Two experiments were conducted to investigate the relationship between nodal distance, response accuracy, and response latency during testing for emergent relations. In both experiments, undergraduate subjects first learned A-B, B-C, C-D, and D-E constituent relations of six 5-member equivalence classes. In Experiment 1, only selected tests of trained and of 0-, 1-, and 2-node tests of emergent relations were carried out in order to avoid testing of 0-or 1-node relations that might form constituents of the 2-node relations which were tested. In Experiment 2, all possible trained and derived relations were tested in random order. Although considerable individual variability was observed in both response times and accuracy for the 14 subjects completing Experiment 1, latencies for correct responses generally increased and response accuracy decreased as a function of nodal distance. There was no nodal distance effect for latencies of incorrect responses. In Experiment 2, these relationships between response times, response accuracy, and nodal distance were observed for 3-node relations in 5 out of 6 subjects. Analysis of error response latencies for 2 subjects who made sufficient errors revealed a nodal distance effect for 1 subject but not for the other. In both experiments, response times decreased as testing progressed, but response accuracy increased during testing only in the second experiment.In experiments on stimulus equivalence human subjects first learn a minimal number of relations between individual stimuli in a set. Each stimulus has a trained relation with at least one other in the set, so that all the stimuli are minimally interlinked. If the subjects are then able to demonstrate, without further training, additional relations between the Garside, B. Rogers, and S.Turner who contributed to the collection of data for Experiment 1. We also thank Tom Critchfield and William Dube for helpful comments on an earlier draft of this paper.

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In order to study brain activation during the formation of equivalence relations, 12 subjects underwent fMRI during matching-to-sample (MTS) tests of (1) previously trained arbitrary relationships between iconic stimuli and the untrained, emergent relations of (2) symmetry, (3) transitivity, and (4) symmetry with transitivity, plus a test of verbal fluency (VF). Brain activation was similar in all MTS tasks and in the VF task. In particular, both types of task activated dorsolateral prefrontal cortex (DLPFC) and posterior parietal cortex bilaterally. However VF, but not the MTS tasks, activated Broca's area. In three of the four MTS tasks, behavioural accuracy was significantly correlated with left lateralisation of DLPFC activity. Brain activation patterns during equivalence thus resembled those involved in semantic processing underlying language, without involving regions concerned with the simple sub-vocal articulation of stimulus names.

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