1416 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



bilateral amysjdaloid or other rhinencephalic lesions 

 fail to produce deficits of the very autonomic and 

 somatomotor functions which are so clearly influenced 

 by rhinencephalic stimulation. The rhinencephalon 

 cannot therefore he essential for their integration. 

 Most of these functions are known to be integrated in 

 the hypothalamus and brain stem. Despite the great 

 similarity ol rhinencephalic stimulation responses to 

 those obtained from stimulations of the hypothalamus 

 and brain stem, a great difference in functional 

 significance is thus revealed between the rhinencepha- 

 lon and those subcortical areas. This noninvolvement 

 of the rhinencephalon in the integration of basic 

 autonomic and somatomotor mechanisms makes it 

 easier to accept the absence of a mosaic of topographi- 

 cal representation of function in this system. It also 

 makes more acceptable the potentially dual character 

 of many rhinencephalic stimulation responses with 

 opposite effects upon the same function apt to occur 

 from the .same locus of stimulation. 



This unessential character of rhinencephalic func- 

 tion for the integration of basic autonomic and somato- 

 motor mechanisms stresses the importance of the 

 behavioral and mental alterations produced by rhin- 

 encephalic stimulations, epileptic discharges and 

 lesions. However here again the situation is not 

 clear-cut and simple. In animals, electrical stimula- 

 tion brings forth not only reactions of fear and anger 

 but also behavioral patterns of an opposite character, 

 suggesting that a 'rewarding' e.\perience can be 

 elicited by stimulation. Furthermore bilateral lesions 

 have produced placidity as well as its opposite — 

 savage, angry behavior. From the study of the experi- 

 mental records it seems unlikely that differences in the 

 anatomical location of the site of stimulation or of the 

 lesion could adequately account for these opposite 

 effects. 



All these findings suggest a great lability of rhin- 

 encephalic function based upon flexible neuronal 

 mechanisms, as revealed by electrographic studies of 

 the amygdaloid projections system (97). It has there- 

 fore been suggested that the amygdala and other 

 parts of the rhinencephalon modulate activities inte- 

 grated in subcortical structures (97, 99, loi). 



This modulatory influence is probably of great 

 importance in the organization of complex behavioral 

 mechanisms. Olds (192, 194) advanced the theory that 

 the amygdala and other rhinencephalic structures of 

 the 'second system' may be essential for the elabora- 

 tion of the reinforcing effect of 'reward' upon instru- 

 mental behavior. In view of the clear evidence that 

 other than rewarding experiences, e.g. lear and rage, 



can be produced by amygdaloid stimulation it .seems 

 necessary to broaden this hypothesis in order to 

 include motivational forces other than reward. One 

 may thus conceive of this system as more generally 

 concerned with motivational reinforcement of be- 

 havioral patterns. The behavioral alterations pro- 

 duced by amygdaloid lesions support such a view. 

 The basic defect produced by these lesions could then 

 be described as a disturbance in those motivational 

 mechanisms which normally allow the selection of 

 behavior appropriate to a given situation (192, 252). 

 Disturbances in these mechanisms would then ac- 

 count for the indiscriminate fearless approach to any 

 object, animal or person, the indiscriminate reaction 

 to any environmental stimulus described as hyper- 

 metamorphosis, the indiscriminate tendency to ingest 

 food and nonfood objects, and the indiscriminate 

 attempts to derive se.xual satisfaction from any poten- 

 tial source of gratification. The behavioral disturb- 

 ances displayed by many psychomotor epileptics give 

 further support to this concept. Constant irritation of 

 this system by an epileptogenic focus seems to produce 

 a tendency for insufficiently motivated outbursts of 

 rage as if the rage responses were triggered by a nor- 

 mally 'subthreshold' motivational stimulus. This 

 broadened concept would also be in accord with the 

 more traditional theories ascribing to the rhinenceph- 

 alon the role of integrating emotional experience and 

 expression (70, 72, 73, 138, 167-170, 195). The in- 

 volvement of this system in 'motivational selection' 

 of behavioral patterns would render more meaningful 

 the close anatomical and functional relationship of 

 the amygdala with the hippocampus (96-98), the 

 important role of which in memorv recording becomes 

 more and more apparent (182, 228), for it seems 

 evident that the reinforcing effect of such motiva- 

 tional forces as emotion, as well as the laying down 

 of memory traces, are equally essential for the selec- 

 tion of behavioral patterns (194). A close functional 

 association between the nervous substrata subserving 

 these two functions seems therefore to be a necessary 

 prerequisite for their correct interplay. 



The functional concept here developed presup- 

 poses that the structures supporting these functions 

 show a great flexibility of action and a broad spectrum 

 of influence upon neuronal mechanisms integrating 

 somatomotor and autonomic functions. The experi- 

 mental data accumulated over the past years clearly 

 show that the amygdaloid complex and other parts 

 of the rhinencephalon would meet these requirements. 



