The anatomy of spatial neglect after posterior cerebral artery stroke

Left- and right-side unilateral spatial neglect: Hemispheric differences

Neglect of one side of space, typically contralateral to a lesion of one cerebral hemisphere, is a multicomponent neurologic syndrome. In humans, left neglect after right brain damage is more frequent, severe, or both, than right neglect after left brain damage. Right neglect is behaviorally like left neglect. In the monkey, such a functional asymmetry is not present. In humans, left hemisphere-based spatial systems are weaker, likely due to the coexistence of language and spatial processes. This may account for the lateral asymmetry of neglect, which is present at birth. Except in a few patients, there is no global functional reversal of language and spatial cognition. Left brain-damaged patients often show both aphasia and right neglect, as many right brain-damaged patients with crossed aphasia show left neglect. Lateralized sensory stimulations temporarily improve both left and right neglect. Damage to the posterior parietal lobe (inferior parietal lobule), the temporo-parietal junction, the superior and middle temporal, and to the premotor and prefrontal cortices is associated with contralateral neglect; also, lesions in white matter fiber tracts and subcortical nuclei bring about neglect, with no definite left-right asymmetries.

Severity-Dependent Interhemispheric White Matter Connectivity Predicts Poststroke Neglect Recovery

Left-sided spatial neglect is a very common and challenging issue after right-hemispheric stroke, which strongly and negatively affects daily living behavior and recovery of stroke survivors. The mechanisms underlying recovery of spatial neglect remain controversial, particularly regarding the involvement of the intact, contralesional hemisphere, with potential contributions ranging from maladaptive to compensatory. In the present prospective, observational study, we assessed neglect severity in 54 right-hemispheric stroke patients (32 male; 22 female) at admission to and discharge from inpatient neurorehabilitation. We demonstrate that the interaction of initial neglect severity and spared white matter (dis)connectivity resulting from individual lesions (as assessed by diffusion tensor imaging, DTI) explains a significant portion of the variability of poststroke neglect recovery. In mildly impaired patients, spared structural connectivity within the lesioned hemisphere is sufficient to attain good recovery. Conversely, in patients with severe impairment, successful recovery critically depends on structural connectivity within the intact hemisphere and between hemispheres. These distinct patterns, mediated by their respective white matter connections, may help to reconcile the dichotomous perspectives regarding the role of the contralesional hemisphere as exclusively compensatory or not. Instead, they suggest a unified viewpoint wherein the contralesional hemisphere can - but must not necessarily - assume a compensatory role. This would depend on initial impairment severity and on the available, spared structural connectivity. In the future, our findings could serve as a prognostic biomarker for neglect recovery and guide patient-tailored therapeutic approaches.

Exploring the role of COX-2 in Alzheimer's disease: Potential therapeutic implications of COX-2 inhibitors

This review highlights the potential role of cyclooxygenase-2 enzyme (COX-2) in the pathogenesis of Alzheimer's disease (AD) and the potential therapeutic use of non-steroidal anti-inflammatory drugs (NSAIDs) in the management of AD. In addition to COX-2 enzymes role in inflammation, the formation of amyloid plaques and neurofibrillary tangles in the brain, the review emphasizes that COXs-2 have a crucial role in normal synaptic activity and plasticity, and have a relationship with acetylcholine, tau protein, and beta-amyloid (Aβ) which are the main causes of Alzheimer's disease. Furthermore, the review points out that COX-2 enzymes have a relationship with kinase enzymes, including Cyclin Dependent Kinase 5 (CDK5) and Glycogen Synthase Kinase 3β (GSK3β), which are known to play a role in tau phosphorylation and are strongly associated with Alzheimer's disease. Therefore, the use of drugs like NSAIDs may be a hopeful approach for managing AD. However, results from studies examining the effectiveness of NSAIDs in treating AD have been mixed and further research is needed to fully understand the mechanisms by which COX-2 and NSAIDs may be involved in the development and progression of AD and to identify new therapeutic strategies.

A case study of left visual neglect after right pontine lesion: pathophysiological evidence for the infratentorial involvement in human visual attention

Hemispatial neglect, the inability to attend to the contralesional side of space, is the most common disturbance of visuospatial attention. Both hemispatial neglect and visuospatial attention are typically associated with extended cortical networks. Nevertheless, recent accounts challenge this so-called corticocentric view and postulate the participation of structures well beyond the telencephalic cortex, in particular advocating the role of the brainstem. However, to the best of our knowledge, hemispatial neglect after a brainstem lesion has not yet been described. We describe, for the first time in a human, the occurrence and remission of contralesional visual hemispatial neglect after a focal lesion in the right pons. Hemispatial neglect was assessed by means of video-oculography during free visual exploration, a very sensitive and established method, and its remission was followed up until 3 wk after stroke. Moreover, by means of a lesion-deficit approach complemented by imaging, we identify a pathophysiological mechanism involving the disconnection of cortico-ponto-cerebellar and/or tecto-cerebellar-tectal pathways passing through the pons. Our findings offer, for the first time in a human, causal, lesion-based support for recent seminal accounts postulating the role of infratentorial structures participating in the activity of cerebral cortical attentional networks mediating attentional processes.NEW & NOTEWORTHY Visuospatial attention and its most common disturbance, hemispatial neglect, are typically associated with extended cortical networks. However, recent accounts challenge this corticocentric view and advocate the role of infratentorial structures. We describe, for the first time in a human, the occurrence of contralesional visual hemispatial neglect after a focal lesion in the right pons. We provide causal, lesion-based evidence for a pathophysiological mechanism involving the disconnection of cortico-ponto-cerebellar and/or tecto-cerebellar-tectal pathways passing through the pons.

The neuroanatomy of visuospatial neglect: A systematic review and analysis of lesion-mapping methodology

While visuospatial neglect is commonly associated with damage to the right posterior parietal cortex, neglect is an anatomically heterogenous syndrome. This project presents a systematic review of 34 lesion-mapping studies reporting on the anatomical correlates of neglect. Specifically, the reported correlates of egocentric versus allocentric, acute versus chronic, personal versus extra-personal, and left versus right hemisphere neglect are summarised. The quality of each included lesion-mapping analysis was then evaluated to identify methodological factors which may help account for the reported variance in correlates of neglect. Overall, the existing literature strongly suggests that egocentric and allocentric neglect represent anatomically dissociable conditions and that the anatomy of these conditions may not be entirely homologous across hemispheres. Studies which have compared the anatomy of acute versus chronic neglect have found that these conditions are associated with distinct lesion loci, while studies comparing the correlates of peripersonal/extrapersonal neglect are split as to whether these neglect subtypes are anatomically dissociable. The included studies employed a wide range of lesion-mapping analysis techniques, each producing results of varying quality and generalisability. This review concludes that the reported underlying anatomical correlates of heterogeneous visuospatial neglect vary considerably. Future, high quality studies are needed to investigate patterns of disconnection associated with clearly defined forms of visuospatial neglect in large and representative samples.

Splenial Callosal Disconnection in Right Hemianopic Patients Induces Right Visual-Spatial Neglect

Posterior cerebral artery (PCA) territory infarction involving occipital cortical damage can give rise to contralateral homonymous hemianopia. Here, we report two rare cases of patients with lesions in the left hemisphere PCA territory who developed right visuo-spatial neglect. One patient suffered right hemianopia and right visuo-spatial neglect after a stroke that damaged the left primary visual cortex and the callosal splenial fibers. The other unique case is of a patient who had a brain tumor in the posterior cerebral region in the left hemisphere and initially exhibited only right hemianopia that developed into right visuo-spatial neglect after tumor resection that included the splenial fibers. These cases indicate that, as in cases with damage in the right PCA territory, lesions in the left PCA yield visuo-spatial neglect when the damage produces contralateral hemianopia and concomitant disconnection of the splenium of the corpus callosum, which interferes with the arrival of visual inputs from the intact right to the lesioned left hemisphere. These results also emphasize the necessity of sparing the splenial fibers in surgical interventions in patients who exhibit hemianopia.

Lesion topography of posterior cerebral artery infarcts

This study analyzed the topography of acute ischemic stroke in the posterior cerebral artery (PCA) territory. We studied 84 patients with unilateral ischemic PCA stroke. Patients were classified according to lesion levels as cortico-subcortical (superficial), combined (cortical and mesodiencephalic) or isolated thalamic. To receive a lesion map, data from acute MR and CT imaging were normalized and labelled automatically by mapping to stereotaxic anatomical atlases. Cortical lesions accounted for 41.7%, combined for 36.9%, and isolated thalamic lesions for 21.4%. The maximum overlay of ischemia and, thus, highest occurrence of PCA ischemic stroke was found in the ventral and medial occipito-temporal cortex and adjacent white matter association tracts. Dorsal and peripheral segments of the occipito-temporo-parietal region were only rarely lesioned. This configuration was similar in both hemispheres. Consistent with this lesion pattern, visual field defects (VFD) were the most frequent signs, followed by sensorimotor signs, dizziness and sopor, cognitive and oculomotor deficits, and ataxia. The three vascular subgroups differed not only by their anatomical lesion profile and lesion load, but also by their clinical manifestation; although patients with combined and thalamic lesions were sigificantly younger, they were more disabled than participants with cortical lesions. VFD were only found in cortical and combined, and oculomotor deficits only in mesodiencephalic lesions. White matter lesions were common in the cortico-subcortical and the combined group. Basal occipito-temporal and calcarine regions, and neighbouring white matter tracts have the highest risk of ischemia in acute PCA stroke.

Number space is made by response space: Evidence from left spatial neglect

Whether the semantic representation of numbers is endowed with an intrinsic spatial component, so that smaller numbers are inherently represented to the left of larger ones on a Mental Number Line (MNL), is a central matter of debate in numerical cognition. To gain an insight into this issue, we investigated the performance of right brain damaged patients with left spatial neglect (N+) in a bimanual Magnitude Comparison SNARC task and in a uni-manual Magnitude Comparison Go/No-Go task (i.e. "is the number smaller or larger than 5?"). While the first task requires the use of contrasting left/right spatial codes for response selection, the second task does not require the use of these codes. In line with previous evidence, in the SNARC task N+ patients displayed a significant asymmetry in Reaction Times (RTs), with slower RTs to number "4", that was immediately precedent to the numerical reference "5", with respect to the number "6", that immediately followed the same reference. This RTs asymmetry was correlated with lesion of white matter tracts, i.e. Fronto-Occipital-Fasciculus, that allows prefrontal Ba 8 and 46 to regulate the distribution of attention on sensory and memory traces in posterior occipital, temporal and parietal areas. In contrast, no similar RTs asymmetry was found in the Go/No-Go task. These findings suggest that while in the SNARC task numbers get mentally organised from left-to-right as a function of their increasing magnitude, so that N+ patients display a delay in the processing of number-magnitudes that are immediately smaller than a given numerical reference, in the Go/No-Go task no left-to-right organization is activated. These results support the idea that it is the use of contrasting left/right spatial codes, whether motor or conceptual, that triggers the generation of a spatially left-to-right organised MNL and that the representation of number magnitude is not endowed with an inherent spatial component.