OPN4 and Alzheimer disease: Subsequent investigations into the AD retina have found thinning of the NFL, reduced density of melanopsin-containing RGCs, GCL cell loss, and elevated apoptotic markers, along with intraneuronal Aβ oligomers and other Aβ species within RGCs in these patients [5–7, 23, 26, 38, 57, 63, 64, 69, 70, 73, 77].