fMRI, along with other imaging techniques brought to bear on research into the brain/mind relationship, can often seem like the only reliable way to get hard data out of a very messy area. In principle it seems neat, clean and rigorous: expose a subject to some stimuli, or ask them to think particular thoughts or perform a particular action, then check what brain activity correlates with this stimulus. Having done this, and observed the fMRI readings, Bob's your uncle- the areas of the brain showing most activation are involved in generating the particular feeling, action or thought in question.
However, while brain scanning can no doubt teach us an awful lot, the most valuable thing that many papers using such readings in their data can tell us is that their authors are rather poor at statistics.
Inevitably, researchers make certain assumptions about fMRI.
First, it must be assumed that the alterations in blood flow that fMRI measures truly indicate that simultaneous brain activity causing feelings or thoughts is occurring in the regions showing increased blood flow. This of course is by no means certain.
Second, there is a related assumption made that correlation implies causation.The putative validity of fMRI as a research tool depends upon these assumptions, and while they do present problems, we can acknowledge them and get on with things regardless: brain scan studies have, beyond doubt, driven significant advances in our understanding of minds and brains.
However, having accepted the validity of the technique and that correlation/causation inference based on fMRI is warranted, there remains a more fundamental problem. Shoddy use of statistics and misuse of numbers, detailed by Rebecca Goldin and Cindy Merrick here, threaten to undermine the validity of many inferences, and perhaps to corrode faith in studies using fMRI altogether.
One of the principal errors highlighted in the article is the "nonindependence error", or "double dipping", whereby a researcher makes use of non-independent data sets.
The Texan gunman, for instance, who fills the wall of his barn with buckshot, observes the damage and then draws a target around the best looking cluster of holes to demonstrate his marksmanship is guilty of double dipping. He is creating his data-set, choosing his target based on the data set, then using the same data set to prove his hypothesis that he is a good marks-man.
In the context of fMRI, a researcher who uses fMRI data to decide what area of the brain should be focused upon, and then makes use of the SAME data set to compute correlations and calculate his results, is guilty of using nonindependent data. In using observed correlations to decide where to look for correlations and then using the very same data to calculate the extent of these correlations, the researcher is guaranteed a positive result.
In the worst cases, double-dipping can cause correlations to be observed where none exist, and at the very least it will cause exaggeration of any correlations that do exist.
Such statistical errors are by no means rare. In one literature search of 134 fMRI papers, by Kriegeskorte et al., the authors suggested that 42% of these papers was guilty of at least one non independence error, and that a further 14% did not provide sufficient evidence to judge the quality of their statistical work.
These numbers are shocking: consider the idea that the results of over 50% of the fMRI papers read should be doubted.
If nothing else, this article underlines the importance of reviewing the "methods" section of any fMRI study claiming to show significant results.