A Picture of Not Thinking

One reason that I'm so riveted by neuroscience is the way it can blow the lid off of philosophical conundrums that have dogged Western thought for centuries. Case in point: in a recent study, scientists at Dartmouth asked subjects about something that was on their mind--an exam, a girlfirend, and so on. Then, while scanning their brains with an MRI machine, they told their subjects NOT to think about that thing. We're all pretty comfortable with the idea that thoughts are the product of neurons, electrical impluses, and neurotransmitters. But if that's all that thought is, then what (or who) suppresses those thoughts? This is a paradox that has bedeviled Western thought for centuries. Neuroscience has its roots in the scientific revolution in the 1600s, when natural philosophers set out to reinterpret the world as a machine. Rene Descartes saw the brain as a set of tubes, strings, and pulleys. His anatomy was still stuck in medieval misconceptions, and it would be for another 30 years until it was set right by the English physician Thomas Willis, who's generally considered the founder of modern neurology. But both Descartes and Willis got tangled up in a paradox. On the one hand, they saw movement, memory, and lots of other faculties of the mind as mechanical changes in particles of the brain. But when it came to something like the will--the ability to control your own thoughts, for example--they hung up their scientific spurs. That had to be the result of an immaterial, rational soul above the laws of nature. Only a soul could be the source of something like will; nature, after all, was purely passive. (This is one of the themes of my upcoming book, Soul Made Flesh.) Research like this study from Dartmouth (in press at Neuropsychologia) can help resolve this material/immaterial paradox. They found that one key region in the brain becomes more active when you're trying to keep a particular thought out of your mind as opposed to just thinking in an ordinary way. It's known as the anterior cingulate cortex (ACC), located in the cleft of the brain. A lot of earlier research has suggested that the ACC is a conflict monitor, seeing how well the brain's outputs match up with its goals. If the goal is to not think of a girlfriend, thinking about her rouses the ACC. Earl Miller at MIT and Jon Cohen at Princeton have proposed a model for what happens once the ACC sense a conflict. They argue that it sends signals to the front of the brain, known as the prefrontal cortex. The prefrontal cortex is basically a very complicated set of track switches. Signals coming into the brain activate neurons in pathways leading from input to output--some kind of action, in other words. The neurons in the prefrontal cortex mingle their nerve endings with every step in these pathways, and they can boost some signals while squelching others. In the process, they can produce a particular output for any given input. If everything is working smoothly--if inputs produce the right outputs--the prefrontal cortex is pretty quiet. But when the ACC kicks in, the prefrontal cortex neurons responds by boosting and squelching signals in new ways until they get rid of the conflict. In the case of the Dartmouth study, unwanted thoughts appear to switch on the ACC, triggering a rearrangement of the prefrontal cortex until the thought goes away. The Dartmouth researchers put a twist on this study that was very interesting: they then told their subjects not to think of anything at all. The ACC became active again, but so did other regions, most notable a strip on the side of the brain called the insula. The insula is known to be associated with pain and distress. It's possible that suppressing thoughts so strongly is hard work, and the insula represents the struggle. This study may help in a practical, psychiatric way, by shedding some light on what happens when unwanted thoughts intrude on our minds uncontrollably. But it also has a side benefit of a more philosophical streak. When the Dartmouth scientists put their subjects in a scanner and told them not to think about an exam or a girlfriend, the images they took didn't show some homunculus crouching in a ventricle of the brain, issuing commands. ("You there--you thought! Disappear immediately!") Instead, they saw evidence of a network continually reconfiguring itself.

A Picture of Not Thinking

Discover how neuroscience reveals philosophical conundrums about thoughts and the anterior cingulate cortex function in this enlightening article.

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Carl Zimmer

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