1 488 



IIVVlmilllK OF PHYSIOLOGY 



XEIROPHYSIOLOGY III 



available show some of the ways in which drugs may 

 eventually prove of great analytic value. 



A given drug (in this case, reserpine) may have 

 no effect upon acquisition of a C'.ER (229) but may 



attenuate its expression remarkably. 



b) When employed in trained rats displaying three 

 different learned responses, a drug (again reserpine 1 

 may severely depress two of these without affecting 

 the third, or interfere with one of them and enhance 

 another even though both presumably create 'anxiety' 

 in the animal (218). Such specific differential effects 

 upon behavior mean, we must presume, differential 

 influence upon the underlying neural events. 



c) We have already seen that animals will operate 

 a switch in order to procure shocks in the brain, and 

 that the highest response rates are attained with 

 shocks delivered to the limbic and hypothalamic 

 structures implicated in emotional behavior (19, see 

 Chapter LXIII by Brady in this volume). If an animal 

 behaving in this manner is given a tranquilizer (res- 

 erpine or chlorpromazine), a sharp decline in rate of 

 self-stimulation ensues, while with pentobarbital no 

 drop is seen (178). The tranquilizer would appear 

 somehow to reduce the reward value of the brain 

 shocks; how, if at all, this is related to the processes of 

 learning is an interesting matter Tor speculation and 

 further study. 



<li Drugs, finally, act differently upon the neural 

 processes in discrimination. Blough (15) has shown 

 that pigeons are better able to make a certain visual 

 discrimination under the influence of LSD and less 

 able under chlorpromazine, although in neither case 

 is their ability to peck at the visual targets employed 

 different from the control. It is not possible as yet to 

 state where in the nervous system the effects required 

 to explain these results could be exerted. 



Summm v 



Any discussion relating the budding held of psycho- 

 pharmacology in problems of learning must neces- 

 sarily be inconclusive at the present time, for until 

 recently the available data have settled few, il any, 

 questions. However, much activity along new and 

 promising lines is now in progress. 



1 iPHYSIOl OGII VI till oRIES 



We propose in ibis section and the next to state 

 when- we stand in our understanding of the learning 

 process. In the preceding sections the facts thus fai 



w rested from nature by much hard work —both intel- 

 lectual and experimental — have been assembled. Can 

 anything be done with them toward synthesizing a 

 concept of the essential features of the process? We 

 will first outline the ideas others have had and then 

 proceed to some of our own. 



Change in Central Synapses 



The inference that the learning process is to be ex- 

 plained by a specific change in central synapses has 

 had a particular appeal. According to t his idea some- 

 thing called s\ naptic resistance' is lowered, or, put 

 another way, the efficiency of synaptic action is some- 

 how increased during learning at those places where 

 the temporary connections are made and this increase 

 may become more or less permanent. There are two 

 main classes of such ideas; one holds that the changes 

 are chiefly anatomical, the other that they are pri- 

 marily biochemical. 



anatomical theories. The anatomical explanations 

 are numerous. Some of these stem from the observa- 

 tions upon embryos that led Rappers to offer his 

 theory of neurobiotaxis as an explanation of the means 

 whereby neurons come to make their proper connec- 

 tions in the first place (ill). He supposes electrical 

 or metabolic gradients, or both, to be developed in 

 tissues and to attract growing nerve fibers. In learn- 

 ing, similarly, foci of activity are supposed to be 

 created in certain neurons by the stimuli, these foci 

 attracting neurons or parts of neurons A formal 

 statement of this idea has been given bv Holt (09). 



Recent discoveries about the anatomy and phvsi- 

 ologv of synapses have led 10 more specific variants 

 of the neurobiotaxis theme. Thus 1 lebb (881 vis- 

 ualizes the axon terminals of one presynaptic ele- 

 ment to multiply in number during stimulation, an 

 .111.1tomie.il change that would enable the element to 

 increase iis contribution lo the depolarization of the 

 postsynaptic neuron. Konorski ( 1 jo, p. 86) con- 

 siders "101111,111011 and multiplication ol new synaptic 

 junctions between the axon terminals of one nerve 

 cell and the soma (i.e., the body and dendrites) of 



die other" to be the responsible factor in the elabora- 

 tion of conditioned reflexes, with "fading or atrophy 

 ol synaptic connections" occurring when the US is 

 withheld. Eccles (56) holds dial axon terminals swell 

 in size during activiiv, thus increasing their area of 

 coin. ul with and capacity 10 influence the post- 

 ed, ipiic neuron. Finally, die neurohistologist Sar- 

 kisov (212) reports seeing much variation in the 



