| dc.description.abstract | This study compares psychophysical and EEG responses to 3 types of tactile stimulation (brush stroke, hand stroke, and massage-like tapping) to investigate whether affective afferent only response to slow, gentle brush stroke. The classical definition of affective touch involves CT afferent and A-beta afferent pathways, with the CT pathway characterized by activation in the orbitofrontal and insular cortex and deactivation in the primary (SI) and secondary
somatosensory cortex (SII). However, recent evidence showed that the spinothalamic ablation in humans does not make difference in rating the pleasantness of the CT touch paradigm before and after cordotomy, and even, compared to the control groups (Marshall et al., 2019). Therefore, the
current study has investigated if the properties of the tactile information were judged while relying on ascending A-beta low threshold mechanoreceptor afferent inputs in affective touch using EEG theta oscillation.
This study involved a learning phase followed by three test phases. Participants were asked to memorize button locations and practice rating hedonic responses during the learning
phase with open eyes. During the test phase, participants closed their eyes and rated with brush stroke, hand stroke, and massage-like tapping with two velocities (CT-optimal 5 cm/s and CTsuboptimal 18 cm/s, 3 styles x 2 velocities). We investigated theta oscillations in EEG analysis and replicated previous findings in affective touch. Specifically, we performed a time-frequency analysis to identify channels and frequency bands showing significant differences in power before and after each tactile stimulation style and velocity. Once we identified these frequency bands, we applied a filter-Hilbert transform to calculate the instantaneous power of the selected
bands for statistical comparison. We compared the EEG theta power before and after tactile stimulation, considering the temporal sequence of theta power bands, and found significant differences only when accounting for temporal sequence. The hand stroke with CT-optimal velocity was rated as the most pleasant touch condition. EEG analysis focused on the theta oscillation changes which were associated with the most pleasant hand stroke in both FP1 and CP4 channels. Brush stroke only showed significant theta oscillation increases in the FP1 channel, while massage-like tapping showed significant changes across most channels, including
FP1, indicating activation close to the orbitofrontal cortex.
This study, along with previous research on affective touch, suggests that the ＂dual pathway＂ theory may not provide a complete understanding of how affective and discriminative afferent respond to different types of tactile stimulation. Our results showed that changes in the
theta oscillations, resulting from the least pleasant massage-like tapping, could be found across multiple channels, including FP1 and CP4, indicating that mechanic touch may induce responses beyond the somatosensory cortex. The hand stroke condition, which facilitated comparable hedonic ratings as the brush stroke, also induced significant theta oscillations at FP1 and CP4 channels, potentially reflecting activation close to the somatosensory cortex and the orbitofrontal cortex. Together, these findings indicate that affective and discriminative touch cannot be dissociated by CT-afferent activation.
Consequently, affective touch may be primarily determined by a top-down cognitive mechanism, making it a higher-order process than just processing the mechanical characteristics of touch. | en_US |