Prefrontal Compensatory Mechanism May Enable Normal Semantic Memory Performance in Mild Cognitive Impairment (MCI)

Mild cognitive impairment (MCI) is considered as a potential transitional state between normal aging and dementia. Studies addressing semantic memory in patients with incipient dementia and MCI show inconsistent results. In the current report we focused on MCI and examined memory performance (semantic, episodic, and working memory) in addition to structural and functional magnetic resonance imaging (fMRI) measurements. We studied 6 MCI, 6 normal controls, and 4 Alzheimer disease (AD) patients. MCI participants demonstrated normal semantic memory performance while fMRI examination revealed distinct patterns of activations between MCI and normal aged subjects. According to our previous study, time courses were taken from parietal, dorsolateral-prefrontal-cortex (DLPFC), hippocampal formation, and fusiform gyri. A small number of activated voxels in parietal regions were depicted in MCI participants and were correlated with structural changes in this region. In contrast, significantly higher activation (intensity and number of voxels) was observed in DLPFC of MCI subjects. The overactivity seen in DLPFC of MCI may represent a compensatory mechanism that enables them to perform normally. These preliminary results suggest that functional imaging may be useful for early diagnosis of dementia and call to develop reliable tests and criteria that will enable using such methods clinically.