A major determinant of multisensory integration, derived from single-neuron studies in animals, is the principle of inverse effectiveness (IE), which describes the phenomenon whereby maximal multisensory response enhancements occur when the constituent unisensory stimuli are minimally effective in evoking responses. Human behavioral studies, which have shown that multisensory interactions are strongest when stimuli are low in intensity are in agreement with the IE principle, but the neurophysiologic basis for this finding is unknown. In this high-density electroencephalography (EEG) study, we examined effects of stimulus intensity on multisensory audiovisual processing in event-related potentials (ERPs) and response time (RT) facilitation in the bisensory redundant target effect (RTE). The RTE describes that RTs are faster for bisensory redundant targets than for the respective unisensory targets. Participants were presented with semantically meaningless unisensory auditory, unisensory visual and bisensory audiovisual stimuli of low, middle and high intensity, while they were instructed to make a speeded button response when a stimulus in either modality was presented. Behavioral data showed that the RTE exceeded predictions on the basis of probability summations of unisensory RTs, indicative of integrative multisensory processing, but only for low intensity stimuli. Paralleling this finding, multisensory interactions in short latency (40-60ms) ERPs with a left posterior and right anterior topography were found particularly for stimuli with low intensity. Our findings demonstrate that the IE principle is applicable to early multisensory processing in humans.
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