Often, Alzheimer's patients appear to have difficulty seeing, but the ophthalmologist cannot find anything wrong with their eyes. The reason is visual agnosia ... the eyes are functioning as they should, but the brain is unable to process the signals properly. This can manifest itself in many ways.

'Motion Blindness,' Not Just Poor Memory, Causes Alzheimer's Patients To Lose Their Way
ScienceDaily (Mar. 23, 1999) – A previously unobserved condition in some Alzheimer's patients that physicians are calling "motion blindness" is a big reason such patients become disoriented and lose t
'Motion Blindness,' Not Just Poor Memory, Causes Alzheimer's Pati...
Motion Blindness video
For years doctors generally have thought that Alzheimer's patients become lost simply because they forget directions or where they're going. But the Rochester team has found that while Alzheimer's pat
http://www.webmd.com/video/motion-blindness-alzhei...
Road Skills Hint At "Motion Blindness" Of Alzheimer's
Doctors have added to the evidence that patients with Alzheimer's disease lose their way not simply because their memory is failing but because they are subject to a unique form of brain damage that c
http://www.urmc.rochester.edu/pr/news/story.cfm...
Vision and dementia: Ocular effects and examination
Dementia is a loss of mental function in two or more areas such as language, memory, visual and spatial abilities, or judgment severe enough to interfere with daily life. Dementia itself is not a dise
http://www.optometry.co.uk/clinical/details...
Vision Screening for Alzheimer's Disease
Recent evidence suggests that memory impairment and vision impairment are closely linked in Alzheimer's disease and that special testing for vision impairment can improve early detection and treatment
http://xnet.kp.org/permanentejournal/winter04/visi...