DISCRIMINATION OF SIZE AND FORM IN THE RAT 325 



able that animals which were attending to the stimuli and 

 reacting to differences in size might be trained without great 

 difficulty to attend to differences in form. 



Two forms, a square of 1600 square millimetres area and a 

 circle of 706 square millimetres, were substituted for the forms 

 used with rat No. 6 in experiment 9. The animal (No. 6) dis- 

 criminated between these immediately, choosing the square. 

 The area of the circle was gradually increased to 1200 square 

 millimetres at which point the rat failed to react. This process 

 of starting with a wide difference in area and gradually decreas- 

 ing it was repeated three times, but whenever the area of the 

 circle became more than three-fourths that of the square the 

 rat ceased to react to the difference. After 600 trials there was 

 no evidence that she could discriminate form. 



With rat No. 4 a six pointed star was used as the positive 

 stimulus, a circle of varying size as the negative. No. 4 chose 

 the star so long as the difference in size was great, but when 

 the area of the circle became more than three-fourths that of 

 the star, the reaction became uncertain or broke down entirely. 



With rat No. 5 two circles of a combined area of 706 square 

 millimetres area (see experiment 4) w T ere used as the negative 

 stimulus, and a square of varying area as the positive. Start- 

 ing with a square of 3600 square millimetres area, the area was 

 reduced to 1225, at which point discrimination failed. This 

 was repeated twice but the reaction always became uncertain 

 when the difference in size became less than the area of the 

 smaller form. 



Later, with rat No. 6, a 40 millimetre square was used as the 

 positive stimulus, and the negative stimulus was varied in form, 

 its size being kept less than three-fourths that of the square. 

 Circles, triangles, and various other forms were used. After the 

 first few changes the rat was no longer disturbed by alterations 

 in the form of the stimulus and chose the larger without hesita- 

 tion. But whenever the difference in size was reduced below 

 the threshold (experiment 9) she fell back upon position asso- 

 ciations and failed to discriminate. In several of these series 

 the rat chose the form of greater area, even when the other 

 appeared larger when viewed as a unit of light and shade. 



From these experiments it appears that the rats, in discrim- 

 inating between objects of different size, do not perceive or do 



