Magnetoencephalography MEG

In recent years, a new technique known as magneto-encephalography or MEG has been developed. It involves using a superconducting quantum interference device (SQUID), which measures the magnetic fields produced by electrical brain activity. The evidence suggests that it can be regarded as "a direct measure of cortical neural activity" (Anderson et al., 1996, p. 423). It provides very accurate measurement of brain activity, in part because the skull is virtually transparent to magnetic fields. Thus, magnetic fields are little distorted by intervening tissue, which is an advantage over the electrical activity assessed by the EEG.

Anderson et al. used MEG in combination with MRI to study the properties of an area of the visual cortex known as V5 (see Chapter 2). They found with MEG that motion-contrast patterns produced large responses from V5, but that V5 did not seem to be responsive to colour. These data, in conjunction with previous findings from PET and fMRI studies, led Anderson et al. (1996, p. 429) to conclude that "these findings provide strong support for the hypothesis that a major function of human V5 is the rapid detection of objects moving relative to their background." In addition, Anderson et al. obtained evidence that V5 was active approximately 20 milliseconds after V1 (the primary visual cortex) in response to motion-contrast patterns. This is more valuable information than simply establishing that V1 and V5 are both active during this task, because it helps to clarify the sequence in which different brain areas contribute towards visual processing.

Evaluation

MEG possesses several valuable features. First, the magnetic signals reflect neural activity reasonably directly. In contrast, PET and fMRI signals reflect blood flow, which is assumed in turn to reflect neural activity. Second, MEG supplies fairly detailed information at the millisecond level about the time course of cognitive processes. This matters because it makes it possible to work out the sequence of activation in different areas of the cortex.

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Business Correspondence

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