Acute visual neglect and extinction: distinct functional state of the visuospatial attention system.
ABSTRACT The neural mechanisms underlying spatial neglect are still disputed. Abnormal left parietal hyperactivation is proposed to lead to the rightward attentional bias, a clinical hallmark of neglect. Extinction, another deficit of visuospatial attention, is regarded as either a 'mild' form of neglect or a distinct syndrome. Although both neglect and extinction are typical syndromes of acute right hemispheric stroke, all imaging studies investigating these syndromes were conducted at least several weeks after stroke onset, in a phase when brain reorganization has already progressed. The present study aimed at comparing the activation patterns in acute stroke patients with neglect and extinction during visuospatial processing. Using functional magnetic resonance imaging, we examined the functional state of the attention system in 33 patients with a first ever stroke (53 ± 5 h after stroke onset) and age-matched healthy subjects (n = 15). All patients had embolic infarcts within the territory of the right middle cerebral artery. Patients were divided into three groups: (i) normal visuospatial processing (control patients, n = 11); (ii) patients with visual extinction but with no signs of neglect (n = 9); and (iii) patients with visual neglect (n = 13). While undergoing functional magnetic resonance imaging, patients performed a Posner-like task for visuospatial attention with detection of the targets in the left and right visual hemifields. Patients with neglect showed the expected imbalance in the left versus right parietal activation, which however, was present also in control and extinction patients, thus representing an epiphenomenon of the acute structural lesion in the right hemisphere. Compared with control patients, neglect was characterized by reduced activation in the right parietal and lateral occipital cortex, as well as in the left frontal eye field. In contrast, the activation pattern in patients with extinction differed from all other groups by an increased activation of the left prefrontal cortex. In both patients with neglect and extinction, detection of targets in the left hemifield correlated with an activation in the left prefrontal and parietal cortex. Thus at least in acute stroke, a relative hyperactivation of the left parietal cortex is not a particular characteristic of neglect. The specific signature of neglect is represented by the dysfunction of the right parietal and lateral occipital cortex. The function of the left attentional centres might provide a compensatory role after critical right hemisphere lesions and be relevant for the contralesional spatial processing.
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ABSTRACT: 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. Hum Brain Mapp, 2014. © 2014 Wiley Periodicals, Inc.Human Brain Mapping 03/2014; · 6.88 Impact Factor
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ABSTRACT: Patients affected by right parietal lobe lesion can be severely impaired in sustained attention tasks, particularly in the left visual field. For example, patients with right parietal stroke are commonly limited in their ability to attentionally track multiple moving objects in their left visual field when competing stimuli are simultaneously presented in the right, ipsilesional visual field. This is a hallmark of visual extinction, a failure to respond to contralesional visual stimuli, when competing stimuli are presented in the good hemifield. It has been hypothesized that post-stroke hyperactivity of the undamaged left hemisphere leads to excessive cross-hemispheric inhibition of the damaged right hemisphere, thus exacerbating the attentional deficits. However, there has been no direct physiological demonstration of this hypothesis, as most of the studies are conducted using unilateral tasks, a condition not sufficient to drive inter-hemispheric competition. The inter-hemispheric inhibition hypothesis also raises the possibility that if hyperactivity of the healthy hemisphere were reduced, this could relieve inter-hemispheric inhibition, disinhibiting the damaged hemisphere and potentially restoring some function. To test this hypothesis, and to examine whether we could relieve deficits in sustained attention in right parietal patients, we used repetitive transcranial magnetic stimulation (rTMS) to reduce the activity of the left, healthy hemisphere. Six patients suffering from visual extinction underwent two counterbalanced sessions: low frequency rTMS over the left parietal lobe and sham control stimulation. The patients’ performance in an attentional tracking task significantly improved in the contralesional visual field immediately after rTMS, but not after sham. Performance remained unaltered in the ipsilesional field. We hypothesize that rTMS temporarily releases the damaged right hemisphere from excessive cross-hemispheric inhibition by the hyperactive healthy hemisphere, leading to some cognitive recovery after cortical lesion.Neuropsychologia 01/2014; · 3.48 Impact Factor
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ABSTRACT: The idea of two separate attention networks in the human brain for the voluntary deployment of attention and the reorientation to unexpected events, respectively, has inspired an enormous amount of research over the past years. In this review, we will reconcile these theoretical ideas on the dorsal and ventral attentional system with recent empirical findings from human neuroimaging experiments and studies in stroke patients. We will highlight how novel methods-such as the analysis of effective connectivity or the combination of neurostimulation with functional magnetic resonance imaging-have contributed to our understanding of the functionality and interaction of the two systems. We conclude that neither of the two networks controls attentional processes in isolation and that the flexible interaction between both systems enables the dynamic control of attention in relation to top-down goals and bottom-up sensory stimulation. We discuss which brain regions potentially govern this interaction according to current task demands.The Neuroscientist 07/2013; · 5.63 Impact Factor