BRAIN

The brain has a number of language-related functions. It controls the cognitive processing involved in producing or understanding language; the motor activities involved in articulation (the movement of tongue, lips, vocal cords etc.); and involuntary activities such as breathing which need to be co-ordinated with speech.

The human brain consists of two hemispheres linked by a complex web of nerve connections, the corpus callosum. Generalising somewhat, the left hemisphere in most individuals is associated with analytic processing and symbolisation, while the right is associated with perceptual and spatial representation. The left hemisphere is particularly implicated in language processing (see brain lateralisa tion), though the right contributes as well. The vast network of connections between the two ensures that any operation can draw upon both.

 The hemispheres have a contralateral relationship with the rest of the body: the right side of the brain controls the left side of the body and vice versa. Signals received by the right ear have a preferential link to the left hemisphere and vice versa. However, the situation with the eye is less straightforward: information from the left visual field in both eyes is transmitted to the right hemisphere and vice versa.

 The upper surface of the brain, the cortex, is especially associated with language processing, though the sub-cortical areas also contribute. The cortex is marked by a pattern of hills (gyri) and valleys (sulci). Some of the deeper sulci divide each hemisphere into four (see Figure B1). These are the temporal lobe, which runs from front to back, the occipital and parietal lobes at the back and categorised as ‘posterior’, and the frontal lobe, sometimes described as ‘anterior’. The occipital lobe is associated with visual stimuli, and the temporal lobe with auditory stimuli.

A strip running centrally across the hemisphere controls much motor activity, including operations involving the articulators. Other linguistic functions are associated with some of the lower levels of the brain: the medulla regulates involuntary activities including breathing, and the cerebellum plays a major part in controlling and co-ordinating articulation.

The transmission of information in the brain takes place by means of nerve cells or neurons. The human brain contains about 100 billion of these. The power of the brain derives not so much from the activity of individual neurons, as from the multiple interconnections between them and the way in which they operate in parallel. The human cortex contains about three-quarters of the brain’s neurons, and is proportionately much bigger than the cortex in other mammals.

Information is transmitted within the brain by the discharge of a chemical across a synapse (a gap between neurons), which causes a change in the receiving neuron. The change may involve an increase in the neuron’s electrical potential (termed excitatory) or a decrease (termed inhibitory). It accumulates until it reaches a threshold, when it triggers a pulse (or action potential), which is transmitted along the neuron and on to others.

The transmission can potentially take place on a massive scale, so small effects can become greatly magnified. This pattern of activity is referred to as activation– and has provided an influential model of how words are accessed from the lexicon during listening or reading.

See also: Brain: human vs animal, Brain lateralisation, Brain: localization

 Further reading: Dingwall (1998); Libben (1996); Obler and Gjerlow (1999)

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