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Starting with psychology
Starting with psychology

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2 A brain of two halves

2.1 Introduction

If you are not too squeamish, imagine you have lifted the top off someone's skull and peeled back a thin protective membrane. You are now looking down on the brain sitting in a pool of liquid. You have probably heard the phrase ‘grey matter’ and one of the first things you would see is that the outermost layer of the brain is indeed slightly grey in colour. It also has many dips and folds.

You would also notice that the brain is divided into two halves or hemispheres with the division running from the front to the back of the brain.

These two hemispheres are joined together by a bundle of approximately 200 million nerve cells that pass messages between the two hemispheres. This connecting bundle of cells is called the corpus callosum.

Figure 2
Figure 2 Looking down on the brain

Although these two hemispheres look the same, so they have a similar structure, there are differences in the way they function so they control different responses. For example the left hemisphere controls and receives information from the right side of the body and the right hemisphere controls and receives information from the left side of the body.

The two hemispheres may also differ in the extent to which they control certain functions such as producing speech, daydreaming or recognising someone's face. Some functions may be more under the control of one hemisphere, so that hemisphere will dominate the other. Other functions may be shared equally by both hemispheres. For example our speech area is usually located in the left hemisphere except in some, but not all, left-handed people who may have areas controlling speech on both the left and the right hemisphere. Conversely both hemispheres play a role in vision although it is the right hemisphere that receives information from the left visual field and the left hemisphere that receives information from the right visual field.

You will have noted from the mention of left-handed people above that not all brains are organised in the same way. Another finding in this area is that males, especially right-handed males, have greater left hemisphere dominance for speech than females. If a man suffers damage in the speech area of his left hemisphere this will have a greater impact on his speech compared to a woman who has suffered similar damage.

However, bearing in mind that there will be some differences between people in the way that their brains are organised, we do have a range of evidence that suggests that generally the two hemispheres are dominant in different areas. The left hemisphere dominates for speech, writing, mathematical ability, logic and analysis. The right hemisphere dominates for perception, spatial ability, musical and artistic abilities, imagery and dreaming. The right hemisphere also seems to be more emotional and negative compared to the positive and rational left hemisphere.

Evidence to support the proposal that one hemisphere may dominate the other for a particular function, or hemispherical specialisation, has come from a number of sources. In this section you will consider what has been learned through research with people who have had an operation that splits the left hemisphere of the brain from the right hemisphere of the brain.