It is clear that consciousness is central to understanding human mental processes, and therefore is the holy grail of modern neuroscience. What is less clear is that much of our mental processes are unconscious and that these unconscious processes are as important as conscious mental processes for understanding the mind. Indeed most cognitive processes never reach consciousness.
As Sigmund Freud emphasized at the beginning of the 20th century most of our perceptual and cognitive processes are unconscious, except those that are in the immediate focus of our attention. Based on these insights Freud emphasized 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 the speed of light, fast as the conduction of electricity in copper cables, and found to his surprise that it was much slower, only about 90m sec. He then examined the reaction time, the time it takes a subject to respond to a consciously a perceived stimulus, and found that it was much, much slower than even the combined conduction times required for sensory and motor activities.
This caused Helmholz to argue that a great deal of brain processing occurred unconsciously prior to conscious perception of an object. Helmholtz went on to argue that much of what goes on in the brain is not represented in consciousness and that the perception of objects depends upon "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 experiments were those 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, Libet 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 a person would do before the person was actually aware of having decided to do it.
These experiments led to the radical insight that by observing another person's brain activity, one can predict what someone is going to do before he is aware that he has made the decision to do it. This finding has 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 for what has happened? Freud, Helmholtz and Libet would disagree and argue that the choice is freely made but that it happens without our awareness. According to their view, the unconscious inference of Helmholtz also applies to decision-making.
They would argue that the choice is made freely, but not consciously. 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?