Cortical EEG components that reflect inverse effectiveness during visuotactile integration processing.

Multisensory integration is required to interpret information from multiple senses and then produce the appropriate behavioral output. Inverse effectiveness, a phenomenon in which weaker stimuli induce greater activation of multisensory neurons than do stronger stimuli, is an essential component of multisensory integration. The superior colliculus is especially abundant in multisensory neurons with properties of inverse effectiveness. However, multisensory neurons are distributed throughout the brain, suggesting that a wide range of brain regions are involved in generating multisensory behavior in addition to the superior colliculus. In this study, we aimed to use scalp EEG to elucidate the cortical responses that respond to multisensory stimuli according to the principle of inverse effectiveness. By modulating the intensity of the tactile aspect of a simultaneous visuotactile stimulus, we explored the time-frequency component of scalp EEG waveforms produced during multi-sensory stimulation. Using this method, we determined the relative values and temporal dynamics of the increment of power spectrum (event-related spectrum perturbation) and phase coherence across trials (e.g., inter-trial phase coherency) produced by multisensory stimulation compared to unisensory stimulation. In the somatosensory and anterior cingulate cortices, we observed significant differences in the inter-trial phase coherence of the theta band oscillation 200-400 ms post-stimulus, in response to visuotactile (multisensory) and tactile (unisensory) stimulation, while no differences were found in later time windows and other cortical areas. These results suggest that inverse effectiveness is an important aspect of multi-sensory processing in the somato-sensory and frontal cerebral cortices.