238 Information Storage and Neural Control 



tioned; namely, averaging" not only over time but over neural 

 aggregates, in order to get the profile of a population of neurons. 

 This is of particular importance in the brain because of the demon- 

 strated interaction of units within populations. This second ap- 

 proach necessitates the use of electrodes large enough to record 

 from populations of neurons and thus able to average over space 

 as well as time. 



I will illustrate this approach by brief mention of some examples 

 drawn from our laboratory. Suppose we take the response of an 

 unanesthetized cat to a flash of light that is repeated monoton- 

 ously without any change in timing, or intensity, or in any other 

 of its parameters. 



At the beginning of a train of such stimuli, the message the brain 

 will receive will contain at least three major components; 



1) the stimulus is visual, 



2) the stimulus is repetitive, 



3) the stimulus is novel. 



On prolonged repetition, however, the third of these messages 

 (that the stimulus is novel) is no longer being sent. The probability 

 of its arrival is now very high. 



If the hypothesis is to be regarded as tenable, one of the tests 

 the neurophysiologist must make is a demonstration that the 

 response of the brain to a novel stimulus is difTerent on the average 

 from its reponse to a familiar one. 



What would be demanded by the hypothesis under discussion? 

 Averages of a sample of responses late in the series would be ex- 

 pected to carry two of the same components of the message as are 

 carried by the first set of flashes; namely, that the stimulus is visual 

 and that it is repetitive, but the third component, i.e., that the 

 stimulus is novel, would need some change of signal. 



When the responses to a repetitive train of flashes are recorded 

 from the visual cortex of an unanesthetized cat with permanently 

 implanted electrodes, one finds that the short latency responses 

 that have been identified with transmission in the specific aflferent 

 systems persist for the whole duration of the train. They apparently 

 carry the first two components of the message (that the stimulus is 

 visual and that it is being repeated). 



