Characterizing the Cortical Processes Related to the Perception of Long-Lasting Warm and Cool Sensations

Aim of Investigation. Long-lasting periodic sensory stimulation is increasingly used to study, using electroencephalography (EEG), the cortical processes underlying perception in different modalities. Sustained periodic stimulation can elicit synchronized periodic activity in neuronal populations responding to the stimulus, referred to as a steady-state response (SSR). The recording of SSRs has been proposed as an alternative to classical event-related potentials (ERPs) to probe sensory perception, since it can reveal new aspects of sensory information processing. Periodic radiant heat stimulation of the skin has already been used to characterize the cortical processes underlying the perception of sustained burning pain. Here, we examined whether SSRs related to the perception of noxious heat and innocuous cold can be obtained by periodically heating or cooling the skin using a contact thermal stimulator. Furthermore, because repeated stimulation of skin afferents could induce peripheral adaptation or receptor fatigue, we assessed the effect of displacing the stimulated skin surface. Methods. We recorded 64-channel scalp EEG on 15 healthy subjects exposed to long-lasting (75 s) periodic cool and warm stimuli delivered to the right volar forearm. Stimulation was applied using a prototype contact stimulator made of micro-Peltier elements, capable of warming or cooling the skin at rates of up to 300°C/s. The elements were organized in five independently-controlled zones of 24 mm2, allowing to vary the stimulated skin surface without displacing the stimulation probe. The stimulation consisted in a 0.2 Hz sinusoidal cooling (up to 14°C) or warming (up to 48°C) of the skin. Twelve trials of each condition (warm vs. cool; constant vs. variable stimulation surface) were presented in a randomized order. Results. Frequency analysis of the EEG recordings showed significant periodic EEG responses to all stimuli, except cool stimulation using a fixed stimulation surface. The SSRs to periodic heat stimulation delivered using a variable stimulation surface were more pronounced than the SSRs to periodic heat stimulation delivered using a constant surface. The responses to heat stimulation were of greater magnitude than the responses to cool stimulation. Time-domain analysis of the EEG recordings showed a strong habituation of both heat- and cold-evoked responses along the stimulation cycles. This habituation was stronger when stimulation was delivered using a constant surface, and most pronounced for cool SSRs. Conclusions. The novel stimulator used in our study offers new alternatives to study the spinothalamic system and the perception of long-lasting heat and cold. However, the elicited responses can exhibit a strong habituation over time, especially when the stimulation is applied to the same skin area. This response reduction is most pronounced during cool stimulation, and probably due to peripheral adaptation