one is that his homosexual lovers were misleading him ~ or, “early onset” Alzheimer’s
...naaaah, I think he popped too many viagra while cruising the DC bathhouses every night... that will cloud your memory from all the teabagging.
ymmv
The other form is brought about with overindulgence in Viagra, et al.
Airline pilots get their color vision checked regularly because they really do need to see blue.