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BACKGROUND AND PURPOSE
Plasticity in extended, parallel, or reciprocal operating networks is well recognized. Changes in neuronal activity after lesions to distinct localized structures, such as the primary visual cortex, are less well characterized. We investigated the cortical reorganization in patients with poststroke visual field defects using blood oxygen level-dependent functional MRI.

METHODS
Brain activation was measured in 7 patients with a single occipital cortical lesion and partially recovered hemianopia and in 7 age-matched control subjects. Differences in activation between rest and visual hemifield stimulation were assessed with statistical parametric mapping (SPM'99).

RESULTS
In normal subjects, significant activation was found in the contralateral primary visual cortex and bilaterally in the extrastriate cortex. During hemifield stimulation of the unaffected side of stroke patients, a similar pattern was found compared with that seen in control subjects. During stimulation of the hemianopic side, bilateral activation was seen within the extrastriate cortex, stronger in the ipsilateral hemisphere. The primary visual cortex was not significantly activated in either hemisphere during stimulation of the hemianopic side.

CONCLUSIONS
Visual field defects after stroke are associated with bilateral activation of the extrastriate visual cortex. This pattern of activation indicates altered neuronal activity in the visual system. Further investigation is necessary to determine the relationship between functional reorganization and recovery of lost visual function after poststroke hemianopia.