DRIVE AND MOTIVATION 



r 507 



arouse these diencephalic mechanisms directly or in- 

 directly. On the one hand, as Beach suggests, these 

 sensory influences are additive in their effects, so 

 that it is the sum total of sensory input to these dien- 

 cephalic mechanisms that determines the amount of 

 arousal or satiation. On the other hand, sensory in- 

 fluences must contribute highly specific information, 

 for motivated behavior may be highly discriminative 

 and selective. Furthermore, as Beach points out, 

 sensory influences may be classed as learned or un- 

 learned, for previously ineffective stimuli can come to 

 arouse or satiate as a result of past experience, b) 

 Chemical and physical properties of the internal en- 

 vironment, operating through the circulators system 

 and through the cerebrospinal fluid system, pre- 

 sumably can contribute directly to the activity of 

 these excitatory and inhibitory mechanisms and, 

 therefore, to the arousal and satiation of motivated 

 behavior. Again, while these humoral influences may 

 have general arousing and depressing effects, there is 

 evidence to suggest that their effects may be highly 

 specific to particular types of motivated behavior, 

 implying the possible existence of highlv selective 

 central 'receptors,' sensitive to changes in the internal 

 environment, c) Finally, central neural influences, 

 arising elsewhere in the nervous system, particularly 

 the neocortex and the rhinenccphalon, may contrib- 

 ute excitatory or inhibitory effects to the control of the 

 diencephalic mechanisms and, through them, play a 

 role in the serial organization and patterning of mo- 

 tivated behavior, as well as its arousal and satiation. 



While the factors operating in the control of motiva- 

 tion can be listed simply, their mode of action is 

 undoubtedly complex. In the first place, all of these 

 factors probably interact in their influences, being 

 interdependent and perhaps equipotential in the 

 arousal of the basic diencephalic mechanisms. Thus, 

 their combined influences might be thought of as addi- 

 tive such that an increase in one influence would com- 

 pensate for a decrease in another. For example, how 

 effective a given sensors stimulation will be in 

 arousing motivation will depend upon the concom- 

 itant influence of the internal environment or upon 

 the nature of previous stimulation whether learned or 

 unlearned. 



In the second place, it is possible that each of these 

 factors could have two kinds of influence on the dien- 

 cephalic mechanisms; thev could either activate or 

 depress either excitatory or inhibitory diencephalic 

 structures. This results in an enormous increase in the 

 possibilities for refined control and for complexity 

 of mechanism. A sensors stimulation, a neocortical 



or rhinencephalic influence, a hormone, or a drug, 

 for example, could decrease motivated behasior by 

 activating the inhibitory mechanism or depressing 

 the excitatory mechanism. 



A third complexity in this mechanism derives from 

 the fact that the execution of the motivated behavior 

 itself can provide important sensory changes and 

 changes in the internal environment (see the lower 

 part of fig. i ). Where materials are ingested by the 

 organism as in hunger and thirst, there is a continuous 

 change in the internal environment due to absorption 

 once the behasior starts, and throughout, there is 

 obviously a new source of stimulation of the ali- 

 mentary tract upon ingestion. In addition, even svhere 

 there is no ingestion, as in pain asoidance or in nest 

 building, there is feedback of response-produced stim- 

 ulation occasioned l>s the execution of the behavior 

 as well as changes in stimulation resulting from 

 changes in the ens ironment produced by the behavior. 



As you ssill see later, there is much experimental 

 csidence for many parts of this conceptualization, 

 but there are still many gaps in our knowledge and it 

 is not clear that all the details of this general mecha- 

 nism appls tn .ill cases of motivated behasior. For ex- 

 ample, we cannot be certain esen at present that the 

 major focus of the neural system controlling motiva- 

 tion actually is in the hypothalamus (69). Esen in 

 cases where the hypothalamus is implicated our in- 

 formation is still incomplete. There seems to be no 

 humoral factor in the motivation to avoid noxious 

 stimuli; no inhibitory mechanism in the diencephalon 

 has set been discovered for thirst, although one has 

 been shown for hunger; very little is known about 

 cortical mechanisms in hunger, although a great 

 deal has been worked out relative to sexual behasior, 

 maternal behasior and emotions. Furthermore, there 

 is a real question of boss- much of this theoretical 

 mechanism mas be operatise in learned motivation, 

 and in certain complex instances of human social and 

 personal motives. As Beach has shown in the case of 

 sexual motivation, it is clear that the rclatise contribu- 

 tions of the factors controlling motivated behasior 

 change in phylogeny. Obviously, the specific physio- 

 logical mechanism for each kind of motisation, and 

 for each species, svill be unique in some way; but the 

 point here is that all motisation should ha\-e the same 

 general, multifactor physiological mechanisms at its 

 basis. 



Before going into the experimental evidence sum- 

 marizing our knowledge of the physiology of motisa- 

 tion, it will be helpful to digress in order to specifs 

 more precisely sshat sve mean by motisation in be- 



