Although the neuroscientific investigations on the other tactile perception dimensions have advanced, little is known about the perception of toughness. Considering this fact, an investigation that elicits sticky feeling without friction force is needed to elucidate the neural responses for the tactile perception of stickiness. In the fMRI experiment session, sticky silicone stimuli were tested to find the brain regions responsible for the tactile perception of stickiness.
From the analysis of the constant stimuli test method, we could find an absolute threshold for the tactile perception of stickiness. Infra-threshold contrast to detect the brain regions that play a role in perceiving the intensity of stickiness.
Introduction
- Introduction to fMRI
- Human somatosensory system
- Human tactile dimensions
- Research aim
First of all, fMRI can measure functional brain activation in a completely non-invasive way. This is possible since fMRI samples brain activity in 3-Dimensions, the unit of which is the voxel (Huettel et al. 2009). Unlike the MRI technique, whose data is collected when the pulse sequence is tuned to hydrogen nuclei, fMRI data is recorded using blood oxygenation level-dependent (BOLD) brain responses (Huettel et al. 2009).
For two reasons, it makes sense to record a STRONG signal to capture the functional activity of the brain. Of the two gamma functions, one represents the peak of the BOLD response, while the other models the trough or initial decline ( Fig. 5 ) ( Glover 1999 ; Büchel et al. 1998 ). The three processes on the left (realignment, normalization and smoothing) show a representative preprocessing procedure.
Since the voxels in fMRI data are also assumed to be independent, the application of GLM to the analysis of fMRI data is possible. In this study, comparison of absolute peak BOLD PSC between conditions was assessed. Typically, the positive BOLD PSC value is considered activation, while the negative value is considered to be induced by the inhibitory mechanism of the brain region (Kastrup et al. 2008).
In particular, Merkel cells, which lie in the epidermis, envelop the end of the unmyelinated SA1. These properties are due to the shape and distribution of the Meissner corpuscles, the endings of RA fibers. This inconsistency may be partially due to the use of the frictional force to create the effect of skin stretching.
Experimental Design
- Participants and ethics approval
- Stimuli
- Experiment procedures
- Behavioral experiments
- fMRI experiment
- Data acquisition and preprocessing
- Data analysis
In the first training task, participants were trained to consistently exercise finger pressure when touching the surface of the stimuli. With this visual feedback, participants could monitor in real time the amount of pressure they exerted on the sensor. The first experiment used the method of constant stimuli to find the absolute threshold of the intensity of our stimulus set for the tactile perception of stickiness.
On the other hand, the stimuli with a wider range of the ratios were used for the size estimation method to enable participants to feel various intensities of stickiness within a limited experimental duration. Then, when participants heard the verbal instruction of the 'Ready ('Jun-be' in Korean)', they attached their right index finger to a given stimulus and maintained the posture for 3 seconds until they heard a short beep sound. Each of the 7 stimuli was presented 10 times in a random order, and thus a single scanning session consisted of 70 trials.
Specifically, we looked for variance in behavioral responses during the magnitude judgment task by first transforming all participants' responses into z-scores for each stimulus and then setting upper/lower limits by adding 3 times the interquartile range (IQR) to the third quartile or by subtracting it from the first quartile. Analysis of the first behavioral experimental data using the constant stimuli method estimated the absolute threshold for the perception of stickiness. Analysis of other behavioral experimental data examined differences in magnitude estimation responses between stimuli.
Group labels activated in SPM were determined using the Automated Anatomical Labeling (AAL) toolbox ( Tzourio-Mazoyer et al. 2002 ).
Results
Behavioral responses to sticky stimuli
The analysis with the method of constant stimuli revealed the absolute threshold for the tactile perception of stickiness. The mean threshold value across participants was determined at 7.47% of catalyst ratio point (SD = 1.31%), and the mean sigma value for cumulative guassian distribution was 1.03 (SD = 0.42). Participants experienced a sticky feeling almost every time (98.89%) when they touched the stimulus with 5% of catalyst ratio.
The "supra-threshold" group contained stimuli with 7% catalyst or less (5, 6, and 7% . stimuli) and were considered to evoke the perception of stickiness in participants. Although stimuli from the Infra-threshold group did not sufficiently elicit the perception of stickiness, the average chances of perceiving stickiness across participants' responses were non-zero (Appendix 2). This grouping of stimuli (Supra-threshold, Infra-threshold and Sham) was used in the analysis of the functional imaging data to find the brain areas responsible for perceiving stickiness.
The average corrected magnitude estimation responses for different stimuli showed a decrease in the estimated stickiness intensity as the catalyst ratio increased (Figure 16). The one-way ANOVA test revealed that participants experienced significantly different stickiness between the stimuli (F p < 0.0001). The post-hoc t-test showed that the perceived intensity of the stimulus with a catalyst ratio of 7% was significantly different from that with 5 and 6% stimuli or that with 8, 10 and 30% stimuli (p < 0.0001 for all ) .
The absolute threshold for stickiness perception was determined at between 7 and 8% catalyst ratio (7.47% across all participants).
Brain responses to sticky stimuli
Infrathreshold contrast, the basal ganglia region, the insula, and the middle and superior temporal gyri were activated.
Correlations between perceived intensity and BOLD responses
Infra-threshold, 6 regions showed significant relationship between the mean corrected BOLD peak values and the mean corrected magnitude estimate values (ps < 0.05). 2 out of the 8 ROI regions, ipsilateral caudate (Left) and middle temporal cortex (Right), failed to show a significant relationship between the mean-corrected BOLD peak values and the mean0-corrected magnitude estimate value (ps > 0.1).
Discussion
- General discussion
- Behavioral responses in two psychophysics experiments
- Brain responses in Supra-threshold vs. Sham and Infra-threshold vs. Sham contrasts
- Brain responses in Supra- vs. Infra-threshold contrast
- Correlation between perceived stickiness and BOLD responses
- Limitations and future work
The aim of this study was to investigate neural responses to the tactile perception of stickiness using fMRI. Second, as the magnitude estimation result clearly shows, the 7% catalyst ratio stimulus has a perceptibly different stickiness intensity than the 5% and 6% stimuli. Thus, it is likely that the activation of contralateral S1 and ipsilateral DLPFC is attributed to the perception of stickiness of the stimuli, rather than to the material difference between silicone and acrylic.
Infra-threshold stimuli were investigated in the brain regions involved in the perception of different intensities of stickiness. All stimuli from the same silicone material produced stickiness perception either stably or not, depending on the catalyst ratio. Infra-threshold contrast would reveal brain regions involved in detecting differences in stickiness intensity between the two groups.
Infra-threshold contrast may be related to different perception in intensity of stickiness between the two stimulus groups. Consistent 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, especially in discriminating subtle differences of the perceived intensity of stickiness. The result supports the importance of the activation of the subcortical areas, including the basal ganglia, thalamus and insula, to distinguish the differences between the perceived intensities of stickiness.
Above all, we did not record behavioral responses about the perceived intensity of stickiness from participants during the fMRI experiment.
Conclusion
The study aimed to investigate neural responses to the tactile perception of stickiness, and is significant because for the first time the neural responses to stickiness are accessed with the adhesive stimuli. Regarding the fact that supra- and infra-threshold stimuli are made of the same substances, S1 and DLPFC are thought to process the detection of stickiness. In particular, the activation in DLPFC implies that the tactile perception of stickiness requires not only tactile processing mechanisms but also higher cognitive processes.
In contrast, the basal ganglia, thalamus, insula, and superior and middle temporal cortex areas were activated. Furthermore, the BOLD peak value of 6 of the 8 activated areas showed a significant relationship with the perceived intensity of stickiness. The finding confirms the results of previous studies that the subcortical region also plays a role in tactile perception discrimination, and extends the scope of tactile discrimination to the perceived intensity of stickiness.
Brodmann Area
22 Superior temporal gyrus, the caudal part of which is usually considered to contain Wernicke's area. 38 Temporopolar area (most rostral part of the upper and middle temporal gyri) 39 Angluar gyrus, considered by some to be part of Wernicke's area. 44 Pars opercularis, part of inferior frontal gyrus and part of Broca's area 45 Pars triangularis, part of inferior frontal gyrus and part of Broca's area 46 Dorsolateral prefrontal cortex.
Responses of the method of constant stimuli task
Responses of the magnitude estimation task
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