Loci of discriminanda and discrimination proficiency

Three groups of nine juvenile rhesus monkeys (Macaca mulatta) whose mothers were maintained on diets affording 3.35, 6.70, and 13.4 percent of calories as protein during gestation were tested in a series of discrimination-learning tasks in which the critical variable was the loci of the relevant cues. The stimuli used were two 75-mm x 75-mm plastic plaques. The loci of the discriminanda (colored dots on the plaque) varied according to a 3 x 3 grid. No significant differences were found among the diet groups. A gradient of discriminability appeared that accorded with the location of the discriminanda: The stimulus region closest to the monkey made for the least and the farthest region the most difficulty of discrimination, also discriminanda on the right part of the stimulus were easier to discriminate than those on the left. Animals rewarded for displacing the plaque containing a colored dot learned the discriminations faster than those reinforced for displacing the blank plaque. The differences in salience of different parts of a stimulus plaque, though systematic, are modest, not dramatic.