232 Information Storage and Neural Control 



Taking" the examples just given, neurophysiologists are finding 

 closer correlates with the states of sleep and anesthesia from studies 

 of the coupling between the cortex, the thalamus and the brain 

 stem than they have found in their measurements of energy-transfer 

 reflected in arterio-venous differences between carotid and jugular 

 blood. In the limbic system there is now evidence for re-routing 

 taking place during the learning process in animals being trained 

 in a T-box (1). Many other examples could be quoted. 



The second attraction that I mentioned was the way information 

 theory handles the problem of signals-in-noise; but here, neuro- 

 physiologists generally use this term in the vernacular rather than 

 in its critically defined sense. This is because we do not usually 

 apply the criteria for randomness when speaking of biological noise. 

 As a matter of fact, many use the term 'noise' in quite the opposite 

 sense from that defined by mathematical theory. In the neuro- 

 physiological journals we frequently find 'noise' used to describe 

 disorderly, unpredictable activity in which no regularity can be 

 detected. 



On the other hand, the mathematical approach (5, 11) has a 

 very clear-cut set of criteria for random processes — criteria based on 

 probability distributions that effectively result in statistical regu- 

 larity, statistical orderliness and statistical predictability. 



The whole gamut of criteria for a mathematical model of random 

 processes would be very difficult to apply to the nervous system, but 

 already some consideration has been given to this problem (6). 

 The probability functions that have seemed to be the least difficult 

 to carry from the mathematical model into the 'real' nervous 

 system have been those of means, spectra and correlation functions. 

 These comparatively simple factors bring us only to a limited and 

 fractional descriptive usefulness, and hence an increasing number 

 of neurophysiologists are exploring this approach. 



The statistical regularity of a random process bears considerable 

 interest for the neurophysiologist because of his familiarity with 

 the concept of the statistically steady state that has earned itself the 

 name of homeostasis. The fact that the brain, in its evolution, has 

 reached a stage in man where the neuronal mechanisms for homeo- 

 static control of his milieu interieur are handled by his medullary 

 brain stem, frees the cortex from these concerns and reserves it for 



