Two Directions in a Search for Symmetry in Rats

Two methodologies that have been successful in producing emergent symmetry in some nonhuman species include multiple exemplar training (sea lions) and successive conditional discrimination training of both arbitrary and key identity relations (pigeons). Two experiments were conducted to examine these procedures in rats using olfactory stimuli. In Experiment 1, sets of arbitrary conditional discriminations were trained in eight rats using a successive (go, no-go) procedure, followed by symmetry tests and then direct training of symmetry relations for four of the rats. Acquisition of bidirectional conditional discriminations was slow and limited the number of exemplars that could be trained. However, this multiple exemplar training did not result in emergent symmetry. Experiment 2 was a systematic replication of a study by Prichard et al. (Prichard et al., Journal of the Experimental Analysis of Behavior 104:133–145, 2015) that failed to find symmetry in an extension of Urcuioli’s (Urcuioli, Journal of the Experimental Analysis of Behavior 90:257–282, 2008) theory of stimulus class formation that better controlled for complications in presentation of odor stimuli. Five rats were trained on identity and arbitrary conditional discriminations with odors, then tested for emergent symmetry across eight probe sessions. Although there was some evidence for symmetry in most rats, it was ephemeral, being limited to the first few probe sessions. The search for symmetry in nonhumans continues to be elusive but using successive procedures that train arbitrary and key identity relations appears to be the more fruitful avenue in rats.