Exploring the time course of nociceptive remapping

Details

Supervisors

Legrain, Valéry

Faculty

Faculté de pharmacie et des sciences biomédicales

Degree label

Master [120] en sciences biomédicales, à finalité approfondie

Abstract

To respond efficiently to somatosensory stimuli, whether tactile or nociceptive, the brain relies on two spatial systems: the somatotopic system, which codes the anatomical location of the stimulus, and the spatiotopic system, which codes the relative position of the stimulated body part in external space. This dual mapping has been demonstrated using Temporal Order Judgment (TOJ) tasks, where participants discriminate the order of two successive stimuli on the hands in uncrossed or crossed-arm positions. Crossed arms reduce performance due to conflicting somatotopic (e.g., "my left hand was stimulated first") and spatiotopic (e.g., "the hand stimulated first was in the right space") information. Psychophysical and neurophysiological studies have shown that, for tactile stimuli, this dual coding is serial: somatotopic coding occurs first in the primary somatosensory cortex, followed by spatiotopic coding in secondary somatosensory, parietal, and prefrontal regions. However, the mechanisms of the time course of this dual mapping in nociception remain unclear. This study investigated the timing of nociceptive spatial representations using electroencephalography (EEG). Pairs of thermo-nociceptive stimuli were applied to each hand dorsum, and participants performed a TOJ task with hands crossed or uncrossed. Contact-heat Evoked Potentials (CHEPs) were recorded, and their magnitude was compared between the two hand postures for three components: N1 (~250-400ms, lateral scalp topography), N2 (~350-450ms, vertex), and P2 (~400-700ms, vertex topography). We hypothesized that the earliest component impacted by hand posture would reflect the time frame of the two mapping systems' activation. While no significant differences in CHEPs peak amplitudes were found, point-by-point cluster analysis revealed significant differences as early as 60-120ms and around 200ms post-stimulus in occipital, frontal, and parieto-temporal areas. These findings, along with potential dipole sources, are discussed, as well as suggestions for future analyses, such as time-frequency analysis.