An excerpt from The Mind’s Past, by Michael Gazzaniga
Starting in the mid-1950’s, although not publishing his results until the mid-1960’s, Benjamin Libet tweaked the scientific world with the question, “How long must a stimulus occur before we become conscious of it?”… Libet determined the time is five hundred milliseconds, which is half a second between when the stimulus starts in the cortex and the subject reports the presence of a sensation.
Yet we think we are almost instantly aware of stimuli and can react more quickly than half a second. When we are at our best, we can react within a tenth of a second or thereabouts. The concept here is tricky. We have to distinguish between when a stimulus can initiate cerebral processes that move the hand and body and when we become aware of the movement – two different time frames. A single pulse delivered to the skin can evoke an electrical impulse in the cortex in twenty milliseconds. But after that single pulse gets to the cortex, it takes up to five hundred milliseconds for us to become aware of the stimulus. Again, those early signals can begin to recruit other neural processes that might organize a motor response to withdraw from the stimulus. But we aren’t aware that the event is going on until about five hundred milliseconds have elapsed. Even though Libet found a half-second delay, we like to believe that we become aware of the stimulus earlier.
Libet’s research team came up with the idea that, for subjective time, we automatically refer the beginning of an event back closer to the onset of the stimulus. Think of it this way: If I sneak into your head and stimulate the part of your brain that represents your pinkie, you don’t feel the stimulation in your head. You spatially refer it to your pinkie, a full three feet away. Spatially referring events goes on all the time. Libet proposed that we refer the onset of an experience back in time, and he proceeded to test the idea – the sine qua non of good science. First he applied a half-second stimulus directly to the cortex, and then, four hundred milliseconds later, he applied a second single pulse stimulus to the skin. Amazingly, all subjects reported that they felt the skin stimulus first. Apparently there was no subjective delay for the skin stimulus. Libet carefully observed the cortical response to these two stimuli. There was no locally evoked response to the cortical stimulus train, but the peripheral skin stimulus produced a quick response in ten to twenty milliseconds.
When a stimulus is applied directly to the skin the neural message from there arrives in the cortex with a bang called the evoked response. All the neurons involved in transmitting the information from the periphery to the cortex sum up their action, and out comes the evoked response. But when a stimulus is applied directly to the cortex, there is no big bang, no signal heralding the response from the periphery. Stimulating the cortex directly eliminates this phase of the processing chain.
Libet proposed that this early signal from the skin becomes a reference point for a trick the brain uses to make our sensory experience seem like it occurs instantly. The brain refers the stimulus to the skin back to the quickly arriving local potential. Since that local potential occurs before the first stimulus (to the cortex) has gone through its necessary five hundred-millisecond sequence to become known to the conscious system, it seems to the subject that the second, skin, pulse is actually first. Now that is a big idea, so Libet ran a test on it.
He tested the idea by altering the experiment a bit. Instead of delivering one of the stimuli to the cortex first, he delivered it to the subcortical pathways – a neural system that leads to the cortex from the midbrain. Here, Libet was studying patients suffering from dyskinesias, a set of movement disorders. Part of a therapeutic attempt to help this disoder was the insertion of electrodes into their sensory pathways. This stimulus took a half-second to develop a raw feel, but it also had an early component, just like the peripheral skin stimulus. It, too, had an early marker to refer back to in judging which came first. As predicted, the subject now judged the real first stimulus as the first stimulus and the real second stimulus as the second stimulus.
Libet has provided us with an intriguing possible mechanism for explaining why we think we are doing in real time things that we have in fact already done. His second major observation builds on the work of Hans Kornhuber and Luder Deecke of Germany. They made recordings from the scalp and determined that a certain brain wave begins to fire up to eight hundred milliseconds before a self-paced movement is made. Using another method of recording, Libet determined that brain potentials are firing three hundred fifty milliseconds before you have the conscious intention to act. So before you are aware that you’re thinking about moving your arm, your brain is at work preparing to make that movement!
Libet’s work suggests that the brain doesn’t use the timing of its own firing to represent timing in the real world. When you think about it, this is clever and not too surprising. After all, impulses from the eyes, ears, and cheeks arrive in the brain more quickly than impulses from the distant big toe. We wouldn’t want a system that keeps us up to date on all information coming in through time, piece by piece. It would be weird to be aware that things happen in our head before our feet. So the brain makes all kinds of computations that determine when we experience things; we don’t just listen to our brain firing directly – any more than we think John is upside down just because his head is at the bottom of our retina. The only thing that counts is information about the world in the brain, not whether the brain resembles the world.
This research has raised many tantalizing questions that range across issues of volition, free will, the necessary cerebral events associated with conscious events, and so on. In the present context, however, it tells a significant truth. Major events associated with mental processing go on, measurably so, in our brain before we are aware of them. At the same time these done deals do not leave us feeling we are only watching a movie of our life. Because of temporal referral mechanisms, we believe we are engaged in affecting these deals…
Michael Gazzaniga is the Director of the SAGE Center for the Study of Mind at the University of California, Santa Barbara. He has published many books accessible to a lay audience, such as The Social Brain, Mind Matters, Nature’s Mind, The Ethical Brain, Human and Who’s in Charge? Free Will and the Science of the Brain. Dr. Gazzaniga’s teaching and mentoring career has included beginning and developing Centers for Cognitive Neuroscience at Cornell University Medical Center, University of California-Davis, and Dartmouth College He founded the Cognitive Neuroscience Institute and the Journal of Cognitive Neuroscience, of which he is the Editor-in-Chief Emeritus. Dr. Gazzaniga is also prominent as an advisor to various institutes involved in brain research, and was a member of the President’s Council on Bioethics from 2001-2009. He is a member of the American Academy of Arts and Science, the Institute of Medicine and the National Academy of Sciences. His new book is Tales from Both Sides of the Brain.
*All of the excerpts on my blog are from books that have stayed with me for some reason—because the concept was awe inspiring, changed how I view the world, was beautifully expressed, or all three. I personally curate all of the book excerpts, and I always obtain the author’s final approval before posting their work on my blog.