Abstract
Humans tend to make illogical inferences, that is, after 'if A then B' (AB) relation was learned, subjects readily infer 'A' from 'B' (BA relation), which animals rarely exhibit. Such an illogical, although not necessarily wrong, cognitive bias may be essential to human highly intelligent functions including language faculty. In this study, we conducted an fMRI experiment to investigate the neuronal correlates of such illogical inference. The subjects were trained with five sets of AB and BC relations to reach the criteria of 90% accuracy before the MRI scans. During scan sessions, subjects were tested with, in addition to trained relations, four untrained and emergent relations; illogical relations of symmetry (BA and CB), their combination (CA), and a logical relation of transitivity (AC). Behaviorally, correct rate were significantly higher than chance level in all emergent relations. Imaging result detected activities in the brain areas both distinctive and common for logical and illogical inferences. The left anterior PFC was activated for logical AC inference, which may contribute to combination of trained relations. The posterior cingulate cortex was activated only for illogical CA inference, which may be involved in the combination of symmetrically emergent relations. The commonly activated areas, right PFC, medial frontal cortex and intraparietal area, may be involved in the process of an abstract category formation which should be made by manipulating relational orders and the categorical discrimination based on the emergent relations. [J Physiol Sci. 2008;58 Suppl:S144]
