Michael Corballis: Mind Wandering

This is an extract from the talk on "Mind Wandering" Emeritus Professor of Psychology Michael Corballis gave at Brain Day 2013

Emeritus Professor of Psychology Michael Corballis

“You may think that there are other more important differences between you and an ape, such as being able to speak, and make machines, and know right from wrong, and say your prayers, and other little matters of that kind; but that is a child’s fancy, my dear. Nothing is to be depended on but the great hippopotamus test.”
from The Water Babies, by Charles Kingsley


In this extract, Kingsley intended the “great hippopotamus test” to be a satirical reference to the view, expressed by the distinguished anatomist Sir Richard Owen, that only humans possess a brain structure called the hippocampus minor. Owen thought that this disproved Darwin’s theory that humans share a common ancestry with apes. Kingsley, a minister of the church as well as a historian and novelist, was a friend and supporter of Darwin, and his scepticism was rewarded when T.H. Huxley showed that great apes also possess a hippocampus minor.

The hippocampus minor is now largely a relic of history, but its larger companion the hippocampus major, now generally referred to simply as the hippocampus, has assumed great importance in modern neuroscience. It is the hub of a large network known as the default-mode network, which is active when people disengage from the present and indulge in mind wandering. Brain imaging shows that the brain to be almost as active when people in the scanner are supposedly at rest as when they are engaged in a designated task. The activity during the resting state also covers much wider brain areas than when the mind is focused.

The hippocampus coordinates personal orientation in space and time. Brain-imaging shows it to be active when people recall past episodes and imagine future ones. If it is destroyed, the individual is effectively stuck in the present, unable to recall past episodes or imagine future ones. A striking case is the English musician Clive Wearing, an expert in early music who built up a distinguished musical career with the BBC; he was responsible for the musical content on Radio 3 on the day of the royal wedding of Prince Charles to Diana Spencer. In 1985, at the height of his career, he was stricken with a form of Herpes simplex (the cold-sore virus) that attacked his central nervous system, destroying his hippocampus. His plight is captured in the title of the book Forever Today, written by his wife Deborah. Although locked in a personal here-and-now, he can still play the piano and conduct a choir.

The hippocampus is also a cognitive map, coding one’s location in space. Spatial mapping is especially critical to London taxi drivers, who must decide the quickest route to a passenger’s destination immediately, without looking at a map, consulting a GPS system, or asking a controller by radio or cellphone. Brain imaging shows their hippocampi to be enlarged relative to those of London bus drivers, who follow fixed routes.

Even rats pass the great hippopotamus test. Recordings from so-called place cells in their hippocampi code where the rat is located in an environment such as a maze. But even when the rat is out of the maze, and either asleep or otherwise motionless, place cells are often active in fast “ripples,” sweeping out trajectories in the maze. These trajectories need not correspond to trajectories the animal actually took while it was in the maze. Sometimes they are the reverse of an actual trajectory, and sometimes they correspond to trajectories the rat never actually took. The rat, it seems, is mind wandering.

Mind wandering in humans, though, no doubt includes elements other than places. We construct episodes that include things, actions, emotions, people—even Jeanie with the light brown hair. We even wander into the minds of others. Mind wandering is the source of stories, imaginary tales of heroism, love, and death. Language itself may have evolved precisely so we could share the wandering of our minds.