P300 and positive slow waves reveal the plausibility in inductive reasoning.

Category-based induction is an advanced cognitive function that is based on our category-level knowledge. Previous findings have recognized the distance effect in category-based induction: Inductive strength is affected by the hierarchical distance between the premises and conclusions. However, the neural mechanisms underlying this effect require elucidation. In the present study, we investigated the neural mechanisms of the distance effect by using EEG technology and a new experimental paradigm-category-based induction. In this paradigm, we used three hierarchical levels of categories-the subordinate category, the basic category, the superordinate category-and an irrelevant category. We further used these categories to create four types of trial that varied in the hierarchical distance between the premise and the conclusion: the subordinate-basic, the basic-superordinate, the subordinate-superordinate, and the irrelevant-superordinate trials. In each trial, participants judged the probability that the conclusion category had the same property as the premise category. Our behavioral results revealed that people responded more slowly in the irrelevant-superordinate trials than in the basic-superordinate and the subordinate-basic trials. Our ERP results showed that the irrelevant-superordinate trials elicited smaller P300 (250-500 ms) amplitudes than did the subordinate-basic and the basic-superordinate trials. In addition, the subordinate-superordinate trials elicited smaller P300 and PSW (700-998 ms) amplitudes than did the subordinate-basic and the basic-superordinate combinations. These findings indicate that the amplitudes of P300 and PSW may reflect the distance effect in inductive reasoning: The further the premise-conclusion hierarchical distance, the lower the inductive strength, and thus the smaller the P300 and PSW amplitudes.