DRIVE AND MOTIVATION 



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thalamus. (See also Sawyer's account of this topic in 

 Chapter XLIX of this Handbook.) 



The role of the hypothalamus has been less exten- 

 sively investigated in other kinds of motivation, but 

 the findings available are quite suggestive. Increased 

 emotionality in cats has been demonstrated following 

 lesions in the vicinity of the ventromedial nucleus of 

 the hypothalamus (167), suggesting an inhibitory 

 mechanism. Posterior hypothalamic lesions result in 

 reduced emotionality and placidity (121) and stimu- 

 lation of these posterior regions produces many of the 

 signs of rage (70, 123), suggesting an excitatory 

 mechanism. Strong and persistent maternal behavior 

 has been elicited in both male and female rats by 

 injection of male and female sex hormones respec- 

 tively into the medial preoptic area (56). Reductions 

 of gross bodily activity have been reported following 

 lesions near the ventromedial hypothalamus of the 

 rat, even without concomitant hyperphagia (71 ). 



Lacking in all of these experiments, unfortunately, 

 is any precise anatomical work specifying precisely 

 what structures must be involved to give these various 

 effects on motivated behavior. It is not clear in either 

 the ablation or the stimulation studies whether ii is 

 cell bodies or fiber tracts or both thai must be de- 

 stroyed or stimulated. The most we can say is that 

 the lesion or the tip of the stimulating electrode or 

 pipette must be in a particular region of the brain to 

 yield significant effects on the particular kind of 

 motivated behavior that is measured. 



A further criticism of many studies is that only one 

 kind of motivated behavior is measured upon stimu- 

 lation or ablation of a particular locus. The physio- 

 logical studies by Hess show a great deal of overlap- 

 ping and intermingling of points as far as the kinds of 

 effects he found upon stimulation are concerned. It 

 has also been found that lesions in the vicinity of the 

 ventromedial hypothalamus, designed to produce 

 hyperphagia, will also produce hypoactivity in some 

 rats and emotionality in others. But examination for 

 these additional effects has only been casual in most 

 studies, and we know nothing about whether such 

 lesions would also produce noteworthy changes in 

 sexual behavior, pain avoidance, etc. A number of 

 studies give some idea of the multiplicity of functions 

 that can be revealed from experimentation with one 

 locus. For example, lateral hypothalamic lesions 

 eliminate hunger and thirst simultaneously and often 

 produce transient somnolence (157). In his experi- 

 ments, Fisher (56) found that injection of sex hor- 

 mones into the anterior hypothalamus could elicit 

 maternal and sexual behavior simultaneously, and in 



addition could sometimes produce changes in respira- 

 tion, exploratory behavior, digging, leaping, etc. 



Thus, only the crudest questions about localization 

 of function within the diencephalon can be answered 

 on the basis of the present data on motivated be- 

 havior. At present, it appears that there are different 

 foci in the diencephalon which can be manipulated 

 experimentally by stimulation and ablation to produce 

 marked changes in different kinds of motivated be- 

 havior. Some of these foci can be characterized as 

 excitatory and others as inhibitory. But much more 

 has to be done experimentally, on the anatomical 

 side and on the behavioral side, before we can con- 

 clude much beyond these two points. 



Other Central Mechanisms 



Investigation of other central neural structures out- 

 side the diencephalon has revealed much about their 

 role in motivation. Again, it is possible to characterize 

 the function of many of these regions of the brain as 

 excitatory or inhibitory in terms of the effects of 

 lesions and stimulations on the arousal and satiation 

 of motivated behavior. Perhaps the most thoroughly 

 studied kind of motivation is emotional behavior, 

 although the picture we have at present is not en- 

 tirely clear. (Its present si. mis [s considered in Chap- 

 ter I. XIII by Brady in (his work.) Decortication, as 

 Bard showed, leads to "sham rage, 1 suggesting an 

 inhibitory role of the cortex in emotion (8); while 

 Bard & Mounteastle (()) found that ablation of 

 parts of the rhineneephalon, particularly the amyg- 

 dala and transitional cortex of the mid-line, produced 

 great increase in the ferocity of cits. Removing only 

 the neocortex, on the other hand, resulted in extremely 

 placid cats. In other studies on monkeys and cats, 

 rhinencephalic ablations, including parts of the tip of 

 the temporal lobe and the hippocampus in some 

 cases, led to placidity rather than ferocity (81, 135). 

 So far this disagreement is unresolved, but it is clear 

 that certain parts of the cortex may exert an inhibitory 

 effect and others an excitatory effect on emotional 

 behavior. 



Contributions from other parts of the brain to 

 emotional behavior have been revealed in a number 

 of other studies, a) An inhibitorv role is suggested for 

 the septal area where lesions produce a transient, but 

 greatly exaggerated, emotional response to tactile 

 stimuli in rats (27). b) There is evidence for an excita- 

 tory contribution from the brain-stem reticular system 

 and mid-line thalamic nuclei (76, 91, 92). c) There is 

 an inhibitory contribution from the anterior nuclear 

 complex of the thalamus, ablation of which causes 



