In evolutionary terms, why, oh why, do we pay the enormous biological cost of consciousness? Biological costs imply biological functions.
What is the cost? The human brain uses 20% of the body’s oxygen intake and 25% of its glucose demand. In the depth of anesthesia it consumes half as much, still 10-12%, a sizable part of the total energy budget. The other 50% is used for actual mental work as opposed to just maintaining life in the brain. But in ordinary conscious operation the brain uses nearly the same energy whether it is concentrating on a task or resting, the amount of work does not seem to be that important to the use of energy. Therefore, it is safe to guess that maintaining consciousness itself is an important fraction of this other 10-12% of the body’s total energy use. Let us say that in the region of 5% of the body’s energy consumption goes towards the process of consciousness. That is a high price in biological terms and would certainly be eliminated in evolutionary history if there were not an important function, or functions, being ‘paid for’.
So what is the function of consciousness? On the surface it appears to be a frill. Sleep walking shows that consciousness is not that necessary for normal activity over short periods. A person can get up from bed, get dressed, go outside, get in their car and drive it without accident and without waking or being aware. Well learned actions and responses do not need consciousness for their execution – so the important function must be more than just routine living. The reasons for consciousness are not known but there are a number of candidates, any or all might be important in explaining why we have consciousness. This is the mystery that attracted me.
This is the first of a series of posts on possible functions of consciousness.
My original idea was that consciousness was part of memory. And I remember the time in the late 70′s when that idea came to me. I thought how do I know I am conscious and the only answer I found was that I remembered what came before ‘now’. If I had no reason at the time to question the continuity of my memory then I was and had been conscious. This seemed to me a good hint that there might be a connection between consciousness and memory. There had to be a way to form a model of reality that could be the source of material for memory and, equally important, there had to be a model structure that could hold the recalled past or imagined future in the same form that is used for the present.
The way I saw it then, I wrote down a couple of years later, emphasized the importance of remembered experience to cognition and learning. Other ideas have been added later but I still associate consciousness with memory. Here is some writing from 30 years ago:
“The simplest nervous system amounts to a number of sensitive sensory neurons contacting a number of effecting motor neurons, to give a fixed set of reflex responses to environmental change. The sensory neurons notice certain changes in the environment and trigger particular motor neurons to respond to the changes. The only learning that is possible in this structure is habituation, the temporary loss of a reflex when it is triggered at a high frequency. A more flexible behavior is possible when inter-neurons are interposed between the sensory set and the motor set. With small numbers of inter-neurons, the behavior is still reflexive but the inter-neurons allow more complex and discriminating patterns of movement. A small amount of non-habituation learning is possible. Further, the inter-neuron net can also maintain spontaneous rhythms. The spinal cord and much of the lower brain stem is such a network. It generates the rhythms of heart beat, breathing and alternate limb movements; it mediates the reflexes of avoidance movements; it maintains postural muscle tone. But its ability to learn is limited.
In order to learn, in the sense of gaining from past experiences, it is necessary to have experiences, to store them and evaluate them. When inter-neurons increase in number and become organized into large sheets and nuclei, it is possible to deal in experience. The inter-
neurons function to build a coherent model of experience, store an edited version of this model as a sequential memory, simulate future events with the model and compare results with expectations. Now there is the ability to learn from experience, there is thought. It is important at this stage to point out that in this concept of thought, there is a difference between the model (and its production and use) and our consciousness of it. The phenomenon of consciousness is associated with the formation of an edited form of the model and the storage of this summary. Consciousness is not the modeling activity itself.
If we ask the question, “How do I know that I am conscious at this very instant of time?” it is difficult to answer without referring to the continuum of past, present and future experience. Our awareness of having been without consciousness is simply a discontinuity in the memory. Consciousness can be pictured as the leading edge of memory, having the same detail and structure as recent memory. It is one of the finished products of thought, not the process of thought. Far from being uniquely human, consciousness will occur in all animals with brains of the same memory fabricating type, at least the higher vertebrates. The elaborateness of the edited model would dictate the elaborateness of the consciousness of it. ”
It seems to me that the feeding of working memory and short-term memory is one of the important functions of consciousness; and, the acceptance of recalled memories and of imaginings constructed from memory fragments is another important associated function. Consciousness is the creation of experience that can be stored, retrieved and manipulated.