316 K. S. LASHLEY 



the interrupted light or, at first, even into the discrimination 

 compartment. In accordance with the custom of training against 

 a preference the rats were required to select the interrupted 

 light. After the first 20 trials the animals gave no further signs 

 of attention to the flickering stimulus. Increase or dimunition 

 in speed of rotation produced no change in their behavior. 

 After 200 trials rat No. 2 became stubborn under punishment 

 and had to be removed from the experiment. 



As shown in table II, there was no decrease in the per cent 

 of error after 500 trials. The rats did not learn to react to the 

 stimulus although at first they had seemed to be disturbed by it. 



EXPERIMENT 3 



The size of the sector and its speed of rotation were next 

 increased so that a marked difference in the brightness of the 

 forms was produced.* For rat No. 1 a sector of 270 degrees 

 was used. She learned to react to the difference in brightness 

 and a change to the original speed of rotation did not affect 

 the reaction. The size of the sector was then reduced to 180 

 degrees and later to 90. Each change was followed by a sudden 

 increase in the per cent of error, then a gradual improvement 

 in discrimination. When the sector was reduced to 75 degrees 

 the rat failed to discriminate and soon ceased to attend to the 

 stimulus (table III). The chief interest of this experiment lies 

 in the fact that by a gradual increase in the similarity of the 

 stimuli the rat could be trained to respond to a difference to 

 which it had formerly given no attention. 



EXPERIMENT 4 



Since the animals did not distinguish between the square and 

 circle, another experiment, apparently involving a greater differ- 

 ence, was undertaken. A circle, 30 millimetres in diameter, was 

 used as the positive stimulus and two circles, 21.2 millimetres 

 in diameter and 40 millimetres apart in a horizontal line, as 

 the negative. Rats Nos. 3 and 7 were given this problem. 



Table IV gives the percentage of error for each 100 trials and 

 shows that there was no discrimination of the stimuli, even 

 after 1000 trials. The animals ran through the discrimination 

 compartment without a pause and never attended to the stimuli. 



* Hoge and Stocking (1912), have shown that differences in brightness are dis- 

 criminated by both albino and black and white rats. 



