Structural MRI Correlates of Episodic Memory Processes in Parkinson's Disease Without Mild Cognitive Impairment

Abstract

Background: Changes in episodic memory are common early in Parkinson's disease (PD) and may be a risk factor for future cognitive decline. Although medial temporal lobe (MTL) memory and frontostriatal (FS) executive systems are thought to play different roles in distinct components of episodic memory impairment in PD, no study has investigated whether different aspects of memory functioning are differentially associated with MTL and FS volumes in nondemented patients without mild cognitive impairment (PD-woMCI).

Objectives: The present study investigated MRI markers of different facets of memory functioning in 48 PD-woMCI patients and 42 controls.

Methods: Regional volumes were measured in structures comprising the MTL and FS systems and then correlated with key indices of memory from the California Verbal Learning Test.

Results: In PD-woMCI patients, memory was impaired only for verbal learning, which was not associated with executive, attention/working memory, or visuospatial functioning. Despite an absence of cortical atrophy, smaller right MTL volumes in patients were associated with poorer verbal learning, long delayed free recall, long delayed cued recall, and recognition memory hits and false positives. Smaller right pars triangularis (inferior frontal) volumes were also associated with poorer long delayed cued recall and recognition memory hits. These relationships were not found in controls.

Conclusions: The findings indicate that MTL volumes are sensitive to subtle changes in almost all facets of memory in PD-woMCI, whereas FS volumes are sensitive only to memory performances in cued-testing formats.

Keywords: Parkinson’s disease; cognitive functions; frontostriatal system; magnetic resonance imaging; medial temporal lobe system; memory; neuropsychology.

Fig.1

Regional volumes of interest and their association with memory in Parkinson’s patients. 1) Left panel: Colored areas designate regions of interest (ROI) in the frontostriatal and medial temporal systems. ROIs on sagittal surfaces are displayed for one hemisphere, but were analyzed for homologous areas of both hemispheres. The caudate/putamen and the hippocampus are respectively shown on axial and coronal views. 2) Right panel: Right hemisphere cortical (lateral/medial sagittal surfaces) and hippocampus (coronal view) volumes that significantly correlated with various memory measures. For CVLT learning (top row), the left hemisphere medial surface displays the significant correlation with the temporal pole. The p value for correlation coefficients is designated by the color bar; p≤0.01 (dark blue) and p≤0.005 (light blue).

Fig.2

Scatter plots showing significant correlations between measures of memory and regional volumes in Parkinson’s patients. For most variables, age and gender adjusted residuals for MRI volumes and CVLT measures are plotted. An exception is CVLT learning, for which levodopa dosage equivalence, age, and gender adjusted residuals are plotted. Solid and dotted lines display the best-fitting linear regression line and 95% confidence intervals.