Raine, A, Buchsbaum, M & LaCasse, L. (1997)
Brain abnormalities in murderers indicated by positron emission tomography. Biological Psychiatry, 42 (6), 495 - 508
PET stands for POSITRON EMISSION TOMOGRAPHY. PET scans allow researchers to examine the relationship between activity in the brain and mental processes. PET works by measuring the level of metabolic activity occurring within the brain. Someone having a PET scan is first injected with a small amount of harmless radioactive material ‘bonded’ to a substance such as glucose. Since the brain’s primary form of energy is glucose, the areas which are most active absorb more of it. The glucose is broken down by the brain but the radioactive material is not, and as it decays it emits positively charged particles called positrons which are detected by the scan. This information is then fed to a computer which produces different coloured images of the level of activity occurring throughout the brain, different colours indicating different levels of activity.
PET scans can be used to:
¨ provide images of what is going on in the brain DURING various BEHAVIOURS (an advantages over other scanning techniques such as CAT and MRI scans).
¨ identify those areas of the brain that are active when we are THINKING (for example: different brain activity has been found in response to the instructions: ‘move your right hand’ and ‘think about moving your right hand’).
¨ locate tumours and growths (to provide vital information about the likelihood of essential brain structures being damaged by surgery).
¨ reveal possible differences between the brains of people with and without psychological disorders (some differences have been found between the brains of schizophrenics and non-schizophrenics - supporting the notion that this disorder has a physical cause).
¨ explore differences between the sexes (Gur et al, 1995) showed that men have a more active metabolism than women in the primitive brain centres that control sex and violence).
In this study PET scans are used to investigate the notion that there are differences in brain activity between 41 murderers (39 male & 2 female, mean age = 34.3) pleading Not Guilty by Reason of Insanity (NGRI) and 41 controls (screened for mental and physical health).
The participants had to work at a continuous performance task based around target recognition for 32 minutes and then given the PET scan. Evaluate the selection of task remembering what the aim of this study is!
The researchers found that there was less activity in the corpus callosum of the NGRI group compared to the controls. What characteristics are associated with low levels of activity in this part of the brain?
The researchers found that there was an imbalance of activity in the amygdala (less activity in the left side and more in the right side of the NGRI group). What characteristics are associated with this kind of imbalance?
What other differences did the researchers find in the brain activity of the NGRI group compared to the controls?
There are a number of problems associated with the data produced by PET scans. Outline THREE of these problems.
The researchers offer two tentative conclusions:
1. They suggest that the research supports previous findings relating to the role of the amygdala in violent behaviour (imbalance in activity = unusual emotional responses and lack of fear).
2. They suggest that the difference in corpus callosum activity may have similarities with research conducted on people with ‘split-brains’ (see Sperry) which shows them to have inappropriate emotional expression and the inability to grasp the long term implications of a situation.
However, the researchers are very cautious indeed about the implications of their findings. There are FOUR key points here:
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2.
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It should also be noted that imaging techniques are still being developed and any data produced in this way should be treated with caution. The relevance of the task has already been noted. Finally the issue of violent and non-violent murder is raised: is murder always a violent act? This has not been controlled for.