The physical basis of consciousness is perhaps the greatest mystery and problem in modern science. There can be little doubt that consciousness is a physical phenomenon but we cannot even begin to explain how it arises in brains. The simple – and doubtless simplistic – medical model of consciousness is that the cerebral hemispheres, where thinking and feeling goes on, are like millions of light bulbs. Consciousness is the brightness with which they shine. If you progressively damage the cerebral hemispheres, consciousness dims. Small areas of damage to the hemispheres have little effect on consciousness: many neurosurgeons have seen patients who walked into hospital with a knife or a nail, for instance, stuck in their brains, and yet who are fully conscious.
The hemispheres are powered, in ways we do not understand, by the brainstem, the part of the brain between the hemispheres and the spinal cord. In the medical model, the brainstem is equivalent to an electric cable supplying the millions of light bulbs. Small injuries to the brainstem can cause profound coma – all the light bulbs will be dimmed at once.
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