[8 



II VXDBOOK OF l-MYMol I k.\ 



SI I ROI'IIVSIOLOGY III 



i .ii> id solicit petting and respond to it with heightened 

 'pleasurable 1 reactions (137). d) There have been re- 

 ports of reduced emotionality following cingulectomy 



1 ii(|, ijoi and the frontal lobes have been impli- 

 cated, although their contribution is not entirely 

 clear (59). 



The situation is somewhat clearer in the case of 

 sexual behavior. Here it has been found that neo- 

 cortical lesions lead to a reduction in sexual motiva- 

 tion, the larger the lesion in the male rat the greater 

 the effect without regard to locus (12); females are 

 less affected than males by cortical ablations; and the 

 higher the animal on the mammalian scale, the 

 greater the effect (15, 17). In addition to this excita- 

 tory role of the neocortex, there is evidence for an 

 inhibitor) role of the amygdala (135, 136) and over- 

 lying pyriform cortex (43 1, in male cats at least. 

 After some delay following ablation of these regions, 

 there may be exaggerated sexual responses in male 

 cats and monkeys characterized by strong drive and 

 poor discrimination of sexual objects. (See also 

 Chapter XI.IX on reproductive behavior and Chap- 

 ters LVI, LVII and LVII1 on the limbic system in 

 this Handbook. 1 



In the case of sleep, it has been found that decorti- 

 cation in dogs is followed by an inability to postpone 

 sleep and maintain wakefulness for long periods of 

 time with the result that such dogs sleep and wake in 

 short cycles over the 24-hr. period (80). Similar 

 excitatory contributions are revealed in human 

 cases with restricted cortical and thalamic lesions 

 (46-48). Most striking of all, however, is the excita- 

 tory contribution from the brain-stem reticular forma- 

 tion (91, 92). Lesions here are followed by marked 

 somnolence, and central stimulation leads to prompt 

 arousal from sleep. 



Little is known about the role of central neural 

 structures outside the diencephalon in hunger and 

 thirst, although overeating has sometimes been re- 

 ported as a result of frontal lobe damage (132, 13 ;) 



In maternal behavior and in food-hoarding, it has 



been reported thai destruction of mid-line cortex in 



the ral leads to a reduction in motivation .is well as a 



disintegration of the organization of these patterns 

 ol behavior (148, 149). Finally, some suggestion 

 about an excitatory role oi the frontal cortex in the 

 motivation to avoid pain (nmes from the psycho 

 mrgii al studies dune for the relief ol intrai table pain 

 In these cases, the patients reporl postopera- 

 tively that while thev still feel the pain, it no longer 

 bothers them .1- b 

 Because <>l the lack "i good anal ical data rele- 



vant to these problems, it is impossible to say defi- 

 nitelv by what route these structures exert their ex- 

 citatory and inhibitory effects on motivated behavior. 

 I he most common hypothesis is that thev work 

 through the integrating mechanisms of the hvpo- 

 thalamus, for many pathways to this structure have 

 been described. Thus it has been suggested that the 

 contribution of the rhinencephalic cortex to emotional 

 behavior is mediated by the amygdala which in turn 

 functions through the ventromedial nuclei of the 

 hypothalamus (9). While such hypotheses are reason- 

 able in the light of a concept of hierarchical organiza- 

 tion of the central nervous system and are part of the 

 theoretical viewpoint followed here, it will remain for 

 future experimental work to supply the detailed 

 evidence needed for their support. For example, 

 although we may conclude clearly that a part of the 

 cortex has an excitatory or an inhibitory effect on 

 motivated behavior, we do not know whether its 

 physiological role is excitatory or inhibitory, for 

 under the present viewpoint, it may exert its influence 

 on either an excitatory or inhibitory diencephalic 

 mechanism, or have a reciprocal influence on both. 

 Or by alternative anatomical and physiological routes, 

 it may bypass these mechanisms. 



Sensory Factors 



From both anatomical and physiological evidence, 

 it is quite clear that the hypothalamus is under ex- 

 tensive sensory control, receiving afferents directly 

 from all the modalities over the specific pathways 

 and, through collaterals, over the nonspecific path- 

 ways ol the reticular system (60, 90, [51). Further- 

 more, on the basis of extensive behavioral evidence, a 

 prhnarv role must be given to sensorv factors in the 

 control of motivated behavior. It has already been 

 pointed out that mammalian sexual motivation mav 

 be destroyed bv peripheral surgical reduction or 

 elimination of two or more sensorv avenues (16). In 

 this case, the sensorv contribution to arousal is non- 

 specific since any one sensorv path can be eliminated 

 without significant effect. In his analvsis ol' sleep, 

 Kleitman (79J draws the similar conclusion that it is 

 the sum total of afferent input that controls wakeful- 

 ness rather than anv specific sensorv svstem, and this 

 notion is inherent in Magoun's conception (9a) of the 

 role of the brain-stem reticular formation. Direct 

 support lor these v iew s coines from Bremer's prepara- 

 tions in which waking EEG patterns survive brain 



sei linn until there is a sufficient reduction in alter- 

 ent input I 28, -'<l 



