Comparison of brain functional response to mechanical prickling stimuli to the glabrous and hairy skin

Abstract

Background: A kind of prickle sensation, which is a composite feeling of pain and itch, can be evoked by mechanical stimulation of fiber ends from fabric surface against to human hairy skin, rather than glabrous skin. Now, a functional magnetic resonance imaging (fMRI) study was conducted to investigate the cognitive differences in the brain for mechanical prickling stimuli to the two types of skin.

Materials and methods: A nylon filament with the diameter of 205 μm and the length of 8 mm was used to deliver mechanical prickling stimuli respectively to two skin sites, fingertip (glabrous skin) and volar forearm (hairy skin), of eight healthy male subjects. Simultaneously, the technology of fMRI was adopted to acquire BOLD (Blood Oxygen Level-Dependent) signals of brain functional response of the subjects.

Results: Somatosensory areas, emotional areas, and the posterior parietal cortex (especially the precuneus) are important brain regions that distinguish between the two conditions. The representation of mechanical prickling stimulation to glabrous skin in the brain favors much more the tactile information of the stimulation and contains no itch, while the key brain area, precuneus, involved in itch was activated by the same mechanical prickling stimulation to hairy skin, and brain response for the condition of hairy skin contains more emotional information, which plays an important role in pain processing.

Conclusion: Therefore, it can be inferred that a kind of stronger prickle sensation, which contains both pain and itch, was evoked by mechanical stimulation to hairy skin than glabrous skin.

Keywords: brain response; functional magnetic resonance imaging; glabrous skin; hairy skin; itch; mechanical prickle; pain.

FIGURE 1

Brain activation map for mechanical prickling stimuli to fingertip of the right index (glabrous skin). The number indexing each slice at the lower left corner means the MNI z coordinate of the slice. The specific location in the brain of each slice is indicated in the lower right 3D image.

FIGURE 2

Brain activation map for mechanical prickling stimuli to the right volar forearm (hairy skin).

FIGURE 3

Images of common brain regions activated in both conditions of skin types (glabrous skin and hairy skin).

FIGURE 4

Mean signals (beta value) in the primary somatosensory cortex (S1). S1_L refers to ROI of the whole region of S1 of the left hemisphere. S1_L* refers to the sphere ROI (MNI coordinates: −66, −21, 24, 10‐mm radius) of the common activated region of S1 of the left hemisphere in both conditions of skin types. S1_R* refers to the sphere ROI (MNI coordinates: 51, −27, 48, 10‐mm radius).

FIGURE 5

Mean signals in the secondary somatosensory cortex (S2). OP4_L* refers to the sphere ROI (MNI coordinates: −66, −18, 21, 10‐mm radius).

FIGURE 6

Mean signals in emotional areas. IC_L* refers to the sphere ROI (MNI coordinates: −48, −21, 15, 10‐mm radius).

FIGURE 7

Mean signals in motor areas. M1M2_L* refers to the sphere ROI (MNI coordinates: −60, −18, 36, 10‐mm radius).

FIGURE 8

Mean signals in posterior parietal cortex. BA40_R* refers to the sphere ROI (MNI coordinates: 42, −36, 60, 10‐mm radius). Pre: precuneus.