Thursday, May 8, 2008

"The Neural Data is More Sensitive than the Behavioral Data"

Before I get back to the "Four Ages of Functional Neuroimaging" I'd like to take a brief detour and talk a little bit about a phrase -- or a slogan, perhaps -- that one hears more and more in the neurosciences, namely, that: "the neural data is more sensitive than the behavioral data".

I'll give a little context. A speaker has just presented some data, say, on the relationship between hippocampal volume and a genetic polymorphism, or the effect of some drug on dopaminergic activity in the striatum. Impressive bar graphs are displayed, with big effects and little error bars. There is no doubt that the the finding is Real, that such and such drug or such and such genetic polymorphism is having a measurable biological impact, and that it's interesting and worth studying, etc., etc.

Sometimes these biological data are presented along side lots of "scatterplots" showing that the effects are also correlated with some behavioral measure, say, working memory capacity, or performance on the Wisconsin Card Sorting Task. If you have a biological finding and a scatterplot showing a relation to behavior, then you're golden. Everybody in the room is happy, even the cranky behavioral psychologist in the back.

But what if the speaker just presents the biological measure without the scatterplot, without the link to behavior? This is usually fine, provided no claims are made about behavioral relevance. Sometimes it really isn't that important to link the two. One is just trying to get a handle on the relationship between two neural variables (say gene X, and hippocampal volume) and no strong claims are made about causal links to some behavioral state. Someone else will figure that out, later. Sometimes, however, the speaker wants to make these strong claims, even without the scatterplot. Of course, the speaker would have liked to show the audience a nice brain-behavior correlation, and he or she almost certainly collected some behavioral index, but as occurs in science from time to time, the correlation failed to reach significance. And, thus, no scatterplot.

The talk concludes, the speaker having argued forcefully for the importance of drug X, because of its effect on brain system Y. The speaker goes on to say that the drug allows subject to focus attention better and enhances working memory and general fluid cognition.

Hand goes up in the back -- it's the cranky behavioral psychologist. He has a kind of a gravelly voice and one has the distinct impression that he was asleep for most of the talk. Here's what he asks: "Did you measure any behavioral variables? Did administration of the drug have any effect on cognition, as measured by standard measures or memory, reaction time, etc?"

The speaker is ready for this. He is indeed smiling. He's been handling this question for years, and frankly, he's rather amused at the naivete of the questioner.

"Well", he or she says, "of course we had our subjects perform a whole battery of neuropsychological tests, cognitive tasks, and personality inventories, including the WCST, N-Back, Trails A, B, C, and D, the Simon task, the TPQ, the Sensation Seeking scale, the impulsivity scale, locus of control, etc. etc. but none of these measures were significantly correlated with our biological finding. Of course, this is no surprise, because as everybody knows the neural data is more sensitive than behavioral data." The cranky psychologist offers a slight grimace, but does not follow up with another question. Once again, the response worked its charm. After all, who is to argue? There was a big neural effect and no behavioral effect -- therefore, surely the neural data is indeed more sensitive than the behavioral data. Right?

But wait, one might ask what is the neural data more sensitive to?. That is surely an important question. Let's think. The neural data is more sensitive to neural differences (e.g. hippocampal volume) than the behavioral data is. That is true -- perfectly trivial but perfectly true. The converse is also -- trivially -- true: "The behavioral data is more sensitive to behavioral differences than the neural data is".

A more interesting statement would be as follows: "The neural data is more sensitive to behavioral differences than the behavioral data is". That would be a strong claim, but one that is rarely made. Instead, we get the stock "neural data is more sensitive than behavioral data" without any context or qualification. The problem is that this phrase, this slogan, this stock reply to to the crabby behavioral psychologist, is empty of content and specificity.

Just to drive the point home, what if I told you that a stethoscope is more sensitive to differences in heart rate then any behavioral measure. Would you be surprised? But what if I went on to say that my heart rate measurements, because they are so sensitive, indicate that subjects with a faster heart rate live longer. But wait, asks the old guy in the back of the room, where is your behavioral evidence for that assertion (e.g. measure of longevity)?. Don't need any, because biological measures are more sensitive than behavioral measures.

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