Frequency-tagging of steady-state evoked potentials to isolate and compare the cortical processing of of noxious and innocuous sensory input.

The sustained periodic modulation of a sensory stimulus has been shown to induce an entrainment of cortical neurons responding at the frequency of stimulation, appearing as a steady-state evoked potential (SS-EP) in the EEG frequency spectrum. Recently, we showed that SS-EPs can be elicited by the selective activation of skin nociceptors. Because, in other sensory modalities, these responses have been shown to reflect activity originating mainly form primary sensory cortices, nociceptive SS-EPs constitute a potential mean to characterize nociceptive cortical processing in humans. Here, we used SS-EPs to frequency tag the cortical activity elicited by concurrently presented nociceptive, tactile and visual stimuli to examine (1) whether the elicited responses originate from distinct neuronal populations and (2) whether this gives rise to additional cross-modulation SS-EPs indexing the activity of neurons involved in multisensory integration. Ten subjects took part in the experiment. 5-s trains of nociceptive, tactile, and visual stimuli were concurrently delivered to the left and right hand. All three stimuli elicited consistent and distinct SS-EPs, appearing as three separate peaks in the EEG frequency spectrum, with distinct scalp topographies. The nociceptive SS-EP was maximal over the scalp vertex, whereas the tactile SS-EP was maximal over the parietal region contralateral to the stimulated side and the visual SS-EP was maximal over occipital regions, thus indicating that all three responses originate from distinct neuronal populations. No significant cross-modulation SS-EP could be identified. Here, we show that distinct SS-EPs can be reliably recorded following concomitant nociceptive, tactile and visual stimulation. This approach could be used in future studies to explore crossmodal interactions and multisensory integration of nociceptive and non-nociceptive sensory inputs