With the spread of Head Mounted Display (HMD), Virtual Reality (VR) technology is becoming more familiar, and VR technology is developing. Currently, the main developments and research on haptic stimuli are vibrations. However, there are other pain sensations in the human tactile sensation. Therefore, in this study, we proposed employing a spatiotemporal display method to control the thermal grill illusion (TGI). With the method, the pain sensation can be presented more universally during the VR experience.
This illusion induces a burning sensation under a spatial display of warmth and coolness of approximately 40 °C and 20 °C. The sensation was observed by Thunberg in 1896 [1
]. The presented warm and cold stimulus temperatures are safe for humans. In this temperature range, the warm receptors (TRPV3, TRPV4) and the cool receptors (TRPM8) work, although neither of them causes a pain sensation [2
]. Studies vary with regard to the frequency of painful and non-painful paradoxical sensations obtained, and one possible reason for the discrepancy is the experimental paradigm. The ’classic’ combination of 20 and 40 °C [4
] and the combination of 15 and 45 °C [7
] produced a non-painful heat sensation. However, 20 and 40 °C induced painful heat elsewhere [8
In previous research, Stevens et al. reported that temperature perception is based on spatial weighting in thermosensation and that the wider the stimulus range, the stronger the warming sensation that occurs and that doubling the stimulus range half the temperature threshold [9
]. Green [10
] reported that the phenomenon of referral and domination occurred when they presented thermal stimulation to the index, middle, and ring fingers of subjects at the same time with hot and cold stimuli. In the thermal referral phenomenon, even though the middle finger touches a room-temperature object, the middle finger is perceived as warm when the index and ring fingers are presented with a warm stimulus at the same time. On the other hand, in the domination phenomenon, even when a cold stimulus is presented to the middle finger under similar conditions, it is perceived as ‘warm’.
Hsin-Ni et al., reported detailed results of the thermal referral phenomenon. They examined the perception of the intensity of the sensation resulting from thermal referral to human participants. They found that the sensation was uniform between the three fingers, but its apparent intensity was always lower than the physical intensity applied to the outer two fingers [11
]. They considered the reason for the effect to be the link between thermal sensation and tactile somatosensory perception. Furthermore, the physical-perceptual correspondence was not consistent between warm and cool stimuli, suggesting that warm and cool stimuli have different temporal filtering properties and that cool stimuli are more transient than warm stimuli [12
Ahmad et al. also found that when multiple temperature stimuli are used to rapidly cool some of the actuators and slowly heat the rest, the slow-heating actuators are not perceived, suggesting that fast temperature changes are perceived at a much lower threshold than slow temperature changes, and that temperature perception is a non-linear phenomenon [13
]. Arai et al. found the inverse thermal sensation caused by the presence of hot and cold stimuli, which they called hot-cold confusion [14
]. The opposite thermal stimuli applied at multiple locations affect each other, and participants sometimes perceive the hot stimulus at the outer location as cold even when the two of the three stimuli are hot, and vice versa. Several researchers modeled the TGI phenomenon [8
], though the whole underlying mechanism of the ’grill illusion’ is still ambiguous.
In summary, TGI, thermal referral, and dominance have similar stimulation conditions, though the felt sensations vary. In several thermal referrals and dominance, changes in outer temperature are limited to +10 °C or −10 °C, and the center of the stimuli is set at room temperature. These stimuli induce a similar feeling to that of other fingers, even on room temperature stimulation. On the other hand, under the TGI condition, the adjacent temperature changes to +10 °C and −10 °C simultaneously. The stimuli induce a sensation of pain.
Sato et al. [19
] evaluated that spatiotemporal control induces a faster thermal change illusion. In previous studies, thermal stimulation was arrayed in a row (Figure 1
a). However, they arrayed the stimulation with the 2 × 2 grid to generate spatially separated stimuli (Figure 1
b). They revealed that spatially separated thermal stimuli are perceived as one thermal stimulus and that spatial stimulus change induces faster thermal change perception than a single thermal stimulus change (Figure 2
a). On the basis of the result, we proposed employing the spatiotemporal display method to control the thermal illusion. In this research, we treated TGI with our spatiotemporal display method (Figure 1
c) and evaluated the effect of the proposed method. With this method, we enhance the effect of the illusion.
Here, we proposed a spatiotemporal control method to realize a variable TGI and evaluated the effect of the method. First, we examined whether there was a change in the period until pain occurred due to the spatial temperature distribution of pre-warming and pre-cooling and verified whether the period until pain occurred became shorter as the temperature difference between pre-warming and pre-cooling increased. Next, we examined the effect of the number of grids and their size on the illusion and verified that increasing the number of grids induces a larger pain sensation and its area simultaneously.
To evaluate the pre-cooling/pre-warming effect and the spatial distribution effect of the TGI, we conducted the following experiments.
3.1. Experiment with the Pre-Cooling/Pre-Warming Effect
In this section, to evaluate the pre-cooling/pre-warming effect, we carried out the following psychophysical experiment. In this experiment, we evaluate the periods (
in Figure 2
b,c) until the pain feeling among the difference between the pre-cooling/pre-warming conditions. Figure 3
b shows an overview of the experiment system.
The participants were seven healthy men aged 22 to 24 years. We recruited the participants from our university students. They were paid $10 upon completion of the experiment. During this experiment, they used a noise canceling headphone to block external sounds and listened to pink noise. They were all right-handed and used their right index fingers to feel the stimuli. To eliminate the influence of the order effect, every pattern was presented in random order for each subject.
We compared seven rendering methods of cooling/warming patterns, with , , and /without pre-cooling/pre-warming. They felt the stimuli ten times for each pattern in random order. In ‘without pre-cooling/pre-warming patterns,’ the temperature of part A changes from 33 to 40 °C, and that of part B changes from 33 to 20 °C. In ‘with pre-cooling/pre-warming patterns 1, 2, and 3,’ the temperature of part A had been set at 33 + 1, +2, and +3, and was changed to 40 °C each. According to part A, part B had been set to 33 − 1, −2, and −3 and changed to 20 °C. The temporal patterns are as follows;
±0 for pre-cooling/pre-warming temperature (control)
±1 for pre-cooling/pre-warming temperature
±2 for pre-cooling/pre-warming temperature
±3 for pre-cooling/pre-warming temperature
In all conditions, the participants placed their fingers at the crossing point of the Peltier devices and the cutaneous temperature of their fingertips was controlled to 33 °C by another Peltier device. After that, they pressed the start button of the experiment. When they felt a pain sensation, they were asked to press the button. After each experiment, they answered the 5 stage Likert scale questionnaires about the amount of pain felt (1: No pain, 2: Mild pain, 3: Moderate pain, 4: Severe pain, 5: Intense pain). After the 3 minute intervals, they repeated the experiment 10 times for each condition. In the experiment, the type of stimuli is not announced. To eliminate the sound effect, they listened to pink noise during the experiment.
3.2. Questionnaires with Spatial Distribution Effect
In this section, to evaluate the effect of spatial distribution, we conducted the following psychophysical experiment. In this experiment, we compared the spatial distribution effect of the TGI. To compare the effect, we measured the periods (), the area of pain sensation, and the estimate of the magnitude of subjective pain sensation under each condition.
The participants were five healthy men aged 22 to 24 years. We recruited the participants from our university students. They were paid $10 upon completion of the experiment. Participants were partially different from the previous research. During this experiment, they used a noise canceling headphone to block external sounds and listened to pink noise. They were all right-handed and used their right forearms to feel the stimuli. To eliminate the influence of the order effect, every pattern was presented in random order for each subject.
We compared four spatial and three temporal combinations of cooling/warming patterns. The spatial patterns were as follows;
2 × 2 display with warm stimuli in part A, and cool stimuli in part B
2 × 2 display with warm stimuli in part B, and cool stimuli in part A
3 × 3 display with warm stimuli in part A, and cool stimuli in part B
3 × 3 display with warm stimuli in part B, and cool stimuli in part A
The temporal patterns were as follows;
±0 for pre-cooling/pre-warming temperature
±1 for pre-cooling/pre-warming temperature
±3 for pre-cooling/pre-warming temperature
In all conditions, the participants placed their forearms on the tiled Peltier devices, and the cutaneous temperature of their forearms was controlled to 33 °C by another Peltier device. After that, they pressed the start button of the experiment. When they felt a pain sensation, they were asked to press the button. After each experiment, they asked about the amount of pain felt using the magnitude estimation method, and these data were normalized to 0.0–1.0. In the previous experiment, we used a Likert-scaled questionnaire. This time, to equalize the maximum and minimum responses between participants and to evaluate the pain sensation more precisely, we held a magnitude-estimation-based questionnaire. Using the estimation, we normalized the answers between participants. In addition, they sketched the area of pain in their forearms. The area information was scanned, evaluated, and normalized by the squared thermal area (Figure 4
c). After the 3 min intervals, they repeated the experiment 3 times for each condition. In the experiment, the type of stimuli is not announced. To eliminate the sound effect, they listened to pink noise during the experiment.
In this paper, we proposed a spatiotemporal control method to realize the variable TGI and evaluated the effect of the method. First, we examined whether there was a change in the period until pain occurred due to the spatial temperature distribution of pre-warming/pre-cooling and verified whether the period until pain occurred became shorter as the temperature difference between pre-warming/pre-cooling increased. Then, we examined the effect of the number of grids and their size on the illusion and verified that increasing the number of grids induces a greater pain sensation and its area simultaneously.
To evaluate the spatiotemporal control on TGI, we performed two experiments on the pre-cooling/pre-warming effect and the spatial distribution effect. Through the experiments, we found that our hypothesis was correct. Thus, in terms of the period, , pre-cooling/pre-warming induced a shortening of the period. Furthermore, the larger the pre-cooling/pre-warming temperature, the shorter the period. Furthermore, the larger thermal area induced a shortening of the period.
In terms of the pain area, the larger thermal area increased the pain area. Pre-cooling/pre-warming did not affect the pain area. In terms of the pain magnitude, the larger thermal area magnified the magnitude of the pain sensation. Pre-cooling/pre-warming did not affect the pain magnitude. The amount of pain caused by the stimulus of 3 × 3 was twice greater than that of 2 × 2. The pain area of 3 × 3 was also twice larger than 2 × 2. Here, the device’s area of 3 × 3 is 2.25 times larger than 2 × 2. Thus, the reason could be the difference of the area. However, everyone felt a pain sensation in a part of the cooling/warming area. That is, it was possible that the sensation of pain was not induced by stimulation of the whole thermal area, but rather its borders. In the near future, we want to consider the causes more deeply. Furthermore, the layout of the cooling/warming position did not affect the period, the area of pain, and the sensation of pain.
From the findings of our research, we have got a basic design method for the pain display. By controlling the thermal device spatially and temporally, we can control the amount of pain sensation and pain area without any physical damage. Pre-warming and pre-cooling temperature control induces temporal changes until the sensation of pain. To cause the wider pain sensation area, the stimulation area control is effective. To induce the pain sensation more strongly, the stimulation area or pre-warming and pre-cooling temperature control is effective. In the future, to reveal its mechanism, we will carry out deeper spatial/temporal research.