We are visual animals. Although we certainly use our hearing and other senses, the cliché, seeing is believing, resonates with us because we use our eyes to make sense of the world. Like other brain processes, we never give sight a thought until the system is malfunctioning in some way. Only then might we consider how amazing it is that electromagnetic energy can bounce off the environment, into our eyes, and onto our two-dimensional retina, creating neural impulses that travel through the brain and ultimately paint a three-dimensional image of our world. In this article and the next two we will discuss how the visual system works and some of the ways in which it can break down in dementia, leading to difficulty seeing or to strange phenomenon such as hallucinations, neglecting the left side of the world, or believing that someone has been replaced by an imposter.
Impaired vision may lead to getting lost and difficulty recognizing people and places
When we discussed memory in a prior article we mentioned that poor memory often leads to getting lost and not recognizing people or places. These same difficulties with navigation and recognition can also occur from—or be exacerbated by—difficulty seeing. So, if one has poor vision from any cause, it will be easy to get lost because it is hard to see street signs, landmarks, maps, GPS devices, and phone apps. It may also be difficult to recognize people’s faces and places that one knows—even if they remember them.
Vision difficulties may be due to eye problems
The eyes themselves are a complicated bit of machinery. Let’s say you are looking at a dog. In order for you to see it, the dog needs to be illuminated by a light source, perhaps the sun. The image of the dog then passes through your pupil and reaches the lens. The lens of your eye focuses the reflected light onto the back of your eye, hopefully producing an accurate representation of the dog. If, however, you are wearing glasses, that means that the lens of your eye needs a little help focusing the image. Once the image is on the back of your eye, the light receptors there turn the image into neural impulses that are transmitted into your brain. There are many treatable eye diseases that can affect vision, including cataracts, glaucoma, and macular degeneration. Make sure that your loved one sees an eye doctor regularly.
Vision occurs in the occipital lobes in the back of the head
After the eye has turned the image of the dog into neural impulses, the impulses travel through nerves to your occipital lobes in the back of your brain. The left occipital lobe recreates the images on the right side of your vision (the dog’s head, for example, if it is facing to the right), and the right occipital lobe recreates the images on the left side of your vision (the dog’s body, for example). Once the image of the dog has been recreated by the occipital lobe, the image is transferred to the temporal and parietal lobes.
The parietal lobes focus attention and tell you where things are and what they are doing
When the image of the dog reaches the parietal lobes, you will know where the dog is, what direction it is facing, and whether it is walking, running, or standing still. The parietal lobes also help to focus attention. Here parietal lobe function is asymmetric: the right parietal lobe can focus attention on either the left or right side, whereas the left parietal lobe only focuses attention on the right. This asymmetry means that if there is damage to the left parietal lobe, attention can still be focused on both sides, but when there is damage to the right parietal lobe, attention cannot be focused on the left side, and things on the left are neglected.
My wife is getting lost all the time. I know her vision isn’t very good. How can I tell if it is her eyes or her memory that is the problem?
- It can be difficult to sort out whether impairments in memory, vision, or both are the cause of someone getting lost. One thing to do is to have them describe how they should travel on a route that they have become lost on. If they are able to describe the route perfectly yet they failed in life, that could be a sign that vision is the main problem.
My mother’s dementia is in the moderate stage. But half the time I think her biggest problem is that she can’t see well. Is it worth getting her eyes evaluated at this stage?
- In general, we always recommend correcting eye problems, if possible. Make sure that eyeglass prescriptions are current, and purchase extra, inexpensive pairs of glasses in case one (or more) becomes broken or misplaced. Speak with the doctor about the potential benefits and risks of eye surgery if that is recommended.
More on vision in my next article!
© Andrew E. Budson, MD, 2019, all rights reserved.