146 Inside the Living Cell 



impulses, so that the patterns which are frequently excited tend to 

 become permanent. Hebb thinks that the change is to be regarded 

 as a kind of growth process, as a result of which the terminations of 

 the axons increase in size as the result of the passage of impulses. This 

 means that although a short-lived memory might be maintained by 

 a reverberating circuit of neurones, if stimulation is frequently re- 

 peated, it will become very much easier to produce the stimulated 

 pattern. The memory will then be, as it were, 'built in' to the cortical 

 structure and will not be destroyed by electric shocks and other 

 disturbances. 



These proposals are obviously very sketchy and incomplete and 

 readers will have no difficulty in concluding that no really adequate 

 models of brain action exist — except for perhaps the very simplest 

 types of action. 



The existence of memory obviously requires that patterns of some 

 sort based on the original sense stimuli are stored somewhere in the 

 brain. The form in which this storage is effected is not really known. 

 It is not necessary to suppose that the original impulses go on circu- 

 lating in the brain for ever. What is required is a mechanism which 

 will store them up in such a form that they can be repeated. This 

 could occur, as Hebb suggested, by structural changes in the con- 

 nections between the neurones, as a result of which a particular pat- 

 tern of stimulation is easily re-established. Another possibility is that 

 the impulses are stored in the neurones themselves, e.g. when a series 

 of impulses reaches a neurone, it may bring about permanent changes 

 in its structure of such a kind that the neurone will be capable of 

 emitting the same series of impulses at a future time. Thus the 

 neurone may not only be a receptor of signals and a relay; but a re- 

 peater unit. It will store up patterns of impulses and repeat them at a 

 later stage. However such a process has not been demonstrated. 



As I have explained, the memory mechanism is quite fundamental 

 to brain action. Without memory there can be no recognition and no 

 learning. But it is also probable that the memory is not a simple 

 memory of the actual original sensations but of the interpretation 

 which is made of them. It is the meanings which are remembered, 

 rather than the sensations themselves. The situation is therefore a 

 very complex one and it is difficult to separate one aspect from 

 another. But the very least we can say is that behind the neurones in 

 which the immediate sense impressions are received, there must be 

 others in which they are compared and interpreted in terms of the 

 memory record of previous experiences. That is, the immediate sen- 

 sations find their places in the continuing record of the structure of 

 experience which has been gradually built up during all our lives. 



