The sticky brain: A fMRI study of stickiness

Abstract

The human tactile perception is generally known as having 4 different dimensions: Roughness, warmness, hardness and stickiness. Although the neuroscientific investigations on the other tactile perception dimensions have been progressed, little is known about the perception of stickiness. Moreover, even the studies that examined stickiness feeling and its neural responses induced the tactile sensation from the grip or friction force. However, the sticky sensation is aroused when the skin is stretched as people detaching their part of the body from adhesive substances, as well as the skin stretch is assumed to be perceived by the Ruffini corpuscles while the grip force is mediated by the Meissner corpuscles. Considering this fact, an investigation that generates sticky feeling without frictional force is necessary to elucidate the neural responses for the tactile perception of stickiness. Hence, the present study aims to find the brain activities when perceiving sticky stimuli via the functional magnetic resonance imaging (fMRI). To achieve our goal, we created silicone-based sticky stimuli by mixing catalyst with a different ratio. Participants in our experiment underwent two behavioral tests with the silicone stimuli, the method of constant stimuli and the magnitude estimation. Each test aims to find a threshold for perceiving stickiness and measure the perceived intensity of stickiness. In the fMRI experiment session, sticky-silicone stimuli were tested to find the brain regions responsible for the tactile perception of stickiness. Also, an acrylic sham stimulus (‘Sham’) was employed to present a condition that the sticky feeling is not aroused. From the analysis of the method of constant stimuli test, we could found absolute threshold for the tactile perception of stickiness. Based on this threshold, the silicone stimuli were divided into two groups: One arousing the perception of stickiness significantly, named as ‘Supra-threshold’, and the other failing to arouse sticky feeling constantly, or ‘Infra-threshold’. The analysis of the fMRI data revealed that contralateral primary somatosensory area (S1) and ipsilateral dorsolateral prefrontal cortex (DLPFC) were significantly activated in Supra-threshold vs. Sham contrast, while no significant activation was observed in the Infra-threshold vs. Sham contrast. Since the stimuli of Supra- and Infra-threshold were all made of same silicone material, it is reasonable to attribute the result to the perception of stickiness. We further examined the Supra- vs. Infra-threshold contrast to detect the brain regions playing a role in perceiving the intensity of stickiness. Two clusters in basal ganglia-thalamus region as well as one cluster spanning from insula to temporal cortex were significantly activated. All three clusters lie in brain areas which have been reported as responsible for the tactile discrimination. Thus, the activation in these area is assumed to be induced by the subtle difference in the intensity of stickiness between the Supra- and Infra-threshold stimuli.