>33« 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



Typically the damage or stimulation of mediobasal 

 sectors affects intentional behavior by disrupting the 

 more or less orderly recurring sequences of actions 

 which constitute feeding, fighting, fleeing, mating and 

 maternal behavior. None of the elements of the 

 sequence drop out; rather the duration of any one 

 such element of action is altered. The outcome of an 

 action appears, in these damaged animals, to be an 

 ineffective terminant or maintainant of acts in the 

 .sequence (i8). Specifically, animals with mediobasal 

 foreljrain resections continue feeding long after control 

 subjects (with the same amount of deprivation and in 

 the same situation) have stopped eating (34, in). The 

 duration of avoidance behavior is shortened : thus, a 

 monkey will repeatedly grasp a flaming match even 

 though he is burned each time (35). A fighting re- 

 action is not maintained. An animal with a medio- 

 basal lesion may draw blood or have a finger bitten 

 off and within a few seconds sit unconcernedly 

 munching peanuts. This effect, as that on avoidance, 

 is especially easy to discern in measures of extinction 

 (117). Reactions to a 'frustration situation' are also 

 altered along this dimension: the intensity of an 

 animal's reaction to frustration is unimpaired, but the 

 duration of the reaction is shorter than that of a 

 control subject (113). When closely examined, the 

 effects of mediobasal forebrain ablations on hoarding 

 (133), mating (34) and maternal (134) behavior, are 

 on the duration of a particular element of the se- 

 quence, for example, food or an infant is dropped be- 

 fore the nest is reached or, occasionally, carried to the 

 nest and then taken out again to be dropped elsewhere. 



The neural mechanisms whereby the mediobasal 

 sectors affect the outcome determinants of behavior 

 are only beginning to be detailed (109). Essentially, 

 the mediobasal forebrain structures are especially re- 

 lated afferently and efferently to medial mesencephalic 

 and diencephalic structures in which are located the 

 slowly adapting receptors surrounding the third and 

 fourth cerebral ventricles (such as the osmo- and 

 temperature-sensitive elements) as well as to the non- 

 specific diffuse systems. The latter are characterized 

 by networks of short fine-fiber neurons. In such net- 

 works synaptic, dendritic and electrotonic phenomena, 

 especially sensitive to neurochemical influences, are 

 most likely of greater total significance than are 

 rapidly propagated patterns of neural impulses. In 

 fact, the connections between the mediobasal fore- 

 brain and medial mesencephalic and diencephalic 

 structures are so arranged that even when propagated 

 signals are transmitted, the effect on the target site is 



more often a change in local excital)ility than the 

 firing of neurons (44). 



Characteristic interactions between the functions 

 of the mediobasal sectors and those of the diffuse non- 

 specific systems are thus clearly established at the 

 neural level — interactions which can account for the 

 finding that intentional behavior is affected when 

 mediobasal forebrain structures are ablated or 

 electrically excited. An analysis of the effects of these 

 interactions can therefore be undertaken. Changes in 

 the excitability of these neural mechanisms have been 

 correlated with changes in activation, such as sleep- 

 wakefulness, which in the intact organism are cyclic 

 processes. Whether the outcome of any particular 

 action is desirable or not is a cyclic function — for 

 instance, a heaping plate of food is most desirable at 

 the peak of the appetitive cycle but slightly nauseating 

 just after consumption of a large meal. The differ- 

 ences in the effects of outcomes depend therefore on 

 the dispositions of the organism that are only partially 

 (and inadequately) described by the differences that 

 can be found to occur during any one cycle (27, 28, 

 48, 77, 118). More complete description would take 

 into account cyclically recurring regularities. 



The cycles of activation (or deactivation) in be- 

 havior that occur with changes in the excitability of 

 the central system are analogous to conversions be- 

 tween potential and kinetic energy in physical 

 systems — the activity of water at the base of a fall is 

 not properly described in terms of the differences be- 

 tween the 'amount' of energy which exists in the 

 limpid pool at the top of the falls and that which 

 characterizes the excited turbulence at the base. 

 Rather, the difference is measured by reciprocally 

 related quantities — kinetic and potential, in the case 

 of physical systems (such as the waterfall); or anabolic 

 and catabolic, in biological descriptions. Thus, a 

 "need-reduction" formulation, in which the referent 

 against which change is specified is considered to be 

 some basal (that is minimal) level is inadequate. This 

 conceptualization, by insistence on "amount' of need 

 as the basic variable, easily falls into the trap of con- 

 fusing the reciprocally related potential and kinetic 

 manifestations of the energic process with quantita- 

 tive differences in the total amount of energy in the 

 system. 



An added argument against simple need "reduc- 

 tion,' based on the notion of 'physiological need,' is 

 that such a notion does violence to physiological fact. 

 Oxygen deprivation produces little increase in 

 respiratory rate, provided a constant partial pressure 

 of carbon dioxide surrounds the respiratorv receptor 



