What Is Consciousness? | Christof Koch

Christoph, I did my doctorate in neurophysiology, got it some more than 40 years ago, and I've lived in Pasadena for many decades. And certainly combining those two, I would have never imagined that a professor at Caltech would be working on consciousness. One would have thought that that would have been a really an oxymoron, something that would be impossible. What is it about consciousness that allows a professor at Caltech to do such work? Consciousness is a central aspect of my life. As Rene Descartes in the most famous deduction in Western philosophy said, essentially modern language, I am conscious, therefore I am. So I think it's a legitimate subject of scientific inquiry. If we really want to have a comprehensive view of the universe, we have to account for consciousness, given its central aspects. There has been a lot of progress over the last two decades in this sort of nascent science of consciousness. Let me give you one simple example. So 15 years ago, Francis Crick, the co-discoverer of DNA, and with whom I worked for 15 years, he was profoundly interested in consciousness, and we made a prediction. We said that the primary visual cortex, so a cortex is this sheet of neurons at the top of our brain. It's really essentially for our language and intelligence and perception. Consciousness is divided into many different regions, perhaps at least 100 different regions. The best understood one is the one at the back of the brain. It's called primary visual cortex. It's a terminus for the optic nerve. So essentially the information, the visual information leaves our eyes and through a relay station in the thalamus goes to the back of the head. The area is roughly as big as a credit card. It's clearly involved in visual perception, and I can stick you in a magnet, and when you're looking at something, this part of the brain lights up. But now you can ask a question, to what extent are the neurons in this part of the brain actually responsible for generating visual consciousness? And we hypothesized at the time, so 16 years ago, that they are not directly responsible for generating consciousness. And the language that people use, it's that a primary visual cortex is not a neuronal correlate of consciousness. This turns out to be true. There's a number of experiments recently. Why did you first think that? Well, we had a specific idea about the function of consciousness that relates to planning, and we asked where are the planning stages of the brain. They're in the front of the brain, called prefrontal cortex, and there are essentially no neurons in the back, in the visual cortex, that directly project to the planning stages. So we made this sort of very bold hypothesis that primary visual cortex is not part of the mechanism that gives rise to conscious sensation. So it turns out that the evidence seems to be in favor of that. Whether it's for the reasons we advocated, we don't know. But you can now do beautiful experiments. What you can do, you can put people in a magnet. This has also been, similar experiment has been done in monkeys, where you can actually put microelectrodes in, where you can directly look at the neurons, rather than indirectly listen to the blood flow evidence. And you can show that whether or not, so the subject is looking at something. It's a sort of a complicated experiment where the person's looking at something, but sometimes the person is seeing it, and sometimes it's not seeing it. It's similar, you go to a magic show, when the magician makes things disappear in front of your eyes. Very often, the things are actually still there, but you're not looking at them, because he misdirects you. He does this, and you look at the hand, and he quickly makes this hand movement to make something disappear. But if you pay attention, you can see, you can actually see the thing that he's trying to make you not see. So in a sense, you can do the same thing in a magnet, where you're looking at something, but not seeing it. And what you can show, that whether or not you attend to something makes a big difference to the neurons in primary visual cortex. But whether or not you consciously see it makes no difference to the fMRI signal, to the signal in primary visual cortex. In other words, yes, primary visual cortex is involved in processing and taking in that visual information. And if you attend or not attend, makes a difference to the neurons there. But whether you're conscious or not doesn't seem to be the job of neurons in primary visual cortex. That seems to be this conscious experience I have when I see you, or when I see this red of this table, that seems to be generated in a different part of cortex. So it seems then that consciousness is generated only a small part of the brain? We don't know how small. I mean, it may turn out to be that the total part of the brain that's involved may be large. It may only be at any given point in time, a small number of neurons. It may well be that right now, when you're conscious of my voice, only 1% or maybe a thousandth of a percent of neurons are directly responsible for generating this conscious percept. This is new information, because certainly when I was studying the brain, primary visual cortex at the back was assumed to be conscious, because if you had trauma or injury there, you lost the ability to see. And it was a rough kind of analysis, but it seemed to be strong at that point. Yeah, and that's still true. If I lose my eyes also, of course, I won't see, but nobody really believes that the eyes are where my visual consciousness happens. And so, yeah, it's genuine progress. And so here you have at least, so A, the general point is not all parts of your brain are equally important for consciousness. Some parts of the brain have a much more privileged relationship with consciousness than others. And B, that you can make genuine conscious progress on these ancient questions. You're not condemned forever to sit around and do your armchair and do philosophy, that's correct. Yeah, so you have a theory in terms of brain circuitry that deals with consciousness. How does that work? It involves the prefrontal lobes, but it has some interactive circuitry. Does it involve the thalamus in the lower part of the brain or different parts of the cerebral cortex? So I wouldn't call it a theory, it's a model. And the model makes several assertions. A, thalamus is essential. It's sort of, you can think of it as the organ of attention. You know, there's vastly more information out there in the world that I can take in. Of course, we attend, we do it all the time. We take a subset of information and that's a part of the information that we're attending to. And then, yeah, Francis Crick and I believe that you need this feedback. You need access to structures in the front of the brain and then in turn feedback to a higher level visual structures in infratemporal cortex specifically, maybe in posterior parietal cortex. And then in this, that this loop by itself will then be necessary and ultimately sufficient to give rise to conscious sensation. What type of data backs that up? Well, so there was recently an experiment where people showed, and you have two classes of gravely injured brain damaged patients. One is their so-called vegetative, in a vegetative state. You might remember Terri Schiavo, you know, more than a decade ago in Florida who was in the state for 14 years. In other words, she's alive, the patient opens her eyes, sometimes she moans or, you know, moves around like this but there's no any behavioral evidence for consciousness, it's more like a reflex. The brainstem reflexes are still there. Some of these patients are, they're called MCs, minimal conscious state. They may sometimes be conscious or they may all the time be conscious but they only have very, very limited means of communicating because they are so severely injured. And so recently there was an attempt to try to differentiate between the two. You know, if I look at the brain basis, can I, because very often in clinical practice it's not easy to differentiate between these two types of patients. And it's not just one or two. In the U.S. alone, people have estimated there may be roughly 20,000 of these vegetative state patients. It's a great tragedy for everybody affected. And these experiments suggested that if you don't have that patient, if they don't have an intact feedback from the front of the brain to the back, then they're in a vegetative state. While if they do have it, then they tend to be more in a minimal conscious state. That's sort of evidence in favor of this hypothesis.