Equivalence Class Formation and the N400: Methodological Issues

The electroencephalogram (EEG)-based N400 component is often described as an index of a semantic relation. Recent studies suggested that the N400 component can also be used as an electrophysiological measure of equivalence class formation, yet more research is needed to clarify the effects of experimental conditions on the N400 response. In Experiment 1 of the present study, the participants were trained on six conditional discriminations and tested for the formation of three 3-member classes. If they formed equivalence classes with half of the possible emerged relations, the participants were given a priming test with the other half. Related and unrelated stimulus pairs were presented, and the participant had to decide whether the stimuli were related or not. The results showed that a nonsignificant N400 response was observed after unrelated stimulus pairs were presented but not when related stimulus pairs were presented. The strength of the N400 response weakened over the number of stimuli presentations. In Experiment 2, we examined whether changes to the methodology of Experiment 1 would produce stronger N400 responses. The participants also underwent a word priming procedure, which has been shown to produce robust N400 responses. We found more robust N400 responses in Experiment 2 than in Experiment 1, and the N400 response was larger in transitivity/equivalence relations than in symmetry relations. There was also a significant relation effect in the word priming procedure. Together, these findings support the notion that the N400 component can be used as an electrophysiological measure of equivalence class formation and illustrate how experimental conditions can influence the N400 response.