'476 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II! 



Hilgard (97), Hilgard & Marquis (98), McGeoch & 

 Irion (153), Pavlov (181-183) and Konorski (120, 

 1 2 1 I . 



ABLATION STUDIES 



The literature contains so many studies of the 

 effects of damage to the brain upon learning and 

 retention that any selection such as this one will 

 provide limited coverage at best. This section attempts 

 to summarize general trends by citing what are 

 believed to be the most significant articles. More 

 extensive treatments of the subject may be found in 

 Morgan & Stellar (163), Morgan (161 ), Milner (157), 

 Chow & Hutt (41), and in the chapters on physi- 

 ological psychology appearing regularly in Annua/ 

 Review of Physiology and Annual Review 0] Psychology. It 

 is also considered in the chapters in this Handbook 

 dealing with various regions of the brain, especially 

 Chapters LIV and LXVII. 



Sjunal Conditioning 



It still is an open question whether the basic 

 phenomena of conditioning and learning can be 

 demonstrated in the spinal segments of the mam- 

 malian nervOUS system. Attempts by several dilfcrcnl 

 investigators (25, 26, 47, 115, 198) to obtain con- 

 ditioning in the spinal cord have mel with failure. On 

 the other hand, Shurrager and his colleagues have 

 published a scries of experiments (213-216) which 

 the) interpret in favor of spinal conditioning. These 

 experiments, however, arc open to criticism on at 

 least two counts: a) whal seems io be the CR may in 

 fact be a L'R (47); and b) what appears to be con- 

 ditioning may be sensitization (114, ii-,)- The 

 question of spinal conditioning is therefore one that 

 remains to be setded by future research (25, 26). 



Subcortical Factor i in Learning 



Relatively little has been done with lesions below 

 the level of the cerebral cortex. Ghiselli & Brown 

 (27, 77) made stereotaxic lesions in the thalami ol .1 

 large number ol rats, testing them thereafter cm .1 

 i\ ol discriminative and maze learning tasks. 

 Almost an) thalamic injur) outside the anterior 

 nuclei significantl) affected rate of learning, but on 

 the whole the effects were not related to an) sel oi 

 nuclei. Allen I ). j), working with the don, found no 

 change in .1 conditioned diileienii.il response to 



olfactory stimuli after ablations in the hippocampus, 

 amygdaloid nuclei, pyriform areas, or in combinations 

 of these areas. 



Recent advances in our knowledge of the limbic 

 system and its importance in emotion have turned 

 attention to the role of this system in emotionally 

 motivated learning (see Chapter LXIII by Brady in 

 this volume). Septal lesions do not affect the learning 

 of a classical CR to shock but do impair the retention 

 of such a response learned before operation (21). 

 However, learning of a conditioned avoidance re- 

 sponse is more rapid in rats with septal lesions than in 

 normal rats, and slower in rats with amygdaloid than 

 in normal animals (116). On the other hand, retention 

 of such a response is not impaired by amygdaloid 

 ablation in cats (22). Much research of this sort is 

 currently being reported, but it is not possible as yet 

 to state accurately what the results imply about the 

 role of the limbic system in conditioning and learning. 



Conditioning in Decorticate Animals 



Pavlov's theory (181) originally explained con- 

 ditioning in terms of cortical processes. However, 

 many experiments in recent years have demonstrated 

 that learning can take place in decorticate animals. 

 If the learning task is one of simple classical con- 

 ditioning, such animals show little or no impairment 

 (44, 78, 189). However, if the task is one of instru- 

 mental learning, e.g. a conditioned avoidance response, 

 animals have difficult)' with it (44). Such responses 

 can nevertheless be acquired, if the experimenter is 

 careful to see that the animal does not become too 

 'emotional' (187), but the process is .1 slow one and 

 under some circumstances will never be completed. 

 Apparently then, the cortex is not necessary for 

 classic. il conditioning (Type I), but it is important for 

 avoidance conditioning (Type 11) and more complex 

 trial-and-error learning. 



.l/</;< Learning 



The maze is particularh suited to sin, ill animals 

 such as rats, and for this reason Lashle) used them 

 extensively in his carlv studies of cortical factors in 

 learning (125, 133). lbs general procedure w.i^ to 

 make lesions of different sizes and in different locations 

 in a large group of rats, some before learning the 

 maze .md some afterward so that he could stud) 

 e|i.n.iiilv the effects ol lesions on learning and 

 retention, lie also used mazes of varying difficulty. 

 I pon completion of the behavioral work, he sacrificed 

 his animals and reconstructed theii lesions. 



