Abstract-concept learning carryover effects from the initial training set in pigeons (Columba livia).

Among long-standing issues in animal cognition is which (nonhuman) animal species have the cognitive ability to learn abstract concepts (e.g., Premack, 1978). The hallmark of abstract concepts is that they transcend specific features of the training stimuli because they are based upon relationships (i.e., relational learning). In a seminal paper that was instrumental in the modern resurgence of animal cognition, David Premack said “…unlike the primate, the behavior of the pigeon is predominantly controlled by absolute rather than relational factors…” (1978, p. 432). Premack’s claim that item-specific learning (i.e., absolute factors) by pigeons has been the accepted explanation for why pigeons, at that time, had failed to learn abstract concepts (e.g., Cumming, Berryman, & Cohen, 1965; Farthing & Opuda, 1974; Santi 1978). Nevertheless, evidence began to emerge that if a large number of training stimuli (e.g., 152 color cartoons) were used, pigeons actually could learn a matching-to-sample (MTS) abstract concept (Wright, Cook, Rivera, Sands, & Delius, 1988). A somewhat more advantageous procedure than MTS for testing abstract-concept learning is the same/different procedure because the matching and non-matching stimuli are presented on separate trials. Details of currently used S/D procedures used to test pigeon abstract-concept learning vary widely including the numbers of stimuli used in training and testing (e.g., Blaisdell & Cook, 2005; Cook, Kelly, & Katz, 2003; Santiago & Wright, 1984; Wasserman, Hugart, & Kirkpatrick-Steger, 1995; Young & Wasserman, 1997). In consideration of the evidence that some species only accurately transfer their performance to novel stimuli following training with a substantial number of training stimuli, we have regularly made the number of training stimuli a parameter in our experiments. In a series of experiments with rhesus monkeys, capuchin monkeys, and pigeons, we began training with a small set of 8 stimuli and then progressively expanded (doubled) the training set followed by transfer tests with novel stimuli (Katz & Wright, 2006; Katz, Wright, & Bachevalier, 2002; Wright, Rivera, Katz, & Bachevalier, 2003). This regime allowed us to assess whether or not the degree of concept learning would depend on the number of training exemplars and whether or not transfer would eventually reach the level of baseline performance with the training stimuli. We anticipated that large numbers of training stimuli might be necessary to achieve a level of transfer equivalent to baseline performance, and therefore used picture stimuli. We selected pictures so that the training and testing pictures would be distinctive from one another and the transfer stimuli were only tested once, by definition of their being novel. In these experiments when the training set was expanded sufficiently (256 items for pigeons, 128 for monkeys), transfer became equivalent to baseline performance which remained above 80% correct. We termed this full abstract-concept learning because the baseline performance sets an upper limit. This result was the first time that pigeons had shown full concept learning in an S/D task, but pigeons required more training exemplars (i.e., stimulus pairs of same or different) than rhesus or capuchin monkeys. We characterized this species difference as a quantitative difference in contrast to the ability to fully learn the S/D abstract concept which we characterized as a qualitative similarity. The purpose of the experiments of this article was to explore whether or not training pigeons with somewhat larger initial sets of stimuli might produce better transfer than those trained initially with an 8-item set in the prior study. Training with the smaller 8-item set might produce carryover effects that would interfere with generalized relational learning which would in turn be necessary for good performance with the large training sets. To this end, we trained different groups of pigeons with larger initial training sets and compared their transfer to groups trained initially with smaller training sets. Such comparisons, of course, would depend upon pigeons being able to learn the S/D task with large training sets. With these considerations, the first group of pigeons was trained on an intermediate set size of 32 pictures.