A popular method for detecting abnormalities in the shape and size of individual brains is seriously flawed, and is almost guaranteed to find 'differences' even in normal people.
So say Italian neuroscientists Scarpazza and colleagues in an important new report: Very high false positive rates in single case Voxel Based Morphometry.
Voxel Based Morphometry (VBM) is a way of analyzing brain scans to detect structural differences. It's most commonly used to compare groups of brains to find average differences, but some neuroscientists have started using VBM to check for abnormalities in a single brain. Scarpazza et al list 34 pieces of research about that, including 13 since 2010.
So it would suck if there were a problem with individual VBM... but there is. This pic tells the tale:
The authors took 200 normal brains and compared each one of them in turn to a control group of 16 normal brains. Because all of them were healthy, the comparisons ought to show no significant differences.
The technique was set up so that, in theory, only 5% of the brains should have been wrongly labelled as containing an abnormality. But in fact, a full 93.5%of the normal brains gave at least one false positive.
So 5% is more like the rate of not being wrong. Oops.
The image shows that in some brain areas, almost 25% of the normal brains were branded as 'abnormal' just in that region alone - the hotter the colour, the higher the proportion of false 'hits'. The top row is for false reports of brain volume increases, while the bottom row is decreases; false 'increases' were more common.
So what's going wrong? It's not entirely clear and several factors are probably at play, but the authors say that the main issue is that VBM makes the assumption of statistical normality which doesn't in fact hold.
Either way, it's a serious problem, and Scarpazza et al point to one especially worrying implication: some people have proposed using single-subject VBM in a legal context, to reinforce insanity pleas by showing subtle 'brain abnormalities' not obvious to the naked eye. Yet if this paper's right, such evidence could be entirely meaningless, almost guaranteed to give a positive result.
P.S. Last time I posted about this kind of analysis flaw, the internet went crazy because they didn't understand it. So just to be clear,
this is not a problem for clinical scans
- the kind you'd get to check whether you have a brain tumour.
Scarpazza, C., Sartori, G., De Simone, M., and Mechelli, A. (2013). When the single matters more than the group: Very high false positive rates in single case Voxel Based Morphometry NeuroImage DOI: 10.1016/j.neuroimage.2012.12.045