DRIVE AND MOTIVATION 



: 5°5 



subsequent development and extension of his theories 

 by Morgan (107), Beach (13) and Stellar (151), and 

 by the parallel theories developed by the European 

 ethologists, Tinbergen (158) and Lorenz. In his ex- 

 tensive work on sexual motivation, Beach was able 

 to bring together much experimental evidence in 

 support of Lashley's thesis and did much to make his 

 analysis of sexual behavior a model for the under- 

 standing of the physiology of motivation. He was able 

 to show, for example, that sexual motivation actually 

 was under multifactor control, presumably through 

 the joint effects of a number of variables on a central 

 excitatory mechanism which he felt had many of the 

 properties of Sherrington's central excitatory state. 



Beach (14, 16, 17) drew the following conclusions 

 from his extensive research and surveys of the litera- 

 ture, a) No one sensory avenue is indispensable for the 

 arousal of sexual behavior, for any two sensory s\ 5- 

 lems can be interrupted by peripheral nerve or tract 

 section (auditory, visual or olfactory) or by partial 

 denervation (of genital areas or face and mouth 1 

 without destroying sexual motivation in the naive 

 rat. Rather, it appeared that it is a nonspecific mini- 

 mum of sensory input that is important in determining 

 sexual arousal, just as it seems to be in the activation 

 of locomotion (62). h) The neocortex plays a role, 

 but no one part of it is critical in sexual arousal of the 

 male rat, for example, since experiments show that, 

 regardless of locus, the larger the cortical lesion, the 

 greater the deficit in arousal, c) Sex hormones also 

 add their effects, for without them, sexual motivation 

 may be greatly reduced or absent, yet it can be re- 

 stored by hormonal injection, d) Learning also makes 

 an important contribution to the arousal of sexual 

 behavior, for previously ineffective stimuli may, 

 through experience, facilitate the arousal of sexual 

 motivation. In sensory deprivation experiments, for 

 example, it may be necessary to interfere with three 

 sensory systems peripherally in the experienced male 

 rat before motivation is eliminated, compared to two 

 in the naive animal. Furthermore, there is evidence 

 that in the male primate, some sexual experience may 

 be essential for the appearance of adult sexual motiva- 

 tion, e) That these various factors interact in a com- 

 mon subcortical neural mechanism is suggested by tin- 

 fact that sexual behavior, lost as a result of neocortical 

 lesions, may be restored by hormone injections. (See 

 p. 1520 on interaction of factors.) 



Particularly important in this analysis of sexual be- 

 havior are the changes which take place in the con- 

 trol of sexual motivation in phylogeny. Comparing 

 animals from rat to man, there are a decreasing de- 



pendence upon hormones, and an increasing depend- 

 ence upon sensory factors, learning and the neocortex. 

 Sex differences are also important and instructive. 

 The female, for example, is much more dependent 

 upon hormones through the phylogenetic series Beach 

 compared than the male. The male, on the other 

 hand, is much more influenced by changes in sensory 

 stimuli, cortical lesions and learning than is the fe- 

 male. 



A somewhat independent development of these 

 views of Lashley and Beach is seen in the contribu- 

 tion of the European ethologists, Tinbergen and Lorenz 

 (cf. 89, 158), in their study of instinctive behavior. 

 This theory of instincts perhaps suffers because it is 

 not based on modern neurophysiological principles 

 but rather is cast in hydrodynamic terminology. Thus, 

 Tinbergen speaks of neural mechanisms controlling 

 instinctive acts which build up a "reservoir' of 'action 

 specific energy' until released l>\ some appropriate 

 Stimulation. No direct effort at localization or experi- 

 mental manipulation of the neural mechanism is as 

 yet apparent in this work. But this criticism is of only 

 minor concern at the moment, for it would be quite 

 possible to recast Tinbergen's terminology and make a 

 direct experimental approach to the neurophysio- 

 logical problem. 



Like Beach and Lashley, the ethologists propose 

 that changes in the internal environment, such as 

 those produced by deprivation or an increase in sex 

 hormones, contribute, along with sensory influences, 

 to the arousal ol .1 central neural mechanism. In some 

 instances, internal changes may be intense enough to 

 yield instinctive patterns, in the absense of sensory 

 stimuli, in which case the ethologists speak of 'vacuum 

 reactions.' But usually, sensor) stimuli play two essen- 

 tial roles: a) they contribute to the level of excitation 

 in the central neural mechanisms, and b) they 'trigger' 

 the response by releasing the excitatory mechanisms 

 from the control of a postulated inhibitory mecha- 

 nism; in this latter case, the stimuli are called 'sign 

 stimuli' or 'releasing stimuli." 



On the basis of their analysis of complex instinctive 

 acts into behavioral hierarchies, the ethologists fur- 

 thermore assume that there is a hierarchy of neural 

 mechanisms, each built up by internal and sensory 

 influences and each selectively released from inhibi- 

 tion by appropriate stimuli. The release of each neural 

 mechanism not only results in a behavioral expression 

 but also 'primes' the next lower neural mechanism 

 in the hierarchy by contributing to its excitation along 

 with sensory and humoral factors. Thus in the re- 

 productive behavior of fish, for example, the sequence 



