ERIC R. KANDEL is a biochemist and university professor at Columbia University. The essay below is taken from his recent book, In Search of Memory: The Emergence of a New Science of Mind.
Sigmund Freud emphasized at the beginning of the twentieth century that most of our perceptual and cognitive processes are unconscious—except those that are in the immediate focus of our attention—and that unconscious mental processes guide much of human behavior.
Freud’s idea was a natural extension of the notion of unconscious inference, proposed in the 1860s by Hermann Helmholtz, the German physicist turned neural scientist. Helmholtz was the first to measure the conduction of electrical signals in nerves. He had expected it to be as fast as the speed of light, as fast as the conduction of electricity in copper cables, and found to his surprise that it was much slower, only about 90 milliseconds. He then examined the reaction time—the time it takes a subject to respond to a consciously perceived stimulus and found that it was much, much slower than even the combined conduction times required for sensory and motor activities.
This caused Helmholtz to argue that a great deal of brain processing occurred unconsciously prior to the conscious perception of an object, that much of what goes on in the brain is not represented in consciousness, and that the perception of objects depends on “unconscious inferences” made by the brain, based on thinking and reasoning without awareness. This view was not accepted by many brain scientists, who believed that consciousness is necessary for making inferences. However, in the 1970s a number of experiments began to accumulate in favor of the idea that most cognitive processes that occur in the brain never enter consciousness.
Perhaps the most influential of these were carried out by Benjamin Libet in 1986. Libet used as his starting point a discovery made by the German neurologist Hans Kornhuber. Kornhuber asked volunteers to move their right index finger. He then measured this voluntary movement with a strain gauge while at the same time recording the electrical activity of the brain by means of an electrode on the skull. After hundreds of trials, Kornhuber found that invariably each movement was preceded by a little blip in the electrical record from the brain, a spark of free will! He called this potential in the brain the “readiness potential” and found that it occurred one second before the voluntary movement.
Libet followed up on Kornhuber’s finding with an experiment in which he asked volunteers to lift a finger whenever they felt the urge to do so. He placed an electrode on a volunteer’s skull and confirmed a readiness potential about one second before the person lifted his or her finger. He then compared the time it took for the person to will the movement with the time of the readiness potential. Amazingly, he found that the readiness potential appeared not after but 200 milliseconds before a person felt the urge to move his or her finger! Thus by merely observing the electrical activity of the brain, Libet could predict what the subject would do before the subject was aware of having decided to do it.
These experiments have caused philosophers of mind to ask: If the choice is determined in the brain unconsciously before we decide to act, where is free will?
Are these choices predetermined? Is our experience of freely willing our actions only an illusion, a rationalization after the fact? Freud, Helmholtz, and Libet would disagree and argue that the choice is freely made but that it is made without our awareness. Libet, for example, proposes that the process of initiating a voluntary action occurs in an unconscious part of the brain but that just before the action is initiated, consciousness is recruited to approve or veto the action. In the 200 milliseconds before a finger is lifted, consciousness determines whether it moves or not.
Whatever the reasons for the delay between decision and awareness, Libet’s findings now raise the moral question: Is one to be held responsible for decisions that are made without conscious awareness?