Functional imaging studies of neuropsychological patients: applications and limitations.

Functional imaging studies of neuropsychologically impaired patients have not enjoyed the immediate success that was attained by functional imaging studies of normal subjects. This is largely because it has taken time to appreciate some of the deeper issues surrounding study design, analysis and interpretation. The most significant limitation is that functional imaging experiments with patients need tasks that the patients can perform. This precludes direct investigations of the physiological correlates of cognitive deficits. Nevertheless, functional imaging studies of brain-damaged patients who retain task competence can provide information that is not available from structural imaging, behavioural assessments or functional imaging with normal subjects. This is because intact task performance, following a brain lesion, does not necessarily entail normal neuronal responses in undamaged cortical areas. Abnormal neuronal responses, in the context of normal performance, can indicate alternative neuronal and cognitive mechanisms for supporting the same task. This, in turn, has important implications for understanding the mechanisms that mediate recovery and the organizational principles that underlie functional architectures in the human brain.