360 Injormation Storage and Neural Control 



permitted very slow evolution. A great speeding up of modi- 

 fication of the system by environmental impact, i.e., an enhance- 

 ment of response to the information available, allowed a second 

 forward step — the invention of sex. This latter maneuver made 

 it possible to mix the genes in two individuals, to shuffle the cards, 

 and so get an almost infinite number of hands with the same small 

 array of individual items. 



The third major landmark was the invention of multicellularity. 

 This made possible the setting off of groups of cells, tissues, and 

 organs for particular functions, including susceptibility to environ- 

 mental influences. Multicellularity made possible a meaningful 

 nervous system, the appearance and steady improvement of which 

 is the most important invention for us. This evolution over suc- 

 cessive epochs probably involved an initial improvement of the 

 individual unit neurons from decrementing to all-or-none con- 

 duction, from reciprocal to irreciprocal synapses, from lower to 

 higher speeds, from higher to lower thresholds, and all the rest. 

 Then there developed better circuitry between the neurons, 

 including such effective physiological devices as the simple reflex, 

 the reverberating loop, the negative feedback loop, etc. Two of 

 the circuits already mentioned are worth a moment. 



Dr. Brazier, particularly, referred to one as the "inhibitory 

 surround." This term emphasizes recent work by investigators such 

 as Hartline, Hubel, and many others, dealing with the sensory 

 input, but the mechanism really goes back to Sherrington's 

 reciprocal inhibition. This mechanism not only cuts in a clean 

 group of motor neurons to give a shaiply integrated act, very 

 possibly via the feedback inhibition by Renshaw cells, but it also 

 operates all through the nervous system. I have suggested in 

 The Handbook of Neurophysiology that it functions in giving attention 

 to one or another sensory input or thought train and in shifting 

 mood sharply, as well as in selecting a behavior. This device 

 (active units blocking out nearby ones that could have become 

 engaged in the activity but are in this way kept inactive) is the 

 basic mechanism for dissecting a graded continuum into sharp 

 classes. "Nature doesn't come as clean as we can think it," as 

 Whitehead said, but our whole nervous system and our sense 

 organs are designed to clean it up for our thought processes. 



