How Can Models from Injormation Theory be Used in Neurophysiology? 233 



higher functions, thus giving man what Claude Bernard, in his 

 famous phrase, called "la condition de la vie libre." 



This concept of a statistically regular, predictable randomness 

 of 'noise' against which the neurophysiologist emphasizes his 

 'signal' when averaging by computers, has a close relationship to 

 one of the most basic principles of information theory. This prin- 

 ciple is that information is carried by departure from orderliness or, 

 in other words, by departure from the predictable. Even the 

 intuitive concept of information is a change from what you already 

 know and can predict. 



Several neurophysiologists have now invoked this principle to 

 explain such phenomena as "attention" and "habituation" and 

 "the orienting reflex," together with their attendant electrical 

 concomitants. One such example is the model proposed by Sokolov, 

 (12) which envisages novelty, i.e., departure from the statistically 

 expected state, as being the factor that evokes activity in the brain 

 stem and the resultant orienting reflex. The concept of "attention" 

 being related to matching the probability of a neuronal event 

 against the expected distribution of possible events, will be found 

 in the work of many neurophysiologists. * 



This brings us to the third major attraction of information theory 

 for the neurophysiologist: the use of a probabilistic model for the 

 nervous system rather than a deterministic one. I would like to 

 approach this from the neurophysiologist's angle. 



I have spoken earlier about Johannes Miiller and you will re- 

 member his famous "Law of Specific Nerve Energies" by which 

 each of the myriad facets of sensation was assigned its special nerve 

 — how uneconomical, but how simple. A single output would be 

 obtained from a single input. Nothing could be more deterministic. 

 One could design no simpler code. But it was too good to be true. 



In the earlier part of this century, the belief in a ubiquitous 

 all-or-nothing law for the nervous system and the demonstrations of 

 the coding of intensity by frequency of discharge in single fibers 

 of the peripheral nervous system, led eventually to exploration of 

 single cell discharges within the brain itself. 



Iminediately, it became clear that coding was no simple problem. 

 Miiller would have been chagrined to see how many difl'erent 



"For early examples see references (2) and (7). 



