| Many differing views have been expressed with regard to the relation of the state of the brain to the phenomenon of consciousness. There is remarkably little consensus of opinion for a phenomenon of such obvious importance. It is clear, however, that all parts of the brain are not equally involved in its manifestation. For example… the cerebellum seems to be much more of an 'automaton' than the cerebrum. Actions under cerebellar control seem almost to take place ‘by themselves’ without one having to ‘think about’ them. While one may consciously decide to walk from one place to another, one does not often become aware of the elaborate plan of detailed muscle movements that would be necessary for controlled motion. The same may be said of unconscious reflex actions, such as the removal of one's hand from a hot stove, which might be mediated not by the brain at all but by the upper part of the spinal column. From this, at least, one may be well inclined to infer that the phenomenon of consciousness is likely to have more to do with the action of the cerebrum than with the cerebellum or the spinal cord. On the other hand, it is very far from clear that the activity of the cerebrum must itself always impinge upon our awareness. For example, as I have described above, in the normal action of walking, where one is not conscious of the detailed activity of one's muscles and limbs the control of this activity being largely cerebellar (helped by other parts of the brain and spinal cord), primary motor regions of the cerebrum would seem also to be involved. Moreover, the same would be true of the primary sensory regions: one might not be aware, at the time, of the varying pressures on the soles of one's feet as one walks, but the corresponding regions of the somatosensory cortex would still be continually activated. Indeed, the distinguished US Canadian neurosurgeon Wilder Penfield (who, in the 1940s and 1950s, was responsible for much of the detailed mapping of the motor and sensory regions of the human brain) has argued that one's awareness is not associated simply with cerebral activity. He suggested, on the basis of his experiences in performing numerous brain operations on conscious subjects, that some region of what he referred to as the upper brain-stem, consisting largely of the thalamus and the midbrain, though he had mainly in mind the reticular formation, should, in a sense, be regarded as the 'seat of consciousness'. The upper brain-stem is in communication with the cerebrum, and Penfield argued that 'conscious awareness' or 'consciously willed action' would arise whenever this region of brain-stem is in direct communication with the appropriate region of the cerebral cortex, namely the particular region associated with whatever specific sensations, thoughts, memories, or actions are being consciously perceived or evoked at the time… [Penfield’s] view was that consciousness is a manifestation of activity of the upper brain-stem… Other neuro-physiologists have also argued that, in particular, the reticular formation might be taken to be the 'seat' of consciousness, if such a seat indeed exists. The reticular formation, after all, is responsible for the general state of alertness of the brain. If it is damaged, then unconsciousness will result. Whenever the brain is in a waking conscious state, then the reticular formation is active; when not, then it is not. There does indeed appear to be a clear association between activity of the reticular formation and that state of a person that we normally refer to as 'conscious'. However, the matter is complicated by the fact that in the state of dreaming, where one is indeed aware in the sense of being aware of the dream itself, normally active parts of the reticular formation seem not to be active. A thing that also worries people about assigning such an honoured status to the reticular formation is that, in evolutionary terms, it is a very ancient part of the brain. If all that one needs to be conscious is an active reticular formation, then frogs, lizards, and even codfish are conscious! … Another viewpoint seems to be that it is the action of the hippocampus that has more to do with the conscious state. … [T]he hippocampus is crucial to the laying down of long-term memories. A case can be made that the laying down of permanent memories is associated with consciousness, and if this is right, the hippocampus would indeed play a central role in the phenomenon of conscious awareness. Others would hold that it is the cerebral cortex itself which is responsible for awareness. Since the cerebrum is man's pride (though dolphins' cerebrums are as big!) and since the mental activities most closely associated with intelligence appear to be carried out by the cerebrum, then surely it is here that the soul of man resides! That would presumably be the conclusion of the point of view of strong AI, for example. If awareness is merely a feature of the complexity of an algorithm or perhaps of its 'depth' or some 'level of subtlety' then, according to the strong-AI view, the complicated algorithms being carried out by the cerebral cortex would give that region the strongest claim to be that capable of manifesting consciousness. | — Roger Penrose, The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics, Chapter 9 – Real Brains and Model Brains | Indexes/23 |
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