The Power of Combining Techniques
The techniques we’ve described are each quite powerful; but as we’ve noted, each has its own limitations—some techniques provide information about structures but not activities, while others tell us where the activity took place but not when, and so on. In addition, PET scans and fMRI scans can tell us what brain areas are activated during a particular process, but this by itself doesn’t tell us whether those areas are actually needed for the process. Perhaps the brain activity we’re observing is a consequence of the process, much like sweating is a consequence of (and not a cause) of physical activity. In that case, the brain activity would be correlated with a mental process but would not play a role in guiding or supporting that process.
How can we get past these various limitations? The answer draws on a strategy com-monly used in psychology: We seek data from multiple sources, so that we can use the strengths of one technique to make up for the shortcomings of some other technique. Indeed, by collecting data of many different types, we gain the advantages of all the types—and so end up with a compelling package of evidence.
Thus, for example, some studies combine EEG recording with fMRI scans, so that we can learn from the EEG exactly when certain events took place in the brain, and learn from the scans where the activity took place. Likewise, some studies combine fMRI scans with CT data—the first procedure tells us about blood supplies (and therefore brain activity) and the second provides a detailed portrait of the person’s brain anatomy. Together, these techniques can give us enormous precision in identify-ing exactly which regions of the brain are involved in a particular task.
In addition, neuroscientists often combine neuroimaging with studies of brain dam-age, basing this approach on the logic that if damage to a brain site disrupts a function, this is an indication that the site does play a role in supporting the function. In the same way, TMS allows us (temporarily) to “turn off ” a particular brain site. If doing so causes a disruption in some process, this too indicates the site plays a role in support-ing that function. Data like these allow us to go beyond claims about correlation and make stronger claims about cause and effect. The key, though, is that we rely on a vari-ety of techniques and draw our conclusions only from a convergence of evidence gathered from many paradigms.