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HANDBOOK OF l'l 1YS101.0GY 



\I IROPIIYSIOLOGY III 



[and] take part in the formation of neuron chains 

 joining the cortex and the underlying parts of the 

 brain. . . . The entire action of the cortex during 

 both unconditioned and conditioned reflexes is 

 caused by the excitation of certain neuron chains and 

 the more or less considerable inhibition of all the 



rest MTerent impulses (thalamic or association) 



activate the star and other neurons with short axons, 

 on the one hand, and the internuncial and associa- 

 tion pyramidal neurons, on the other. ... In the 

 formation of the neuron chains of the cortex the 

 dendrites of the pyramidal neurons do not take part 

 because they do not conduct excitation to the cells 

 . . . [but] more or less considerable and prolonged 

 potentials may arise [in them]. 



"By combining: two stimulations acting on one and 

 the same or on different analyzers ■two-was' tem- 

 porary connections an- set up . . . among the star 

 neurons . . . by means of the internuncial and asso- 

 ciation pyramidal neurons. . . . The formation of 

 temporal - ) connections supposes both functional and 

 morphological changes in the cellular elements and 

 in the synaptic apparatus. ... A cell of one neuron 

 and the synaptic knobs of another neuron lie closer 

 to one another . . . their excitability is noticeably in- 

 creased and the processes of excitations proceeding in 

 them are intensified. These morphological and 

 physiological changes take place as a result of the 

 interaction of the excitation in all parts of the nervous 

 system but in the cerebral cortex they arise more 

 quickly and last for a longer time, sometimes for 

 life, while in other parts of the brain they are always 

 transilorv and pass ,iw.i\ soon after cessation of the 

 interaction." 



Cognizance does not seem to have been taken in 

 BeritofPs theory oi the experiments by Sperry and 

 collaborators showing that knife cuts and the im- 

 plantation ol win- .mil sheets oi mica -devices thai 

 destroy mii.icortu.il connections and distort what- 

 ever el© 1 1 H .il fields i ortii il neurons generate inter- 

 fere little "i not ,u all with performance of even 

 complicated CR's (226). Konorski (120) has still 

 other objections to it. 



As aln-.nK mentioned, the idea thai stellate ('star 1 

 cells in the cortex .in- ol especial important e for < IR's 

 is supported by s.ukiso\ (212, p tig) who holds 

 that theii structural peculiarities "show their specific 

 role in tin- cortical processes and prim. 11 lis 111 the 



interconnections between the cellular elements of 

 the- cortex " He holds, further, "thai the cells of the 

 hinder parts of the central nervous s\siem and pri- 

 marily ol the cerebral cortex are characterized by 



considerable lability of form, changing under the in- 

 fluence of external and internal stimulations" (212, 

 p. 120). A report of intracellular changes, twisting of 

 apical dendrites and coarsening ol libers in cortical 

 layers I and II following electrical stimulation is 

 actually available (38). The collection of experimental 

 evidence related to both the anatomical and the 

 neuron chain theories of learning marks much of the 

 current Russian work. 



REVERBERATING CHAINS. Another concept of how 

 nerve circuits are rearranged in learning has been 

 called the reverberating chain theory (74, 98, 120). 

 Proceeding from the anatomical fact that neurons 

 are at least potentially connected to other neurons in 

 a reciprocal manner, it supposes that acquisition of a 

 OR consists of setting up a closed circuit of neuronal 

 activity in which neuron A fires neuron B which in 

 turn fires A again. Such chains can include very 

 many neurons. Retention of the CR (or memory) is 

 explained by perseveration of activity in the chain. 

 Hebb (881 supposes that short-term retention may 

 invoke reverberating chains only but, if reverbera- 

 tion persists sufficiently in such a chain, the anatomi- 

 cal synaptic changes required to explain long-term 

 memories occur. Many other variants of these popu- 

 lar schemes have been advanced but space will not 

 permit their discussion (lit, 131, 132 



Theories from EEG Studies 



The most recent theories come from the EEG 

 studies which, as we have seen, arc beginning to re- 

 veal important new facts. We will consider only one 

 of these, the theory of Gastaut t 1 al. (69, 70 1. It re- 

 quires rearrangements in at least six separate cir- 

 cuits to explain habituation and conditioning. Both 

 the CS and the L'S are thought to activate the reticu- 

 lar formation as well as their specific cortical areas, 

 an inference iustilicd by recent ncurophysiological 



findings. Each stimulus, furthermore, activates both 

 a mesencephalic and a thalamic locus in the reticular 

 formation. Habituation consists in the inhibition of 

 both these loci, with c c insequenl disappearance of the 

 alerting response in the scalp EEG. Pairing of a 

 (habituated 1 aiidilnrv CS with ( unhabiluatcd I 

 somesthetic I S leads to formation of .1 temporary 

 link in the thalamic reticular formation thus "pet 

 mining thalamic collaterals borrowed by the sound 

 signal 10 act on the neurons previously only acti- 

 vated by the collaterals ol the somesthetic signal" 

 (70, p. 31). The mechanism by which CS input 



