When a sentence loses semantics: Selective involvement of a left anterior temporal subregion in semantic processing

Although the left anterior temporal lobe (ATL) has been associated with semantic processing, the role of this region in syntactic structure building of sentences remains a subject of debate. Functional neuroimaging studies contrasting well-formed sentences with word lists lacking syntactic structure have produced mixed results. The current functional magnetic resonance imaging study examined whether the left ATL is selectively involved in semantic processing or also plays a role in syntactic structure building by manipulating syntactic complexity and meaningfulness in a novel way. To deprive semantic/pragmatic information from a sentence, we replaced all content words with pronounceable meaningless placeholders. We conducted an experiment with a 2 x 2 factorial design with factors of SEMANTICS (natural sentences [NAT]; sentences with placeholders [SPH]) and SYNTAX (the basic Japanese Subject-Object-Verb [SOV] word order; a changed Object-Subject-Verb [OSV] word order). A main effect of SEMANTICS (NAT > SPH) was found in the left ATL, as well as in the ventral occipitotemporal regions. The opposite contrast (SPH > NAT) revealed activation in the dorsal regions encompassing Brodmann area 44, the premotor area, and the parietal cortex in the left hemisphere. We found no main effect of SYNTAX (OSV > SOV) in a subregion of the left ATL that was more responsive to natural sentences than meaningless sentences. These results indicate selective involvement of a subregion of the left ATL in semantic/pragmatic processing.