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.
- SourceAvailable from: Silvia Savazzi[Show abstract] [Hide abstract]
ABSTRACT: Spatial neglect is traditionally explained as an imbalance of the interhemispheric reciprocal inhibition exerted by the two hemispheres: after a right lesion, the contralesional hemisphere becomes disinhibited and its enhanced activity suppresses the activity in the lesioned one. Even though the hyperexcitability of the left hemisphere is the theoretical framework of several rehabilitation interventions using non-invasive brain stimulation protocols in neglect, no study has yet investigated directly the actual state of cortical excitability of the contralesional hemisphere immediately after the brain lesion. The present study represents the first attempt to directly assess the interhemispheric rivalry model adopting a novel approach based on the induction of neglect-like biases in healthy participants. Applying repetitive transcranial magnetic stimulation (rTMS) over the right posterior parietal cortex while concurrently recording the EEG activity allows to measure specific neurophysiological markers of cortical activity (i.e. TMS-evoked potentials, TEPs) both over the stimulated right hemisphere and over the contralateral homologous area. Besides the effectiveness of the protocol used in modulating behavior, our results show an inhibition of the cortical excitability of the directly stimulated parietal cortex (right hemisphere) and, most importantly, a comparable reduction of cortical excitability of the homologous contralateral (left) area. TEPs and additional electrophysiological measures reliably provide strong evidence for a bilateral hypo-activation following TMS induction of neglect-like biases. These results suggest that the parietal imbalance typically found in neglect patients could reflect a long-term maladaptive plastic reorganization that follows a brain lesion. Copyright © 2015. Published by Elsevier Ltd.Neuropsychologia 04/2015; 72. DOI:10.1016/j.neuropsychologia.2015.04.010 · 3.45 Impact Factor
- Brain 09/2011; 135(Pt 2):e207; author reply e208. DOI:10.1093/brain/awr227 · 10.23 Impact Factor
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ABSTRACT: Unilateral spatial neglect involves a failure to report or orient to stimuli in the contralesional (left) space due to right brain damage, with severe handicap in everyday activities and poor rehabilitation outcome. Because behavioral studies suggest that prism adaptation may reduce spatial neglect, we investigated the neural mechanisms underlying prism effects on visuo-spatial processing in neglect patients. We used functional magnetic resonance imaging (fMRI) to examine the effect of (right-deviating) prisms on seven patients with left neglect, by comparing brain activity while they performed three different spatial tasks on the same visual stimuli (bisection, search, and memory), before and after a single prism-adaptation session. Following prism adaptation, fMRI data showed increased activation in bilateral parietal, frontal, and occipital cortex during bisection and visual search, but not during the memory task. These increases were associated with significant behavioral improvement in the same two tasks. Changes in neural activity and behavior were seen only after prism adaptation, but not attributable to mere task repetition. These results show for the first time the neural substrates underlying the therapeutic benefits of prism adaptation, and demonstrate that visuo-motor adaptation induced by prism exposure can restore activation in bilateral brain networks controlling spatial attention and awareness. This bilateral recruitment of fronto-parietal networks may counteract the pathological biases produced by unilateral right hemisphere damage, consistent with recent proposals that neglect may reflect lateralized deficits induced by bilateral hemispheric dysfunction.Cortex 11/2011; DOI:10.1016/j.cortex.2011.10.009 · 6.04 Impact Factor