Pleasant Deep Pressure: Expanding the Social Touch Hypothesis

Abstract

Neuroscientific research on pleasant touch has focused on the C-tactile pathway for gentle stroking and has successfully explained how these sensory fibers transmit information about affective social touch to the brain and induce sensations of pleasantness. The C-tactile social/affective touch hypothesis even proposes that C-tactile fibers form a privileged pathway underlying social touch. However, deep pressure is a type of touch commonly considered pleasant and calming, occurring in hugs, cuddling, and massage. In this paper we introduce a paradigm for studying pleasant deep pressure and propose that it constitutes another important form of social touch. We describe development of the oscillating compression sleeve (OCS) as one approach to administering deep pressure and demonstrate that this touch is perceived as pleasant and calming. Further, we show that deep pressure can be imaged with functional magnetic resonance imaging (MRI) using the air-pressure-driven OCS and that deep pressure activates brain regions highly similar to those that respond to C-tactile stroking, as well as regions not activated by stroking. We propose that deep pressure constitutes another social touch pathway of evolutionary importance signaling the close proximity of conspecifics.

Keywords: C-tactile; fMRI; pressure; social touch; somatosensation.

Figure 1.. Design of Oscillating Compression Sleeve (OCS).

a). Commercially available lower limb lymphedema compression sleeve. Darker area shows the two chambers that were inflated in Study 2. b.) Custom built pressure regulation device. Device converts USB signaling into electric current to drive the regulator and delivers information from the sleeve back to the USB. c.) Freestanding compressed air tank to supply air to the sleeve. d.) Matlab computer connected via USB to the flow regulator and used to regulate pressure level and flow rate.

Figure 2.. Pleasantness ratings for pressure on five body locations.

Box plot of pleasantness ratings for static pressure pulses showing the third quartile and first quartile range of the data. Lower and upper error lines display the 5th and 95th percentiles. Pressure pulses of 30, 50, 70, and 90 mmHg were applied to the wrist, forearm, bicep, ankle, and calf on the non-dominant side using commercially available lymphedema compression sleeves. Healthy adult participants provided pleasantness ratings for each body location/intensity combination. The VAS scale ranged from 0 (no pleasantness) to 100 (most extreme pleasantness imaginable).

Figure 3.. Pleasantness ratings for brushing and pressure

Box plot of pleasantness ratings of gentle brushing (Study 2), oscillating compression (Study 2), and static compression (Study 1) on the left calf. Lower and upper error lines display the 5th and 95th percentiles, and filled circles display data falling outside these percentiles. Gentle brushing was applied at two velocities (~3cm/s and ~30cm/s) using a soft goat-hair brush. Pressure was applied at two intensities. Oscillating pressure rose from a baseline of 10–15mmHg to a peak of 30mmHg (low) or 65mmHg (high). Static pressure rose to hold at 30mmHg (low) or 70mmHg (high). Trials of each stimulus lasted 15–30 seconds. Participants rated the pleasantness of each sensation on a VAS scale that ranged from 0 (no pleasantness) to 100 (most extreme pleasantness imaginable).

Figure 4.. Effects of brushing and pressure and mood on anxiety/calm.

Box plot of ratings of anxiety/calm and mood before and after affective stimuli. Lower and upper error lines display the 5th and 95th percentiles, and filled circles display data falling outside these percentiles. Gentle brushing was applied to the left calf at two speeds (~3cm/s and ~30cm/s) using a soft goat hair brush. Oscillating pressure was applied to the same area at two intensities (rising from a baseline of 10–15mmHg to a peak of 30 or 65mmHg) using the Oscillating Compression Sleeve. Trials of each stimulus lasted 15–30 seconds, with a 20 second inter-stimulus-interval. At baseline and after each trial, participants rated their current mood and level of anxiety on a VAS scale with anchors from −100 (Extremely bad mood/Extremely anxious) to 0 (neutral) to 100 (Extremely good mood/Extremely calm).

Figure 5.. Affective ratings of brushing and pressure.

Participants completed the Touch Perception Task (17), rating their affective experience of brushing (green) and pressure (blue). Participants were not asked to distinguish between slow/fast brushing or high/low pressure but to rate the most strongly they experienced each adjective.

Figure 6.. BOLD response to brushing and compression

BOLD images were acquired using a 3T MRI scanner while participants received brushing at two speeds (~3cm/s and ~30cm/s) and oscillating pressure at two levels (a peak of 30mmHg or 65mmHg) in separate blocks on their lower left calf. Contrasts were conducted to identify the brain regions activated by brushing > rest (fast and slow brushing combined) and compression > rest (low and high pressure combined). Activation maps were thresholded at Z = 3.1 and cluster corrected at p < 0.001 unless noted otherwise. Response to brushing was observed in contralateral S1, bilateral S2 and SMG, and contralateral insula. Response to compression was observed in contralateral S2 and insula, and subthreshold in contralateral S1. Insert on far right displays brushing and compression representation overlaid in the insula. Coordinates are in the MNI 152 space.

Figure 7.. BOLD response to brushing > compression and compression > brushing

A masked analysis was conducted to identify significantly greater responses to brushing or compression, within areas significantly activated by brushing or compression. Greater response to brushing was found in ipsilateral S1/SMG and greater response to compression was observed in contralateral SMG. Coordinates are in the MNI 152 space.