Imagining an apple vs. a banana

I’ve written previously about my skepticism regarding the use of fMRI to localize a function in the brain. Multivariate pattern analysis (MVPA) is commonly used to detect subtle differences in patterns of brain activity in fMRI data.

This post articulated the misleading potential of MVPA really clearly.

In the new paper, Todd et al make a very simple point: all MVPA really shows is that there are places where, in most people’s brain, activity differs when they’re doing one thing as opposed to another. But there infinite reasons why that might be the case, many of them rather trivial.

The authors give an example of two similar tasks, imagining apples and imagining bananas. Although standard fMRI analysis might find no significant differences in activation, an MVPA might reveal one area in which the different tasks produced differing patterns of activity (differences in blood flow). At first, it might seem that the MPVA revealed the area of the brain responsible for encoding specific fruits, but this conclusion overlooks context. Every person has had different experiences with apples and bananas. Maybe the participant likes one and not the other. Maybe he ate a banana right before participating in the study and therefore has bananas on his mind. The potential contextual differences in experiences that any individual has had with the two fruits is endless, and would lead to an apparent difference in individual activation patterns for different tasks.

Apples and bananas are linked to different experiences and thoughts for everyone.  Image:
Apples and bananas are linked to different experiences and thoughts for everyone.

Here’s another way of looking at the hypothetical task (image below). For Participant A (the dark line), task A (imagining the apple) requires more concentration than task B (imagining the banana) does. For Participant B, the difficulties are reversed. Normally, these individual differences would cancel each other, demonstrating no significant difference between the brain activity when people imagine apples and the activity when they imagine bananas, but MVPA is designed not to allow individual differences to cancel each other out.

MVPA confound
Image from Todd et al.:

The authors of the original paper suggest that MVPA confounds can be controlled by classical Generalized Linear Model (GLM) analysis or by post hoc linear regressions, so  I don’t take this as a sign that MVPA or fMRI data aren’t meaningful, but more of a warning to be very cautious when interpreting them. Further, I take it as a reminder of the importance of context- we’re all such different creatures and our vastly different experiences inevitably color our cognition and behavior.

Brain Porn

This article by Sally Satel really hit the nail on the head for me by articulating some fMRI feelings I’ve had more eloquently than I could do myself. Brain scans are undoubtedly pretty:


And the idea that we might be able to understand what’s going on when we have certain thoughts and emotions and even to induce them is really seductive.

But it leaves out context, the most crucial ingredient in understanding the mind. fMRI scans are necessarily done in a lab, specifically in a really noisy, claustrophobic machine. Personally, most of the thoughts and feelings I have in life don’t occur in that environment. They occur in real-life situations, with other people, and in situations in which I’m not aware that I’m being scrutinized. Without a doubt, fMRI data teach us a lot about the human brain and some correlates of thoughts and emotions, but it’s not the single explanation for all that goes on in our minds, as many people wish and expect it to be. Satel writes that “mechanism is not meaning. The brain creates the mind through the actions of neurons and circuits, yes, but it cannot reveal its nuanced contents.”

If we want to truly understand the thoughts and feelings that make us human, we have to look beyond the pervasive pretty rainbow pictures of “brain porn” that may at first seem enticing, but in the long run won’t bring the satisfaction we’re looking for.