DRIVE AND MOTIVATION 



'5'9 



Our knowledge of hunger and thirst suggests that 

 the arousal of motivated behavior in these cases 

 should be a joint function of sensory impulses arising 

 from gastric contractions or dryness of the throat, and 

 taste, tactile and temperature receptors in the mouth. 

 There are no sensory deprivation experiments to 

 provide a good test of this point, but everything we 

 know about the acceptability of foods and fluids of 

 different temperatures, consistencies and flavoring 

 suggests the joint operation of many stimuli in the 

 control of these types of motivation. 



In addition to sensory contributions leading to the 

 arousal of motivated behavior, it is clear that some 

 stimuli can be inhibitory and lead to the reduction of 

 motivation. Most striking are the so-called aversive 

 taste stimuli like quinine which are strong enough to 

 inhibit eating or drinking (ioo). In hunger and thirst, 

 there also seems to be an inhibitory sensory effect 

 arising from the stomach, judging from the effects of 

 loading food and fluids into the stomach and mechani- 

 cally stretching it (i, 75, 106, 153). Furthermore, the 

 fact that animals with esophageal hstul.is stop rating 

 and drinking at some point even though nothing 

 enters the stomach (21, 140, [53) suggests that the 

 stimuli that feed back from consummatoiy behavior 

 might have a net inhibitory effect on hunger and 

 thirst. 



Many other studies shed light on the role of sensory 

 factors in motivation. Some of these ue will discuss 

 below when wc take up the interaction of factors con- 

 trolling motivation. Others we must omit because at 

 the present stage of knowledge, we have no idea 

 through what neurophysiological mechanism they act. 

 What is needed, obviously, is direct experimental evi- 

 dence on the relation between peripheral sensory 

 factors and central neural mechanisms. A start has 

 been made in the finding that changes in the electric. tl 

 activity of the lateral part of the anterior hypothala- 

 mus may be induced by vaginal stimulation of the 

 estrual cat (118). But until we have more information 

 of this sort, any specific notions about the mechanisms 

 whereby excitatory and inhibitory sensory contribu- 

 tions are made to the arousal and satiation of motiva- 

 tion must remain rather speculative. 



Internal Environment Factors 



Because the hypothalamus is highly vascularized 

 (44) and borders on the third ventricle, it has been 

 strongly suspected in the past that it may be a major 

 site of interaction between the internal environment 

 and the nervous system. Fortunately, we now have 



some direct information on this point in the study of 

 motivated behavior. One striking line of investigation, 

 already mentioned, involves the direct introduction of 

 substances into the brain in an effort to imitate ex- 

 perimentally the role of the internal environment in 

 motivation. Minute quantities of sex hormones or 

 hypertonic solutions that would be totally ineffective 

 if introduced systemically can produce marked and 

 persistent changes in motivated behavior when intro- 

 duced directly into the hypothalamus through 

 chronically implanted pipettes. Out of this and related 

 work has grown the notion that there are special 

 'receptor' cells in the central nervous system, selec- 

 tively sensitive to certain chemicals or hormones or 

 physical changes in the blood and cerebrospinal fluid. 

 Thus, it has been suggested that there are tempera- 

 ture (31) and glucostatic receptors (93, 94) important 

 in hunger, osmoreceptors (3, 160) critical in thirst, 

 temperature receptors (122) playing a role in body 

 temperature regulations and perhaps motivated be- 

 havior related to it, and possibly hypothalamic cells 

 especially sensitive to such drugs as amphetamine 

 (32), an appetite depressant. 



The evidence from direct injections is not entirely 

 clear at this point, however. Preliminary results have 

 shown that sex hormones arouse sexual behavior, but 

 they also arouse maternal behavior, changes in 

 respiration, exploration and digging. In the case of 

 thirst, the evidence for osmoreceptors derives from the 

 fact that hypertonic solutions arouse drinking and 

 water does not. As Larsson has shown, however, 

 hypertonic solutions will also elicit eating, rumina- 

 tion, chewing and licking in goats when injected into 

 other parts of the hypothalamus (85). Perhaps these 

 solutions have rather widespread or general stimu- 

 lating effects similar to those of electrical stimulation. 

 Thus, while the present evidence is suggestive, it is b\ 

 no means conclusive. 



I here is, however, much additional evidence to 

 support the general notion that the hypothalamus is 

 critical in the mediation of the effects of the internal 

 environment on motivated behavior. In the case of 

 sexual behavior in the ovariectomized female cat (33) 

 and in the guinea pig (51), for example, section of the 

 nervous system below the hypothalamus made the 

 contribution of sex hormones ineffective in the elici- 

 tation of estrual behavior by sacral and vaginal stimu- 

 lation. If the hypothalamus were included below the 

 section, however, sex hormones were effective in 

 rendering the response mechanisms reactive to stimu- 

 lation. Supporting the conclusion that the site of action 

 of the sex hormones may be in the hypothalamus is 



