Positron emission tomography PET

Of all the new methods, the one that has attracted the most media interest is positron emission tomography or the PET scan. The technique is based on the detection of positrons, which are atomic particles emitted by some radioactive substances. Radioactively labelled water is injected into the body, and rapidly gathers in the brain's blood vessels. When part of the cortex becomes active, the labelled water moves rapidly to that place. A scanning device next measures the positrons emitted from the radioactive water. A computer then translates this information into pictures of the activity levels of different parts of the brain. It may sound dangerous to inject a radioactive substance into someone. However, only tiny amounts of radioactivity are involved.

Raichle (1994b) has described the typical way in which PET has been used by cognitive neuroscientists. It is based on a subtractive logic. Brain activity is assessed during an experimental task, and is also assessed during some control or baseline condition (e.g., before the task is presented). The brain activity during the control condition is then subtracted from that during the experimental task. It is assumed that this allows us to identify those parts of the brain that are active only during the performance of the task. This technique has been used in several studies designed to locate the parts of the brain most involved in episodic memory, which is long-term memory involving conscious recollection of the past (see Chapter 7). There is more activity in the right prefrontal cortex when participants are trying to retrieve episodic memories than when they are trying to retrieve other kinds of memories (see Wheeler, Stuss, & Tulving, 1997, for a review).

Evaluation

One of the major advantages of PET is that it has reasonable spatial resolution, in that any active area within the brain can be located to within about 3 or 4 millimetres. It is also a fairly versatile technique, in that it can be used to identify the brain areas involved in a wide range of different cognitive activities.

PET has several limitations. First, the temporal resolution is very poor. PET scans indicate the total amount of activity in each region of the brain over a period of 60 seconds or longer, and so cannot reveal the rapid changes in brain activity accompanying most cognitive processes. Second, PET provides only an indirect measure of neural activity. As Anderson, Holliday, Singh, and Harding (1996, p. 423) pointed out, "changes in regional cerebral blood flow, reflected by changes in the spatial distribution of intravenously administered positron emitted radioisotopes, are assumed to reflect changes in neural activity." This assumption may be more applicable at early stages of processing. Third, it is an invasive technique, because

MRI scan showing a brain tumour. The tumour appears in bright contrast to the surrounding brain tissue. Photo credit: Simon Fraser/Neuroradiology Department, Newcastle General Hospital/Science Photo Library.

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