Functional neuroanatomy of three-term relational reasoning

Abstract

In a recent study we demonstrated that reasoning with categorical syllogisms engages two dissociable mechanisms. Reasoning involving concrete sentences engaged a left hemisphere linguistic system while formally identical arguments, involving abstract sentences, recruited a parietal spatial network. The involvement of a parietal visuo–spatial system in abstract syllogism reasoning raised the question whether argument forms involving explicit spatial relations (or relations that can be easily mapped onto spatial relations) are sufficient to engage the parietal system? We addressed this question in an event-related fMRI study of three-term relational reasoning, using sentences with concrete and abstract content. Our findings indicate that both concrete and abstract three-term relational arguments activate a similar bilateral occipital–parietal–frontal network. However, the abstract reasoning condition engendered greater parietal activation than the concrete reasoning condition. We conclude that arguments involving relations that can be easily mapped onto explicit spatial relations engage a visuo–spatial system, irrespective of concrete or abstract content.

Introduction

An important question that concerns cognitive models of reasoning is whether logical reasoning is inherently sentential or spatial. Sentential (mental logic) theories of reasoning claim that deductive reasoning is a rule governed syntactic process [20] where internal representations preserve structural properties of linguistic strings in which the premises are stated. This linguistic hypothesis predicts that language processing mechanisms mediate human reasoning processes. Mental model theories claim that deductive reasoning is a process requiring spatial manipulation and search where internal representations preserve the structural properties of the world (e.g. spatial relations) that the sentences are about. The spatial hypothesis suggests that the neural structures for visuo–spatial processing contribute the basic representational building-blocks used for logical reasoning [11].

In a recent fMRI study [7] we demonstrated that both linguistic and spatial processing mechanisms are engaged in syllogistic reasoning processes, but under different circumstances. Reasoning involving concrete syllogisms (e.g. ‘all dogs are pets; all poodles are dogs ∴ all poodles are pets’) engages a left hemisphere temporal linguistic system, while formally identical reasoning tasks involving abstract syllogisms (e.g. ‘all P are B; all C are P ∴ all C are B’) recruit a parietal spatial network.

The involvement of a parietal visual–spatial system in the abstract syllogism condition, raises the question whether argument forms involving three-term relational items (e.g. ‘the apples are in the barrel; the barrel is in the barn; the apples are in the barn’ and ‘apples are more expensive than pears; pears are more expensive than oranges; apples are more expensive than oranges’) are sufficient to engage the parietal system? One rationale for thinking this might be the case is subjects’ reported phenomenological experience of using a visuo–spatial strategy during these tasks. Secondly, neuroimaging studies have shown the involvement of the parietal system in the encoding of relational spatial information [15], [16]. To resolve these issues we carried out a single-event, fMRI study of three-term spatial and nonspatial relational arguments with sampling of the BOLD signal during the reasoning component of the task. Our findings indicate that in three-term relational arguments both the concrete and abstract arguments activate a similar occipital–parietal–frontal network.