Explanation of the functions of the brain

As dementia progresses, some functions of the brain  can become affected, making some tasks much more difficult, such as vision, language and behaviour.

Dementia and the brain
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The brain carries out a huge variety of tasks every day. This includes receiving and processing information about the environment and taking actions based on thoughts, feelings and perceptions. The brain also handles a lot of tasks that a person is unaware of, such as telling the heart how fast to beat.

As dementia progresses, some tasks become much more difficult. Aside from memory, functions that are particularly affected in dementia include:

  • executive function (the ability to plan, organise and complete tasks)
  • vision
  • language
  • emotion and behaviour.

Executive function

Executive function is the ability to plan, organise and complete tasks. It also includes solving problems, setting useful goals and making rational decisions.

For executive function to happen, information needs to be held in working memory long enough for the person to be able to perform the task – for example, remembering how many teaspoons of sugar they have added to the cup so far, or the reason for going into a room.

Executive function includes organising and planning a sequence of actions. Things that most people might consider to be simple, everyday tasks involve a series of pre-planned steps, all of which need to be done in the correct order. For example, the task of getting dressed involves putting several layers of clothing on in the correct order.

Loss of executive function can make communication difficult, as a person may struggle to maintain their attention on what is being said. They may find it hard to hold a conversation while other things are going on in the background.


Vision is a complicated process that involves different parts of the brain, not just what is seen with the eyes. Different parts of the brain process different information to make sense of what a person is seeing.

When the brain is damaged, although the eyes may be healthy, there may still be problems with vision. For example, the temporal lobes match up what is being seen with memories of things the person has seen before – such as a face. If dementia disrupts these processes the person may have problems recognising faces or objects (known as visual agnosia).

Problems with understanding what is being seen can also contribute to a person with dementia seeing things that are not real – known as visual hallucinations.

Visual hallucinations occur frequently in dementia with Lewy bodies, often in the early stages. In most other types of dementia hallucinations tend to happen more during the later stages. 


Language is a process that allows a person to understand and communicate thoughts and ideas, whether through spoken sounds (speech), visual patterns (writing), or gestures (including sign languages).

When a person listens to someone talking, reads words written on a page or reads sign language, the information is understood as language. If dementia disrupts this process, it can affect how a person uses language.

If a person’s temporal lobes are damaged, they may not be able to understand the meaning of words – for example, that the word spelled ‘K-E-T-T-L-E’ means ‘kettle’ and also that its purpose is for boiling water. A person may understand the meaning of words but be unable to find them when needed – for example, they will know what a kettle is, but will be unable to find the word when talking or signing.

Less commonly used or more recently gained words tend to be lost first. Basic words that the person learned at an earlier age are kept for much longer. When a person speaks more than one language, they may go back to communicating in the language they first learned as a child.

Emotion and behaviour

The way a person feels and behaves depends on communication between the limbic system (which deals with emotions) and the frontal lobes (which deal with rational thoughts and judgements).

If these parts of the brain are damaged by dementia, however, the person may feel anxious without good cause, or react aggressively to a threat which isn’t really there. Problems with processing emotions may also lead to inappropriate or confusing behaviour – for example a person laughing when they are told about a sad event.

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