Cortical network underlying audiovisual semantic integration and modulation of attention: An fMRI and graph-based study.

Many neuroimaging and electrophysiology studies have suggested that semantic integration as a high-level cognitive process involves various cortical regions and is modulated by attention. However, the cortical network specific to semantic integration and the modulatory mechanism of attention remain unclear. Here, we designed an fMRI experiment using "bimodal stimulus" to extract information regarding the cortical activation related to the effects of semantic integration with and without attention, and then analyzed the characteristics of the cortical network and the modulating effect of attention on semantic integration. To further investigate the related cortical regions, we constructed a functional brain network for processing attended AV stimuli to evaluate the nodal properties using a graph-based method. The results of the fMRI and graph-based analyses showed that the semantic integration with attention activated the anterior temporal lobe (ATL), temporoparietal junction (TPJ), and frontoparietal cortex, with the ATL showing the highest nodal degree and efficiency; in contrast, semantic integration without attention involved a relatively small cortical network, including the posterior superior temporal gyrus (STG), Heschl's gyrus (HG), and precentral gyrus. These results indicated that semantic integration is a complex cognitive process that occurs not only in the attended condition but also in the unattended condition, and that attention could modulate the distribution of cortical networks related to semantic integration. We suggest that semantic integration with attention is a conscious process and needs a wide cortical network working together, in which the ATL plays the role of a central hub; in contrast, semantic integration without attention is a pre-attentive process and involves a relatively smaller cortical network, in which the HG may play an important role. Our study will provide valuable insights into semantic integration and will be useful for investigations on multisensory integration and attention mechanism at multiple processing stages and levels within the cortical hierarchy.