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HANDBOOK OF I'lIYSIOl-dGY 



NEUROPHYSIOLOGY III 



student) that pays 'attention' to a problem is more 

 likeK to solve it than one whose attention wanders. 

 What is the role of these poorly defined and somewhat 

 elusive concepts in the learning process? 



mim:ii wisms in motivation We know thai a hungry 

 rat readily learns to run a maze for food, and that a 

 well-fed one learns the maze imperfectly or not at 

 all. It is generally accepted that both of these observa- 

 tions are related to the fact that, so far as food and 

 water at least are concerned, the animal possesses a 

 built-in mechanism that detects what substance is 

 required for continued well-being and institutes motor 

 activity appropriate for restoring an equilibrium with 

 respect to it. So far as learning goes, we are concerned 

 not with the detection aspect of this process but 

 rather with the way in which such innate mechanisms 

 promote motor activity, for in doing so, they also 

 somehow produce conditions that are favorable for 

 learning to occur. 



The hypothalamus is known to contain 'centers' 

 for hunger and thirst. These are defined by the fact 

 that certain hypothalamic lesions produce animals 

 that are continuously ravenous while other lesions 

 produce animals that starve to death in the presence 

 of abundant food (162, 230, 233). For thirst, on the 

 other hand, stimulation of the hypothalamus elec- 

 trically (83) or with salt solutions (7) elicits dramati- 

 cally increased drinking behavior. Such observations 

 make it clear that hypothalamic structures connect 

 with the motor apparatus and, under certain condi- 

 tions, organize and control behavior. 



Animals are motivated to learn, of course, other- 

 wise than by hunger or thirst. Electric shocks, for 

 instance, offer powerful motivation and such shocks 

 act, if we can believe our introspections, through 

 pain and the emotional response to pain that they 

 produce. Learning is a No commonly accomplished 

 when a pleasant emotional experience is associated 

 with the termination of the training procedure. Thus 

 the emotional repertoire of the animal seems to be 

 implicated in learning in a manner similar to that of 



"simple' factors such .is hunger and thirst. The ques- 

 tion for which we would like an answer is how .ill 

 these motivating factors hunger, thirst, emotional 

 mechanisms and the like prepare the brain for the 

 specific 1 hanges of learning. 



mechanisms in \iiinhmn. Before proceeding to fur- 

 ther consideration ol ibis, however, le( us examine 

 the concept ol 'attention.' Ii is a common observa- 

 tion in human behavioi that we can deliberately pa) 



attention to one stimulus rather than another in a 

 constellation of stimuli. The related ability to empha- 

 size relevant stimuli and exclude irrelevant ones is 

 also clearly an important item in some human learn- 

 ing at least. Similar attentive functions appear to 

 operate in animals as well, and selection of the rele- 

 vant stimuli and objects in the learning situation may 

 be the first thing an animal does in learning a maze, 

 a discrimination or even a simple classical CR. 



In recent times this capacity to attend has been 

 brought into the experimental realm by the studies 

 on the reticular system which are described in detail 

 in Chapter LI I by French in this Handbook (see also 

 139, 141). Excitation of the ascending reticular acti- 

 vating substance exerts a general alerting or activat- 

 ing effect on many parts of the brain and particularly 

 on the cerebral cortex. Depression by sleep or drugs 

 leads, among other things, to the dropping out of 

 learned reactions, although unconditioned reactions 

 may still remain (113). Many other examples could 

 be given to show its apparent direct involvement in 

 attentive processes. The reticular formation is not 

 the only part of the brain involved in the reactions 

 we call attention, nor is there any reason to believe 

 that 'causing attention' is its sole function. There is 

 not much question, however, that it plays a major 

 role in the process 



limbic-midbrain CIRCUIT. How then do the motivat- 

 ing and attentive functions prepare the brain for the 

 specific changes in learning? Recent experiments on 

 the limbic system provide what may be a key to the 

 answer. It is becoming increasingh clear that emo- 

 tional mechanisms are largely the consequences of 

 activities in the limbic system and hypothalamus 

 (e.g. 146, 1471. According to anatomical data re- 

 cently analyzed by Nauta (173, 174), these structures 

 receive .1 substantial afferent supplv from the nuclei 

 of Gudden and Bechterew in the midbrain reticular 

 formation; the limbic structures project, in turn, back 

 to these same midbrain regions. Thus a major input 

 to, and outflow from, the limbic system involves the 

 midbrain reticular substance which is somehow con- 

 1 nned with attention and alertness, and which also 

 transmits, and modifies the transmission of, impulses 

 passing to and from the cortex. II, .is serins reason- 

 able, the limbic and hypothalamic structures are 

 concerned with innate- mechanisms in behavior — 



emotional and Otherwise then the midbrain reticu- 

 lar substance into which they discharge is potentially 



the place where such neural aclivilv is brought into 

 contact with the neural consequences of current 



