasoning has a relevant role in mathematics and helps daily computational activities. It is widely assumed that in cultures with left-to-right reading, numbers are organized along the mental equivalent of a ruler, the mental number line, with small magnitudes located to the left of larger ones. Patients with right brain damage can disregard smaller numbers while mentally setting the midpoint of number intervals. This has been interpreted as a sign of spatial neglect for numbers on the left side of the mental number line and taken as a strong argument for the intrinsic left-to-right organization of the mental number line. Here, we put forward the understanding of this cognitive disability by discovering that patients with right brain damage disregard smaller numbers both when these are mapped on the left side of the mental number line and on the right side of an imagined clock face. This shows that the right hemisphere supports the representation of small numerical magnitudes independently from their mapping on the left or the right side of a spatial-mental layout. In addition, the study of the anatomical correlates through voxel-based lesion-symptom mapping and the mapping of lesion peaks on the diffusion tensor imaging-based reconstruction of white matter pathways showed that the rightward bias in the imagined clock-face was correlated with lesions of high-level middle temporal visual areas that code stimuli in object-centred spatial coordinates, i.e. stimuli that, like a clock face, have an inherent left and right side. In contrast, bias towards higher numbers on the mental number line was linked to white matter damage in the frontal component of the parietal-frontal number network. These anatomical findings show that the human brain does not represent the mental number line as an object with an inherent left and right side. We conclude that the bias towards higher numbers in the mental bisection of number intervals does not depend on left side spatial, imagery or object-centred neglect and that it rather depends on disruption of an abstract non-spatial representation of small numerical magnitudes.

Allocentric neglect strongly associated with egocentric neglect

Following brain injury, many patients experience egocentric spatial neglect, where they fail to respond to stimuli on the contralesional side of their body. On the other hand, allocentric, object-based neglect refers to the symptom of ignoring the contralesional side of objects, regardless of the objects' egocentric position. There is an established tradition for considering these two phenomena as both behaviorally and anatomically dissociable. However, several studies and some theoretical work have suggested that these rather reflect two aspects of a unitary underlying disorder. Furthermore, in a recent large study Yue et al. [Archives of Physical Medicine and Rehabilitation 93 (2012) 156] reported that acute allocentric neglect is only observed in cases where substantial egocentric neglect is also present. In a new sample of right hemisphere stroke patients, we attempted to control for potential confounds by using a novel continuous measure for allocentric neglect (in addition to a recently developed continuous measure for egocentric neglect). Our findings suggest a strong association between egocentric and allocentric neglect. Consistent with the work of Yue et al. (2012), we found allocentric behavioral deficits only in conjunction with egocentric deficits as well as a large corresponding overlap for the anatomical regions associated with egocentric and with allocentric neglect. We discuss how different anatomical and behavioral findings can be explained in a unified physiologically plausible framework, whereby allocentric and egocentric effects interact.