2o6 BRAINS OF RATS AND MEN 



must be considered in its physiological setting. It is 

 probable that the visual cortex plays some part other 

 than mere facilitation of subcortical activities which 

 has not yet been brought to light. In the acquisition 

 and execution of a brightness-discrimination habit 

 there are many factors other than the time employed 

 and the number of trials and errors, and at present 

 these alone have been reported in detail. Lashley sug- 

 gests, "that more complex habits involving diverse 

 sense organs may demand coordination of distant por- 

 tions of the cerebrum and that the visuo-motor habit 

 dealt with in this study is too simple to give a typical 

 picture of cerebral function'* (1921^, p. 277; see also 

 1924, p. 270, n. 47). Elsewhere he calls attention to 

 the fact that even in these simplest brightness-dis- 

 crimination habits other than visual systems are in- 

 volved. This perhaps will provide the key to the prob- 

 lem.^ 



In this connection we must bear in mind that even 

 the simplest discriminative reaction involves the use 

 of a very complex neuromuscular mechanism, and the 

 cortical activity is perhaps a relatively small part of 

 the learning process here under consideration. The 

 brightness-discrimination habit can be acquired in the 



^ Study of human cases (like those of Holmes and Lister, 1916, and 

 Riddoch, 191 7) has revealed the great complexity of the cortical repre- 

 sentation of vision, with evidence of separate differentiation of cortical 

 apparatus for the different sectors of the visual field and for form, color, 

 and movement within this field. How far such differentiation has gone 

 in the brain of the rat is unknown. 



