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Brain Scans

Figure 3.1. The subtraction method in functional neuroimaging experiments. The figure displays the pattern of brain activity when generating words in a second language in two different groups of subjects. The conditions were a second language to which subjects of the study were less exposed (on the left) and a second language to which a different group of subjects were highly exposed (in the middle). As may be observed, more extensive brain activation in the left dorsolateral frontal cortex is found when generating words in a less exposed second language. These findings suggest that a second language associated with lower environmental exposure is in need of additional neural resources.

In order to know where these additional neural resources are located, the subtraction method can be used: the brain activity pattern of the well-exposed group is subtracted from the pattern found in the group of low-exposed subjects' brain activity. The result is illustrated on the right, and the pattern of brain activity shows those areas necessary for supporting a second language to which subjects are relatively less exposed (modified from Perani et al., 2003). Note that the subtraction method may be used either between different groups or between different conditions in the same group (that is, word generation in L1 versus L2). In our example, subtraction is used between groups.

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