Spatiotemporal Thermal Control Effects on Thermal Grill Illusion

Abstract

The thermal grill illusion induces a pain sensation under a spatial display of warmth and coolness of approximately 40 °C; and 20 °C. To realize virtual pain display more universally during the virtual reality experience, we proposed a spatiotemporal control method to realize a variable thermal grill illusion 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 on the illusion and verified the following facts. In terms of the pain area, the larger the thermal area, the larger the pain area. In terms of the magnitude of the pain, the larger the thermal area, the greater the magnitude of the sensation of pain.

Keywords: pre-warming and pre-cooling; spatiotemporal control; thermal grill illusion.

Figure 1

Thermal change patterns of each method. (a) Thermal stimulation on TGI. The stimulation was arrayed in a row. (b) Thermal stimulation on Sato’s method. Peltier elements are grid-arrayed, and a part of the thermal pattern is changed. (c) Thermal stimulation on the proposed method. Peltier elements are grid-arrayed, and both parts of the thermal pattern are changed.

Figure 2

Modelized thermal histories of the conventional and proposed method. (a) Thermal history of Sato’s method. The red and blue line shows stimulated temperature on each grid-arrayed Peltier element. The solid black line shows the perceived temperature. The spatially separated pre-warmed or pre-cooled thermal stimulation induces faster recognition of thermal change. (b) Thermal history of normal TGI stimulation. The red and blue line shows stimulated temperature on arrayed Peltier elements in a row. The solid black line shows the perceived temperature. The thick dotted line shows the threshold of the pain sensation. $\mathsf{\Delta }t$ shows the period until pain sensation. (c) Thermal history of the proposed method. The spatially separated pre-warmed and pre-cooled thermal stimulation could induce faster recognition of the pain sensation. $\mathsf{\Delta }{t}^{\prime }$ shows the period until pain sensation.

Figure 3

Experiment setup of pre-cooling/pre-warming effect. (a) 2 × 2 tiled Peltier devices. The total size of Peltier devices was 36 mm${}^2$ (b) System overview of the experiment about pre-cooling/pre-warming effect (c) Examples of both A and B parts’ thermal histories. As an adaptation temperature, we set 33 °C. To adapt to the temperature, the participants kept their hands on the device for three minutes. As a cooling/warming temperature target, we set 20 and 40 °C for each.

Figure 4

Experiment setup of spatial distribution effect. (a) 3 × 3 tiled Peltier devices. The total size of Peltier devices was 45 mm${}^2$ For 2 × 2 stimuli, the total size was 30 mm${}^2$ (b) System overview of the experiment about spatial distribution effect. As an adaptation temperature, we set 33 °C. To adapt to the temperature, the participants kept their hands on the device for three minutes. As a cooling/warming temperature target, we set 20 and 40 °C for each. (c) Participant’s drawing examples of the pain area.

Figure 5

Results of pre-cooling/pre-warming effect. (a) Each participant’s period until pain sensation and their average (b) The period of average and significant difference (except the outlier) (c) An average of 5 stages Likert-scaled questionnaires about the sensation. The * and ** indicated a significant difference of $p<0.05$ and $p<0.01$ for each.

Figure 6

$\mathsf{\Delta }t$’s result of spatial distribution effect.

Figure 7

$\mathsf{\Delta }t$’s results of spatial distribution effect (except the outlier). The ** indicates a significant difference of $p<0.01$. (a) Effect of layout of Peltier device and pre-cooling/pre-warming condition on time until pain. (b) Averaged effect of pre-cooling/pre-warming condition on time until pain. (c) Averaged effect of layout of Peltier device on time until pain.

Figure 8

Area of the pain sensation result of spatial distribution effect. The * and ** indicates a significant difference of $p<0.05$ and $p<0.01$ for each. (a) Effect of layout of Peltier device and pre-cooling/pre-warming condition on pain area rate (b) Averaged effect of pre-cooling/pre-warming condition on pain area rate (c) Averaged effect of layout of Peltier device on pain area rate.

Figure 9

Magnitude estimation of the pain sensation result of spatial distribution effect. The ** indicates a significant difference of $p<0.01$. (a) Effect of layout of Peltier device and pre-cooling/pre-warming condition on magnitude estimation rate (b) Averaged effect of pre-cooling/pre-warming condition on magnitude estimation rate (c) Averaged effect of layout of Peltier device on magnitude estimation rate.