Co-stimulation-removed audiovisual semantic integration and modulation of attention: An event-related potential study.

The integration of multisensory objects containing semantic information involves processing of both low-level co-stimulation and high-order semantic integration. To investigate audiovisual semantic integration, we utilized bimodal stimuli (AV, simultaneous presentation of an auditory sound and a visual picture; An, simultaneous presentation of an auditory sound and a visual noise; Vn, simultaneous presentation of a visual picture and an auditory noise; Fn, simultaneous presentation of an auditory noise and a visual noise) to remove the effect of co-stimulation integration and extract data regarding high-order semantic integration. Electroencephalography with a high temporal resolution was used to examine the neural mechanisms associated with co-stimulation-removed audiovisual semantic integration in attended and unattended conditions. By comparing the (AV + Fn) and (An+Vn), we identified three effects related to co-stimulation-removed audiovisual semantic integration. In the attended condition, two semantic integration effects over bilateral occipito-temporal regions at 220-240 ms and over frontal region at 560-600 ms were observed. In the unattended condition, only one semantic integration effect over centro-frontal region at 340-360 ms was observed. These effects reflected the semantic integration processes of pictures and sounds after removing the co-stimulation caused by spatiotemporal consistency. Moreover, the discrepancy in these effects in temporal and spatial distribution implied distinct neural mechanisms underlying attended and unattended semantic integration. In the attended condition, the audiovisual semantic information was initially integrated based on the semantic congruency (220-240 ms) and then reanalyzed according to the current task (560-600 ms), which was a goal-driven process and influenced by top-down attention. Contrastingly, in the unattended condition, no attention resources were allocated and the semantic integration (340-360 ms) was an unconscious automatic process.