Dynamic diaschisis: Anatomically remote and context-sensitive human brain lesions

Functional neuroimaging was used to investigate how lesions to the Broca's area impair neuronal responses in remote undamaged cortical regions. Four p atients with speech output problems, but relatively preserved comprehension , were scanned while viewing words relative to consonant letter strings. In normal subjects, this results in left lateralized activation in the poster ior inferior frontal, middle temporal, and posterior inferior temporal cort ices. Each patient activated normally in the middle temporal region but abn ormally in the damaged posterior inferior frontal cortex and the undamaged posterior inferior temporal cortex. In the damaged frontal region, activity was insensitive to the presence of words but in the undamaged posterior in ferior temporal region, activity decreased in the presence of words rather than increasing as it did in the normal individuals. The reversal of respon ses in the left posterior inferior temporal region illustrate the context-s ensitive nature of the abnormality and that failure to activate the left po sterior temporal region could not simply be accounted for by insufficient d emands on the underlying function. We propose that, in normal individuals, visual word presentation changes the effective connectivity among reading a reas and, in patients, posterior temporal responses are abnormal when they depend upon inputs from the damaged inferior frontal cortex. Our results se rve to introduce the concept of dynamic diaschisis; the anatomically remote and context-sensitive effects of focal brain lesions. Dynamic diaschisis r eveals abnormalities of functional integration that may have profound impli cations for neuropsychological inference, functional anatomy and, vicarious ly, cognitive rehabilitation.