Capuchin monkeys were trained on a series of learning-set problems which required discrimination between stimulus arrays having ABA and BBA configurations. Transposition was later tested by presenting training configurations with CCC or ABC configurations. Considerable negative transposition and a small amount of positive transposition occurred, a result suggesting that the original learning set was based at least partly on perceived differences in the homogeneity of the ABA and BBA configurations. Fast-learning subjects manifested more persistent random responding and less position responding than did slow-learning subjects. This difference suggests that random responding during the initial stages of learning may be used as an index of attention.During the past 25 years, a highly diversified variety of learning sets have been demonstrated in learning research on nonhuman primates (French, 1965;Miles, 1965). For some varieties of learning sets, such as discrimination learning-set, the positive and negative stimulus within a given problem are differentiated by a constant difference in object quality. However, for other types of learning sets, such as oddity and sameness-difference, the positive and negative stimuli within a problem are not defined by any constant stimulus quality of the positive and negative stimulus but by the relationship either between the positive and negative stimuli, or between 1 he stimuli within the positive or negative complexes. King and Fobes (1975) demonstrated that capuchin monkeys could learn a sameness-difference learning set which required discrimination between simullaneously presented same (AA) and different (AB) object pairs with a new object combination appearing on each trial. Previously, King (1973) found that after learning a sameness-difference learning set, with the sameness pair (AA) positive, chimpanzees and organgutans preferred an AAB stimulus configuration over an ABA configuration. Conversely, after sameness-difference learning with the different (AB) pair positive, the apes preferred the ABA over the AAB configuration. Thus, although AAB and ABA configurations both contained two identical and one dissimilar element. the AAB configuration was apparently perceived by the apes as more homogeneous than the ABA configuration.The above results suggest the question of whether monkeys can learn a modified sameness-difference learning set based upon discriminations between two

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Eight capuchin monkeys were trained on a simultaneous shape discrimination and a subsequent reversal with size, brightness, and position cues irrelevant. One group (HS) was trained with highly salient size and brightness cues, and the other group (LS) was trained with less salient size and brightness cues. The two groups did not differ significantly in learning either the original discrimination or its reversal. However, during reversal, Group HS responded more to its preferred size than did Group LS. This result indicates that the emergence of previously absent preferences along an irrelevant dimension is a function of the salience of the dimension. Responding to irrelevant dimensions was also found to increase from acquisition to reversal for the HS group, but not for the LS group. This increase is not consistent with Mackintosh's recent theory of discrimination learning.A variety of animal learning experiments have amply demonstrated that responses to irrelevant dimensions do not necessarily decrease monotonically as learning proceeds. For example, Turner (1968) reported that position habits increased during acquisition of brightness discrimination by rats in a Lashley jumping stand , and Shepp and Schrier (1969) obtained comparable results during learning set acquisition by macaque monkeys . Increased position responding was also previously observed by the authors (Scanlon & King, 1976) in sameness-difference learning by capuchin monkeys. In addition, previously suppressed position habits and other systematic modes of responding correlated with irrelevant dimensions are potentially prone to elicitation during reversal of a previously learned discrimination, a phenomenon that Harlow (1959) has referred to as "regression." An example of regression was the appearance during reversal of stimulus preference and differential cue error factors on a series of multiple discrimination reversal problems (Harlow, 1950).Increased responding to irrelevant dimensions during acquisition and regression during reversal are inconsistent with a recent theory of discrimination learning proposed by Mackintosh (1975). According to this theory , a dimension-specific learning rate parameter, a, changes for each stimulus dimension according to that dimension's correlation with reinforcement. Increases in aA occur only when Dimension A is correlated with reinforcement and, as in overshadowing, is more salient than another stimulus dimension also correlated with reinforcement, or, as in blocking, has been previously established as the signal for the reinforcer . Conversely, aB decreases only when Dimension B is not correlated