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There are some major technical problems associated with the use of MEG. The magnetic field generated by the brain when thinking is about 100 million times weaker than the Earth's magnetic field, and a million times weaker than the magnetic fields around overhead power cables, and it is very hard to prevent irrelevant sources of magnetism from interfering with the measurement of brain activity. Superconductivity requires temperatures close to absolute zero, which means the SQUID has to be immersed in liquid helium at four degrees above the absolute zero of -273 °C. However, these technical problems have been largely (or entirely) resolved. The major remaining disadvantage is that MEG does not provide structural or anatomical information. As a result, it is necessary to obtain an MRI as well as MEG data in order to locate the active brain areas.

Section summary

All the techniques used by cognitive neuro-scientists possess strengths and weaknesses. Thus, it is often desirable to use a number of different techniques to study any given aspect of human cognition. If similar findings are obtained from two techniques, this is known as converging evidence. Such evidence is of special value, because it suggests that the techniques are not providing distorted information. For example, studies using PET, fMRI, and MEG (e.g., Anderson et al., 1996; Tootell et al., 1995a, b) all indicate clearly that area V5 is much involved in motion perception.

It can also be of value to use two techniques differing in their particular strengths. For example, the ERP technique has good temporal resolution but poor spatial resolution, whereas the opposite is the case with fMRI. Their combined use offers the prospect of discovering the detailed time course and location of the processes involved in a cognitive task.

The techniques used within cognitive neuro-science are most useful when applied to areas of the brain that are organised in functionally discrete ways (S.Anderson, personal communication). For example, as we have seen, there is evidence that area V5 forms such an area for motion perception. It is considerably less clear that higher-order cognitive functions are organised in a similarly neat and tidy fashion. As a result, the various techniques discussed in this section may prove less informative when applied to such functions.

You may have got the impression that cognitive neuroscience consists mainly of various techniques for studying brain functioning. However, there is more than that to cognitive neuroscience. As Rugg (1997, p. 5) pointed out, "The distinctiveness [of cognitive neuroscience] arises from a lack of commitment to a single 'level' of explanation, and the resulting tendency for explanatory models to combine functional and physiological concepts." Various examples of this explanatory approach are considered during the course of this book.

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