Somatosensory event-related potentials to stroking but not tapping are influenced by medium of stimulation

Touch is a multifaceted sense essential for both sensory discrimination and social communication. While most experimental research has focused on mechanically controlled, object-mediated stimulation, the ecological and neural relevance of direct skin-to-skin contact remains underexplored particularly in EEG studies. This study investigated Event-Related Potentials (ERP) to tactile stimuli delivered to the hairy forearm innervated by both Aβ and C-tactile (CT) afferents under two protocols: tapping and stroking. Each protocol included human-delivered tactile stimulations, with a “complex” object-mediated condition and a “social” skin-to-skin condition, and in the tapping protocol an additional automated mechanical condition. Our results demonstrate the methodological feasibility and neurophysiological reliability of using ecological human-delivered touch in EEG research. Robust ERP components (P50, N80, P200) were elicited in the tapping protocol and were not modulated by condition. In the stroking protocol, we found a P300 and an Ultra-Late-Potential (ULP), consistent with CT activation. Unexpectedly, both the P300 and the ULP showed greater amplitudes in the “complex” (object-mediated) compared to the “social” (skin-to-skin) condition. We propose that the salience or textural features of the object may have elicited enhanced perceptive and attentional processing. More generally, this study highlights the importance of accounting for inter-individual variability in the study of touch and emphasizes the need for greater consideration of ecologically valid touch, especially in the context of social touch.