Effect of disease severity on neural compensation of item and associative recognition in mild cognitive impairment.

It is proposed that the prodromal phase of Alzheimer's disease is associated with additional brain activation in key regions involved in memory, reflecting compensatory brain plasticity. Very little is known, however, about the evolution of these compensatory mechanisms as the brain acquires more damages. We conducted an fMRI memory study measuring brain activation related to old/new (item recognition) and intact/rearranged (associative recognition) word-pair recognition paradigms in 26 persons with mild cognitive impairment (MCI) and 14 healthy older adults. The Mattis Dementia Rating Scale was used to divide persons with MCI into those with higher and lower cognitive performances. Results indicated more brain activation in MCIs than in controls but disease severity determined which cognitive process was associated with larger activation: Persons with less severe MCI showed hyperactivation during associative recognition only, whereas persons with more severe MCI showed hyperactivation during item recognition only. These hyperactivations were found mainly in brain areas that are typically associated with retrieval mode (e.g., bilateral prefrontal cortex). These findings indicate that neural plasticity occurs during the entire MCI phase but that it is associated with different cognitive components. As they progress in the disease, individuals with MCI will experience a breakdown in the compensatory mechanisms for associative recognition accompanied by emergence of compensatory mechanisms for item recognition.