Symbolism, Imagery, Allegory
Doc calls himself "the only man on earth who can photograph the past" (13.4).
Classic Doc arrogance.
Of course, Doc is no more photographing the past than he is actually experiencing it when teenage Codi shows up in his doorway, five foot eleven and strangely thirty, or when he remembers the night he "rescued" his daughters from the flood.
Doc's Alzheimer's is a fairly straightforward symbolic device in Animal Dreams, because its effect on his memory is actually pretty similar to how his mind worked before he began to be sick. Doc has always believed in reliving and altering the past, and as the boxes and boxes of mementos from his daughter's life stored up in the attic attest, he has always, in some ways, lived in that past, too. In the process, Doc loses his own identity. Without memory—or without real memories—we aren't who we are.
Alzheimer's is a real disease, of course, but in Doc's case it's like a physical manifestation of what are also deeply psychological issues.
Sometimes life mimics art, especially in art.