Links to previous posts:
Challenges in applying neuroscientific data to education.
Day 1: Basic architecture
Day 2: Single cell inspiration
Day 3: Reliable neuro-knowledge
Today I'll tackle what is probably the most common misinterpretation of human brain-imaging data.
It's almost irresistible to interpret brain imaging results as making visible and thereby, confirming , some abstract construct you use to account for behavioral data. By abstract construct I mean some entity that you've invented that's meant to account for data.
I suggest that there is a mental structure called a short-term memory system, which can store information for about 30 seconds. Short-term memory is an abstract construct; it's a proposed mechanism of the mind, which I think will help explain behavior.
Now it's clear that I've simply invented this idea of a short-term memory and that seems like a problem. "Oh people remember things for 30 seconds? That must mean you've got a remember-for-30-seconds mechanism in your mind!" I need something better to persuade people (and myself) that this entity actually helps explain how people think.
But now suppose I use functional brain imaging during that 30 seconds. I test 20 people and find that the same network of three brain areas is always active. Haven't I now seen short term memory in action? And doesn't this support my theory?
No and no.
Here's the problem. Finding activation is not interesting because mental activity is going to cause brain activity somewhere. Some part of the brain is always going to "light up" during a task. It proves nothing.
A more reasonable interpretation of my cafeteria study is this: people have brain systems that support vision, decision making, movement, spatial processing, etc. When given a complex task (e.g., cafeteria navigation) they recruit the appropriate systems to get the job done. The "cafeteria navigation system" is a dumb theory because it applies to just one task.
How do we know what the real brain systems are then, if "finding" them via brain imaging doesn't work?
Well, if we think systems ought to support lots of different tasks, that's a clue. This is a general desideratum of science, not particular to psychology. It's okay to make up theoretical entities that can't be observed if they can account for a lot of data.
Likewise, it's legitimate for me to propose something like "short term memory" if it's part of a theory that accounts for a lot of data. But the mere fact that some part of the brain is active during what I claim to be a task tapping short-term memory doesn't help my case. I need to show that "short term memory" helps to account for data.
So can brain imaging do anything to help verify that a theoretical construct is useful? Yes. It can serve as a dependent measure.
Here's a problem I face in persuading you that my proposed construct, short-term memory, is legitimate. I need to show that short-term memory participates in lots of tasks (so its not like the cafeteria navigation task). But how do I know that short term memory is at work during a task? Presumably there would be some sign in your behavior that it's at work. But in addition, if I've previously shown that three brain areas, A, B, and C, support short-term memory, then A, B, and C ought to be active during any task that requires short-term memory. Now I have a way of verifying that short-term memory contributes to a task, and that's useful to me, because one of my goals is to show that it's important in many different tasks.
Further, I can use this fact (A, B, and C will be active) to show that my theory of short-term memory is well developed. I can devise two tasks that look very very similar, but that I (with my terrific theory in hand) can predict differ in the extent to which they tap short-term memory. So one task will make the three areas active and the other task won't even though the tasks look very similar. Or I can devise two tasks that look wildly different but that my theory predicts both tap short-term memory and so will show overlapping activation in areas A, B, and C.
Tomorrow: A highly practical application, and the big wrap-up.