STUDIES OF CEREBRAL FUNCTION IN LEARNING. VII 37 



We have taken it for granted that the location of the visual 

 function in the occipital region is due to the massing in this 

 area of projection fibers from the optic nuclei of the thalamus, 

 and this view is doubtless correct, but there remains to be 

 explained the fact that these fibers are of no especial impor- 

 tance for the learning of visual habits. Since the habit can 

 be established equally readily in the absence of the occipital 

 projection fibers and yet normally has cerebral representation 

 there, it seems as though these must normally restrict the 

 habit mechanism to the occipital region, not only by conduct- 

 ing impulses to this part of the cortex, but also by exerting 

 some inhibitory action upon those parts which take over the 

 habit function when the occipital areas are destroyed. 



A similar example is offered by the observation of Gold- 

 stein ('23) that a pseudofovea develops only in patients with 

 complete hemianopsia, and not in hemiamblyopia. Here the 

 functioning of part of the visual area of the injured hemi- 

 sphere seems to prevent the reorganization of the visual 

 mechanisms, whereas the complete destruction of the area 

 permits the shift of fovea. The cases are not parallel, for 

 in the human patients there is only a reorganization within 

 the intact field, and not, as in the rat, a vicarious function for 

 the hemianopic field, yet both conditions suggest that func- 

 tional areas somehow actively restrict similar functions to 

 themselves. 



Mutual facilitation of cerebral areas 



We do not know what is lost when a habit deteriorates — 

 whether there is a dropping out of some essential elements 



in front of the occipital region (the anterior two-thirds) was destroyed after 

 training in brightness discrimination. The whole occipital region with its thalamic 

 connections remained intact. These animals lost the habit as a result of the 

 operation and relearned it in no more trials than were required for their 

 original learning, thus showing that the loss was no more due to a general 

 deterioration than it is in the case of occipital lesions. More work must be 

 done before the significance of these cases will be clear, but they suggest that 

 an extensive lesion in one part of the cortex may have the same effect as a more 

 limited one in another part — a further indication of the dynamic relations of 

 the cortex as a whole. 



