THE NEURAL BASIS OF LEARNING 



1489 



structure of cortical synapses and suggests that 

 morphological changes in the stellate cell synapses in 

 particular may accompany the learning process. 



biochemical theories. There are biochemical ex- 

 planations also for how synaptic 'resistance' might 

 be reduced in learning. Most of these stem from the 

 basic observation that chemical substances like epi- 

 nephrine and acetylcholine are involved in the trans- 

 mission of nerve impulses across some synapses; they 

 suppose these or similar chemical events can be made 

 to operate more effectively and that the learning pro- 

 cess is explained by the fact that they do so. There is 

 some recent experimental evidence for this view ( 1 23). 



The post-tetanic-potentiation (PTP) theory should 

 be considered here because many physiologists cur- 

 rently hold that all synaptic transmission is effected 

 through chemical agencies. The physiological defi- 

 nition of PTP is as follows: after prolonged high- 

 frequency (tetanic) excitation of a synaptic region, a 

 test stimulus of fixed strength evokes a larger (po- 

 tentiated) postsynaptic response. This PTP may last 

 for hours in the animal preparations, although ii 

 usually does not, and it has sometimes been sug- 

 gested that a process similar to that underlying PTP 

 may produce the durable changes in learning (but 

 see 56). 



Another suggestion (112) rests on the remarkable 

 advances that have been made in recent years in 

 understanding the biochemical aspects of heredity 

 and neural function. Pointing to the similarities be- 

 tween heredity and memory, its sponsors suggest that 

 the essential feature of learning (or in their words, 

 memory trace) is the formation of geometrically pat- 

 terned protein molecules in the neurons of the cere- 

 brum. 



glia cell possibilities. In some quarters interesl has 

 been revived in the possibility that the glia cells of 

 the brain may play some important role in learning. 

 These cells outnumber the neurons by at least two 

 to one in the cortex. Very little is known about their 

 function but they are commonly believed to provide 

 mechanical and metabolic support for the neurons. 

 Ramon v Cajal (see 175) thought they might be 

 capable of some movement in which case they could 

 insinuate processes, in an ameboid manner, into the 

 space between pre- and postsynaptic elements at 

 synapses. Presumably this would hinder synaptic 

 transmission. Their retraction might reduce 'synap- 

 tic resistance' and enhance conduction. Some recent 

 investigations do show glia cells to be capable of 



movement while other studies show that they contain 

 a cholinesterase different from that of nerve cells 

 (49). What these facts concerning glia cells have to 

 do with learning remains to be demonstrated. 



Rearranged Neural Circuits 



The second category of theories to explain learning 

 conceives the essential change to be a new arrange- 

 ment of the way neurons excite one another. It is 

 clear that innate factors cause stimuli to activate 

 certain neural circuits preferentially, but it is also 

 clear that in learning two such preferred circuits be- 

 come linked in the CR. What is the nature of the link 

 and the new circuit? 



Russian ideas. Pavlov's concept of cortical irradiation 

 attempts to deal with this problem. According to this 

 theory excitations arise from CS and L'S in the cortical 

 areas to which the afferents project (auditory ana- 

 lyzer, optic analyzer, etc.). These excitations irradi- 

 ate, like the spokes of a wheel, from their points of 

 arrival in the cortex, diminishing in intensity as they 

 spread. The excitations initiated l>\ the CS are the 

 weaker of the two excitations. For this reason they 

 flow toward, or are drawn toward, the center of 

 stronger excitation, i.e. the place where excitations 

 from the L'S are generated. Then, as a consequence 

 of the repeated presentation of CS and US, a path is 

 worn, so to speak, from the CS center to the US 

 center and the CS comes thereby to bring about the 

 same neural effects as the US. 



A modern Russian theor) cm be illustrated by the 

 ideas of Beritofl 11 ;. 1 jo, p. j(>). This will be re- 

 counted here in detail because it is not likely to be 

 readily available elsewhere t<> the reader, although 

 Konorski (120, p. 56) has discussed it. Like Pavlov's 

 notion, its central concept is that excitation irradi- 

 ates in all directions from the cortical areas excited 

 by L'S and CS and preferentially along the shortest 

 line between them. However, according to Beritoff, 

 "two-way" connections come to be formed, specifi- 

 cally between "star cells' of the two cortical areas and 

 l>\ way of subcortical white matter through the 

 pyramidal association neurons. Ultimately the motor 

 pyramidal cells are influenced to produce move- 

 ments. "All the star and other neurons with short 

 axons and also all the small (internuncial) and 

 medium sized (association) pyramidal neurons with 

 descending and ascending axons form closed chains 

 of neurons both vertically and horizontally. . . . Neu- 

 rons forming the pyramidal and extrapyramidal 

 tracts . . . are joined together by internuncial neurons 



