Altered Brain Activities Associated with Neural Repetition Effects in Mild Cognitive Impairment Patients

Abstract

Older adults with mild cognitive impairment (MCI) manifest impaired explicit memory. However, studies on implicit memory such as repetition effects in persons with MCI have been limited. In the present study, 17 MCI patients and 16 healthy normal controls (NC) completed a modified delayed-match-to-sample task while undergoing functional magnetic resonance imaging. We aim to examine the neural basis of repetition; specifically, to elucidate whether and how repetition-related brain responses are altered in participants with MCI. When repeatedly rejecting distracters, both NC and MCI showed similar behavioral repetition effects; however, in both whole-brain and region-of-interest analyses of functional data, persons with MCI showed reduced repetition-driven suppression in the middle occipital and middle frontal gyrus. Further, individual difference analysis found that activation in the left middle occipital gyrus was positively correlated with rejecting reaction time and negatively correlated with accuracy rate, suggesting a predictor of repetition behavioral performance. These findings provide new evidence to support the view that neural mechanisms of repetition effect are altered in MCI who manifests compensatory repetition-related brain activities along with their neuropathology.

Keywords: Delayed-match-to-sample task; functional MRI; mild cognitive impairment; repetition; repetition suppression.

Figure 1

For each memory trial in this delayed match-to-sample task, two sample objects in green borders were initially presented, followed by 12 (or 13) successive test pictures (target or distracter object). D1 stands for the initial presentation of a distracter stimulus (i.e., non-match to either of the sample targets), and so forth for D2, D3, and/or D4.

Figure 2

Group differences in RTs and accuracy during distracter repetition and in explicit memory performance. (a) RT for the distracters with repetitions in NC and MCI group respectively; (b) Accuracy rate for the distracters with repetitions in NC and MCI group respectively; (c) Total RT for the DMS task in NC and MCI group; and (d) Pr for the DMS task in NC and MCI group, respectively.

Figure 3

Regions showing activation (in red) and deactivation (in blue) during repetition in NC and MCI group (p < .05, corrected).

Figure 4

Activation regions showing significant group differences during 3^rd&4^th repetition (p < .005, voxel size > 26, AlphaSim corrected; in D1& D2, the NC and MCI group did not have significant activation or deactivation differences).

Figure 5

Mean activations (in red) or deactivations (in blue) within the ROIs from (a) the middle occipital gyrus (MOG), (b) middle frontal gyrus (MFG), and (c) inferior frontal gyrus (IFG) for the D1, D2, and D3&4.

Figure 6

Correlational analysis of brain responses in middle occipital gyrus (MOG) and behavioral performance. (a) Activations in the left MOG were found to positively correlate with RT, whereas (b) negatively correlate with accuracy rate.