Investigations of human cortical processing of gentle touch. A study with time-resolved electro-magnetic signal analysis

Abstract

The present work summarizes investigations of the temporal correlates of brain activity elicited by gentle, moving touch on the hairy skin in healthy participants and in epilepsy patients. Light touch to the hairy skin activates two distinct afferent classes: fast conducting, Aβ afferents and slowly conducting C-tactile (CT) afferents. Aβ afferents signal discriminative aspects of touch, whereas CT afferents are proposed to play a role in affective touch. Using complementary neuroimaging methods with high temporal resolution, we aimed to distinguish between brain responses evoked by Aβ and CT afferents. In Papers I and IV, electroencephalography (EEG) showed that brush stroking on the hairy skin evoked an ultra-late potential, presumably driven by CT afference to the brain. Source localization indicated the cingulate cortex, and the precuneus as the underlying sources of this ultra-late potential. In Paper II, using magnetoencephalography (MEG) to spatially track brain activations in response to brush stroking over time, we showed that Aβ afference rapidly activates a well-defined network, including operculo-insular and cingulate regions. In Paper III, time- frequency analyses of MEG recordings from healthy participants were complemented with analyses of stereotactic EEG (SEEG) recordings from epilepsy patients. Here, we showed that naturalistic stroking touch induced spectral changes in alpha, beta, and gamma frequencies in sensorimotor regions and the posterior insula, similar to what has been described previously in studies using less naturalistic stimuli such as electrical median nerve stimulation. The present work contributes new information about the spatiotemporal evolution of the brain’s responses to caress-like touch and highlights the importance of considering both Aβ and CT afferents in gentle touch processing.