The Brain and What It Does 127 



impressions by recognizing something you have seen before. You 

 could not recognize objects as squares unless you had previously 

 seen and made a note of this feature. So picking out the features of a 

 pattern and recognizing them involves memory and also to some 

 extent learning about them. So we see that we do not spontaneously 

 interpret our sense impressions. The use we make depends on our 

 memory of similar experiences and the meanings we have found in 

 them. 



MEMORY 



There is every reason to think that the stimulation of brain cells 

 leaves a permanent trace of some kind. This is not very surprising, 

 as all living cells are to some extent influenced permanently by 

 external agencies acting on them. The state of a cell at any one 

 moment depends on what has happened before and everyone who 

 has worked even with simple organisms such as bacteria knows how 

 hard it is to reproduce a particular condition exactly. This amounts 

 to saying that all living cells have some kind of a memory of what 

 has happened to them, so that it is not surprising to find, perhaps 

 to a much more highly developed degree, that brain cells are perma- 

 nently influenced by the sensory images they received. But the exact 

 nature of the memory record, whether it is proved by changes in 

 molecules or structures, or whether permanently circulating currents 

 of electricity are set up, is hardly known. 



The memory record made by the brain must, however, have one 

 characteristic if it is to be of any use. It must be available for inspec- 

 tion. It is no use having a gramophone record unless we can play it. 

 So our memory impressions wifl be useless unless they can at least 

 be used for recognition. There must be some way of comparing the 

 sensation of the moment, with what is remembered of previous 

 sensations. Exactly how this is done is not known. We might think 

 that the sense impressions circulate among the cefls until they find 

 something with which they correspond — holes into which they fit. 

 Alternatively it is possible that the memory record is able to send 

 out again, perhaps rather weakly, a copy or repetition of the elec- 

 trical impulses which caused it — so that we could have a direct 

 comparison between the fresh impulses arriving from the senses and 

 the fainter images coming from the memory record. 



DEALING WITH SENSE IMPRESSIONS 



It will be obvious from this that the reception of a sense impression 

 in the brain is not an isolated act, but something which is part of a 



