Why Do Hallucinations Occur in Dementia?

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Not seeing the left (or right) side can occur with occipital or parietal lobe damage.

In my last post, I mentioned how the parietal lobes help to focus attention, and that it is asymmetric. Although the right parietal lobe attends to both the left and right sides of the world, the left parietal lobe attends only to the right. For this reason, if there is damage to the right parietal lobe, the ability to pay attention to things on the left is lost, and so things on left may not be observed unless they are explicitly pointed out. Right parietal lobe damage can be from a stroke as part of vascular dementia, although it can also occur from many other types of dementia, including Alzheimer’s disease. Lastly, occipital lobe damage from stroke or dementia can cause loss of vision on either side.

The temporal lobes tell you what you are looking at and your emotional connection to it.

When an image—a dog, for example—reaches the temporal lobes, you will be able to identify the image as a dog, what color its hair is, and which breed it is. Also in each temporal lobe are the emotional centers of the brain, almond-shaped structures called amygdala. When the image reaches the amygdala, it produces the appropriate emotion: affection if it is your dog and wariness if you have never seen the dog before.

Illusions occur when there are misperceptions.

Have you ever looked at something quickly and thought it might be a person or an animal, and then you look more closely and you realize it was something else, perhaps a tree or a bush? If so, you’ve experienced an illusion. We use the term illusion when you are perceiving something that is there, but you don’t perceive it correctly, and so you make a mistake as to what it is. Illusions are common when vision is poor. Note that illusions are different from hallucinations. As we will discuss below, hallucinations occur when one sees something even though there is nothing there to see.

Hallucinations may be due to problems in the visual system—or a sleep disturbance.

We explained that illusions were misperceptions—when there really is something there but it is not perceived correctly, such as mistaking a tree for a person. A hallucination, by contrast, is when something is seen by the individual despite the fact that there is nothing there. Hallucinations of people or animals is a common feature of dementia with Lewy bodies. In fact, although we used to think that hallucinations could occur in any type of dementia, if we separate true hallucinations from illusions and false memories, the presence of hallucinations may be an indication that the individual has at least a bit of dementia with Lewy bodies in their brain. No one is exactly sure why these hallucinations occur, but they are likely due to the combination of damage to the occipital lobes impairing the visual system plus a sleep cycle disturbance, such that the hallucinations may be dreams breaking into waking consciousness. If your loved one is having hallucinations, do not despair; there are several ways to treat this disturbing symptom, which we will discuss in future posts.

Key questions:

• OK, so we put my father through the cataract surgery and got him new glasses, but he still can’t seem to see very well, particularly on the left side.
  • There are a number of vision problems that are directly caused by the effects of dementia on the brain. Parietal or occipital lobe damage can cause difficulty seeing on either or both sides of the world, particularly the left.
• My husband told me he saw his mother last night in our bedroom. How do I know if this is a hallucination, an illusion, or a false memory?
  • The only way to know for sure that it is a true hallucination is if you (or another person) actually witness your loved one pointing out or interacting with something or someone who clearly isn’t there. If, on the other hand, they think another woman is their mother that would be an illusion—a misperception—rather than a true hallucination. (In this case, your husband might have seen you last night and thought you were his mother.) If the individual reports that they saw their mother and there was no witness, another possibility is that it is a false or distorted memory. They may be recalling a real time that they did see their mother 30 years ago, but the memory is distorted such that they believe it occurred yesterday.

In my next post, we will discuss what to do when your loved one doesn’t recognize you—or thinks you have been replaced by an imposter.

© Andrew E. Budson, MD, 2019. All rights reserved.