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Generalized identity in a successive matching-to-sample procedure in rats: Effects of number of exemplars and a masking stimulus.

Two experiments examined the emergence of generalized identity matching in rats using a successive discrimination procedure with olfactory stimuli. Trials consisted of the presentation of two odors separated by a 1-s interstimulus interval. Responses during the second odor presentation were reinforced only if the two odors were identical. In Experiment 1, rats were trained with two odors and then exposed to sessions that included unreinforced probe trials with novel odors. There was evidence of higher response rates on matching probe trials in some rats, but matching did not approach baseline levels. Additional training with four exemplars produced transfer to novel odors that was equivalent to baseline levels. Experiment 2 tested the possibility that detection of stimulus change, rather than generalized identity, was responsible for the transfer seen in Experiment 1. Thus, a masking odor was inserted during the 1-s interstimulus interval so that stimulus change occurred on all trials. Although response rates on probe trials were lower than baseline rates, above chance transfer to novel stimuli was still observed in four of the five animals tested. These findings support the hypothesis that transfer of matching to novel odors in this successive matching-to-sample paradigm is based on a generalized identity relation.