A critical look at what fMRI can tell us.
FMRI only measures the secondary physiological correlates of neural activity, it is not a direct measure. This means it is not a truly quantitative measure of mental activity - when comparing the FMRI response between individuals it is impossible to say whether the differences are neural or physiological in origin.
Although not a direct measure of neural activity FMRI is still a causal step closer to what is happening in the brain than the behavioural correlates psychologists have traditionally depended on. Although currently qualitative, FMRI is a more objective measure of a person’s mental state than a tick-box questionnaire.
The relationship between the FMRI signal and the underlying activity is an active area of research. A variety of techniques have been developed to calibrate an individual’s response in order to obtain a quantitative measure of neural activity.
Relative to other brain imaging techniques, FMRI has unequalled spatial resolution – at 7T activity can be mapped down to 1mm. However the temporal resolution of FMRI is inherently limited by the slow blood flow response it depends. FMRI cannot uncover the dynamics of mental activity on the sub-millisecond timescale on which neurons operate.
FMRI can be combined with high temporal resolution techniques, such as EEG to combine the different strengths of each technique. An increasingly popular method is combined EEG-FMRI where the two measurements are made simultaneously.
Critics of the technique complain that FMRI overlooks the networked or distributed nature of the brain’s workings, emphasizing localised activity when it is the communication among regions that is most critical to mental function. This, along with FMRI being an indirect measure of brain activity has led to the charge that FMRI is no more than modern day phrenology - a 19th century pseudo-science which purported to character-type by examining the shape of a person’s skull.
Any FMRI experiment is only as good as its hypothesis, design and interpretation. Silly FMRI experiments, for instance one showing men’s amygdala’s (which play a key role in generating emotion) light up when viewing Ferraris, are not difficult to find. But such work doesn’t prove any fatal flaw in FMRI, merely poor use of a good tool. Most FMRI investigators seek not to localize brain function but to map the parts of the system that act in different combinations for different tasks.
As FMRI has begun to address more questions ethical concerns have arisen regarding who should have access to FMRI data.
Ethical approval, which is required for functional imaging experiments, is only granted if satisfactory procedures are in place to protect the privacy of those taking part in the study.