PATTERN-DISCRIMINATION IN VERTEBRATES 



TABLE 5 

 Record of Chick 2 



Width of individual striae on both fields: 1.561 mm. 

 Mean brightness, both fields: 6.24 candles per sq. meter 

 Striae on positive (standard) field: horizontal 



2(tt 



AN APPARENT EFFECT OF " CONTRAST " 



After I had obtained the threshold for deviation in the posi- 

 tive direction for Monkey 2, I gave him 40 trials — 20 trials each 

 on the 9th and 10th of November — in which the striae on the 

 positive field were horizontal and those on the negative field 

 deviated from the horizontal by 5° in the negative direction. 

 Only 12 responses out of the 40 were correct, the animal per- 

 sistently tending to choose the negative field against punish- 

 ment. In this connection it should be noted that when the 

 difference in direction is small and when the striae on one field 

 are horizontal, if the two fields are regarded simultaneously, the 

 striae on both fields appear to be inclined from the horizontal, 

 but in opposite angular directions. For about six weeks previous 

 to these control tests, the striae on the positive field, although 

 really horizontal, appeared to the human observer to deviate in 

 the negative direction, owing to the influence of the other field, 

 whose striae were inclined slightly in the positive direction. If 



