Brain Imaging Technologies

Key Terms Structural - showing the structure of the brain Functional - showing the activity of different parts of the brain Spatial resolution - the resolution of the picture produced Temporal resolution how long it takes to take a frame so it is concerned with how accurately changes in the brain can be tracked

EEG 1924 Electroencephalography (EEG) is the measurement of the electrical activity of the brain by recording from electrodes placed on the scalp. The resulting traces are known as an electroencephalogram (EEG) and represent an electrical signal from a large number of neurons. EEGs are frequently used in experimentation because the process is non-invasive to the research subject. The EEG is capable of detecting changes in electrical activity in the brain on a millisecond-level. It is one of the few techniques available that has such high temporal resolution.

Invasive or Non-Invasive An advantage of EEG is that it is non-invasive. Unlike other scanning techniques, such as Positron Emission Tomography (PET), EEG does not use radiation or involve inserting instruments directly into the brain and are therefore virtually risk-free. Furthermore, EEG and ERP are much cheaper techniques in comparison with fMRI scanning and are therefore more readily available. Consequently, this should allow more patients/participants to undertake EEG/ERPs, which could help psychologists to gather further data on the functioning human brain and therefore develop our understanding of different psychological phenomena, such as sleeping, and different disorders like Alzheimer's. Spatial Resolution However, one disadvantage of EEG/ERP is that these techniques have poor spatial resolution. Spatial resolution refers to the smallest feature (or measurement) that a scanner can detect, and is an important feature of brain scanning techniques. Greater spatial resolution allows psychologists to discriminate between different brain regions with greater accuracy. EEGs/ERPs only detect the activity in superficial regions of the brain. Consequently, EEGs and ERPs are unable to provide information on what is happening in the deeper regions of the brain (such as the hypothalamus), making this technique limited in comparison to the fMRI, which has a spatial resolution of 1-2mm.

Temporal Resolution: An advantage of the EEG technique is that it has good temporal resolution: it takes readings every millisecond, meaning it can record the brain’s activity in real time as opposed to looking at a passive brain. This leads to an accurate measurement of electrical activity when undertaking a specific task. However, it could be argued that EEG is uncomfortable for the participant, as electrodes are attached to the scalp. This could result in unrepresentative readings as the patient’s discomfort may be affecting cognitive responses to situations. fMRI scans, on the other hand, are less invasive and would not cause the participants any discomfort, leading to potentially more accurate recordings. EEG: Another issue with EEG is that electrical activity is often detected in several regions of the brain simultaneously. Consequently, it can be difficult pinpoint the exact area/region of activity, making it difficult for researchers to draw accurate conclusions.

CT Scanners 1972 Computed tomography (CT) scans are x-ray slices that show the density of brain structures. During a CT scan the subject lies on a table that slides in and out of a hollow, cylindrical apparatus. An x-ray source rides on a ring around the inside of the tube, with its beam aimed at the subjects head. Images made using x-rays depend on the absorption of the beam by the tissue it passes through. Bone and hard tissue absorb x-rays well, air and water absorb very little and soft tissue is somewhere in between. Thus, CT scans reveal the gross features of the brain but do not resolve its structure well.

Problems: Prolonged or repeated exposure to ionising radiation can cause tissue damage.  Thus, as with x-rays, CT scans are used sparingly. Additionally, only structural information about the brain can be gathered.  This is perfect for identifying problematic brain tissue, but gives us little insight into how the brain functions during cognition.

MRI 1977 The MRI (Magnetic Resonance Imaging) gives a three-dimensional picture of the brain structures. An older method of brain imaging A powerful magnetic field is placed around the brain. It temporarily holds the nuclei of the brain's atoms in one direction. When released, the atoms "wobble" back to their original positions and emit a weak radio frequency signal that can be picked up by a sensitive receiving device. It shows structural changes in brain matter and is used to investigate tumours or any possible brain damage.

MRI scans are limited to only showing structural changes and need careful interpretation to prevent false positives. Furthermore, due to the nature of the scanner itself, MRI scans are not good for people with claustrophobia which can prevent some people from taking part in MRI studies. This limits the effectiveness of this technique for measuring the relationship between biological factors and behaviour in people with claustrophobia Different areas of the brain have different densities (and so different amounts of water in their tissues). These emit differing amounts of radio waves, producing different amounts of shading on the image produced. The image produced is very detailed (it has high spatial resolution), but can not show brain activity, only structure (so it has very low temporal resolution).

Maguire (2000) Aim: Maguire hypothesised that taxi drivers would show significantly higher volumes of grey matter in their hippocampus, a structure associated with navigational skills. Method: used MRI scans to compare the volume of grey matter in the brains of London taxi drivers compared to a pre-existing sample of matched controls. Findings: By using an MRI scan, Maguire found increased grey matter was found in the brains of taxi drivers compared with controls in both the right and left hippocampi. The increased volume was found in the posterior (rear) hippocampi. Conclusion: The results of the MRI allowed Maguire to investigate the relationship between biological factors (the size of the hippocampi) and the behaviour (spatial navigation).

Discussion While the results of Maguire indicate a relationship between biological factors and behaviour, MRI scans and experiments like this are often criticised for lacking ecological validity. However, until a procedure is developed whereby brain scanning can take place during an everyday activity in the real world, then it must essentially remain lab bound. Furthermore, an MRI image can take several minutes to form, and the slightest movement can affect the validity of the findings. Consequently, while an MRI is useful for establishing a relationship between biological factors and behaviour, its inability to show causation, low ecological validity and susceptibility to distortion via movement make this scanning technique limited in demonstrating a causal relationship with any certainty.

fMRI 1990 The most prominent neuroimaging technology over the last two decades. Unlike the MRI which shows the structure of the brain, the fMRI indicates activity in the brain. fMRI tracks changes in blood flow and oxygen levels to indicate neural activity. When a particular brain area is more active, it consumes more oxygen and blood flow increases.

Problems: fMRI is expensive, and has poor temporal resolution (whole brain images can typically only be collected every 2 seconds).

PET 2000 Positron Emission Tomography Type of nuclear medicine imaging which use a small amount of radioactive material to diagnose and determine the severity of brain diseases, including cancers and neurological disorders Involves the injection of a radioactive tracer which appears as a bright colour on the scan, indicating which areas of the brain are most active in metabolising glucose during a task The brighter the colour, the more active that part of the brain during that particular task PET scans can not be used on everyone due to allergies, children or pregnant women

Problems: Expense, inaccessibility, lack of temporal (40 seconds) and spatial (1 cm) resolution.

Raine (1997) Aim: used PET scans to demonstrate a biological correlation between impulsive behaviour and lack of pre-frontal cortex (PFC) activity. Method: used a sample of 41 murderers (39 men and 2 women) who had pleaded NGRI (not guilty by reason of insanity) and 41 age and sex-matched controls. Findings: Raine found that the NGRI participants had lower glucose metabolism in their PFC in comparison to the controls. Conclusion: It might be inferred from these findings that NGRI murderers do not use their PFC to interpret and respond to non-emotional stimuli (in this case the continuous cognitive task), reacting instead in an emotional manner.

Discussion The use of a PET scan allowed Raine to investigate the link between a biological factor (lack of pre-frontal cortex activity) and behaviour (impulsive behaviour). Such findings would not have been possible without the use of a PET scan which could be used in clinical and forensic settings to inform rehabilitation programmes and to go some way towards preventing crimes from taking place. However, such research does not provide a full explanation of all possible influences on the behaviour in question. The results of such studies are correlational, and therefore researchers are only able to conclude that the two factors (pre-frontal cortex activity and impulsive behaviour) are linked Consequently, there is no real evidence to show conclusively that the NGRI murderers’ crimes are caused by a lack of PFC activity; there could be a huge range of other influences that produced the behaviour e.g. alcohol abuse, upbringing, etc. Therefore, PET scans are limited as it can only tell one part of a complex story when highlighting a link between biological factors and behaviour.