fMRI Subraction Analysis and Why it Matters

Accessibility:  Basic

Let’s say you wanted to do an experiment about color processing. We could do the following:

1. Roll someone into the scanner.
2. Show them two colors
3. Have them press the button corresponding to the color they prefer.
4. Look at the resulting activations, and voila, we have the “color preference area.”

But it’s not that simple. The brain is very active, even when supposedly at rest. While performing the task described, the subject is also breathing, processing ambient noise, thinking about grocery shopping, as well as who knows what else. How do you tell what activation is due to the color judgment, and what is due to other processes?



The traditional fMRI solution is to compare activation with a baseline condition. For our example experiment, we may want a comparison condition where the subject sees the same images, but presses a random button rather than picking a color. We then take the activation from this comparison condition and subtract it form the condition we’re interested in. The assumption (and it’s an assumption, meaning that it may not always be true) is that we’re subtracting out irrelevant brain activation– for example, brain activation due to seeing colors, pressing buttons, being inside a scanner, etc.

This is important to keep in mind when evaluating fMRI results. If someone tells you that brain region X is active during a certain task, you always want to ask what the comparison condition is. If region X is active during task Y, but the comparison condition is super simple (say just laying there in the scanner, for example), that’s not very impressive – lots of other regions will be active in that comparison, and it may not be simply due to that task.