'5"4 



HANDBOOK OF PHYSIOl nl.Y 



NEUROPHYSIOLOGY III 



chorda tympani (116) and a study of conditioned 

 responses to salt (41) both show that the normal and 

 adrcnalcctomizcd rat have the same very low sensory 

 thresholds for salt in solution. Therefore, the greater 

 salt ingestion of the adrenalectomized rat, even at 

 threshold concentrations but without a change in 

 sensory threshold, suggests that the physiological need 

 must reflect itself elsewhere in the nervous system 

 than in the local, peripheral sensory mechanisms of 

 taste. 



Regardless of the mechanism of its influence on 

 the nervous system, the role of the internal environ- 

 ment in motivated behavior is obviously an important 

 one. Vet a number of questions should be asked. 



a) Are there some physiological needs and deficits 

 that do not lead to adaptive motivated behavior? 



b) Are there changes in the internal environment 

 which are not needs or deficits but which can lead to 

 motivated behavior? c) Can there be motivated 

 behavior in the absence of internal environment 

 influences? The answer to all three questions is 'yes.' 

 For example, all rats do not grow well in cafcteria- 

 type feeding experiments where they select their own 

 diets (117), and thus far, no specific hungers have 

 been shown for vitamins A and D (67, 168). Actually 

 of course, there is no logical reason why organisms 

 should have specific hungers for every identifiable 

 nutrient in order to survive. In regard to the second 

 and third questions, it is quite evident that motivation 

 ran occur without specific deficit or need. There is no 

 interna] deficit in sexual and maternal behavior, and 

 the survival of the individual is not at stake, although 

 there are important changes in the internal environ- 

 ment associated with these motivations. Finally, in 

 Cases such as the motivation which an animal shows 

 for a nonnutritive substance like saccharin (20) or the 

 motivation to avoid pain, or to manipulate objects or 

 explore .1 new environment (65), there are no asSO- 

 c iati d 1 hanges in the internal environment known to 

 be important in the arousal and maintenance of the 

 motivation. 



Thus, not all motivated behavior is self-regulators 

 behavior, nor is it always of particular adaptive sig- 

 nificance in the survival of the individual. The in- 

 iriii.il environment, important .is it is in main re- 

 markable cases of sell-regulation, is onl\ one of the 

 factors contributing to the control ol motivated be- 

 havior. When it does operate, the pi i\ siological 



tion becomes: how do changes in the internal 

 environment influence the nervous s\siem and, there- 

 Ion', behavior? Although local peripheral mechanisms 

 may be the critii al targets in certain cases, as Richter 



and Cannon suggest, it is clear we must look elsewhere 

 in the nervous system for the major effects, presumably 

 in some central neural mechanism. 



MIT IT FACTOR CENTRAL NEURAL THEORY 



It was Lashley (86) who first put the problem of 

 motivation on a modern neurophysiological basis in 

 his classic paper, 'The experimental analysis of in- 

 stinctive behavior.' Unlike Cannon and Richter, 

 Lashley gave no special emphasis to local sensory 

 factors in motivation and did not concern himself 

 directly with homeostatic mechanisms, regulatory 

 behavior or needs. Rather he approached the problem 

 of motivation as a student of the central nervous sys- 

 tem with the multifactor theory that motivation was 

 the outcome of the joint contribution of many sensory 

 and humoral influences to some central neural mecha- 

 nism. Although he offered no direct suggestion as to 

 the locus and nature of the central neural mechanism, 

 he made a thoroughgoing analysis of motivated be- 

 havior, departing radically from the simple stimulus- 

 response theories of the behaviorists; thereby, he 

 described what the major properties of this central 

 neural mechanism might be. 



In his behavioral analysis, Lashley made three im- 

 portant points, a) Instincts and motivated behavior 

 are not simply complex chains of reflexes and arc not 

 represented by stereotyped acts. I'he detailed re- 

 sponses involved in mating, nesting, retrieving, etc. 

 vary from individual to individual and occurrence to 

 occurrence. One cannot, therefore, specify a particular 

 motor sequence thai characterizes the behavior, for 

 the same result inav be achieved bv different be- 

 havioral means on different occasions. In Similarly, 

 motivated behavior is not dependent upon anv single 

 stimulus, confined to a particular receptor locus. 

 Usually, a number of stimuli are elle, live in a par- 

 ticular sensory Or perceptual pattern across several 

 modalities. While a single stimulus might be sufficient 

 to arouse motivated behavior, the adequacy and the 



intensity of the response are determined bv the com- 

 pleteness of a complex pattern of stimulation the ani- 

 mal receives. , i Whether stimuli are effective and how 

 effective they are may depend upon sensitization of 



the organism to particular stimuli bv changes in its 

 internal environment. For example, the chronically 

 castrated male rat will not be aroused by the usually 

 effective pattern presented bv the female in heat until 

 injected with SfX hormones. 



1 hat I .ash lev w as on t lie right track is shown bv the 



