Humans exhibit associative symmetry in the absence of backward training and stimulus overlap.

A recent survey of the evidence on associative symmetry in humans revealed that nearly all the demonstrations either unintentionally trained backward stimulus pairings and/or had a temporal overlap between the stimuli being trained. We consider these criticisms and improve on our own method of "associative networks." In this method, participants learn multiple stimulus pairings via arbitrary matching-to-sample tasks in which the stimuli are concurrently presented as sample and comparison stimuli. In Experiment 1, human participants learned a bidirectional network (in which symmetry was synergistic) and a unidirectional network (in which symmetry was antagonistic) or two unidirectional networks (removing explicit reinforcement of backward stimulus pairings). In Experiment 2, participants learned two unidirectional networks; however, we removed the temporal overlap between sample and comparison stimuli by imposing a 1-s delay between them. Both experiments showed robust evidence of symmetry, suggesting that the expression of symmetry in humans survives the most common confounds in published research.