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To reach a decision, the brain processes information as it arrives — on the fly. But scientists didn't understand why a person would change their mind after making a decision. Michael Shadlen, a neuroscientist at the University of Washington in Seattle, and his colleagues have now discovered that a lag of several tenths of a second from the time a decision is made to the time the brain finishes processing all available information has a key role in the process. During this time, if new information in the brain's processing pipeline contradicts the original decision, a person is likely to change their mind (see page 263). Shadlen tells Nature more.

Under what conditions is someone most likely to change their mind?

When they're correcting an error they made in their original decision, and when the difficulty level of that original decision was intermediate. You might think you'd change your mind when the decision is most difficult, but here the processing pipeline doesn't furnish the contradictory information you need to reverse your initial choice. And when the difficulty level is low, you're confident in your first choice, so you don't change your mind then either.

How did this project come about?

My co-author Daniel Wolpert studies the computational principles that underlie the control of movement — for example, how we move body parts when we are uncertain about their position or goals. The mathematics involved in such studies are similar to those I use to explain inference, perception and decision-making. So we decided we'd spend a year working on connecting movement with decision-making. The change-of-mind study naturally followed; it's the cognitive equivalent of revising an action after initiation.

Where was the study conducted?

At Daniel's lab at the University of Cambridge, UK, while I was on sabbatical there. During my stay, I was appointed as a visiting fellow commoner at Trinity College. I didn't understand what a great privilege this was until Daniel described the benefits, including, 'You get to walk on the grass!'

Will your finding benefit human health?

It may help us to understand brain disorders. If you have leg weakness, neurologists can determine the nerves involved, how they control the muscles and so on. But if you can't follow a book or conversation, we don't know enough about the brain's normal function to understand what's going on. By studying relatively simple forms of decision-making, we hope to gain insight into principles and neural mechanisms of higher brain function.