The blogosphere has begun debating the merits of fMRI. That’s a good thing. The debate began with Paul Bloom’s excellent editorial in Seed, in which he argued that “fMRI imagery has attained an undue influence, and we shouldn’t be seduced.” It continues here and here.
I used to work in a neuroscience lab grounded in molecular biology, and there was no shortage of fMRI bashing. A typical complaint went like this: “I’m here struggling with my damn Western blot [or Southern, or PCR, etc.], trying to be a good reductionist, while all those fMRI researchers just pick a sexy question, stuff some people in a magnetic tube, and get their technicolor pictures on the cover of Nature. It isn’t fair.”
Of course, that complaint wasn’t fair either. fMRI isn’t that easy. But I also wonder if there isn’t a germ of truth behind the kvetching. Molecular biology is hard because reductionism is hard. It isn’t easy to parse life into its parts, to break a brain into a collection of identifiable shards. As a result, deciphering a cascade of acronyms can occupy a researcher for their entire career. Gels fail, primers don’t work, proteins misfold…but now I’m complaining again.
So how does this relate to fMRI? Well, I’m not entirely convinced that fMRI has earned its reductionist conclusions. Bloom does a commendable job criticizing fMRI for “turning bad explanations into satisfactory ones.” But he never discusses some of the serious imperfections in the technology itself that are often glossed over. For example, in 2001, Professor Nikos Logothetis, of the Plank Institute in Germany, published a paper in Nature in which he simultaneously recorded the electrical signals of neurons and measured blood flow using fMRI. No one had ever done this before. Logothetis found that the increases in blood flow measured by fMRI do not necessarily parallel increased neural firing rates. In fact, increased blood flow can also parallel a constant, or even a decreasing neural firing rate. In 2004, Logothetis’ lab found something even stranger . Neurons that had been chemically silenced – they could no longer become active – could still generate an fMRI signal that appeared active. As Logothetis notes, “This dissociation [between multi-unit activity and local field potential] could be observed in about 25% of the responses.”
It gets worse. A 2002 study by Robert Harrison at the University of Toronto showed that fMRI signals “emanated only from areas endowed with a rich vascular network, and [that] no signals were obtained from adjacent regions in which the vasculature was less dense.” Unfortunately, Harrison also discovered that in many cases the density of blood vessels in our brain has little, if any, relationship to our neural activity. Furthermore, blood also moves slower than the electricity in our neurons, so it’s always difficult for fMRI to decipher what thought process the blood flow actually correlates with. Finally, whole parts of our brain remain invisible to fMRI machines. The base of our frontal lobe – a brain area crucial for consciousness – is too close to our nasal ducts to be visualized. The magnetism of air interferes.
To be sure, Logothetis and others think fMRI remains useful for measuring many different mental events, especially as imaging technology continues to improve. Even if our blood doesn’t always correlate with neural activity, it can still be used as a pretty good proxy for neural processing. But we should not treat these anatomical portraits as definitive proof of anything. Pictures have a way of seeming more durable than most other types of data. The irony is that the opposite is probably true. fMRI isn’t a window into the brain; it’s just a crude map of our anatomy. The important thing is to not confuse the map of a place for the place itself.
Update 12/22/07: Comments have been closed due to an attack of spam.