Multisensory processing and oscillatory gamma responses: effects of spatial selective attention.

Here we describe an EEG study investigating the interactions between multisensory (audio-visual) integration and spatial attention, using oscillatory gamma-band responses (GBRs). The results include a comparison with previously reported event-related potential (ERP) findings from the same paradigm. Unisensory-auditory (A), unisensory-visual (V), and multisensory (AV) stimuli were presented to the left and right hemispaces while subjects attended to a designated side to detect deviant target stimuli in either sensory modality. For attended multisensory stimuli we observed larger evoked GBRs approximately 40-50 ms post-stimulus over medial-frontal brain areas compared with those same multisensory stimuli when unattended. Further analysis indicated that the integration effect and its attentional enhancement may be caused in part by a stimulus-triggered phase resetting of ongoing gamma-band responses. Interestingly, no such early interaction effects (<90 ms) could be found in the ERP waveforms, suggesting that oscillatory GBRs may be more sensitive than ERPs to these early latency attention effects. Moreover, no GBR attention effects could be found for the unisensory auditory or unisensory visual stimuli, suggesting that attention particularly affects the integrative processing of audiovisual stimuli at these early latencies.