Dementia and sight


Registered User
Jan 31, 2004
near London
In the car the other day, I think it was the Steve Wright Show on Radio 2 :D , in the regular titbits of information they give, the question was "The eye is responsible for how much of a person's sight?"

The answer they gave was 25%.

This was on the grounds that the eye is primarily just a lens.

The image projected by that lens needs to be processed by the brain, in order to have some meaning and recognition.

This seems sensible to me, and explains why some people with dementia - my Jan included - lose their sight, when their eyes are perfectly healthy. Damage to their brain prevents it deciphering the images.

Amazing where these things come out.


Registered User
Mar 22, 2007
This may also offer some explanation as to my mums rather strange sight problems.About a year ago when she still lived at home, she had a sight test and, according to her, had cataracts developing which would be removed after six months.
Once the six months had gone by, she was here in the nursing home , complaining she could hardly see. however, somewhat oddly she seemed to be able to see some things ok and I rather cynically thought this was selective sight on her part!
She then had a furthur eye test and was told there were no cataracts, she simply needed new glasses, which we obtained. However these didnt seem to help much, according to her, she can see a little bit then everything just fades away. Maybe her brain can only process information sometimes, or for a short period of time or something?


Registered User
Jan 15, 2007
I often wondered what was wrong with mums sight she kept saying she could not see anything her eyes were tested at the home, and she had new glasses but it didn't make any difference, she just used to say I wish I could see. But sometimes she could see certain things its very strange.


Registered User
Jul 28, 2005
south london
Alzheimer's effects all the senses, taste as well as the brain does not process the information correctly, or something like that.

My mum had poor periforal vision, which the eye doctors blamed on her alzheimer's.


Registered User
Aug 9, 2005
The term for this sort of blindness is "cortical blindness". It refers to an inability of the brain to process the information taken in by the eyes. The eyes themselves can be in perfect condition!

When working with severely disabled children, I often came across cases of cortical blindness, and many of these children had perfect eyes. Such a tragedy - no matter what age it occurs.


Registered User
Jun 27, 2006
My mother has hemianopsia, which is the loss of half the visual field, as a result of her strokes. Her eyes weren't/aren't perfect, but this problem is entirely due to the brain damage.



Registered User
Sep 16, 2005
Weird stuff

Something to keep in mind, on the sight and "visual perception" side of things...Dad seemed to lose the ability to perceive what was on the television a few years back...but now he regularly watches it with interest. Can't tell if he knows what he is looking at, but something about watching it catches his attention....whereas for about 2 years...he never even used to notice it. Don't know whether it was 'sight' related or understanding related...but whatever it is we are grateful that he has something he likes to do again! :)


Registered User
Jul 2, 2006
Newport, Gwent
Bruce and everyone, thank you for this information, it explains a lot.

Mum recently had an eye test, and I was convinced she would need a new stronger prescription, I was so surprised, her sight has changed very little over the last 3 years.

She can walk quite well considering, but she appears to have problems ‘seeing’ where she is putting her feet, she ‘sees’ steps that are not there, especially when there is a change in floor surface e.g. from path to carpet.

She used to be an avid reader of novels, but hasn’t read anything for a number of years, yet when presented with a passage to read by the optician, she read it no problem at all, I was so surprised. This is probably more to do with, I think, she cannot remember what she has just read, not sure, so it probably makes little sense to her.

About 8 years ago the optician then detected the start of a cataract, nothing showed up this time. Also mum seems to have a turn in her one eye, not there before.

Thanks for the information, very useful


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