The brain and dementia

3. Functions of the brain

Many of the complicated things that the brain does - memory, language, vision - need several parts to work together. Signals move along pathways of nerves that connect different lobes.

Vision

Vision involves several steps. Information from the eye is processed first in the occipital lobes. Signals are sent to the parietal lobes, to work out the object's location. They are also sent to the temporal lobes, to match up with memories of previous experiences (for example of someone's face or an object). For someone with dementia, disruption of these signals causes problems such as not being able to recognise faces or objects (known as visual agnosia).

As part of visual recognition, signals are also passed to the amygdala. This is where we make a quick emotional response to the thing we have seen. Problems here can lead to extreme emotions or a loss of emotional reaction to people.

Language

When we listen to and understand speech, sounds are processed in an area towards the back of the temporal lobes called the auditory cortex. Signals are then passed through the temporal lobes, where the meaning of the words is processed.

When we talk, our vocal cords are controlled by areas in the frontal lobe in the left side of the brain. Talking also involves the temporal lobe (where concepts are converted into words) and the auditory cortex, because we rely on hearing our own voice when we speak.

Memory

Different things we remember - events, faces, facts or skills - are stored and recalled by different types of memory.

Episodic memory is our personal memory of events at a certain time and place. For example: 'I ate eggs for breakfast this morning in my kitchen'. These memories are specific to each of us and can have an emotional aspect.

Semantic memory is our general knowledge about objects, word meanings, facts and people. For example: 'Eggs have a shell and are laid by hens'

Procedural memory is our memory for skills we have learned. Examples include tying shoelaces, brushing our hair or riding a bike.

These different types of memory involve different parts of the brain working together. They can be affected by dementia in different ways.

Episodic memory

Our recollection of an event may have several parts: where we were, what we saw or heard, how we felt, for example. These parts are put together to create the memory.

When we experience something, information from our senses initially goes into the hippocampus. Over time, it is thought that the hippocampus begins to transfer memories into long-term storage in the cerebral cortex.

The memory is stored in the cortex as a network of nerve cells. Recent memories, which have just entered long-term storage, still need the hippocampus to retrieve them.

But memories from further back (such as a wedding day) that have been thought about more often, become more firmly established in the cortex. Recall of these memories from longer ago seems to need the hippocampus less. Retrieval of an episode may be triggered by just one part of the memory, such as a particular smell or piece of music.

Emotions have a large influence on what we remember. An experience that is highly emotional is more likely to be stored in long-term memory. We are also more likely to recall the emotional aspects of an experience. The amygdala is the centre for emotional memories.

Other forms of memory

The hippocampus is also involved in forming semantic memories. These are then stored as long-term memories in the cerebral cortex.

With procedural memory, we use the frontal lobes to concentrate, allowing us to first learn a skill. But once the skill has been learned it is stored in the basal ganglia (a group of structures between the cerebrum and brain stem), as well as in the motor cortex and cerebellum.

Emotion and behaviour

How we respond to the world around us - how we feel and how we behave - depends on signals passed between the limbic system (dealing with emotions) and the frontal lobes (dealing with rational thoughts).

Emotions are generated in the limbic system in response to sensory information. For example, our amygdala responds to danger by generating signals for fear.

Emotions are analysed in the frontal lobes. For example, the frontal lobes allow us to check that something really is a threat - perhaps we misread the situation - and so may stop us from reacting aggressively.

In dementia, different forms of damage to these two emotional centres in the brain can cause someone to become either over-emotional or lacking in feelings.