Sedentary behavior associated with reduced medial temporal lobe thickness in middle-aged and older adults

Abstract

Atrophy of the medial temporal lobe (MTL) occurs with aging, resulting in impaired episodic memory. Aerobic fitness is positively correlated with total hippocampal volume, a heavily studied memory-critical region within the MTL. However, research on associations between sedentary behavior and MTL subregion integrity is limited. Here we explore associations between thickness of the MTL and its subregions (namely CA1, CA23DG, fusiform gyrus, subiculum, parahippocampal, perirhinal and entorhinal cortex,), physical activity, and sedentary behavior. We assessed 35 non-demented middle-aged and older adults (25 women, 10 men; 45-75 years) using the International Physical Activity Questionnaire for older adults, which quantifies physical activity levels in MET-equivalent units and asks about the average number of hours spent sitting per day. All participants had high resolution MRI scans performed on a Siemens Allegra 3T MRI scanner, which allows for detailed investigation of the MTL. Controlling for age, total MTL thickness correlated inversely with hours of sitting/day (r = -0.37, p = 0.03). In MTL subregion analysis, parahippocampal (r = -0.45, p = 0.007), entorhinal (r = -0.33, p = 0.05) cortical and subiculum (r = -0.36, p = .04) thicknesses correlated inversely with hours of sitting/day. No significant correlations were observed between physical activity levels and MTL thickness. Though preliminary, our results suggest that more sedentary non-demented individuals have less MTL thickness. Future studies should include longitudinal analyses and explore mechanisms, as well as the efficacy of decreasing sedentary behaviors to reverse this association.

Fig 1. Methods for producing flat maps.

Oblique coronal images are acquired to cover the long axis of the hippocampus as shown in image A. This image is cropped over the area of interest (red square) and shown in greater detail in image B. The gray matter ribbon (shown in bright blue) is segmented out from the surrounding MTL area. C. Boundaries between subregions are demarcated on each slice and projected to the corresponding location in flat-map space (shown in E). Boundaries between these subregions are shown in C and projected to flat-map (or 2D) space as shown in Fig E. D. Thickness in gray matter space is calculated in-plane space by taking the maximum distance value of the corresponding 3D voxels across all layers and multiplying by two to arrive at a thickness value for each voxel. E. The subregions are labeled as follows: cornu ammonis (CA) fields 1, 2, 3 and the dentate gyrus (DG) subiculum (Sub), entorhinal cortex (ERC), perirhinal cortex (PRC), parahippocampal cortex (PHC) and fusiform cortex (Fus). Boundary colors in C correspond to the same color scale in E and are labeled according to the Boundary Demarcation color bar.

Fig 2. Correlation of average number of hours spent sitting per day to thickness.

(A) Total medial temporal lobe (MTL) (r = -0.37, p = .03) and (B) parahippocampal (r = -0.45, p = .007) thickness correlated inversely with hours of sitting/day, controlling for age.