Exploration of the EEG response to periodic thermal and vibrotactile stimuli

Under certain conditions, a stimulus applied at a given frequency will lead to a periodic variation of neural activity at the same frequency. Taking advantage of this periodicity, it is possible to tag this response in the EEG frequency spectrum. Frequency tagging of sustained periodic noxious heat stimuli led to the recording of phase-locked and non-phase-locked responses whose functional significance remains unclear. This study aimed at assessing whether such responses can also be recorded during the repetitive presentation of brief innocuous cold, noxious heat and vibrotactile stimuli. Comparison between the responses obtained with different stimulation modalities should inform us on the nature of the neural processes underlying these responses (modality aspecific, somatosensory, thermosensory, nociceptive). Comparison between upper and lower limb stimulation should inform us on the somatotopic organization of these responses and, therefore, on their potential sources. Based on our results, on one hand, trains of brief innocuous cold, noxious heat and vibrations can elicit phase-locked and non-phase-locked responses which appear highly similar to those evoked by sustained periodic noxious heat stimuli when frequency tagged. On the other hand, when analysed in the time domain or using time-frequency decomposition, these responses appeared highly similar to those that can be recorded following isolated brief noxious heat or tactile stimuli. These responses consisted in phase-locked activity corresponding to the vertex potential, thought to reflect modality non-specific attentional processes, and in an alpha-to-beta ERD originating in the S1/M1 area contralateral to the stimulated hand, probably reflecting non-specific somatosensory activity.