Words may come to acquire their relational/semantic functions on the basis of participation in derived stimulus relations, such as stimulus equivalence, and evoke the N400 ERP. The present series of experiments were designed to examine the behavioural and ERP correlates of stimulus equivalence relations using a novel, stimulus-pairing yes/no procedure. In Experiment 1, 30 right-handed participants were trained using a many-toone training design to form 2, 4-member relations (A-D, B-D, C-D) consisting entirely of pseudo-words. On every trial, a sample stimulus appeared for 1 s before being replaced by a fixation cross and then a comparison stimulus, which remained on screen until response keys marked ‘Yes’ or “No’ were pressed. Within (i.e., A1-D1-Yes) and between (i.e., A1-D2-No) discriminations were trained to a high accuracy before feedback was faded and symmetry (D-A, D-B, D-C) and equivalence relations (A-B, A-C, B-A, B-C, C-A, C-B)
subsequently tested. EEG recording from 32 sites was undertaken during participants’ only test exposure. Results indicated 21/32 passed the equivalence test. Grand average ERPs indicated no significant separation during the N400 epoch. In Experiment 2, a two-stage training and testing procedure was adopted, and only participants who passed the first phase progressed to the second. First, 2, 3-member relations were trained and tested using a linear-series design (A-B/B-C). Symmetry (B-A/C-B) was first tested, followed by transitivity (A-C) and equivalence (C-A). If participants passed all tests, they proceeded to a training phase in which expanded relations were trained (C-D). Acquisition of these relations was followed by tests for symmetry (D-C), 1-node transitivity (B-D), 1-node equivalence (D-B), 2-node transitivity (A-D), and 2-node equivalence (D-A). Overall, of 35 participants recruited, 7 failed to proceed beyond the preliminary phases, 4 failed the expanded relations phase and 24 passed. Analyses of the EEG data, however, failed to reveal any significant differences between the trial types. This highlights that the procedure, while effective for establishing arbitrary word webs, was not effective for evoking the N400 ERP. It is argued that the stimulus pairing yes/no procedure explicitly reinforces both between and within class discriminations. That is, participants have a direct history of learning that stimuli go together and which stimuli do not. N400 is typically evoked during violation of semantic expectancy; training participants that some stimuli go together and some do not mitigates the potential violation. In conclusion, the findings, which are currently being written up for publication, indicate that equivalence relations may be formed using the stimulus pairing yes/no procedure but that a separate task may be necessary to measure the ERPs evoked by the arbitrary, pseudo-words.