THE SENSORY CENTERS OF THE BRAIN 877 



assign a very important part in the origin of certain aspects of sensation 

 to a center which he terms, the essential organ of the optic thalamus. 



According to this view, afferent impulses may affect consciousness in 

 two distinct ways on arriving in the optic thalamus. They may act upon 

 this sensory center of the thalamus, or they may pass on to the sensory 

 areas of the cerebral cortex. The presence of a sensory center in the 

 thalamus, and the nature of the sensations aroused as the result of its 

 activity, are indicated by the sensations which are experienced by in- 

 dividuals in whom the sensory areas of the cortex have been destroyed. 



In their path through the cord and brain stem afferent impulses have 

 been grouped on a strictly physiological basis, all impulses arising from 

 a common type of receptor traveling together. On reaching the optic 

 thalamus, a regrouping occurs on a psychological basis, the subsequent 

 course of the impulse depending on the kind of appeal which it is to 

 make to consciousness. The thalamic organ is the center for "aware- 

 ness," responding to all stimuli capable of producing sensations of a 

 change of 'state. The cerebral centers, on the other hand, are recipients 

 of impulses which give rise to the discrimination of the detailed qualities 

 of a sensation. 



A patient suffering from the destruction of the cerebral sensory area 

 will exclaim, "Something is happening to me, I am being hurt," instead 

 of "You are sticking a pin into me," because he fails to recognize the 

 distinctive characters of the stimulus in question. The cerebral cen- 

 ters, on the other hand, are concerned with the recognition of fine detail 

 in sensation, enabling us to perceive not only the presence of a stimulus 

 and its gross quality, but also to discriminate between stimuli of differ- 

 ent intensities, to recognize the shape, size, weight, and texture of the 

 stimulating object and to recognize the position of the hand as it explores 

 its surface. Thus, to compare the sensations evoked by stimuli of each 

 of the primary qualities in individuals who have lost the function of the 

 cerebral centers with normal individuals, it is found that contact is rec- 

 ognized, but the distinction of differences in /the intensity of the stimulus 

 cannot be made by the unaided thalamus. The special aspects of tactile 

 sensation, including the impulses involved in one, two, and three dimen- 

 sional localization make no thalamic appeal, so that neither the location 

 of the point stimulated, nor the position of the parts of the body are 

 recognized. Painful stimuli affect the thalamic center powerful^, giving 

 rise to sensations of discomfort, deprived of the distinctive qualities 

 which we recognize in painful sensations when the cortex is intact. The 

 recognition of gradations of pain cannot be accomplished by the thala- 

 mus alone. The thalamus can distinguish between heat and cold, as 

 such, but makes no distinction between various degrees of warmth or 

 coolness. Thus if a glass of hot water is placed in the hand of an in- 



