Summary and Ge?ieral Discussion 361 



Perception of an object comes through clean and sharp, and an 

 act comes through clean and sharp without conflict or blurring 

 by opposing" elements. Sometimes we err grievously by over- 

 commitment to a typology, as did the scholastic philosophers; but 

 without such a commitment we could not think at all, and with 

 sophistication we can return to graded or probabilistic thinking. 

 The mechanisms are standard orthodox neurophysiology; their 

 behavioral consequences are still being explored. 



The second neural circuitry worth mentioning — it has received 

 much attention here — is the double system, discrete and diffuse. 

 The diffuse system gives the metasignals which are the set. It acts 

 like the basic adjustments of the television set that make a picture 

 possible: adjusting brightness and discrimination, locking in the 

 vertical and horizontal, etc., but not giving the actual picture. 

 The discrete system presents the picture, the particular pattern 

 that receives our attention. I have probably oversimplified this 

 (an example of oversharpening nature) but there is much evidence 

 for it. The diffuse system can modulate thresholds and responses 

 of the cortical neurons that are thrown into action initially by 

 the discrete system; and the diffuse system does affect mood, set, 

 emotional background, even level of conscious awareness and 

 attention. The whole question of novelty, stress, anxiety, and 

 performance has been discussed (Gerard, R. W.: Neurophysiology; 

 an integration, in, Handbook of Physiology — Neurophysiology III, 

 Victor E. Hall et al., eds., Amer. Physiol. Society, I960, p. 1919) 

 in relation to the interaction of the two systems in modifying the 

 size of a "physiological neuron reserve." 



Returning to the overall evolution of the nervous system, the 

 third stage, after improved units and organized circuits, is increase 

 in number. The great rise in capacity of the vertebrates, and 

 particularly of the mammals, is attended, so far as I know, neither 

 by improvements in the neurons and their connections nor by 

 any better circuitry. It is a remarkable consequence of simply 

 adding more of the same. While this is surprising at first, a little 

 thought recognizes that more of the same can add entirely new 

 dimensions of richness in performance. In fact, I was struck by 

 the, I am sure accidental, parallel in the number of base pairs 

 in genes and of neurons in brains. The small virus has about 



