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Abstract

Introduction: Tactile sensation is essential for humans to manipulate objects by hands. During object manipulation, many different physical properties of an object are sensed and processed by the human somatosensory system, supporting exquisite perceptual sensitivities [1]. Tactile sensation of different physical properties can be depicted in the several tactile perceptual dimensions, including roughness, hardness, stickiness and warmth [2]. To date, a number of human neuroimaging studies have unveiled neural mechanisms underlying roughness [3] and warmth perception [4]. Yet, relatively little is known about how the human brain subserves the perception of tactile hardness. Previous studies have suggested that slowly adapting type-1 (SA1) afferents are primarily responsible for perceiving hardness from the surface of an object [5] and the Brodmann areas (BA) 3b and 1 may contribute to perceiving hardness [6]. However, it remains elusive how the different levels of hardness are represented in the human brain during the dexterous manipulation of an object. Therefore, this study aims to investigate neural responses to tactile stimuli with the same shape and surface texture but different levels of hardness when people grip on the object with their hand. Functional magnetic resonance imaging (fMRI) is used to identify brain regions related with tactile hardness. Methods: Twelve right-handed subjects (8 female, mean age 23.1 years old) participated in the study. Experimental protocols were approved by the ethical committee of Ulsan National Institute of Science and Technology (UNISTIRB-15-16-A). Tactile stimuli with the same shape (oval) were prepared and grouped into four sets according to their hardness levels (level 1 to 4). Participants first performed a behavioral task in which they were given a pair of stimuli with eyes closed and asked to report the degree of a difference in hardness between them. Afterward, participants performed the fMRI experimental task in which they repetitively gripped on and released a given object (used in the behavioral task) for fifteen seconds followed by a nine-second rest. There were also trials in which participants executed the same grip-and-release motion without objects as a control task. Functional images (T2*-weighted gradient EPI, covering the whole depth of somatosensory area, TR = 3,000 ms, voxel size = 2.0 × 2.0 × 2.0 mm3) were obtained during the fMRI task using a Siemens 3T scanner (Magnetom TrioTim). The functional image analysis was performed using the general linear model (GLM) in SPM8 with a canonical hemodynamic response function to estimate blood-oxygen-level-dependent (BOLD) responses to each stimulus. Results: The analysis of behavioral experimental data showed that participants could correctly find differences in hardness levels among stimuli. The GLM analysis for individuals revealed activations in the contralateral postcentral gyrus in most participants modulated with different levels of hardness (p<0.001 uncorrected). Also, a random-effect group analysis of fMRI data revealed a cluster in the Rolandic operculum activated by the perception of tactile hardness (p<0.001 uncorrected). In addition, the cluster size and maximum activation peak was increased as the hardness level increased. Conclusions: Our study demonstrated that brain regions over the postcentral gyrus (S1) and Rolandic operculum might be related to the perception of tactile hardness. We also observed that the degree of activation in these regions, reflected by the size of the activated area (cluster size) and the level of activation (maximum peak) was proportional to the level of tactile hardness. Our results suggest that neural assemblies in the contralateral S1 and Roland operculum may play a role in sensing tactile hardness during dexterous object manipulation.