Locus Coeruleus Contrast and Diffusivity: Effects of Age and Relations to Memory

Neurocognitive aging researchers are increasingly focused on the locus coeruleus, a neuromodulatory brainstem structure that degrades with age. With this rapid growth, the field will benefit from consensus regarding which magnetic resonance imaging (MRI) metrics of locus coeruleus structure are most sensitive to age and cognition. To address this need, the current study acquired magnetization transfer- and diffusion-weighted MRI images in younger and older adults who also completed a free recall memory task. Results revealed significantly larger differences between younger and older adults for maximum than average magnetization transfer-weighted contrast (MTC), axial than mean or radial single-tensor diffusivity (DTI), and free than restricted multi-compartment diffusion (NODDI) metrics in the locus coeruleus; with maximum MTC being the best predictor of age group. Age effects for the MTC and NODDI metrics interacted with sex such that larger age group differences were seen in males than females. Age group differences were also larger for DTI metrics in the rostral, and NODDI metrics in the caudal, locus coeruleus subdivision. Within older adults, however, there were no significant effects of age on any measure of locus coeruleus structure. Finally, independent of age and sex, higher restricted diffusion in the locus coeruleus was significantly related to better (lower) recall variability, but not mean recall. Whereas MTC has been widely used in the literature, our comparison between the average and maximum MTC metrics, and inclusion of DTI and NODDI metrics, make important and novel contributions to our understanding of the aging of locus coeruleus structure.