Christof Koch:

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German neurobiologist Christof Koch has written a book that not only summarizes progress so far made in understanding the physical events in the brain that correspond to consciousness, but also advances the equivalent for the "mind sciences" of Descartes' "scientific method".
By focusing on practical experiments rather than theoretical speculation, Koch's contribution to framing of the problem and plotting a course is significant.
He doesn't know where consciousness is created in the brain, and doesn't know what causes the feeling of consciousness, but he knows enough of how the brain works to advance working hypotheses on how to find out.
So, first of all, the scope of the book is to explain the relationship between consciousness and the neural activity in the brain. One of the first clues (although apparently disconcerting) is that much of the neural activity of the brain is not conscious at all: we are not aware of most of what our brain does. Most of the time (e.g., habits, instinct, etc) the brain makes key decisions without "us" being conscious of those decisions.
Koch and his research partner Francis Crick (who is not listed as co-author but is credited throughout the book) focus on the "neuronal correlates of consciousness" (NCC): the "thing" in the brain that corresponds to states of awareness. Because most neuronal activity does not yield a state of awareness, they are led to believe that multiple forms of neuronal activity exist (this is almost a tautology). They believe that one could potentially be conscious of many competing views, but only one "wins" the competition and results in awareness of the corresponding view.
The first clue to find these NCCs is the way they represent the external world. All body cells are, to some extent, influenced by what happens to the body, but only a minority of the body cells represent external stimuli in an "explicit" manner: Koch and Crick believe that one is only conscious of features that are encoded "explicity" by some neuronal assembly.
The other way to find them is to listen to the way they oscillate. The electric potential of the brain as a whole exhibits oscillatory behavior in different frequency bands: the dominant rhythm for resting individuals is in the "alpha" band (8-12 Hz); the rhythm for normal cognitive activity is in the "beta" band (15-25 Hz) or, for more complex operations, in the "gamma" band (30 Hz or higher); sleep is in the "delta" band (1-4 Hz). Each of these "oscillations" is caused by some synchronous behavior ("firing") of many neurons.
The "binding" problem is the problem of how the various features are integrated in the brain into the perception of the object as a whole, especially when the same brain is integrating other features of other objects. Christoph von der Malsburg originally proposed that synchronization could be the solution to the "binding" problem: the neurons working on one object are synchronized, and they are not synchronized with other populations of neurons that are working on other objects.
There is one oscillatory behavior by neurons that seems to be associated with awareness, and it is in the 30-70 Hz range, with a peak around 40 Hz: Koch and Crick claimed in 1990 that this oscillation accounts for consciousness (the set of those synchronized neurons "is" the NCC for the current state of awareness).
Koch and Crick believe that several such "coalitions" of neurons exist at every point in time, and a sort of Darwinian selection determines which one (and only one) wins and results into awareness.
Another clue to finding the NCC is the cholinergic system: consciousness only occurs when there is an adequate supply of acetylcholine neurotrasmitters, which are regulated by the brainstem (people whose brainstem is damaged lose consciousness).
The brain has a convoluted structure, and the way it represents an experience is even more convoluted, but we perceive an experience as a sequential of events. Koch thinks this has to do with the fact that, at every point in time, only one coalition is the winning one. It may change all the time, but we perceive an ordered sequence of events, because every other coalition that is active at the time is suppressed. We don't perceive the convoluted activity of the brain, that is analyzing an overwhelming amount of data, but only those events that correspond to the winning coalition.
Koch elegantly divides short and long term memory based on the underlying mechanism: long-term memory is caused by a physical rewiring of the brain (strengthening of connections), whereas short-term memory is caused by a sustained firing pattern by an assembly of neurons. Koch proves that consciousness depends on the latter, not on the former. Short-term (or, better, working) memory could provide a sort of "Turing test for consciousness": any being that displays a working memory is likely to be conscious.
Koch also provides an answer to the mystery of why we are conscious at all. After all, we don't need consciousness: the brain makes most of the key decisions in an unconscious way. The autonomic system directs the organs to do their job, and "instict" helps the body survive. In his opinion, qualia (the qualitative aspects of things) are "symbols" that help the brain synthesize, summarize, huge amounts of data. Seeing red or feeling pain are shortcuts to handle huge amounts of sensory data. Qualia are symbols that summarize the state of the world (including the body itself). This is the "executive summary hypothesis".
Koch believes in a nonconscious homunculus, residing in the front of the forebrain, that handles the information stored in the back of the cortex (the sensory regions) and that does all of the "thinking" (i.e., takes all the decisions). The front of the cortex is looking at the back. This homunculus is beyond consciousness, the same way that automatic, zombie-like behavior is beyond consciousness, although one is "above" it (supramental) and the other is "below" it (submental). Consciousness is an intermediate level, which is conscious not of the homunculus and its work (its "thoughts") but only of representations of the homunculus' work in the form of inner speech.
Thus our consciousness resides at an intermediary level. We are not conscious of the homunculus that is making decisions for us, and we are not conscious of the real world. We are only conscious of our mental representation of the world.
Koch does not completely clarify the relationship between the qualia (the symbols) and this homunculus. It sounds like it is the homunculus that uses the qualia to analyze and plan, i.e. to "think". But it is not clear why it needs qualia (since it is a neuronal area that has direct access to the neuronal activity), and how the qualia are consistent with the assumption that the homunculus is not conscious.
The only area that Koch's book does not treat in detail is the story about neurotrasmitters. Other than a short introduction to the cholinergic system, Koch does not analyze the reason that so many different kinds of neurotrasmitters exist, and how they coexist.
One also wishes that Koch had mentioned the fact that, as far as we can remember, we are not born conscious. We become conscious a little bit at a time, although the brainstem and everything else needed for Koch's theory of consciousness is already in place. Have we just forgotten that we were conscious from the first moment we were born, or is there something else that has to happen in order for a human brain to yield consciousness, something that happens later in the child's development?