'5 22 



IIANDHiiiiK HI- l'lIYSlnl.lKiY 



NEUROPHYSIOLOGY III 



abnormally to the addition of quinine and glucose to 

 the diet or to the change in texture. But they arc 

 sensitive to cellulose adulteration. It is possible, then, 

 that obesity makes some indirect contribution to the 

 'finickiness' Teitelbaum reports, although this finding 

 may he partly the result of the "heedless' eating the 

 nonobese hyperphagics do. 



In similar studies, rats with lateral hypothalamic 

 lesions accept no food postoperatively. But as recovery 

 occurs, mi a regime of forced feeding, it comes about 

 in a very specific way. Alter a week or two of complete 

 starvation, the operated rats will accept chocolate and 

 evaporated milk, but they cannot be aroused to drink 

 water or eat powdered food, glucose, meat, etc. 

 Within a few days to a few weeks later, they will accept 

 water and, last of all, they will eat powdered food. 

 While there are undoubtedly many chemical and 

 nutritive differences among these substances in addi- 

 tion to sensory ones, the suggestion is strong that these 

 rats are changed in their reactions to the stimuli pro- 

 vided by the diet following operation and throughout 

 the course of recovery. 



The Role nf Learning 



Our discussion of the physiological mechanisms of 

 motivation would not be complete without considera- 

 tion of the role of learning. Unfortunately, we are 

 hampered in our thinking by our ignorance of the 

 physiology of learning, but there are experiments on 

 motivation, involving learning, which are particu- 

 larly instructive at the behavioral level and give us 

 some valuable ideas about the general nature of the 

 underlying mechanisms. Three different kinds of ex- 

 periments are of interest here: a) studies of the influ- 

 ence of learning in the modification of biologic, illy 

 adaptive motivated behavior, /» ) comparisons of the 

 effects of a number of physiological variables on mo- 

 tivation in which learned performance .is opposed to 

 unlearned eonsummalnrv response is used as the 

 measure of motivation; and r) direct physiological 

 studies <>l reward and punishment, and the reinforce- 

 ment of learning and learned performance. 



Illustrative of the first kind of stud) is the postop- 

 erative ingestion of sodium chloride bv the adrenal- 

 ectomized rat. Epstein & Stellar (-,-,) have shown 

 thai the rat requires no special postoperative experi- 

 ence to increase ils sodium chloride intake following 



adrenalectomy. < >n the other hand, Harriman (66) 

 has demonstrated thai rats given experience with 



glucose and sodium chloride solutions before opera- 

 tion, unlike naive rats, will prefer glucose to sodium 



chloride postoperatively and will die because they fail 

 to ingest sufficient amounts of salt. In a similar type 

 of experiment. Young (172) has shown that protein- 

 deficient rats will prefer sucrose to protein in a test 

 situation where they had been in the habit of selecting 

 sucrose before any deprivation; if placed in a new 

 test situation, however, where they have no previous 

 habits, they show an immediate preference for protein; 

 if tested in the two situations on alternate trials, they 

 alternate between sugar and protein preference. 

 Similar interfering and facilitating effects of previous 

 habits have been shown in many other studies (67, 

 138, 139), and they hint at how complicated the 

 mechanism of motivation can become when the op- 

 portunity for learning is introduced. This problem 

 has been solved in most of the experiments we have 

 discussed so far by keeping the possibilities of learning 

 at a minimum by using as a measure of motivation a 

 simple, uncomplicated consummatory response, 

 natural to the animal. But as you will see in a mo- 

 ment, this may be a misleading practice, and our con- 

 clusions about the physiology of motivation should 

 properly be based on measures of learned performance 

 as well as consummatory measures. 



A number of experiments have been done compar- 

 ing results with the measure of learned performance 

 and the consummatory response. In a striking study 

 of rats with lesions of the ventromedial hypothalamus, 

 Miller et al. (102) were able to show that while hyper- 

 phagic rats ate more than normals in a free-feeding 

 situation, they showed much less hunger motivation 

 than normals when required to perform learned re- 

 sponses, to work or to overcome the taste of quinine 

 to get food. In the face of these contradictory findings, 

 the authors suggest that ventromedial lesions mav not 

 release hunger motivation for easier arousal but simplv 

 interfere with the mechanism for stopping eating. 

 However, it is well to point out that the hyperphagics 

 used in this study were obese and, judging from 

 Tcitelbaum's findings cited above, the results reported 

 here mav be as much a matter of obesity as they are 

 of hy pothalamic lesions. 



In a series of different experiments bv Miller and 

 his students (22, 82, 101, 103, 104), much better 

 agreement was found between the consuiumatorv 

 measure and the learned-response measure. In these 

 studies, iliev used rats with gasiric fistulas so that they 

 could compare the effects of saline, milk or water 

 introduced directly into the stomach or taken bv 

 mouth. Using both hungrv and thirsty rats, they 

 found that taking fluid bv mouth had greater satiating 

 ellcels whether they used as .1 measure of motivation 



