Design for a brain

Abstract

Behaviour, reflex and learned 1/2.The activities of the nervous system may be divided more or less distinctly into two types.The dichotomy is perhaps an over-simplification, but it will be sufficient for our purpose.The first type is reflex behaviour.It is inborn, it is genetically determined in detail, it is a product, in the vertebrates, chiefly of centres in the spinal cord and in the base of the brain, and it is not appreciably modified by individual experience.The second type is learned behaviour.It is not inborn, it is not genetically determined in detail (more fully discussed in S. 1/9), it is a product chiefly of the cerebral cortex, and it is modified markedly by the organism's individual experiences.With the first or reflex type of behaviour we shall not be concerned.We assume that each reflex is produced by some neural mechanism whose physico-chemical nature results inevit- ably in the characteristic form of behaviour, that this mechanism is developed under the control of the gene-pattern and is inborn, and that the pattern of behaviour produced by the mechanism is usually adapted to the animal's environment because natural selection has long since eliminated all non-adapted variations.For example, the complex activity of ' coughing ' is assumed to be due to a special mechanism in the nervous system, inborn and developed by the action of the gene-pattern, and adapted and perfected by the fact that an animal who is less able to clear its trachea of obstruction has a smaller chance of survival.Although the mechanisms underlying these reflex activities are often difficult to study physiologically and although few are known in all their details, yet it is widely held among physiologists that no difficulty of principle is involved.Such behaviour and such mechanisms will not therefore be considered further.1/4.It is with the second type of behaviour that we are con- cerned : the behaviour that is not inborn but learned.Examples of such reactions exist in abundance, and any small selection must seem paltry.Yet I must say what I mean, if only to give the critic a definite target for attack.Several examples will therefore be given.A dog selected at random for an experiment with a conditioned 2 THE PROBLEM 1/5 reflex can be made at will to react to the sound of a bell either with or without salivation.Further, once trained to react in one way it may, with little difficulty, be trained to react later in the opposite way.The salivary response to the sound of a bell cannot, therefore, be due to a mechanism of fixed properties.A rat selected at random for an experiment in maze-running can be taught to run either to right or left by the use of an appro- priately shaped maze.Further, once trained to turn to one side it can be trained later to turn to the other.A kitten approaching a fire for the first time is unpredictable in its first reactions.The kitten may walk almost into it, or may spit at it, or may dab at it with a paw, or may try to sniff at it, or may crouch and ' stalk ' it.The initial way of behaving is not, therefore, determined by the animal's species.Perhaps the most striking evidence that animals, after training, can produce behaviour which cannot possibly have been inborn is provided by the circus.A seal balances a ball on its nose for minutes at a time ; one bear rides a bicycle, and another walks on roller skates.It would be ridiculous to suppose that these reactions are due to mechanisms both inborn and specially per- fected for these tricks.Man himself provides, of course, the most abundant variety of learned reactions : but only one example will be given here.If one is looking down a compound microscope and finds that the object is not central but to the right, one brings the object to the centre by pushing the slide still farther to the right.The relation between muscular action and consequent visual changeis the reverse of the usual.The student's initial bewilderment and clumsiness demonstrate that there is no neural mechanism inborn and ready for the reversed relation.But after a few days co-ordination develops.These examples, and all the facts of which they are representa- tive, show that the nervous system is able to develop ways of behaving which are not inborn and are not specified in detail by the gene-pattern.1/5.Learned behaviour has many characteristics, but we shall be concerned chiefly with one : when animals and children learn, not only does their behaviour change, but it changes usually for the better.The full meaning of i better ' will be discussed in THE PROBLEM 1/10 the survival of thefittest) is fixed, yet its influence has an infinite variety when applied to an infinite variety of particular organisms in particular environments.A final example is given in the body by the process of inflammation.The function-rules which govern the process are gene- tically determined and are constant in one species.Yet these rules, when applied to an infinite variety of individual injurie