Tuesday, July 17, 2012

A trick question: What has functional neuroimaging told us about the mind?

I'm not actually going to try and answer the question posed in the title, which is taken from Coltheart's (2006) legendary critique of functional neuroimaging in a special issue of the journal Cortex. To summarize, Coltheart concluded that, no, functional neuroimaging hadn't told us anything about the mind so far; and he challenged others to prove him wrong. Others have taken the bait and made heroic and important efforts to meet Coltheart's challenge. Rather, I'm simply going to question the question. Because it's a tricky question. Indeed, it's a trick question.

This may seem obvious and elementary, but to answer Coltheart's question one first has to know what his question means. And the critical word in his question that we need to define is "mind". What has functional neuroimaging told us, he asks, about the mind.  As good reductionists, we might say: "wait, the mind is the brain, they denote the same object". The morning star is the evening star. The two are synonymous. So, substituting, "brain" for "mind", we rephrase the question as follows: "What has functional neuroimaging told us about the brain".  And then the answer is trivial, because novel information about the brain gotten from functional neuroimaging answers the challenge. Case closed.

If only it were so easy. In fact, when Coltheart uses the word "mind" he's not talking about the "brain". He's talking about something else. Is the mind a thing? Or is it an idea? Can we touch it? Can we define it?  Although Coltheart uses the word "mind" 11 times in his essay, he never actually provides a definition. I'm not going to try and define it, either.

If we look elsewhere in the paper for clues about what is meant by "the mind", however, we find that Coltheart is really concerned with psychological theories and the ability to adjudicate between them. But what are psychological theories about? 

From the first paragraph of the Introduction (emphasis added):

There are numerous different reasons for doing [functional neuroimaging]. I will consider only one of these reasons, namely, to try to learn more about cognition itself.

And then:

Although there exists a huge volume of recent literature reporting the results of cognitive neuroimaging studies, there are surprisingly few papers which have evaluated this technique as a way of studying cognition itself.

OK. We can infer that psychological theories are about cognition itself. And we can further infer that "cognition itself" is separate and conceptually distinct from the "brain". And so what is the "mind"? It's cognition itself. It's not the brain. It's apart from the brain -- it's itself.

Another hint comes further down on page 1 (emphasis in original):

My paper, like Henson’s, is concerned solely with the impact of functional neuroimaging on the evaluation of theories that are expressed solely at the psychological level.

So the mind is akin to cognition itself, and cognition itself is described solely at the psychological level. In other words, don't mix up the brain with cognition itself. Stay at the psychological level. Any reference to the brain in a pure psychological theory is verboten. That would be mixing levels. Mixing metaphors with molecules.

The question, then, is whether functional neuroimaging can adjudicate between competing psychological theories that are about cognition itself. And what sort of predictions do these (pure) psychological theories actually make? They make predictions about behavior. Coltheart uses as an example two theories of reading that posit serial or parallel processing, respectively. For instance, in serial processing:

When a word contains an irregular grapheme-phoneme correspondence, the later in that word that correspondence is the less the word’s reading-aloud latency will be affected by its irregularity.

Here, the dependent variable is a measure of response time, or latency, and whether it depends on spelling irregularity.  In all of Coltheart's examples of the predictions made by psychological theories the dependent variable is always a behavioral measure (i.e. reaction time, accuracy, etc.) and never a brain measure.

But that follows perfectly from Coltheart's stipulations. Theories are expressed at the psychological level. They don't make reference to the brain. And because they don't make reference to the brain, they don't make predictions about the brain.  And because they don't make predictions about the brain, ipso facto, functional neuroimaging cannot adjudicate between said theories.

Coltheart was right!

But Coltheart's conclusion is not an empirical one, based on an evaluation of the functional neuroimaging literature. It is simply axiomatic. He defines a psychological theory as that which does not refer to the brain ("expressed solely at the psychological level") and which makes predictions about variables (reaction time, accuracy) that cannot be measured by functional brain imaging. So we know, a priori, that functional neuroimaging cannot tell us anything about these particular psychological theories. And all the articles in the special issue of Cortex that attempted to meet Coltheart's challenge were doomed to failure, a priori, on the basis of a simple logical deduction.

Coltheart was right, but it was a trick question.

Suppose that we loosen up Coltheart's definition of psychological theories to allow for those that make contact with the brain? And if such theories make predictions about states of affairs in the brain, then, guess what, all of sudden functional neuroimaging can adjudicate between two competing psychological theories that make different predictions about brain activation.

And is there any reason a psychological theory, other than for reasons of ideological purity, should not make contact with brain? Or is that some sort of contradiction? Can psychology be mixed with neuroscience? Isn't that what's called ..... cognitive neuroscience?

My answer is, of course, that "psychological theories" can certainly make reference to the brain. Indeed, such theories can be fundamentally neuroscientific. They may borrow concepts and terminology from multiple traditions of inquiry, including cognitive psychology, neurology, neuroscience, psychiatry, neuroanatomy, genetics, physiology, sociology, economics, and so on.  Buchsbaum & D'Esposito (2008) made a similar point in a book chapter a few years ago and I'll quote it before a brief conclusion:

A hypothetical philosopher of metaphysics might ask the question: ‘What has physics told us about metaphysics?’ to which he might answer that because metaphysics is the science of the non-physical, physics by definition has nothing to say about metaphysics. Unlike metaphysics and physics, however, most would agree that the study of the mind and the study of the brain are fundamentally related if not, indeed, one and the same endeavour. There is therefore absolutely no reason why psychological theories should not refer to and make explicit predictions about brain function, nor is there any reason to think such theories would, upon making contact with neuroscience, somehow cease to be ‘psychological’. 

To conclude, in 2006 Coltheart asked whether functional neuroimaging had told us anything about the mind. He concluded "no". There have been many earnest  and technically innovative efforts over the years to persuade he and other skeptics otherwise. Cognitive ontologies, Bayesian probabilistic reverse inference, forward inference, reverse inference, structure-function association, etc. etc. etc. All of this stuff is fantastic and welcome and is enormously useful to cognitive neuroscience. But none of it answers Coltheart's challenge, in the way it was framed. Because it's impossible. The game was rigged.

So here's my answer: we simply must concede the point. No, functional neuroimaging does not and cannot adjudicate between theories expressed solely at the psychological level that make no predictions about the brain. How could it? It was a trick question all along.