How Can Adodels from Information Theory be Used in Neurophysiology? 239 



At the beginning of such a train of flashes there are also long 

 latency responses in the visual cortex. These have been shown to 

 reach the cortex by the nonspecific afferent systems of the midline 

 brain stem and thalamus. It would seem possible that tlie third 

 major component of the message, the one signalling 'novelty' in 

 the stimulus, may be carried by these nonspecific afferents, for as 

 repetition continues, this sequence of later waves fades out. Averag- 

 ing of the first sixty to arrive, then the second sixty, the third sixty, 

 and so on, shows this late component of the multiple response to 

 be dropping out as the novelty wears off. 



The effect can be fractionated even farther in the nonspecific 

 system by actually recording in a nucleus of this midline nonspecific 

 system (the centre mechan) where one of the most prominent of its 

 average electrical responses to flash (the late wave) can be seen to 

 fail with repetition of the unchanging stimulus, while the earlier 

 components persist. 



The serial change in the late component of the multiple response 

 is very marked. Whatever the mechanism for this depressed respon- 

 siveness may prove to be, it is tempting to propose that this forms 

 part of the mechanism that conveys presence or absence of novelty. 



This work has been described in detail and illustrated elsewhere, 

 (3, 4) so it will not be given more space here. However, lest these 

 examples appear to suggest too simple a picture of the brain's 

 message-receiving systems, let me add that not only does one find 

 presence or absence of a component of the response, as novelty 

 wears off, as in the foregoing sample, but one also finds situations 

 in which the time-relationships of the components of the brain's 

 electrical responses may change. Possibly it is in the time domain 

 that the neurophysiologist will find the most clues for the solution 

 of this problem. 



I make only a brief allusion here to the laboratory work. This 

 is not intended as the report of a research, but as an example of 

 work carried out with a probabilistic model in mind, and to illus- 

 trate the point that the response probabilities of the nervous system 

 are influenced by the past events it has experienced. 



Returning now to the more general topic of the utilization by 

 neurophysiology of information theory, let us not forget that one 

 of the innovations of information theory was the axiom that in- 



