Neural correlates of relational reasoning and the symbolic distance effect: Involvement of parietal cortex

A novel, five-term relational reasoning paradigm was employed during functional magnetic resonance imaging to investigate neural correlates of the symbolic distance effect (SDE). Prior to scanning, participants learned a series of more-than (E>D>C>B>A) or less-than (A<B<C<D<E) ordered premise pairs. During scanning, inferential tests presented the premise pairs, adjacent, mutually entailed tasks (e.g., D<E and B>A) and nonadjacent one-step (A<C, B<D, C<E, C>A, D>B and E>C) and two-step (A<D, B<E, D>A and E>B) combinatorial entailed tasks. In terms of brain activation, the SDE was identified in the inferior frontal cortex, dorsolateral prefrontal cortex, and bilateral parietal cortex with a graded activation pattern from adjacent to one-step and two-step relations. We suggest that this captures the behavioural SDE of increased accuracy and decreased reaction time from adjacent to two-step relations. One-step relations involving endpoints A or E resulted in greater parietal activation compared to one-step relations without endpoints. Novel contrasts found enhanced activation in right parietal and prefrontal cortices during mutually entailed tasks only for participants who had learned all less-than relations. Increased parietal activation was found for one-step tasks that were inconsistent with prior training. Overall, the findings demonstrate a crucial role for parietal cortex during relational reasoning with a spatially ordered array.