If we were awarding the Almighty with a prize for the marvels of material creation we would surely pick the human brain. It is the highest instrument of rational thought and, through its marvellous operations, we can – with grace – reach out and touch God.
We may think of the brain as a complex of communicating parts, each part with its specialist function which may work on its own or in concert with other parts. And we may roughly separate it into the folded mantle of the cortex, which is the cognitive part. The limbic system, which majors on feelings, instincts and biological drives and the brainstem, providing basic life-support systems.
The whole will weigh about 1.4kg, and will contain around a hundred billion nerve cells. The more recently evolved cortex is very large in our species and, as the homo line developed, the brain increased in capacity from about 400cc to about 1350cc.
The neurons within the brain form and re-form connections at the rate of a million per second and typically each one has between 1,000 and 10,000 connections with other cells. They communicate using transmitting and modulating chemicals which pass through the synapses (connecting junctions between neurons). Others are more widely spread affecting whole brain regions. Deficiencies in these chemicals can have profound effects. For example, absence of dopamine can lead to Parkinson’s disease, just as lack of serotonin can lead to depression, or acetylcholine to Alzheimer’s.
Some neurons carry messages from the body’s sense receptors. Others carry signals from the central nervous system to the muscles and glands. Others, again, provide the neural wiring. Brain structure is directed by genes and by experience. We now know that new neurons are being formed throughout our lives, though this plasticity is marked by accelerated development in, say, the infant and at puberty. The other major inhabitants of the brain are glial cells, which provide support functions for the neurons.
The baby’s brain learns at a tremendous rate, forming and refining its brain connections in response to its environment. This process continues as everyday experience continues to generate connections. For example, the connection between a face and a name requires two areas of the brain to work in concert. We can even watch such connections being made in, for example, the brain of a mouse which is undergoing a new experience. Long term memory is formed by a continuing reinforcement of connections.
And everyone is familiar with the increased physical size of the memory of a London cab driver, as he learns “the knowledge”.
I cannot chart all the functions of the brain for you here, but a good example is provided by memory. Immediate memory is found in the temporal lobes. Basic stored memory is held in the hippocampus, and longer-term memories are posted in other parts of the brain. Near to the hippocampus is the amygdala, which handles emotions. It responds to our sense of fear or to our sense of delight. (See link below)
When the brain of Phineas Gage, a 19th-century railway worker, was penetrated by an iron bar, he survived. This enabled scientists to locate the characteristics of his personality which were damaged by the wound. Incident by incident they were learning how to relate function to brain area. Nowadays they use brain scans.
Although brain scan signals are, at this stage of knowledge, relatively crude, we are now able, for instance, to map the brains which correlate to different personality traits, such as neuroticism or conscientiousness.
In the past we used observational studies for this. For example, we know that people crossing a road, where jaywalking is forbidden, are over three times more likely to follow a man wearing a business suit than one who is informally dressed. (At least it was so in Texas in 1955, when this was studied.) Nowadays we might simply record the different brain activity triggered by an image of a business person.
There are different forms of brain scan. Some measure the electrical activity of neuron signals from outside the scalp. Others measure activity by inserting “tracer” elements which can then be identified, and others track the blood flow associated with activity in areas of the brain. While these methods have revealed much, it is generally agreed that they are still somewhat crude and generalised in their effects. Much research goes into the development of more refined detectors, and these will undoubtedly become more sophisticated. (see link below)
The wonders I describe are such that we should not be surprised at those scientists who believe that the whole story of our humanity can be deduced from a complete understanding of the brain. After all, if every aspect can be traced to cerebral or related activity, why do we need to look further?
To that, I think we must answer that every aspect of a violin concerto can be traced to the vibration of the strings and the sound-box of the instrument, but without the violinist there is no music. Yet the analogy is not exact. The integration of the human spirit and the human body is so much closer than the violin to the violinist that we are hard put to discern the boundaries.
In my next column we will review the commercial and psephological aspects of brain science. It is known as neuromarketing.
Meanwhile, the website Secondsightblog.net, which is available for comments and questions, carries links to brain functions and to brief descriptions of current scanning methods.
Brain functions – interactive chart at http://www.newscientist.com/movie/brain-interactive
Summary of scanning methods at http://psychcentral.com/lib/2007/types-of-brain-imaging-techniques/