Semantic memory activation in amnestic mild cognitive impairment

Cognitively intact older individuals at risk for developing Alzheimers disease frequently show increased functional magnetic resonance imaging (fMRI) brain activation presumably associated with compensatory recruitment, whereas mild cognitive impairment (MCI) patients tend not to show increased activation presumably due to reduced neural reserve. Previous studies, however, have typically used episodic memory activation tasks, placing MCI participants at a performance disadvantage relative to healthy elders. In this event-related fMRI study, we employed a low effort, high accuracy semantic memory task to determine if increased activation of memory circuits is preserved in amnestic MCI when task performance is controlled. Fifty-seven participants, aged 6585 years, comprised three groups (n 19 each): amnestic MCI patients; cognitively intact older participants at risk for developing Alzheimers disease based on having at least one ApoE ε4 allele and a positive family history of Alzheimers disease (At Risk); and cognitively intact participants without Alzheimers disease risk factors (Control). fMRI was conducted on a 3T MR scanner while participants performed a famous name discrimination task. Participants also underwent neuropsychological testing outside the scanner; whole brain and hippocampal atrophy were assessed from anatomical MRI scans. The three groups did not differ on demographic variables or on fame discrimination performance (>87 correct for all groups). As expected, the amnestic MCI participants demonstrated reduced episodic memory performance. Spatial extent of activation (FameUnfamiliar subtraction) differentiated the three groups (Control 0 ml, At Risk 9.7 ml, MCI 34.7 ml). The MCI and At Risk groups showed significantly greater per cent signal change than Control participants in 8 of 14 functionally defined regions, including the medial temporal lobe, temporoparietal junction, and posterior cingulate/precuneus. MCI participants also showed greater activation than Controls in two frontal regions. At Risk, but not MCI, participants showed increased activity in the left hippocampal complex; MCI participants, however, evidenced increased activity in this region when hippocampal atrophy was controlled. When performance is equated, MCI patients demonstrate functional compensation in brain regions subserving semantic memory systems that generally equals or exceeds that observed in cognitively intact individuals at risk for Alzheimers disease. This hyperactivation profile in MCI is even observed in the left hippocampal complex, but only when the extent of hippocampal atrophy is taken into consideration.