An fMRI study on cortical responses during active self-touch and passive touch from others

Abstract

Active, self-touch and the passive touch from an external source engage comparable afferent mechanoreceptors on the touched skin site. However, touch directed to glabrous skin compared to hairy skin will activate different types of afferent mechanoreceptors. Despite perceptual similarities between touch to different body sites, it is likely that the touch information is processed differently. In the present study, we used functional magnetic resonance imaging (fMRI) to elucidate the cortical differences in the neural signal of touch representations during active, self-touch and passive touch from another, to both glabrous (palm) and hairy (arm) skin, where a soft brush was used as the stimulus. There were two active touch conditions, where the participant used the brush in their right hand to stroke either their left palm or arm. There were two similar passive, touch conditions where the experimenter used an identical brush to stroke the same palm and arm areas on the participant. Touch on the left palm elicited a large, significant, positive blood-oxygenation level dependence (BOLD) signal in right sensorimotor areas. Less extensive activity was found for touch to the arm. Separate somatotopical palm and arm representations were found in Brodmann area (BA) 3 of the right primary somatosensory cortex (SI) and in both these areas, active stroking gave significantly higher signals than passive stroking. Active, self-touch elicited a positive BOLD signal in a network of sensorimotor cortical areas in the left hemisphere, compared to the resting baseline. In contrast, during passive touch, a significant negative BOLD signal was found in the left SI. Thus, each of the four conditions had a unique cortical signature despite similarities in afferent signaling or evoked perception. It is hypothesized that attentional mechanisms play a role in the modulation of the touch signal in the right SI, accounting for the differences found between active and passive touch.

Keywords: glabrous; hairy; motor; sensorimotor; skin; somatosensory; stroking.

Figure 1

Overview of the main regions where positive BOLD signal changes were found for active and passive stroking on the palm and arm, compared to the resting baseline. There were clear differences between active and passive touch, as can be seen in the BOLD signal in the left SI and MI (compare the top two panels). There were also body site differences: the right SI had large regions of activity from palm stroking, whereas much less activity was found to arm stroking (see also Table 1). There was also bilateral SII activation to arm stroking, whereas bilateral activity for palm stroking was only found for passive touch (in active touch to the palm, there was only right SII activity). The maps are to neurological convention (left is left).

Figure 2

Beta weights from the right SI palm and arm respective areas. There were significant differences for the palm SI BA03 region and arm SI BA03 region (see Table 1 for area details), where active stroking gave significantly higher beta values than passive stroking. Error bars show ±1 standard error.

Figure 3

Overview of the left SI area that showed significant negative BOLD during passive stroking. The top graph shows the time courses of all the conditions in the left SI BA03 area (see Table 1 for more details); in active touch, there was a significant positive BOLD signal changes, whereas in passive touch, there was a significant negative BOLD signal. The bottom panel shows the negative BOLD signal areas for passive touch to the palm and arm; these regions overlapped greatly. The brain map in the bottom panel is to neurological convention (left is left).