Alzheimer's patients may have great difficulty "seeing" even when their eyes are working normally. This is due to "visual agnosia", a symptom in which the brain is unable to process the signals from the eyes.

The Alzheimer's Eye Sees Things Differently
The changes in the brain caused by Alzheimer's Disease can also cause changes in how the affected person sees things. Although an opthalmologist may not be able to detect any actual changes in the eye
http://www.eldercareteam.com/public/677.cfm
Vision and Alzheimer's Disease -- A Different Way to See the World
For most of us over 40, it's been awhile since our vision was 20:20. But we bought our bifocals and moved on. Unfortunately, for people with Alzheimer's disease, vision problems aren't as easily treat
http://www.alz-nca.org/aboutalz/vision.php
Vision Problems Associated with Alzheimer's Disease
Vision problems are a concern because more than sixty percent of individuals with Alzheimer's will have a decline in some sort of visual capacity. Problems most often arise in four areas: motion blin
http://www.alz.org/centralohio/in_my_community_158...