STUDIES OP CEREBRAL FUNCTION IN LEARNING 



275 



on a day when the animal was too disturbed to eat. If allowance is 

 made for such errors, retention in every case was practically perfect. 



TABLE I.— Numbers op Trials and op Errors preceding the First Thirty Consecu- 

 tive Trials without Error in Learning and Post-operative Retention Tests for 

 Animals operated after Training. 



Training Retention tests 



No. 20 

 No. 21 

 No. 22 

 No. 23 

 No. 24 

 No. 25 



Average 



The small operative field presented by the rat's brain and the 

 necessity for avoiding haemorrhage on the floor of the cranial cavity or 

 injury to the optic nuclei of the thalamus make the complete destruc- 

 tion of the caudate nuclei difficult. It was accomplished only in 

 No. 25. In this animal also the entire motor area, except the pos- 

 terior part of the hind leg region, was destroyed. The animal showed 

 motor and emotional disturbance, but gave certain evidence of reten- 

 tion of the habit. In No. 20 the posterior halves of both nuclei were 

 destroyed ; in No. 24, the anterior half : in the others the lesions 

 were irregular, but even more extensive. The lesions to the cortex 

 in all cases included the neck, fore-leg, and the greater part of the 

 hind-leg regions. 



Earlier experiments [10] had shown that no part of the stimulable 

 cortex is necessary for the performance of the visual habit. The 

 alternative explanations of this fact were: (1) that the striate nuclei 

 function either vicariously for the stimulable cortex when the latter is 

 destroyed, or normally as the source of efferent impulses from the 

 cerebrum ; or (2) that the supposedly motor regions of the cerebrum do 

 not lie in the direct path of conditioned-reflex arcs and are of only 

 secondary importance in the performance of voluntary movements. 



The foregoing results seem to prove that the second hypothesis is 

 correct. In the absence of the stimulable cortex, injury to the caudate 

 nuclei does not affect retention of the visual habit and perfect discrimi- 

 nations may appear, even after complete destruction of both nuclei. It 

 seems clear, therefore, that neither the stimulable cortex nor the 

 caudate nuclei are directly functional in the performance of the visuo- 

 motor habit. 



