By Peter M. Vishton, Ph.D., William & Mary

Edited by Kate Findley, The Great Courses Daily

“I think, therefore, I am.” Are you sure? Join us as we chat with Dr. Peter M. Vishton, Ph.D., about the role of the conscious mind in decision-making.

What Brain Surgery Reveals about the Conscious Mind

Common culture would have you believe that decisions come from the conscious mind, but in the late 1970s, neuroscientist Benjamin Libet’s groundbreaking work proved otherwise.

Libet’s studies dealt largely with patients who were undergoing brain surgery. For many brain surgeries, patients remains awake and are aware throughout much of the procedure. 

When the brain itself is touched or even cut, there’s no sensation of pain. As such, it can be useful for the surgeon to have the patient awake as the procedure takes place.  

You may have seen this in the recent video of a patient playing the violin during brain surgery. By stimulating an area of the brain and observing the patient’s response, the surgeon can be sure about what an area does before making any cuts.

While studying the somatosensory cortex, a region near the top center of your brain that registers touch, Libet discovered that stimulating this area would induce a “phantom touch” experience in participants, even when nothing had physically touched them.

Libet’s experiments further suggested that, in order for a patient to become conscious of something, 500 milliseconds—half a second—of brain activity had to be produced. A brief tap on the hand would produce activity in the brain, but if that signal was in some way interrupted before 500 milliseconds had elapsed, conscious awareness of it would not occur. 

Note here that we can become conscious of events that are far shorter than 500 milliseconds in duration. If a flash of light lasts only 10 milliseconds, you’ll see it and be conscious of it, but when you do, it will be mediated by a burst of neuronal activity at least 500 milliseconds long.

Libet’s Famous Dot Experiment

This timing issue occupied many years of Libet’s research. In 1983, he conducted one of the most fascinating and influential studies of the human brain to date. The design of the experiments was simple. 

Participants were to watch a dot move around a clock face which was numbered one to 60. From time to time, participants would flex their wrist. It was key that they made no decisions but just watched the clock face, and when they felt like it, flexed their wrist.

Scientists had long thought this process began with a conscious decision to move, and once that decision is made, a signal would be sent from some part of the brain to the motor cortex. From there, the signal traveled to the muscles in the forearm that control the flexion of the wrist. Libet’s work altered this thinking forever.

When we perform a movement like a wrist flexion, the motor cortex builds up activity for about 500 milliseconds prior to the onset of an action. When that buildup peaks, the signal is sent to the muscles, and the hand moves. 

But participants in the study only became aware of their decision to act about 200 milliseconds before the action occurred.

Does Your Mind Move Your Muscles?

During the study, participants were wired to an electroencephalograph to record neural activity. A separate set of sensors recorded when the muscles were activated and when the movement of the arm occurred.

After each movement, the participant would report when they first became aware of the intention to act based on where the dot was on the screen at that moment. 

A typical participant experience might be that participants decided to move their wrist when the dot was next to the number 10 on the clock. About 200 milliseconds later, say when the dot was next to the number 15, the hand would actually move. 

But the readiness potential associated with the action occurred earlier than when the dot was by the number 10. By the time the dot was by the number five, as much as 300 milliseconds o.

“The conscious mind, it seems, isn’t driving the decision at all,” said Professor Vishton. “Your conscious experience of making a decision follows an unconscious process that’s actually in control.”

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Dr. Peter M. Vishton is Associate Professor of Psychology at William & Mary. He earned his Ph.D. in Psychology and Cognitive Science from Cornell University. Before joining the faculty of William & Mary, he taught at Northwestern University and served as the program director for developmental and learning sciences at the National Science Foundation.