The brain and behaviour
Alzheimer's disease is a degenerative condition associated with a progressive loss of nerve cells or neurons. The disease gradually pervades most areas of the brain. However, the evolving pattern of damage can vary greatly between different individuals. As a result, each person with the disease may have a complex set of difficulties and experiences that are unique to them.
This factsheet explains which areas of the brain are largely responsible for which skills and abilities, and how the changes in behaviour, memory and thought experienced by people with Alzheimer's may be a direct result of the way the disease has affected the brain.
Parts of the brain
The brain consists of three main sections: the hindbrain, the midbrain and the forebrain (see Figure 1). The hindbrain and midbrain are mainly concerned with basic life support functions, such as blood pressure and respiration. In contrast, the forebrain is responsible for the majority of higher brain functions, such as memory and language. This factsheet concentrates on the parts of the forebrain.
The cerebral cortex
The most important part of the forebrain is the cerebral cortex − a thin shell covering the surface of the forebrain. This thin layer is tightly crumpled and folded, to increase its surface area. As a result, the cortex alone contains an astonishing ten billion brain cells (the 'grey matter'). Beneath this densely packed cortex lie bundles of fibres (the 'white matter'), which transport information around the cortex and to other brain regions (see Figure 2).
To help us describe the different parts of the brain, the forebrain has been divided up into four sections, or 'lobes', shown in Figure 3.
- The occipital lobe is located at the back of the brain, and deals primarily with visual information from the eyes.
- The parietal lobe lies in the upper-rear portion of the brain, and is concerned with information about spatial relationships, perception and magnitude.
- The temporal lobe lies beneath the parietal lobe and is involved in memory and language.
- The frontal lobe can be thought of as our 'executive' or 'management' centre.
In addition to being divided into lobes, the brain is also physically divided into two halves or hemispheres: left and right. The hemispheres are not entirely separate − they are connected by a large bundle of fibres that allows them to talk to one another. While these hemispheres look almost identical from the outside, one hemisphere is usually dominant over the other. In all right-handed people, and the majority of left-handed people, it is the left hemisphere that is dominant.
There are certain principles that appear to govern the organisation of the brain. The division between the hemispheres signals one of these principles: lateralisation. Lateralisation literally means that some functions are best performed by the left side of the brain, while others are mainly supported by the right side. In the human nervous system, the left hemisphere controls the right side of the body and the right hemisphere controls the left side of the body. However, not all functions are shared so equally. For example, language tends to be a function of the dominant hemisphere of the brain (usually the left side), with certain areas involved in language comprehension and language production managed towards the front of the hemisphere.
Some areas of the brain are organised in a map-like order. For example, the motor cortex, which sends movement instructions to the muscles, is organised so that parts of the body that are physically close (such as the hand and the wrist) are controlled by groups of cells that are also near to one another (see Figure 2). Keeping related information in one place in the cortex helps to keep the amount of wiring between cells to a minimum, and so saves on space.
It would be a considerable understatement to say that the structure of the brain is complicated. However, by studying behaviour and cognitive abilities in people with both healthy and damaged brains, scientists have been able to determine the principle functions of different brain areas.
The occipital lobe
Although the eyes are the source of visual information about the world around us, it is actually the brain that does most of the hard work. The eyes convert sensory information about light into electrical impulses, but these are passed to the brain for interpretation. This division of labour means we have to distinguish between visual acuity and perception.
Visual acuity − the ability to see small objects − is achieved largely by the workings of the eye itself. Perception is achieved by the occipital and parietal lobes, with information about colour, shape and movement being processed separately by the occipital cortex before it is passed on to the parietal lobe for combination into a complete 3D picture of the world.
This means that someone with dementia could have difficulty seeing what an object is, despite both eyes being in perfect condition. However, it is still necessary to maximise their eyesight (for example, with correct prescription spectacles) so that the brain has as much information to work with as possible. For more information, see Factsheet 527, Visuoperceptual difficulties in dementia.
The parietal lobe
The functions of the parietal lobe are somewhat more diverse, and there is a significant difference between the dominant and non-dominant sides.
The dominant parietal lobe (usually the left half) can be thought of as being concerned with things we have to put together into an order or structure. So tasks such as reading and writing, which require putting letters and words together, and calculation, which involves ordering and combining numbers, are critically dependent on the dominant parietal lobe. This side of the parietal lobe has also been heavily implicated in a condition known as apraxia (an impairment of learned purposive movements). 'Dressing apraxia' is the form of apraxia most commonly noticed in people with Alzheimer's disease, and reflects not only a lack of co-ordination but an actual forgetting of the movements required to achieve a goal (fastening a button, for example).
As if to compound such problems, the dominant parietal lobe is also responsible for our body sense − that is, knowing our left from our right, and sensing where one limb is in relation to the rest of our body.
The non-dominant parietal lobe (usually the right half), in contrast, could be thought of as our '3D centre'. This part of the parietal lobe receives visual information from the occipital lobe. The function of this area is to combine such information into a 3D representation of the object being viewed. Damage to this area leads to a symptom known as 'visual agnosia' − an inability to recognise objects, faces or surroundings. Because visual information is processed separately from other modes of sensation, it is possible for individuals to fail to recognise a familiar face by sight but to know who they are once they speak.
The non-dominant parietal lobe also contributes to our understanding of space but in a different way from its dominant counterpart. While the dominant parietal lobe deals with our body sense or personal space, the non-dominant portion helps us to locate objects in external space, and to calculate the location of objects relative to one another and ourselves (for example, when we are reaching to pick something up).
The temporal lobes
The temporal lobes deal primarily with memory functions. The dominant temporal lobe is specialised for verbal (word-based) memory and the names of objects, while the non-dominant temporal lobe is used for our memory of visual (non-verbal) material, such as faces or scenes.
There are also different types of memory. Episodic memory, as its name suggests, is our memory of events or episodes, which are recorded with a reference to the time when they occurred (for example: 'I ate eggs for breakfast this morning'). Meanwhile, semantic memory can be thought of as our own encyclopaedia for facts and figures (for example: 'Eggs have a shell, are laid by hens, and can be eaten boiled, scrambled or fried').
In Alzheimer's disease it is the episodic memory that is most commonly damaged. It is not yet clear exactly how or where long-term memories are stored, but it seems that memories are achieved by strengthening the connections between relevant nerve cells. And it does seem clear that some remote or distant memories, such as where one went to school, are stored more deeply than less familiar or recent memories, such as what one did yesterday. This may be because certain poignant or important events have been re-remembered many times over the years, and so are more rehearsed than recent memories. This is why some people with Alzheimer's disease may find it easier to discuss certain often-recalled aspects of their childhood than what has happened that day.
The frontal lobe
The frontal lobe contains several parts, which all act together to form our 'executive' or management' centre. These parts carry out the following activities:
Planning actions and learning new tasks − The lateral or outer surfaces of the frontal lobe appear to be critical for organising and planning our actions and learning new tasks. For example, in learning to drive, these brain areas help us put together a very complex sequence of movements that at first seem difficult and clumsy but gradually become more smooth and automatic. For someone with damage to this area, it is like being a learner all over again. Many multi-stage tasks, such as cooking and shopping, become very difficult because the pattern, or plan of action, has been lost.
Damage to these lateral areas can also cause people to get 'stuck' on what they are doing (known as 'perseveration'). As the frontal lobe interacts with many other brain areas, this perseveration may take the form of using the same word over and over again, or taking one piece of clothing out of a drawer and then unpacking all the other clothes without a specific purpose.
Motivation − Moving towards the division between the two hemispheres, the middle portion of the frontal lobe generates our motivation and general impetus. If this part of the brain is affected, people can lose their 'get up and go', becoming lethargic and reluctant to get out of bed or perform particular activities. It is important to realise that what some might see as laziness could be a direct consequence of the loss of cells in this area of the brain.
Regulation of behaviour − The regulation of our behaviour appears to be governed by a third area of the frontal lobe, the orbitobasal area, which is located in the curvature at the very front of the brain. In healthy people, this part of the brain helps to monitor, control and moderate our behaviour: for example, preventing us from saying something rude if someone has annoyed us. In someone with Alzheimer's disease, this function may be damaged, which can lead to a failure of inhibition.
Other areas and structures
Many areas of the cerebral cortex are concerned not so much with supporting a specific function, but with integrating and associating the products of surrounding areas. A similar role is played by many structures that lie beneath the cortex, such as the limbic system which, among other things, permits memory and behaviour to influence one another. The limbic system also plays a vital role in the generation of emotions and their interaction with our behaviour.
Looking to the future
It is fascinating to observe and learn about the complex ways in which the components of the brain create and control aspects of our being. If we can combine this knowledge with the ever-increasing technology that permits us to see how a brain affected by Alzheimer's disease changes, in future we may be able to predict, treat and manage the various symptoms associated with the disease. For now, a greater appreciation of the relationship between the brain and our behaviour may help people with Alzheimer's disease and those around them to understand and cope with the many changes brought about by the condition.
Last reviewed: November 2010
Next review due: November 2012
Written and reviewed by: Dr Sebastian Crutch at the Dementia Research Centre, National Hospital for Neurology and Neurosurgery
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