THE SENSORY CENTERS OF THE BRAIN 879 



When sensory loss is occasioned on any part of the body as the result 

 of a limited injury to the cortex, all aspects of sensation are not affected 

 equally, with the result that the ability to respond with accuracy is lost 

 more completely and over a larger area in the case of certain tests than 

 in others. Such dissociations in the sensations which depend on cor- 

 tical activity conform to two general principles. First: The more com- 

 plex and difficult the psychical act required for an accurate ansAver to 

 the test, and the more completely the test appeals to the cortical center 

 in contrast to the thalamic 'center, the greater is the area of disturbed 

 sensibility, and the more complete is the sensory loss in any one part of 

 this area. Thus the extent and degree of disturbance is greater in the 

 case of recognition of posture and movement than In the case of two 

 dimensional localization, and is least for one dimensional localization, 

 which obviously involves a simpler psychical judgment. Sensibility to 

 touch is modified more than to temperature, and temperature more than 

 to pain, because these sensations depend in this order on the cortex as 

 contrasted to the optic thalamus. 



Second: The various aspects of cortical activity depend upon the in- 

 tegrity of different parts of the sensory area. These aspects are (1) spe- 

 cial recognition, typified by the sense of posture and movement, which 

 is dependent chiefly on the part of the sensory cortex which occupies the 

 precentral convolution; (2) the recognition of similarity and difference, 

 on which depends the comparison of weights held in the hand, etc., which 

 resides in the postcentral convolution; and (3) the recognition of the 

 intensity of the sensation, whether it be of touch, temperature or pain, 

 which is centered in the foot of the postcentral convolution, and in those 

 parts of the sensory area lying behind this convolution. It appears that 

 the sensory area of the cortex may be divided into certain horizontal 

 zones which are each involved in sensation arising from stimuli acting 

 on various parts of the body and into three nearly vertical zones involved 

 each in the three fundamental aspects of discrimination. The cortical 

 localization is based therefore, not on purely anatomical relationships 

 by which each part of the body would be represented in all its sensory 

 manifestations by a single part of the cortex, but on functional require- 

 ments, according to which each psychical act of sensory discrimination 

 is centered in a different group of cortical cells. Since these processes 

 are distinct for the different parts of the body, these parts have a sepa- 

 rate representation in the cortex, but the magnitude of the area devoted 

 to each part of the body is dependent solely on the degree in which sen- 

 sation arises from it. 



The Olfactory, Gustatory, and Auditory Areas. The cerebral cortex 

 contains in addition to these centers for cutaneous and deep sensibility 

 certain areas on which depend the perception of olfactory, gustatory, 



