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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



, Prejerenu and Competition Among Drives 



As pointed out earlier in connection with thirst, 

 much of the strength of motivation is determined by 

 the competition anions; different drives and the choice 

 among alternative goals. As animals become partly 

 satiated in one motivation, other motivations become 

 prepotent and contribute, by competitive interference, 

 to the decrement of the measured drive. These rela- 

 tionships are seen most clearly in situations where two 

 drives arc pitted against each other. The obstruction 

 box, of course, is a commonly used method for the 

 stuck of competition between hunger, thirst, etc., 

 and pain avoidance. Similarly, experiments have been 

 done where animals are shocked upon eating in order 

 to determine the amount of shock necessary to in- 

 hibit eating (98). Or, in other cases, food or water 

 have been adulterated by quinine or some other sub- 

 stance with aversive properties to determine the 

 interference necessary to reduce ingestion or abolish 

 it completely (100). A few attempts have been made 

 to develop tests to compare the strength of different 

 drives by the direct choice method. In one test, the 

 rat is given the choice between going in one direction 

 to a food goal and another direction to a sexual part- 

 ner (1 mi. In another test, it is determined how much 

 shock lias to I"' delivered to a rat to make it leave a 

 grid and enter cold water, or how cold water has to 

 be to make the rat enter the electrified grid (1 to). 



Several different kinds of methods have been used 

 to study specific hungers and food preferences. 1 he 

 i l.issical method developed by Richtcr (129) allows 

 the animal to drink from two bottles, one containing 

 water and the other some test solution. The rats have 

 access to the bottles for 24 hr. with food ad libitum, 

 and then, to evaluate position habits and solution 

 preference, the hollies are switched from one side to 

 thr other. Thus, an average of 2 days of ingestion 

 gives the preference for the substance tested. For ex- 

 ample, the normal rat may take roughly 15 cc from 

 each bottle if both contain water, but it will take 80 to 

 100 cc from one containing 10 per cent glucose and 

 practically nothing from the one containing watei 



Preference may also be measured by the single- 

 stimulus method (jo, 166) in which .1 different solu- 

 tion is given for .1 i -hr. period on each day of testing 

 after 1 5 hr. of water deprivation. Food is not avail- 

 able during the test but is given later in the day in .1 



j in maintenance period along with plain water. 



This method has the disadvantage ol introducing .1 



constant factor of thirst in the study of specific hun- 



but it has two great .idv antages : a) it makes con- 



tinuous records at the time-course of ingestion quite 

 feasible, and b) it makes it possible to test the effects 

 of short-term physiological variables on preference, 

 like the effects of drugs, intubation, etc. In the single- 

 stimulus method, the measure of preference is simply 

 the relative amount ingested, as opposed to the two- 

 bottle method where choice as well as ingestion make 

 up the preference. 



Choice is even more important in the two-bottle 

 test where two test substances are offered simultane- 

 ously (154). In this test, animals may drink almost 

 exclusively solutions which they can ingest in only 

 small amounts (30 per cent glucose) in preference to 

 solutions they can ingest in large amounts (10 per 

 cent glucose). Thus, preference indicated by choice 

 and by ingestion do not always agree. 



The differences among the four methods may be 

 illustrated by the rat's preference for sugar. In Rich- 

 ter's two-bottle method, 10 per cent glucose is the 

 most preferred, for that is ingested in the largest 

 quantity over water (131). In the one-bottle test, 

 where thirst is a factor, 5 per cent glucose is the most 

 preferred (95). Where two sugars are presented 

 simultaneously, 30 per cent glucose is taken in prefer- 

 ence to all other solutions (154). Finally, in Young's 

 situation, where ingestion is not a factor at all, the 

 peak preference is for the most hypertonic solutions 

 (173). Obviously, in assessing food preferences it is 

 necessary to make the distinction between how much 

 an animal can ingest and what it chooses. 



Learning and Learned Performance 



While it might be expected that the rate .11 which 

 an animal learns a new response should be .1 function 

 of its motivation, it actually turns out that rate of 

 learning is a poor measure of motivation. The rate 

 at which an animal reaches the asymptote of learning 

 is roughly the same over .1 wide range of strengths of 

 drive and over a wide range of incentives (171 ). The 

 performance of .1 learned response, however, is 

 greatly affected by motivation in that the latency of 

 the new response, the number of errors an animal 



makes, etc., at the asymptote of learning are higher 

 with weak motivation and poor incentives than with 

 Strong motivation and highly effective incentives. 

 Thus, it has been shown in a number of studies that 

 performance is affected I mt rale of learning unchanged 

 with variations in degree of hunger (49), the kind of 

 reward (54), the amount of reward (174) and the 

 number of items of reward with amount held con- 

 stant (45) 



