R. W. GERARD 3 1 



cither feed-back inhibitory arcs or a decrease in the general level of 

 facilitation, as interneurones become less active. This latter would follow 

 the diminished irradiation consequent to effective responses. 



Although the effect of continuing or recurring activity has been 

 described in terms of simple feed-back loops, the same results can be 

 obtained by improved synchrony of beating neurones or, especially, by 

 repeated waves of activity passing through a sheet of neurones. This last 

 model, developed by Beurle (1957), depends only on more or less random 

 connections of neurones in a mass, activated only fractionally by con- 

 trolled waves passing through them. Such waves can cross and at the 

 locus o( intersection will leave a group ot lowered-threshold neurones. 

 From such a locus the original waves can reinitiate without external 

 stimulation; such loci thus of^er engrams for memory, recall, planning of 

 action and for the combination of smaller elements of perception and 

 action into larger wholes of conception and of planning. The model and 

 the physiological support for and predictions from it are more fully 

 discussed elsewhere (Gerard, 1960a). 



Reverberation, or some form of continuing reactivation, is probably 

 involved in a number of other mental processes, including perception, 

 attention, repression, anxiety, and the like; but this is not the place to 

 develop these points. Certainly attention is necessary ior learning and for 

 discrimination. A dog, faced with an impossible oval or circle choice, 

 develops a neurosis only after it has already learned to pay attention to this 

 discrimination as a problem. Presumably the structures feeding diffuse 

 system impulses to the cortex, discussed above in connection with the 

 altering of fixation and recall, are involved in such influence of attention 

 on the learning process. 



The nature of the material change in the brain, of its locus, and of the 

 mechanism of fixation that brings it about are not established, but all these 

 questions are clearly on the way to satisfactory solution. The really diffi- 

 cult problem is none of these, however, and it remains as mysterious as 

 ever. This has to do with spccificiry in the selection and discrimination of 

 what is perceived, in the degree of attention given it, in the presence and 

 firmness of fixation of a memory, in its retention and subsequent recall to 

 consciousness. This qualitative aspect is, of course, not limited to the 

 receiving and retention of experience, but is equally present on the 

 behavioural side and is attached to plans, to values and to actual per- 

 formance. How choices are made, how priorities are assigned, how shifts 

 occur from one set of active neural processes to a different one, remains 

 scill a complete mystery. 



