The Chemistry of Logic

I am older than Maggie, by a couple years. If you know this, and you also know that Maggie is slightly older than Susie, you probably don’t even need a pencil and paper to figure out that I am older than Susie. It’s about as simple an exercise in logic as you’ll find. Yet it is nevertheless an exercise in logic, and as such requires reasoning from certain given facts to a novel conclusion.

Psychologists are intrigued even by such rudimentary problem solving, because understanding its cognitive underpinnings could conceivably illuminate higher forms of reasoning—the thinking needed to split an atom, for example, or fix a leaky spigot. The ultimate scientific goal is to pinpoint exactly where such fundamental abilities reside in the brain’s complex tangle of neurons.

The prevailing theory about logical thinking is that it takes place in a tiny seahorse-shaped brain structure called the hippocampus. That’s because reams of research studies over decades have pinpointed the hippocampus as the seat of memory, and even the most simplistic problem requires memory. It’s obvious I’m older than Susie—just look at us--but in order to know this logically you must learn, store and recall knowledge of two relationships: mine with Maggie, and Maggie’s with Susie. Since I don’t have a relationship with Susie, there’s no other way around it.

Or is there? What if it’s not that simple? A growing number of psychologists have been thinking, from an evolutionary point of view, that it may not make sense to have only one kind of logic. Isn’t it likely more that, as the brain evolved over eons, it developed some sort of backup problem-solving machinery, just in case memory and logic fail us? University of Arizona psychologist Michael Frank is among those who think so, and he decided to test the idea in the lab. Indeed, he went a step further, suggesting that failed memory might actually enhance a parallel, highly intuitive kind of problem solving located elsewhere in our gray matter.

To explore this possibility, Frank and his colleagues recruited a couple dozen healthy young people, and temporarily but profoundly disabled their memories. They did this with a powerful drug called midazolam, one of a class of sedative drugs that acts on a key chemical in the hippocampus, causing amnesia. This procedure effectively made memorization impossible.

Once Frank had shut down the participants’ working memory, he gave them a series of problems to solve. The quiz is complicated, but it resembles the simple logic exercise above. That is, the participants could solve it the old-fashioned way, by remembering a few facts and splicing them together. Or—this is what Frank speculated—they could perhaps come to the same correct conclusion without memory, using “gut level” emotional loading of the facts. Such intuitive learning of one fact over another actually has to do with surges of the brain chemical dopamine, in a cluster of neurons called the basal ganglia.

And this is precisely what the study showed. As reported recently in the journal Psychological Science, the induced amnesia diminished performance on logic tasks that required explicit memory, but it enhanced performance on tasks that could be accomplished by dopamine-driven gut level learning alone. These results support the idea that the hippocampus and the basal ganglia make distinct contributions to problem solving, and further that failure of the hippocampus—the first responder, if you will—triggers the response of the basal ganglia, which performs admirably in its backup role.

What all this suggests, as a practical matter, is that when explicit memory begins to falter--with aging, for example--it is probably okay to trust your feelings when it comes to everyday decision making. A dull memory equals sharp intuition. This is certainly good news for me, and my gut tells me it’s good news for Maggie and Susie as well.

For further insights into the quirks of human nature, visit the Association for Psychological Science website at www.psychologicalscience.org/onlyhuman.