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The Transfer of Category Knowledge by Macaques (Macaca mulatta) and Humans (Homo sapiens)

Cognitive psychologists distinguish implicit, procedural category learning (stimulus–response associations learned outside declarative cognition) from explicit-declarative category learning (conscious category rules). These systems are dissociated by category learning tasks with either a multidimensional, information-integration (II) solution or a unidimensional, rule-based (RB) solution. In the present experiments, humans and two monkeys learned II and RB category tasks fostering implicit and explicit learning, respectively. Then they received occasional transfer trials—never directly reinforced—drawn from untrained regions of the stimulus space. We hypothesized that implicit-procedural category learning—allied to associative learning—would transfer weakly because it is yoked to the training stimuli. This result was confirmed for humans and monkeys. We hypothesized that explicit category learning—allied to abstract category rules—would transfer robustly. This result was confirmed only for humans. That is, humans displayed explicit category knowledge that transferred flawlessly. Monkeys did not. This result illuminates the distinctive abstractness, stimulus independence, and representational portability of humans’ explicit category rules.