Multimodal imaging of residual function and compensatory resource allocation in cortical atrophy: a case study of parietal lobe function in a patient with Huntington's disease.

In a case of Huntington's disease (HD) with dementia and pronounced parieto-frontal atrophy, the functional state of the affected regions was investigated using functional magnetic resonance imaging (fMRI) and fluorodeoxyglucose-positron emission tomography (FDG-PET). It was observed that although parietal areas showed extensive atrophy and reduced resting glucose metabolism, the patient performed with similar accuracy but with longer response time in a visuospatial task compared with healthy control subjects. At the same time, the blood oxygen level-dependent (BOLD) fMRI signal in these areas, which are involved in visuospatial processing, showed a similar task-dependent modulation as in control subjects. The signal amplitude (signal percent change) of the task-dependent activation was even higher for the HD patient than in the control group. This residual functionality of parietal areas involved in visuospatial processing could account for the patient's performance in the task concerned, which contrasted with his poor performance in other cognitive tasks. The increased percent-signal change suggests that a higher neuronal effort was necessary to reach a similar degree of accuracy as in control subjects, fitting well with the longer reaction time. We propose that fMRI should be considered as a tool for the assessment of functionality of morphologically abnormal cortex and for the investigation of compensatory resource allocation in neurodegenerative disorders.