Frequency-tagging of steady-state evoked potentials to characterize the cortical integration of nociceptive an non-nociceptive inputs in humans

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. Here, we used SS-EPs to frequency tag the cortical activity elicited by concurrently presented nociceptive, tactile and visual stimuli such as to examine whether this gives rise to additional cross-modulation SS-EPs, indexing the activity of neurons involved in multisensory integration. Indeed, when two distinct neuronal populations oscillate at two distinct frequencies, cross-modulation frequencies, corresponding to the sum or difference of the two main frequencies, may emerge if the signals conveyed by each of the two oscillating populations converge onto another population integrating these inputs. Ten subjects took part in the experiment. The EEG was recorded continuously. Using distinct frequencies, trains of nociceptive stimuli were delivered concurrently to trains of tactile stimuli or visual stimuli stimuli. Participants were asked to detect the occurrence of brief interruptions occurring in one of the two concurrent trains of stimuli. All three stimuli elicited consistent SS-EPs, appearing as 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. However, no significant cross-modulation SS-EP could be identified