BRAIN IMAGING

Methods of obtaining images of activity in the brain. They have been used in order to determine which parts of the brain are associated with particular language tasks.

 In early experiments, electrical stimulation of the brain (ESB) was used in order to map its operations. By applying a low-level electrical current to an exposed area of the cortex, one can temporarily disable its associated functions. In this way, experimenters traced connections between parts of the body (including the articulators) and various points along the central strip of the cortex, which controls motor activities. They also applied currents to the parts of the brain traditionally associated with language, and identified the language functions which were affected.

Today, the brain can be x-rayed in sections using a method known as computerised axial tomography (or CAT scanning); this is useful for locating lesions but only provides a representation of a single state. Other techniques have therefore been developed to show the brain in action from moment to moment.

 One group of methods exploits the fact that blood flow in the brain closely reflects patterns of activity because of the need to supply oxygen to the neurons which are in operation. In positron emission tomography (PET), a small intake of radioactive gas or fluid renders the subject’s blood radioactive. The degree of radiation within the brain (and thus the intensity of blood flow) can then be measured by gamma ray detectors. A computer picture is generated of the metabolic activity in the brain: bright colours showing the areas where it is at its most intense and darker colours those which are less active.

 Even more precise images can be obtained through a technique known as functional magnetic resonance imaging (FMRI), which exploits the fact that neural activity results in an excess of oxygen in the blood in the brain. The oxygen level can be tracked because it results in changes in magnetic susceptibility. Researchers are thus able to monitor the changing patterns of blood flow.

An alternative technique records brain states sequentially by measuring electrical activity occurring in the brain. Electrodes are attached to the subject’s scalp and indicate where a change in activity (‘evoked potential’) takes place in response to a given stimulus such as seeing or hearing a word. The most common method seeks to identify event related potentials (ERPs), small deviations in voltage level from the normal baseline.

See also: Brain: localisation, Event-related potential

Further reading: Deacon (1997); Posner and Raichle (1994)

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