Ways of studying the brain
The earliest method of studying the brain was post mortems, where dead people’s brains were inspected and dissected. However, it was difficult to reach valid conclusions about what roles different parts of the brain played in behaviour. Post mortems on people with brain damage did provide some information about the functioning of the brain. If a person had a specific deficit in functioning when alive, then dissection of the brain might suggest which brain area was related to that type of functioning.
Electroencephalogram (EEG) and event-related potentials (ERPs)
EEGs measure brain activity through electrodes on the scalp. ERPs measure brain activity in response to a stimulus, using the same equipment as that used for EEGs.
The introduction of scanning techniques in the 1980s revolutionised the ways in which the brain was investigated, as for the first time it offered an opportunity to view a live brain actually functioning on everyday activities without the need for invasive techniques.
Magnetic resonance imaging (MRI)
This technique uses a powerful magnetic field and radio waves to record vibrations from various atomic nuclei within neurons. This allows measurements called blood oxygen-level responses to be taken of the blood flow to specific brain areas. This method of scanning produces static (non-moving) pictures of brain activity.
Functional magnetic resonance imaging (fMRI)
fMRIs work in the same way as standard MRIs but additionally show brain activity as it occurs. MRIs work by recording the energy produced by molecules of water, after the magnetic field is removed. With an fMRI the energy released by haemoglobin (the protein content of blood) is measured. When haemoglobin contains oxygen it behaves differently than when it does not contain oxygen. So when a brain area is active and therefore using more oxygen, the difference in the amount of energy released by the haemoglobin is detected by the scanner and the change is measured. This produces a dynamic (moving) picture of activity about 1 second after it occurs and is accurate to within 1—2 millimetres in the brain. fMRI scanning is the current dominant method of brain scanning and is used to map out brain areas involved in many types of behaviour and psychological processes.
Positron emission tomography (PET)
PET scanning is an invasive technique, as it involves injecting a radioactive substance, usually glucose, into the bloodstream. This travels to the brain where it is used as an energy source by neurons. The glucose emits radioactive particles that are picked up by sensors placed around the head, with active brain areas consuming more glucose. A computerised picture is then created of brain activity.
Computerised axial tomography (CAT)
CAT scanning was an early scanning technique. It produces a static three-dimensional picture of the brain using x-rays. It has mainly been replaced by PET, MRI and fMRI scanning as a means of investigating the workings of the brain.
Post mortems: these gave psychologists a method of mapping out the brain in terms of its component parts and gave some insight into its functioning.
EEG and ERPs: both methods are cheaper, and therefore more available to researchers, than scanning.
MRI: MRI scanning is more ethical as, unlike PET scans, for example, which require intravenous injection of radioactive materials, it is a non-invasive method.
fMRI: fMRI scanning is non-invasive and is regarded as producing the most informative and detailed information concerning brain activity.
PET: PET scans have specialised uses, such as using other tracking agents that bind to synaptic receptors to produce a map of serotonin receptors within the brain.
CAT: CAT scanning is a useful technique for identifying structural changes in the brain, such as the detection of brain tumours.
Post mortems: as the brain is non-active after death, a post mortem is a much less informative method of study than scanning.
EEG and ERPs: output needs to be analysed by a skilled technician, limiting which researchers can use these techniques.
MRI: MRI scans are inferior to fMRI scans, as MRI scanning produces only static pictures of brain activity, while fMRI scans display moving pictures.
fMRI: like PET and MRI scans, fMRI scans do not directly measure the electrical activity of neurons in the brain. The technique requires significant changes in glucose uptake for activity to be detected, so important small changes may be missed.
PET: PET scanning is time consuming and its level of detail and speed of response are not as good as fMRI scans.
CAT: compared with other forms of scanning, the technique is relatively uninformative about the workings of the brain, hence it is falling out of favour as a preferred method of investigation.
Aside from providing a method of investigating the brain, scanning techniques are also useful in detecting brain tumours and assessing the degree of brain damage after a stroke, which helps form rehabilitation programmes.