In healthy people, the pupil constricts in response to light. This reflex, referred to as the pupil light reflex, is simple to study with a device called a pupillometer, which both illuminates the eye and measures the extent and speed of the resulting pupillary constriction. Pupillometry provides objective information regarding the integrity of the structures in the eye and brain that mediate the PLR (the retina, optic nerve, midbrain, oculomotor nerve and iris). In people who have Alzheimer’s disease, the same characteristic protein deposits that accumulate in the brain are found in the retina. This is associated with loss of one class of retinal neuron that mediates part of the PLR. Thus, the PLR may be abnormal in patients with AD. Definition of the specific pupillary abnormalities in AD may allow pupillometry to be used as a rapid, objective, inexpensive and noninvasive measure of AD, improving AD diagnosis, monitoring and research.