THE SENSE OF SMELL 



547 



Behavior Studies of Oljactory Mechanisms 



Excision of the olfactory bulb or damage to the 

 anterior limb of the anterior commissure in the rat 

 leads to a distinct impairment of olfactory discrimi- 

 nation. However, lesions involving; the septuin, the 

 hippocampus, the fimbria, the fornix, the amygdala 

 and the pyriform lobes are without efTect on dis- 

 crimination in tests involving the differentiation of 

 wood shavings scented with oil of anise and creo- 

 sote (82, 83). Brown & Ghiselli (28) also failed to 

 find any impairment of olfactory discrimination 

 after a variety of subcortical lesions. Experiments de- 

 signed to test the role in olfaction of pathways through 

 the anterior thalamic nuclei from the hypothalamus 

 to the cingulate cortex have not disclosed impaired 

 discrimination after total bilateral destruction of the 

 anterior thalamic nuclei and their radiations, with 

 additional involvement of the septum (63). In these 

 experiments removal of the olfactory bulbs per- 

 manently abolished the discrimination between the 

 odors of oil of wintergreen and of bread and milk, 

 indicating that the reaction was based upon olfactory 

 and not trigeminal stimulation. Cats are capable of 

 finding food by olfactory cues after lesions destroying 

 almost all the neocortex but leaving intact the an- 

 terior pyriform and periamygdaloid cortex (18). 



Studies by Allen (12, 13) using conditioned re- 

 sponses have provided a more delicate measure of 



olfactory powers than simple discrimination tests. 

 Using dogs, Allen subjected all animals to four tests 

 which in\olved establishment of a conditioned fore- 

 leg response to clove vapor, ability to transfer this 

 refle.x to the opposite foreleg, ability to establish an 

 absence of foreleg response to asafetida (negative 

 conditioned reflex) and differentiation between two 

 olfactory conditioned reflexes and, finally, ability to 

 select by smell w-hen blindfolded a paper package 

 containing meat from three paper packets of like 

 size and texture. Allen found that bilateral extirpa- 

 tion of the pyriform-amygdaloid areas abolished the 

 negative conditioned reflex, the animals raising the 

 foreleg to the odor of both cloves and asafetida after 

 cortical resection. Additional ablation of the hippo- 

 campal formation was without effect on the olfactory 

 performance, and in no case was the abilitv impaired 

 in the blindfold test. 



In view of the close subjective relationship between 

 the senses of taste and smell, it might be expected 

 that these senses would activate the same or adjacent 

 cortical regions. Quantitative tests of the monkey's 

 preference for water over a bitter quinine solution 

 show, however, that the insularopercular cortex, 

 rather than the amygdaloid complex and pyriform 

 cortex, is primarily concerned in taste (17, 24). 

 Human studies indicate that taste perception per- 

 sists after complete destruction of the olfactorv nerves 

 (29)- 



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