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4.1 General discussion
The objective of the present study was to investigate neural responses to the tactile perception of stickiness using fMRI. To achieve our goal, we presented participants silicone-based sticky stimuli to induce tactile feelings of stickiness, along with an acrylic sham stimulus with no stickiness at all. The behavioral responses from participants demonstrated that the silicone stimuli could be grouped into Supra- and Infra-threshold based on the absolute threshold where the stimuli of Supra-threshold clearly induced sticky perception whereas the Infra-threshold stimuli did not. The analysis of the fMRI data revealed that contralateral S1 and ipsilateral DLPFC were significantly activated in the Supra-threshold vs. Sham contrast whereas no significant activation was found in the Infra-threshold vs. Sham contrast. Moreover, bilateral basal ganglia, ipsilateral insula cortex and superior and middle temporal cortex were activated in the Supra- vs. Infra-threshold contrast. The present study successfully modeled a series of silicone stimuli which has a perceptual threshold for the stickiness.
4.2 Behavioral responses in two psychophysics experiments
From the two behavioral experiments, we could notice several key aspects of our stimuli and of the perceived stickiness. Firstly, the perceptual threshold for stickiness across the participants is made at the point of 7.47% catalyst ratio. The result indicates not only that the silicone stimuli was perceptually divided into two groups, Supra- and Infra-threshold, but also shows that our stimuli set captures the point which our participants starting to perceive stickiness.
Secondly, as the result of the magnitude estimation clearly shows, the stimulus with 7% catalyst ratio has perceptually different intensity of stickiness from the 5 and 6% stimuli. In other words, we succeeded in presenting various intensity of stickiness with our silicone stimuli as we intended. It could be claimed that the 7% stimulus is differentiated from the 5 and 6% stimuli because the 7%
stimulus was used as a standard. Thus, we did an additional ANOVA and post-hoc t-test on the method of constant stimuli responses, which again resulted in similar effects: the chance of perceiving stickiness from 7% stimulus (Mean = 68.89%, SD = 29.34%) was clearly different from the 5 (Mean
= 98.89%, SD = 3.33%) and 6% (Mean = 97.78%, SD = 4.41%) stimuli. Therefore, although the fact that the 7% stimulus was used as a standard might affects to the magnitude estimation response, still it is believable that the 7% stimulus aroused different intensity of stickiness from 5 and 6% stimuli to our participants.
Lastly, the perceived stickiness of the Infra-threshold stimuli was not same as of the sham stimulus, which was made of acrylic material and aimed to present a tactile condition of not sticky feeling.
Although the stimuli of Infra-threshold failed to generate sticky feeling significantly, the responses for these stimuli in the method of constant stimuli and the magnitude estimation tests were greater than 0.
In the method of constant stimuli test, the value 0 indicates that participant did not perceive sticky feeling for all trial from a stimulus, while in the magnitude estimation test, 0 was equal to the intensity of stickiness of sham stimulus. Collectively, we can speculate that the Infra-threshold stimuli aroused tactile perception similar to the stickiness, but the sensation was too weak to be considered as sticky feeling.
The study has limit in that the physical intensity of stickiness of each silicone stimuli is unknown, so it was unable to examine the changes in perception of stickiness according to the physical quantity of stickiness. We attempted to measure the physical intensity of stickiness of our silicone stimuli through the peel-strength test, which was unsuccessful because the size of stimuli was not suitable for the test.
Hence, the entire analysis of our study have done based on perceived intensity of stickiness, not on the physical factor of the stimuli, or the catalyst ratio.
4.3 Brain responses in Supra-threshold vs. Sham and Infra-threshold vs.
Sham contrasts
Contralateral S1 and ipsilateral DLPFC, the two significantly activated regions in the Supra-threshold vs. Sham contrast, may be involved in the tactile perception of stickiness. Even though both the Supra- and Infra-threshold stimuli were made of the same silicone substance, only the Supra-threshold vs. Sham contrast showed significant activities in the two brain regions. Thus, it is plausible to attribute the activation of contralateral S1 and ipsilateral DLPFC to the perception of stickiness from the stimuli, not the difference of the materials between the silicone and the acryl.
S1 has been reported to be involved in perceiving tactile sensation in a number of fMRI studies (Hlushchuk & Hari 2006; Pleger et al. 2003; Pleger et al. 2006; Servos et al. 2001; Kim et al. 2015;
Schaefer et al. 2006). In particular, S1 is deemed to participate in the process of discriminative somatosensory perception (Timmermann et al. 2001; Schnitzler & Ploner 2000; Jiang et al. 1997). For example, Schnitzler et al. reported that S1 discriminated painful stimuli from others (Schnitzler &
Ploner 2000). A few previous studies have investigated the involvement of S1 in stickiness perception in indirect ways, by employing the frictional force induced by the precision grip (Brochier et al. 1999;
Johansson & Westling 1987; Johansson & Cole 1992). For example, a non-human primate study which inactivated S1 of a monkey’s brain reported that frictional sensation aroused by a grip was failed to transmitted to S1 and consequently, detailed control of objects was unable (Brochier et al.
1999). Our study demonstrated that S1 is also involved in processing the tactile perception of stickiness in humans, which has hitherto been unexplored. Based on the previous findings, S1 activation in our study might reflect its role in discriminating sticky sensation from the stimuli to support the perception of stickiness.
The activation in DLPFC could be interpreted in multiple ways. First of all, the sticky feeling might provoke affective responses, such as emotion or pain, in addition to mere tactile sensation. DLPFC, with the connection to parietal cortex, was known to process higher-order somatosensory information (Wood & Grafman 2003). Furthermore, Wager et al. reported increases in DLPFC activity when the anticipation for pain was reduced (Wager et al. 2004), and Navratilova et al. attributed DLPFC activity to a reward mechanism by a relief from an aversive state (i.e., pain) (Navratilova & Porreca 2014). As the perception of stickiness is aroused as the skin is stretched when people detach their part of body from adhesive substances (Yamaoka et al. 2008), an excessive tensile force applied during the detachment can cause damage on the body such as abrasion. In other words, there is a potential risk that accompanies with sticky sensation. Therefore, it is plausible that even non-painful perception of stickiness may arouse aversive emotions or relief from pain, which are reflected in the activation of DLPFC in our study.
Moreover, the task of our experiment, perceiving stickiness, might demand the involvement of the brain region for distinguishing the sensation of stickiness between sticky and not sticky materials. The study of Pleger et al. is a clear example for showing that DLFPC is participated in the processes of tactile discrimination (Pleger et al. 2006). In the experiment, participants were asked to compare the frequency of two tactile stimuli, and the researchers found a positive relationship between the BOLD responses in DLPFC and the discrepancy between the two stimuli of the task. The researchers suggested that considering DLPFC is a well known brain region for processing decision making, the result implies the area might be responsible for accumulating all information related to make a decision from stimuli. A similar result was also observed in a study of a patient whose DLPFC was activated only when the tactile discrimination for direction was possible (Lundblad et al. 2010). In line with previous studies, the DLPFC activation in our study was possibly induced during discriminating the clearly different perception between sticky (Supra-threshold stimuli) and not sticky (Sham), which might be extended to a tactile decision making.
4.4 Brain responses in Supra- vs. Infra-threshold contrast
By contrasting brain responses to Supra- vs. Infra-threshold stimuli, we investigated brain regions involved in the perception of different intensities of stickiness. All the stimuli made of the same silicone material produced the perception of stickiness stably or not, depending on the catalyst ratio.
Thus, the Supra- vs. Infra-threshold contrast would indicate brain regions involved in perceiving differences in intensity of stickiness between the two groups.
It is noteworthy that the activated regions were distributed intensively in subcortical areas (i.e., basal ganglia, thalamus, and insula). Of the regions, the activation in basal ganglia and thalamus may represent the function of the basal ganglia–thalamocortical loop. Traditionally, the motor control aspects of this loop have been mainly discussed (Middleton & Strick 2000; Alexander & Crutcher 1990), and the role of the loop in processing somatosensory information has been attributed to its proprioception that allowing exquisite body movement (Kaji 2001). Recent studies, however, have also revealed that the basal ganglia–thalamocortical loop is involved in a tactile discrimination task (Peller et al. 2006), along the pathway that transferring information of the tactile perception extended from thalamus to somatosensory cortex (Vázquez et al. 2013). These findings collectively point that the basal ganglia–thalamocortical loop delivers tactile discriminative information. In this respect, we conjecture that the activation in basal ganglia and thalamus regions in the Supra- vs. Infra-threshold contrast may be related to different perception in intensity of stickiness between the two stimulus groups.
The Supra- vs. Infra-threshold contrast revealed an activated cluster spanning from insula to temporal cortex as well. Several neuroimaging studies revealed activations in these regions in response to tactile stimulation. While some of them reported the results when tactile and visual stimuli were presented simultaneously (Banati et al. 2000; Saito et al. 2003; Cardini et al. 2011), Lundblad et al.
observed when subjects performed a tactile discrimination task (Lundblad et al. 2011). In line with these previous reports, the result of the present study also suggests that insula and superior and middle temporal cortices are related to the tactile perception of sticky stimuli, particularly for distinguishing delicate differences of the perceived intensity of stickiness.
4.5 Correlation between perceived stickiness and BOLD responses
The result of the Supra- vs. Infra-threshold contrast indicated that fine perceptual distinction of stickiness might be attributed to the subcortical and cortical areas including basal ganglia, thalamus, insula and temporal cortices. Hence, we examined a relationship between the estimated intensity of stickiness and the BOLD peak value of each ROI of these areas. Except for ipsilateral caudate and middle temporal cortex, all six ROIs showed a positive relationship between the behavioral response and BOLD signal changes such that these regions were activated more as participants perceived stronger stickiness.
To confirm the significance of our results in subcortical area, we additionally applied the same analysis to the two regions that were activated in the Supra-threshold vs. Sham contrast: contralateral S1 and ipsilateral DLPFC. The analysis revealed there is no significant correlation between the activation in two cortical regions and the behavioral responses (rs < 0.06, ps > 0.66, Figure 20). The result supports the significance of the activation of the subcortical areas, including the basal ganglia, thalamus, and insula, in discriminating the differences between the perceived intensities of stickiness.
Figure 20. Correlation results between mean-corrected magnitude estimation and mean-corrected BOLD peak values of S1 and DLPFC.
4.6 Limitations and future work
There are a few drawbacks in the present study, which should be addressed in the follow-up studies.
Above all, we did not log behavioral responses about the perceived intensity of stickiness from participants during the fMRI experiment. We intended to let participants focus more on stimuli as well as to avoid artifacts from extra movements caused by the response. However, although we were able to find the relationship between the perceived intensity of stickiness acquired outside the scanner and the neural responses acquired inside the scanner, it still remains unclear whether participants felt exactly the same tactile perception from the stimuli during the fMRI experiment. Next, some might consider the results of the present research are not reliable due to the small number of participants.
Not only we tested a few participants, but also eliminated a considerable amount of data from the analysis because of the artifacts. As a result, the effective sample size was reduced and thus might not make our findings firm enough. Moreover, the current experimental design has limits to clearly identify the exact role of DLPFC in perceiving stickiness and of the subcortical regions in discriminating the intensity of stickiness. We suggested presumptive role of each activated region based on the previous studies, thus our interpretation describes the brain functions in terms of tactile perception and unable to sufficiently explains focused on the aspects of perception of stickiness. The present study only reveals the direct responses in human brain when people perceives tactile feeling of stickiness. The research has its meaning as the first attempt to measure brain activity to the perception of stickiness, but yet the issue should be more deeply investigated to extend our knowledge on human tactile perception. Especially, for the comprehensive understanding of the four tactile perception dimensions and its neural mechanisms, it is necessary to make an endeavor to explore several tactile dimensions together and realize the system thoroughly.
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