Equivalence relations do exist before they are tested: Confirmatory evidence revealed by EEG measurements.

We examined equivalence-based N400 effects by comparing EEG data from participants with different experiences with equivalence testing. Before a priming task used in EEG measurement, Group 1 was given only matching-to-sample training trials whereas Group 2 was exposed to matching-to-sample training and equivalence probe trials. We asked whether exposure to the reinforcement contingency was sufficient to bring about an N400 outcome that might indicate potentially emergent equivalence relations or if such a response depended on experience with equivalence tests. Results showed robust N400 in both groups. Experience with equivalence tests did not further increase the N400 effects. Our findings add confirmatory evidence that equivalence relations may originate via the reinforcement contingency alone. Furthermore, complementary EEG data collected from priming tasks involving words from natural language showed functional overlap between laboratory-defined equivalence and natural word-based N400 effects.