The Frontal Cortex

fMRI Redux

Over at Small Gray Matters, there is an excellent critique of my last post on fMRI. Here is the nut graf:

While fMRI certainly has important technical limitations people should be aware of (low spatial and temporal resolution, high costs giving rise to underpowered studies, etc.), I think the issue Lehrer chooses to focus on-namely, the relationship between the BOLD signal (the signal measured by fMRI machines) and underlying neuronal activity-is actually one of the few areas that aren’t controversial.

Well, yes, that was my point. There have been no shortage of philosophical critiques of fMRI, and I’m afraid I can’t add much to those. (See here, here and here, for starters.) What I wanted to point out was that the most basic assumption of fMRI – that blood neatly correlates with neural activity – is actually a wee bit complicated. Small Gray Matters notes that the 2001 Logothetis paper I cite isn’t actually a secret, since “it’s already been cited a thousand times (in just 5 years!)”. True enough, but those citations rarely grapple with some of the “nuances” of Logothetis’ actual data. Instead, the paper is mindlessly cited as “proof” that the BOLD signal always dovetails neural activity. As Logothetis observes, this assumption is usually true. Except, of course, when it isn’t: his papers have repeatedly demonstrated that silenced neurons – inactive cells – can also produce a BOLD signal. Here is Small Gray Matters:

Now it’s certainly pretty interesting that you can get a dissociation between individual neuron spikes and the local field potential at all; but that’s not an issue that concerns fMRI researchers, since it’s pretty clear that it’s a lower-level phenomenon. Put differently, if the dissociation between individual spikes and the LFP is a reason to question imaging results, then a lot of other areas of neuroscience are in trouble, because local field potentials are used all over the place.

But this is what I want fMRI researchers to grapple with. Instead of searching for the neural correlates of romantic love, why not grapple with the fascinating anomalies the technology actually illuminates. In my earlier post, I said that I wasn’t entirely convinced that fMRI has earned its reductionist conclusions. This is why. I’m fascinated and bewildered by the Logothetis data. In the five years since his paper was published, there have been thousands upon thousands of fMRI papers documenting all sorts of really interesting things. But we still don’t understand a significant part of the “nuanced” relationship between neural activity and the flow of oxygenated blood. The devil is always in the details.