Alzheimer’s disease in the pupil: Pupillometry as a biomarker of cognitive processing in Alzheimer’s disease

The pupil size is not only influenced by light, but also by cognitive effort and research has consistently demonstrated how pupil size can index cognitive effort in nonpathological population. We thus offer an original case study in which we evaluate whether pupil dilation can index cognitive processing in a patient with Alzheimer’s disease (AD). We measured, in a patient with mild AD, pupil size during an effortful task (i.e., counting backwards from 100 by seven) and during a less effortful control task consisting of counting aloud from one. We observed a larger pupil diameter during the effortful task than during the control task. By doing this, we demonstrate how pupil size can mirror cognitive processing in AD. Our findings suggest that pupillometry can be used as a biomarker of cognitive processing in AD. The diagnosis of AD is based on biomarkers such as the presence of amyloid-β and tau pathology in cerebrospinal fluid and the reduced volume of the hippocampus as can be observed on structural magnetic resonance imaging. While these biomarkers are widely used to detect AD, pupillometry can be used as noninvasive and inexpensive biomarker, especially that, unlike other biomarkers, pupillometry is intimately associated with cognitive processing.