Background and purpose: Despite similar clinical onset, recovery from stroke can be largely variable. We searched for electrophysiological prognostic indices, believing that they can guide future neuromodulation treatments boosting clinical recovery. Methods: 19-channels resting electroencephalogram (EEG) was collected in 42 patients after 4-10 days (t0) from a unilateral ischemic stroke in the middle cerebral artery (MCA) territory and 20 controls. National Health Institute Stroke Scale (NIHSS) was collected at t0 and 6 months later (t1). Standard spectral band powers and interhemispheric coherences between homologous MCA regions were calculated in both hemispheres. Results: Total spectral, delta and theta band powers were higher bilaterally in patients than in controls and directly correlated with NIHSSt0 in both hemispheres. A linear regression model including each EEG patient's variable differing from those of controls and correlating with effective recovery [ER = (NIHSSt0-NIHSSt1)/(NIHSSt0-NIHSS in healthy conditions)] showed contralesional delta power as the only valid predictor of ER. A further regression model including also NIHSSt0 confirmed that contralesional delta power can add prognostic information to acute clinical impairment. Contralesional delta activity increase was best explained, in addition to the increasing ipsilesional delta activity, by a reduction of interhemispheric functional coupling - which did not explain a significantly portion of effective recovery variability by itself. Conclusions: Contralesional EEG delta activity retains relevant negative prognostic information in acute stroke patients. Present results point to the interhemispheric interplay as a decisive target in setting up enriched rehabilitations
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