Modification by Experience 251 



88. Inhibition involving Discrimination of Successive 

 Stimuli 



In other experiments requiring the inhibition of an 

 instinct, the animal is caused to discriminate between two 

 nearly similar stimuli, to execute the action when one is 

 presented and to inhibit it when the other one appears. 

 Such tests, as we have seen, are very commonly adopted 

 to investigate sensory discrimination. The principle is the 

 same whether the action to be inhibited is an instinct or an 

 acquired habit. The experiments may be divided into two 

 classes: in the first only one stimulus is given at a time, 

 and the animal in consequence is sometimes required to 

 inhibit its response entirely; in the second, two or more 

 stimuli are given simultaneously , and the animal simply 

 has to choose among them on the basis of its past experiences. 

 Experiments belonging in the former class were successfully 

 performed by Thorndike with Cebus monkeys. Both of his 

 subjects learned to come down to the bottom of the cage 

 to be fed when the experimenter took a piece of food in his 

 left hand, and to stay up when he took it with his right hand, 

 by being fed in the first case and not in the second. One 

 of the monkeys learned to discriminate in like manner be- 

 tween cards carrying different figures; the other one failed 

 with the cards and learned to react or inhibit reaction only 

 in connection with different movements of Thorndike's 

 hands (397). Professor Bentley and the writer made some 

 experiments on the chub by this method, which gave wholly 

 negative results. The red and green forceps, each con- 

 taining food, were plunged one at a time into the water; 

 the fish was allowed to get the food from the red forceps, 

 but the green ones were withdrawn before it had a chance 

 to bite. The time which the fish took to rise and snap at 



