Neuroimaging studies may be giving us a misleading picture of the brain, according to two big papers just out.
By big, I don’t just mean important. Both studies made use of a much larger set of data than is usual in neuroimaging studies. Thyreau et al scanned 1,326 people. For comparison, a lot of fMRI studies have more like n=13. Gonzalez-Castillo et al, on the other hand, only had 3 people – but each one was scanned while performing the same task 500 times over.
Both studies found that pretty much the whole brain “lit up” when people are doing simple tasks. In one case it was seeing videos of people’s faces, in the other it was deciding whether stimuli on the screen were letters or numbers.
With all that data, the authors could detect effects too small to be noticed in most fMRI experiments, and it turned out that pretty much everywhere was activated. The signal was stronger in some areas than others, but it wasn’t limited to particular “blobs”.
So conventional fMRI experiments may just be showing us the tip of the iceberg of brain activity. In a small study, only the strongest activations pass the statistical threshold to show up as blobs, but that doesn’t mean the rest of the brain is inactive. It just means it’s less active. The idea that only small parts of the brain are ‘involved’ in any particular task may be a statistical artefact.
In fact, I wonder if the whole idea of treating statistically significant blobs as different from nearly-significant areas is itself a form of the error of interacting effects?
As if that wasn’t enough, Gonzalez-Castillo further show that there are lots of activations in the brain – even to very simple stimuli – that might go undetected in conventional studies, because they don’t follow the time-course predicted by the usual models.
Have a look –
This shows the average neural activation from various regions of the brain during a letter-number task. The two areas I’ve highlighted in red are the primary visual cortex, and they do follow the expected ‘boxcar’ pattern – the brain is active when the stimuli are on the screen, inactive when they’re not. But you can see that all kinds of other brain areas are also responding to the stimuli – just in different ways.
For example, the left primary motor cortex was activated during the task. That area controls the right hand, and that makes sense, as people responded by pressing buttons with the right hand. But interestingly, the same area on the other side of the brain was deactivated at exactly the same time, even though people weren’t doing anything with their left hand.
These papers illustrate the fact that conventional fMRI is a blunt instrument that often only tells us about the most straightforward events that happen in the brain. A bit like how we only hear the shouts and screams from through our neighbor’s walls, not their normal conversations, which aren’t loud enough to reach our ears.
That’s the bad news, but every blob has a silver lining. fMRI is clearly more powerful than most neuroscientists have realized, and this holds out hope for cracking some of the trickiest questions. As Gonzalez-Castillo et al put it
This result helps narrow the gap between thousands of fMRI manuscripts showing limited activation in response to tasks and cognition theories that defend that cognition—understood as the process of “con?guring the way in which sensory information becomes linked to adaptive responses and meaningful experiences”—can only result from the distributed collaboration of primary sensory, upstream and downstream unimodal, heteromodal, paralimbic, and limbic regions… [we were able to] switch from a regime where activity detection relates primary to sensory processing to a more sensitive regime, where activity detection includes also cognitive processes with subtler BOLD signatures.
Link: See also the interesting discussion here: Surely, God loves the .06 (blob) nearly as much as the .05.
Thyreau, B., Schwartz, Y., Thirion, B., Frouin, V., Loth, E., Vollstädt-Klein, S., Paus, T., Artiges, E., Conrod, P., Schumann, G., Whelan, R., and Poline, J. (2012). Very large fMRI study using the IMAGEN database: Sensitivity–specificity and population effect modeling in relation to the underlying anatomy NeuroImage DOI: 10.1016/j.neuroimage.2012.02.083
Gonzalez-Castillo, J., Saad, Z., Handwerker, D., Inati, S., Brenowitz, N., and Bandettini, P. (2012). Whole-brain, time-locked activation with simple tasks revealed using massive averaging and model-free analysis Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1121049109