Posted by: wolf | March 28, 2008

Why brain imaging is (sometimes) overrated

Here, I don’t want to discuss any specific brain imaging study, and I am being careful in the title: I would bet a large amount of money that there are at least as many crappy behavioral studies out there as there are crappy brain imaging studies. Instead, I will try to give a brief explanation why psychologists or behavioral scientists can get rather angry when they have to read, view or hear yet another silly popular media coverage of a scientist in a lab coat with coloured brains flashing in the background stating something like “the brain’s reward system is active when something feels good”. Duh!

In a nutshell: Brain imaging attracts a disproportionate amount of attention while often failing to be innovative. Above that, brain imaging studies are excessively expensive and yet often fail to meet the methodological standards that apply to behavioral experiments.

I will try to give some more background:

First of all, a lot of brain imaging studies simply repeat older behavioral experiments and then show that something somewhere in the brain “lights up”. Of course, this kind of research sometimes lead to valuable results, but often you will just have something like whenever subjects are required to do something involving language and higher order cognition (applying some kind of rule or whatever), then you will have a coloured region in the prefrontal cortex (higher order cognition) and a coloured Broca- and/or Wernecke region — but that’s something we have known for decades.

Then, obviously there are physical limits to what kind of experiments can be done while a person lies inside a huge tube like this one:

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You can not really move inside of a scanner — as a matter of fact, researchers will routinely try to stop you from moving because brain activity connected with movement will distort the results. The only movements typically allowed are pressing of buttons, to indicate some kind of reaction to stimuli presented on a computer screen. And above being prohibited to move, it is incredibly loud inside of scanner — the sound is similar to standing beside a jackhammer. So clearly there are many psychologically interesting experiments that can never be done inside of a scanner.

These first points (repeating older behavioral studies, failing to yield novel results, limitation to pressing buttons inside of a tube whilst having to endure some rather heavy noise) can really make a creative psychologist angry. But there is more:

A brain scan is incredibly (for behavioral scientists) expensive; the costs for examining a single person can be something like several thousands of dollars. These costs restrict the number of subjects for the typical imaging study to something like ten to thirty persons; the claustrophobic atmosphere and noise will typically restrict sampling to healthy young adults; imaging noise induced by unwanted movement and other experimental error may further decrease the sample size when the respective images have to be excluded from further analysis. Of course, in behavioral experiments you will also have to exclude participants, but because of the lesser costs such loss is far less detrimental. In behavioral studies sample sizes are way larger for a reason (well, at least one): Statistical power. With a small number of participants like in the typical imaging study, scientists will only be able to substantiate the really obvious results — like that the prefrontal cortex is involved in higher order cognition — but the more detailed analysis could only be done with several times more of participants. You can not find out much about individual differences when you just look at a very small number of people, because when one or two out of twelve participants show a different kind of brain activity than the others, that deviation has to be attributed to experimental error. I would estimate that you could do 5 to 10 behavioral experiments (with sample sizes of 60-80!) for the money spent on a typical imaging study.

And despite all that, laypeople will still be more convinced by a crappy explanation when there is a coloured brain attached…

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