Aberrant frontostriatal connectivity in Alzheimer's disease with positive palmomental reflex

Background and purpose: Primitive reflexes may reoccur in various neurodegenerative diseases. However, little is known about their structural and functional correlates in the human brain. Notably, the neural mechanisms underlying a positive palmomental reflex (PMR) are poorly understood. As recent studies link Alzheimer's disease (AD)-related primitive reflexes to a dysfunction of the corticostriatal motor circuit (CMC), we conducted the present study to investigate functional and structural correlates of a positive PMR. We hypothesized an involvement of frontostriatal structures and an impairment of the CMC.

Methods: Using whole-brain resting-state functional connectivity (FC), hypothesis and FC result-based probabilistic tractography, and voxel-based morphometry analyses, we compared two groups of AD patients with either positive (n = 12) or negative PMR (n = 12).

Results: No significant differences in grey matter volume or structural connectivity (SC) could be observed between the PMR-positive and PMR-negative groups. In contrast, the PMR-positive group showed a decreased seed-to-voxel FC between the bilateral supplementary motor area and parts of the right-hemispherical caudate nucleus and thalamus and a decreased region of interest (ROI)-to-ROI FC between the left putamen and the left superior frontal gyrus.

Conclusion: Data suggest that dysfunction of the CMC reflected by decreased FC underlies a positive PMR in patients with AD. The lack of significant grey matter or SC differences might reflect that changes in FC appear before changes in SC in the structures of the CMC and brain atrophy.