THE ASSOCIATION CENTERS 235 



cortical fields. The marginal zone is dominated physi- 

 ologically by its own projection system, it may be 

 visual associational, auditory associational, or some 

 other. These primitive associational fields are con- 

 tiguous with perhaps wide areas of overlap. In higher 

 brains, however, they are separated by secondary 

 association centers which are not dominated physi- 

 ologically by any one system of projection fibers. The 

 secondary association centers have emerged within 

 the cortex in accordance with principles not unlike 

 those which came to expression earlier in phylogeny 

 in the emergence of the true correlation centers of the 

 brain stem (thalamus, etc., see p. 50) out of a more 

 primitive and inflexible arrangement of the primary 

 reflex centers. 



In the further analysis of the secondary associa- 

 tional mechanisms these may be considered from two 

 standpoints: (i) On the motor side with reference to 

 the number and variety of the synergic patterns of 

 muscles to be innervated. This factor seems to deter- 

 mine the size, number, and arrangement of the asso- 

 ciation centers in premotor territory. (2) On the af- 

 ferent side with reference to the ways in which the 

 sensory complexes are worked over in the cortex, cor- 

 related, integrated, and resolved into motor com- 

 plexes — in other words, the apparatus of sensual fu- 

 sion and resolution so as to activate the particular 

 motor apparatus adapted to give the response ap- 

 propriate to the situation. This factor seems to de- 



