'474 



ll\M)H(«)K OF I'llYSIOLOGY 



NEUROPHYSIOLOGY III 



about the neural correlates of such learning, insight 

 will not be discussed further in this paper. 



Psychologists have sometimes tried to include 

 perceptual learning with Type I or Type II learning. 

 Main oilier schemes for classifying learning have 

 been proposed, and considerable research and much 

 controversy have revolved around them. There is no 

 reason in uo into these matters here, but the interesied 

 reader can delve into them b\ referring to excellent 

 m sources (45, 96, 97, 224, 236, 237). For our 

 purposes, it is well to keep in mind the distinction 

 between Type I and Type II learning procedures 

 since the neural correlates of the learning phenomena 

 ,i"i iciated with them are si imetimes different 



Phenomena Related to Learning 



In addition to the three general classes of learning, 

 (here are three important phenomena closely related 

 to learning thai nevertheless differ in certain respects 

 from what we ordinarily consider to be learning. One 

 is 'habituation' (236, p. 3881, the gradual waning of a 

 response in the presence of continued stimulation ( aol 

 10 be confused with 'extinction* which will be discussed 

 below). lire a gun and it at first startles or excites the 

 organism. Upon the repetition of such stimulation, 

 however, the organism makes less and less response 

 to it. Similarly, a dog placed in a harness is al first 

 excitable and restless, but in time it settles down and 

 bei omes habituated to the situation. 



Another related phenomenon is "sensitization' (98, 

 p. 41 I. This is an increase in responsiveness to a 

 stimulus because of prior excitement by another 

 stimulus. Shock an animal a few times and it is much 

 more likely to respond excitedly to a bell or light 

 than it would normally. 



Still a third phenomenon, recently studied in some 

 detail b\ ethologists, is 'imprinting' (200, 236-238). 



I his is a relatively rapid 'learning' that takes place 

 optimally al a certain time in the growing organism. 



\ duckling exposed at a particular time after hatching 

 to the call of the mother duck follows her thereafter, 

 since imprinting develops relative to her. One can, 

 however, experimentally evoke imprinting l>\ the 

 diK kling relative to any other soun e oi mo\ ing sound. 



I lii s 1- .1 permanent change in the behavior ol the 



inismand thus qualifies as learning, yet ii involves 



so far as we know neithei an unconditioned stimulus 



inn any iiisliunienl.il response and lienee is nol quite 



like any ol the three basic classes of learning. 



REINFORCEMENT AND EXTINCTION. Both Type I and 



Type II learning require a 'reinforcement.' In classical 

 (Type I) learning, the reinforcement is the US: in 

 instrumental (Type II) learning, it is the reward or 

 punishment that follows some specified response. The 

 process of providing the reinforcement, it should be 

 noted, is also called 'reinforcement.' 



Alter a subject has learned a habit, the experimenter 

 can institute an 'extinction' procedure. This is the 

 omission of the reinforcement while maintaining all 

 other aspects of the training routine. In Type I 

 conditioning, extinction consists in presenting the ( S 

 without the US: in Type II learning, it consists of 

 presenting the CS (in avoidance conditioning) or of 

 permitting the animal to make its learned responses 

 (in a Skinner box, puzzle box or maze) without 

 providing any reward for such responses. The result 

 of an extinction procedure is, of course, an increasing 

 tendency for the organism not to give the CR until 

 eventually the organism is not responding at all. This 

 process, like the procedure, is also called 'extinction.' 



Extinction is to be distinguished from 'forgetting' 

 and its converse "retention." Forgetting is a diminution 

 in the tendency to respond in the absence of further 

 training or of experimental extinction. Teach an 

 animal some particular response, rest him for a few 

 days (or even hours) and he is likely to forget in some 

 degree what has been learned. A lew more reinforce- 

 ments will be required before the animal again 

 responds as consistently as it did at the end of the 

 learning trials. The usual incisure of such forgetting, 

 however, is not the amount forgotten, but rather what 

 is retained. If, for example, an animal was responding 

 correctly 100 per cem of the time at the end of a series 

 of learning trials, and then responds only 70 per cent 

 of the time when tested after 2 weeks of rest, we say 

 that the retention was 70 per cent. In neurophysi- 

 ological studies of learning, we are interested not only 

 in the neural basis of acquisition of learned responses 



but also in the brain events explaining retention of 

 things previously learned. It sometimes happens, as 

 we shall see, that acquisition and retention are 

 separately affected by alterations in neural structure 

 .Hid functioning. 



discrimination By .1 simple modification in learning 



procedure, cue 1 an studv the acquisition and retention 



ol .1 'stimulus discrimination.' This is done by pre- 

 senting two (or inorei stimuli and reinforcing one but 

 not the other \n animal will, for example, acquire a 



Conditioned discrimination between the sounds of a 

 bell and a metronome if the bell only is reinforced 



