30 BRAIN MECHANISMS AND LEARNING 



direction, should slow it. A fall in threshold of cortical neurones, or an in- 

 crease in impulse bc:)mbardment, should hasten tixation. Since epinephrine 

 lowers thresholds, and is released in vivid emotional experiences, such an in- 

 tense adventure should be highly menaorablc. Perhaps during imprinting 

 periods, the relevant neurones arc similarly in a low threshold or 'soft- 

 shell' stage. Strychnine, by enhancing general activity level and lowering 

 thresholds, should also speed up the fixation process. The reticular forma- 

 tion, the amygdala and other components of the limbic system, the hypo- 

 thalamus, etc., act through one another or directly on cortical neurones to 

 alter their dendrite potentials, their thresholds, and their responsiveness to 

 the discrete impulses which reach the cortex by non-diffuse afterents or by 

 spread within it. By raising or lowering thresholds of cortical neurones, 

 associated with lov/ered or increased attention and vigilance, these deep 

 regions could easily modify the ease and completeness of experience 

 fixation even if the nuclei were not themselves loci of engrams. Ether and 

 meprobamate, by raising thresholds, should slow reverberation and 

 therefore make electroshock effects more severe; unless they also 

 decrease the effect of the shock itself due to the increased neurone thresholds. 

 This last possibility is being further tested in the case of meprobamate; it 

 does explain the synergy of rescrpine and shock, and of anoxia and shock, 

 since both anoxia and rescrpine lower convulsive thresholds. 



Fixation would also be modified by changing impulse bombardment; 

 in fact it is the interference with such continued bombardment, by shock 

 or cold or concussion — which latter rather nicely parallels clinically the 

 amnesic effects of electroshock m animals or man — that interrupts the 

 fixatic^n process. Early in any learning experience, as the organism's 

 actions fail to solve the problem and eliminate the disturbing input, there 

 is great central irradiation; muscle tension is increased, there is generalized 

 contraction of irrelevant as well as of the desired muscles, autonomic dis- 

 charges occur, tension and attention are intense, the 'consciousness of 

 necessity' is high, and many neurones m cortex and deep centres show 

 electrical activity on mass or micro-electrode recording. Later, when 

 learned responses have been established, general radiation disappears, 

 muscles relax, there is little tension and maybe not even attention, 

 habituation is evident in performance and experience, and electric activity 

 has disappeared from all neurones except those specifically involved in the 

 response. With errors or other kinds of emergency situations, the electrical 

 activity and other signs of irradiation promptly return (see Gerard, 1960a 

 for references). As an action is learned and certain paths through the 

 nervous system become canalized, irradiation is eliminated. This requires 



