MENTAL LIFE OF MONKEYS AND APES 11 



pletion of a subject's reaction to the group of three keys, a group 

 of seven keys at the opposite end of the keyboard may, for 

 example, be presented. Similarly, the subject is required to 

 discover with the minimum number of trials the correct reaction- 

 mechanism. Thus, time after time, the experimenter presents 

 a different group of keys so that the subject in no two successive 

 trials is making use of the same portion of the keyboard. It is 

 therefore impossible for him to react to spatial relations in the 

 ordinary sense and manner, and unless he can perceive and ap- 

 propriately respond to the particular relation which constitutes 

 the only constant characteristic of the correct reaction-mechan- 

 ism for a particular problem, he cannot solve the problem, or 

 at least cannot solve it ideationally and on the basis of a small 

 number of observations or trials. 



For the various infrahuman animals whose ideational behavior 

 has been studied by means of this method, it has been found 

 eminently satisfactory to use as reaction-mechanisms a series 

 of similar boxes, each with an entrance and an exit door. An 

 incentive to the selection of the right box in a particular test 

 is supplied by food, a small quantity of which is placed in a 

 covered receptacle beyond the exit door of each of the boxes, 

 Each time an animal enters a wrong box, it is punished for its 

 mistake by being confined in that box for a certain period, rang- 

 ing from five seconds to as much as two minutes with various 

 individuals or types of organism. This discourages random, 

 hasty, or careless choices. When the right box is selected, the 

 exit door is immediately raised, thus uncovering the food, which 

 serves as a reward. After eating the food thus provided, the 

 animal, according to training, returns to the starting point and 

 eagerly awaits an opportunity to attempt once more to find the 

 reward which it has learned to expect. With this form of the 

 apparatus, the boxes among which choice may be made are 

 indicated by the raising (opening) of the front door. 



Since with various birds and mammals the box form of appa- 

 ratus had proved most satisfactory, I planned the primate ap- 

 paratus along similar lines, aiming simply to adapt it to the 

 somewhat different motor equipment and destructive tendencies 

 of the monkeys. I shall now briefly describe this apparatus 

 as it was constructed and used in the Montecito laboratory. 



The apparatus was built in room A (figure 12), this room 



