Why fMRI is unsatisfying – a neuronal perspective

461889a-f1.2Each year when the Society for Neuroscience Meeting rolls around, all of the major journals devote extra space to neuroscience, publishing hot articles to attract the attention of the 30,000 plus attendees at the conference.  This year is no exception, and one of the most important articles came out this past week in Nature with the heady title “Intracellular dynamics of hippocampal place cells during virtual navigation“.  The paper, by Chris Harvey, Forrest Collman, Daniel Dombeck & Dave Tank is a tour de force investigation which combines new technology with insightful experimental manipulations and shows, according to an accompanying commentary by Doug Nitz, that “it is not impossible to examine brain correlates of higher cognitive processes and at the same time identify their underlying causes at the cellular level”.

The detailed results are probably too technically specific for most people in the field of neuroethics, but this study highlights some of the reasons that hard-core neuroscientists view fMRI with disdain.  Given the prominence that imaging the human brain has come to play in neuroethical discourse, I encourage readers to take a few moments to at least try to appreciate what the issues might be.

First, let’s take a look at what Dave Tank’s group at Princeton have done.  For over 35 years, neuroscientists have known that the firing rate of a subset of hippocampal pyramidal cells (the so-called place cells) change in predictable fashion as the animals navigate through a spatial environment.  In particular, the firing rate of a place cell reflects both the animal’s present spatial position and the path the animal has taken to reach that position.  Think about that for a second: the output of a single neuron reflects a highly nuanced and information rich algorithm.  But it does not stop there.  When multiple place cells are recorded at the same time, they exhibit a phenomenon called phase precession.  Nitz’ commentary sums it up nicely:

The firing order for a set of hippocampal place cells with partially overlapping place fields is found to match the animal’s physical trajectory corresponding to those fields. Phase precession stands as perhaps the most robust example of temporal coding of information in the mammalian brain.

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