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Central Nervous System Responses to Comfortable Thermal Stimuli to the Soles of the Feet with Simultaneous Presentation of Other Sensory Stimuli

: The current study sought to examine differences in physiological responses to comfortable thermal stimuli applied to the soles of the feet during the simultaneous presentation of other sensory stimuli. As relaxing stimuli, underwater movie footage, classical music, light blue illumination, vibration of the trunk, and the odor of grapefruit oil were each presented simultaneously with a thermal stimulus. Electroencephalography of parietal and cerebral blood flow in the prefrontal cortex were measured. An increase in α -wave activity and a decrease in cerebral blood flow were found for all combinations of stimuli compared with baseline. Additionally, the results revealed differences in response speed depending on the stimulus presentation condition. However, the tendency suggesting a difference in response times depending on the stimulus condition was similar to that of the thermal stimulus alone. The results indicated that thermal stimulation has a substantial effect on the feeling of relaxation. cortex. The results revealed an increase in


INTRODUCTION
The current study was conducted to examine the differences in physiological responses to various stimulus types (such as visual and auditory) presented simultaneously with thermal stimulation of the soles of the feet. Additionally, we investigated whether different stimulus combinations cause different psychological and physiological effects. Controlling passenger comfort while a vehicle driving is an important issue in next-generation automobiles implementing autopilot technology. We investigated a method to induce feelings of comfort from the control of stimuli presented to the passenger from the vehicle. Many stimulus types can be presented from a vehicle. Thermal comfort is an important factor in indoor environments and in-vehicle comfort. In the current paper, we targeted the effects of multisensory integration between comfortable thermal stimuli to the soles of the feet and other sensory stimuli.
There are many possible ways to provide warmth to humans. In the current study, we presented thermal stimulation to the feet. In previous studies, thermal stimulation of the extremities was reported to enhance thermal comfort. Shin et al. [1] revealed that the thermophysiological and psychological responses of the human body differ depending on the part of the body to which thermal stimuli are presented. Kuji et al. [2] reported that simultaneous warming of the dorsum of the hands and feet was the most effective for inducing a feeling of warmth. Moreover, warming of the feet was found to be more effective for providing warmth than warming of the hands [2]. Yamamoto et al. reported that a foot bath is effective relaxation and increase in parasympathetic nervous activity and decrease in sympathetic nervous activity [3].
In the current study, to evaluate integration effects, electroencephalography (EEG) of parietal and cerebral blood flow in the prefrontal cortex were measured as physiological responses of the central nervous system. EEG was measured to examine the activity of the whole brain, and cerebral blood flow was measured to examine the activity of the frontal lobe. When humans are in a relaxed state, α-wave activity (8.0 -13 Hz) typically appears in EEG signals, accompanied by decreased cerebral blood flow in prefrontal cortex. Evaluation of stimuli and comfort using physiological response measurement in the central nervous system is utilized not only in medical settings, but also in the field of engineering.
Regarding previous research investigating responses to thermal stimuli and other types of sensory stimuli, several previous studies presented odors to participants during warm baths [4,5]. Shirakawa et al. [4] presented lavender oil during a footbath, revealing that the combination was subjectively rated as more relaxing than the footbath alone. Additionally, the researchers confirmed that average heart rate significantly decreased after the aroma bath [4]. Fukuzawa et al. [5] reported that oxy-Hb in the frontal cortex decreased immediately after odor presentation when an aroma (lavender, lemongrass, tea tree) was presented during a footbath. In other combinations of stimuli, Fukumitsu et al. [6] provided a combination of hot packs and music for knee osteoarthritis and found significant changes in the EEG α-band (8.0 -13 Hz). These findings suggest that simultaneous stimulation with thermal stimuli and other sensory stimulus types can induce relaxation. However, few studies have examined the effects of these stimulus combinations.
In the current study, we presented stimuli to other sensory organs that were expected to induce the feeling of relaxation simultaneously with a thermal stimulus. Music stimuli, movie stimuli, illumination stimuli, odor stimuli and vibration stimuli (applied to the trunk) were presented. Previous studies evaluated participants' brain activity when presenting stimuli to a single sensory modality, based on physiological responses of the central nervous system [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. However, few studies have examined the combination thermal stimuli with stimulation of other sensory modalities. Based on the findings discussed above, it would be valuable to clarify the psychological and physiological responses when presenting other stimuli in combination with heat stimulation. Therefore, the current study sought to examine the differences in physiological responses to comfortable thermal stimulation applied to the soles of the feet simultaneously with stimulation in another sensory modality, and whether different combinations lead to different psychological and physiological states. The current study was designed to clarify which sensory stimuli besides thermal stimuli can enable multi-sensory integration to induce feelings of comfort and relaxation. We focused on physiological responses of the central nervous system, measuring brain activity using EEG and cerebral blood flow. Additionally, we also investigated how the warmth of the foot sole contributes to the feeling of relaxation.

Selection of presentation stimuli
Before physiological response measurement, we performed a preliminary experiment to select stimuli to be presented. Table 1 shows the stimuli presented to each sensory modality in this experiment. These stimuli were expected to evoke a feeling of relaxation. Additionally, the stimuli were selected from several kinds of stimuli prepared using Scheffe's method of paired comparison (Ura's variation). The selection of odor stimuli was conducted using Nakaya's variation method. A total of 20 participants were recruited (10 men and 10 women). Participants rated their "feelings of comfort", "feelings of relaxation", and "preferences" on a 7-point scale (−3, extremely; −2, very; −1, slightly; 0, neither; +1, slightly; +2, very; and +3, extremely). This paired comparison method revealed the most relaxing stimuli. The methods of stimulus presentation and selection are described below. 1. Thermal stimulation: A silicon rubber heater (Hakko Fine Thermo Co., Ltd.) was fixed under the participant's feet. This sheet is a mat-shaped heater, in which a resistance wire is arranged between two silicon sheets, and the temperature can be set. A brass diffuser and a 100% cotton cloth were placed on the heater, and the thermal stimulus was presented to participants while barefoot. Three presentation temperatures of 36 °C, 40 °C, and 44 °C were used, and the most relaxing temperature was selected using the paired comparison method. The results revealed that 40 °C was selected as the most relaxing temperature. 2. Movie stimulation: Movies were projected on a screen in front of the participants (1.5 m) using a projector (EB-535W, Epson Co., Ltd.). Five types of movie (underwater, sunset, animal, flower, and forest) were presented. Each video included 18 photos displayed for 10 seconds each, resulting in a duration of 180 seconds. The underwater movie was selected as the most relaxing stimulus. 3. Music stimulation: Music was presented via headphones (WH-1000X M2, Sony Co., Ltd.). Many previous studies [9][10][11] have confirmed the relaxation effects of classical music. We prepared five pieces of classical music (Beethoven -"Pathetique Piano Sonata No.8 Op.13-2", Chopin -"Nocturnes Nocturne No.2 Es-Dur Op.9-2", Mozart -"Konzert für Klavier und Orchester Nr.23 A-Dur K.488 Mov.2 Adagio", Satie -"Gymnopédies No.1", Bach -"Orchestral Suite No.3 in D major, BWV 1068"). "Pathetique Piano Sonata No.8 Op.13-2" was selected as the most relaxing. 4. Illumination stimulation: Illumination stimuli were presented using an LED cube (Thouslite Co., Ltd), which was able to present illumination in a range of colors. The LED cube was installed on the ceiling with a 2.0 m square angle, and illumination was presented from above. We prepared five colors of illumination (blue, light blue, green, yellow-green, orange), and all lights were matched to a horizontal luminance of 50 lx at a height of 1.5 m. Light blue illumination was selected as the most relaxing color using the paired comparison method. 5. Vibration stimulation: Participants held a cushion speaker (Shima System Co., Ltd.) and a certain frequency of vibration was presented via the speaker. The speaker vibrated at low frequencies to match the rhythm of the music. The speaker unit was mounted on a soft rubber plate inside the cushion speaker, and the soft baffle plate absorbed the mid-range vibration and transmitted the low-frequency vibration to the front, making the speaker vibrate easily. In the selection experiment, a sine wave was output using MATLAB (Mathworks Co., Ltd.), and vibration stimuli of 40 Hz, 55 Hz, 70 Hz, 85 Hz, and 100 Hz were presented. We programmed the system so that the amplitude randomly changed to 0.25, 0.50, 0.75, and 1.00 times every 2.5 seconds to prevent participants becoming habituated to the stimuli. The vibration signal was sent from a laptop to the cushion speaker through a digital amplifier PDP-SAII (Shima System Co., Ltd.) and presented to participants. As with other stimuli, the most relaxing stimulus was selected using the paired comparison method, and a vibration frequency of 55 Hz was selected. 6. Odor stimulation: Odors were presented using an aroma shooter (Aroma Join Co., Ltd.). This device presents an odor by injecting the odor stored in an internal cartridge, and was aimed in a forward direction at the right side of the participant. Odor injection was performed at 10-second intervals to prevent participants from becoming used to the odor. Grapefruit, citron and orange were prepared, and the most relaxing odor was selected using a paired comparison method (Nakaya's variation). The grapefruit odor was selected as the most relaxing smell. Based on the above results, the following six patterns of stimulation were selected for use in the current experiment: 1. thermal stimulation alone, 2. "movie + thermal" stimulation, 3. "music + thermal" stimulation, 4. "illumination + thermal" stimulation, 5. "vibration + thermal" stimulation, 6. "odor + thermal" stimulation.

Previous Studies on Brain Activity
Several previous studies have examined central nervous system physiological responses when thermal stimuli are presented. Chang et al. [7] compared EEG signals when the nondominant hand was put in warm water (40 -43 °C) or cold water (12 -15 °C). The results confirmed a decrease in EEG power spectra in α1 (8.0 -11 Hz) bands when the hand was placed in both warm and cold water, compared with baseline measurements [7]. Moreover, the researchers reported that cold water significantly decreased activity in the θ (4.0 -8.0 Hz) and α1 bands after presentation, compared with warm water [7]. Regarding the relationship between cerebral blood flow and thermal stimulation, Hou et al. [8] reported that cerebral blood flow in the frontal lobe during the presentation of local hot-cold stimuli to the hand using an aluminum plate exhibited increased or decreased oxy-Hb under specific temperature conditions. With hot stimulation, oxy-Hb decreased at 39 °C and increased at 45 °C [8]. Taken together, the studies described above suggest that brain activity varies depending on the presentation temperature.
Previous studies evaluated other stimuli (music, movie, illumination, vibration and odor) based on physiological responses of the central nervous system [9][10][11][12][13][14][15][16][17][18][19][20][21]. First, listening to relaxing music has been found to increase power spectrum components of the α-wave [9] and θ-wave [10] of the EEG signal and to decrease cerebral blood flow in the frontal lobe [11]. Second, in a study of movie and image presentation, Aamir et al. [12] reported that watching movie clip increased absolute α-wave power in occipital, parietal, frontal and motor regions compared to baseline. Additionally, Sakamoto et al. revealed cerebral blood flow in the frontal lobe is an indicator of video comfort or discomfort [13]. Regarding illumination, several studies have examined color stimulus presentation [14][15][16]. Noguchi et al. revealed that low color temperature illumination lead to decreased brain activity. Yamashita et al. [15] presented red, blue, and green colored light and reported that bright tones induced greater enhancement of the α2 (10 ± 1.0 Hz) EEG band. Oomori et al. [16] suggested that the more a hue turned from red to green and blue, the more α-wave activity was evoked in the occipital lobe. Regarding odor, it has been reported that pleasant odors are perceived in the left hemisphere of the brain, whereas unpleasant odors are perceived in the right hemisphere [17]. Hashizume et al. [18] presented lavender oil, and reported α-waves of the occipital area are increased by aspiration of aroma. Regarding cerebral blood flow, Kobayashi et al. [19] revealed that an unpleasant odor (isovaleric acid) increased cerebral blood flow in the frontal lobe. On the other hand, Haga et al. [20] reported that essential oils for aromatherapy increased or decreased cerebral blood flow in prefrontal cortex depending on the type of odor. Specifically, peppermint reduced oxy-Hb, and lavender, myrrh and grapefruit increased oxy-Hb [19]. Finally, in a study on vibration stimuli, Satou et al. [21] presented vibration to whole-body with frequency (10 Hz) in the seating position, confirming that weakness levels of AAT (Alpha Attenuation Test) were reduced by vibration presentation.

Physiological Measurements
We derived EEG signals and cerebral blood flow as physiological indices. Polymate AP-1000 (Miyuki Giken Co., Ltd.) was used for EEG measurement. EEG electrodes were attached to Pz and A1 according to the international 10-20 electrode system. EEG was derived from the potential difference from the earlobe using a reference electrode method. The frequency (f) of the EEG can be divided into four bands: δ-wave (f < 4 Hz), θ-wave (4 Hz ≤ f < 8 Hz), α-wave (8 Hz ≤ f ≤ 13 Hz), and β-wave (13 Hz < f). Here, α-wave activity was calculated by dividing the α-wave power spectrum by the sum of the θ-wave, α-wave, and β-wave power spectra. The whole activity of the brain was evaluated from the α-wave content. The sampling frequency was 500 Hz.
Cerebral blood flow in the prefrontal cortex was estimated using NIRS, with Hb-132 (Astem Co., Ltd.). Near infrared light is absorbed by hemoglobin when passing through the living body. As a result, NIRS can measure changes in the concentration of oxygenated (or deoxygenated) hemoglobin as a function of time. Blood flow increases locally at a site where neurons have become active, and the oxygenated hemoglobin concentration in the blood increases. Thus, local cerebral blood flow can be used as an indicator of local neuronal activity. In the prefrontal cortex, activity was assessed from cerebral blood flow measurement. The amount of oxy-Hb was calculated relative to a pre-rest reference point. We used a five-channel sensor, and the channels were arranged at 35-mm intervals on the left and right sides. Channel 3 was located at the center of the prefrontal cortex and was set to Fpz. Channels 1 and 2 were attached over the left frontal lobe and Channels 4 and 5 were attached over the right frontal lobe. The sampling frequency was 2 Hz.

Experimental Protocol
The experimental environment is shown in Figure 1, and the experimental protocol is shown in Figure 2. Each trial comprised two time periods: Cognitive load task (180 s), and Stimulus presentation (300 s). Thermal stimulation was constantly presented to the soles of the feet during the trials. Physiological responses were measured throughout each trial. Each participant participated in six trials, one for each of the presentation patterns. Each set of six tests was performed on the same day, and the experiment was performed twice (2 days in total). The data on a day with no data loss were analyzed. The experiment was conducted in a room with constant temperature and humidity (25 °C, 55% relative humidity). Except when the illumination stimulus was presented, white illumination (horizontal luminance: 50 lx) was presented from the LED cube. Participants were seated  to be comfortable for participants [22].
The number of participants for the experiment was 10 (5 males and 5 femalesaged 20-24 years). Participants were called for from students at Shinshu University.
Participants performed the Advanced Trail Making Test (ATMT) as a cognitive load task in each trial to control initial conditions. The ATMT is a cognitive load task in which participants are instructed to use a mouse to sequentially click numbers displayed on the screen (Figure 3). The numbers were from 20 to 75, and 25 numbers were displayed each time. The screen switches each time a number is clicked, and the arrangement of the displayed numbers changes randomly. When the number reaches 75, the number returns to 20 again.

Sensory Test
We also recorded subjective evaluations of each stimulation after the trial. Participants were asked about their comfort/discomfort and arousal/sedation level using the semantic differential (SD) method after each trial. The evaluation scale comprised seven grades (−3, extremely; −2, very; −1, slightly; 0, neither; +1, slightly; +2, very; and +3, extremely). Participants were also asked to rate their state of relaxation and fatigue using a four-point scale (0, none; +1, slightly; +2, very; +3, extremely).
Participants were asked about their respective states before and after stimulus presentation after each trial.

Statistical Analysis
Two-way analysis of variance (ANOVA) was conducted in order to verify the statistical significance of the two main effects, stimulus and time course and interaction between them. Since ANOVA has robustness, we assumed that the scores of each sensory test, calculated α-wave value and oxy-Hb follow a normal distribution. Because there is no non-parametric test applied to two-way ANOVA, parametric test was conducted in this paper. Additionally, Tukey's test was used for multiple comparisons.

Ethical Considerations
This study was conducted with the approval of Shinshu University's ethical committee for research on humans. All participants were informed on the experimental methods and the protection of the privacy, and the experiments were carried out after obtaining their consent. Figure 4 (a)-(d) show the mean scores of sensory test obtained from 10 participants. We performed a two-way ANOVA on the mean value for each evaluation term with factor 1 (six levels, stimulation condition) and factor 2 (two levels, before-after) as factors. The ANOVA revealed a significant main effect of factor 2 (before-after) for all evaluation terms (p < 0.05). Regarding the feeling of comfort, a significant factor 1 × factor 2 interaction was confirmed. Additionally, multiple comparisons using Tukey's test revealed significant differences between the presentation stimuli and before-after each. In the comparison between before and after, the scores for "feeling of comfort" and "feeling of relaxation" increased significantly after presentation, and scores of arousal and fatigue state decreased significantly (p < 0.05). In the comparison between presentation stimuli, we confirmed that "music + thermal" stimulation scores tended to be significantly higher than those for other stimuli in terms of the "feeling of comfort" and the "feeling of relaxation". Based on these results, we concluded that all of the presented stimulus modalities induced feelings of relaxation and sedation, and that the "music + thermal" stimulation condition was subjectively evaluated as the most relaxing. Figure 5 shows the average EEG α-wave activity, which was calculated every minute. The elapsed time on the x-axis represents the end time of the calculation section for 1 minute (i.e., the calculation interval 0 -1 minute is expressed as 1 minute). Two-way ANOVA was performed on the mean value of α-wave content for factor 1 (six levels, stimulation condition) and factor 2 (eight levels, elapsed time). The results revealed significant main effects of both factors, and a significant interaction between them (p < 0.05). We also performed multiple comparisons (Tukey's test) for the elapsed time at each presentation stimuli level and for the presentation stimuli at each time point. EEG α-wave activity was significantly increased in all stimulation conditions compared with during the cognitive load task (p < 0.05). During stimulus presentation, α-wave activity in the "movie + thermal" and "illumination + thermal" conditions were significantly small compared with the other conditions (p < 0.05); however, this difference was significantly smaller in the final 7 -8 minute time period. Regarding the differences between the stimulation conditions at each time point, the "odor + thermal" and "vibration + thermal" conditions were larger than the only thermal stimulation at 5 -6 minutes (p < 0.05). Additionally, at 6 -7 minutes, stimuli other than music were associated with higher α-wave activity than "movie + thermal" stimulation (p < 0.05), and at 8 minutes, "thermal stimulation alone induced higher α-wave activity than the "illumination + thermal" and "movie + thermal" conditions (p < 0.05). These findings indicate that the movie/illumination stimuli, including visual stimulation, tended to induce less α-wave activity, possibly because of the occurrence of α-attenuation. Conversely, vibration and odor stimuli tended to induce larger α-wave activity. Figure 6 (a)-(e) show the mean oxy-Hb scores on all channels from nine participants. One participant's data was removed because of missing data on both measurement days. The average oxy-Hb in each minute was calculated, and the difference from the oxy-Hb amount from 0 -1 minute is shown in Figure 6. We performed a two-way ANOVA examining the mean value of change in oxy-Hb in all channels for factor 1 (six levels, stimulation condition) and factor 2 (seven levels, elapsed time excluding 0-1-minute data). The ANOVA revealed a main effect of factor 2 (elapsed time) in all channels (p < 0.05), but no significant factor 1 × factor 2 interaction. We also performed multiple comparisons (Tukey's test) for the elapsed time in each stimulus presentation condition, and for the stimuli at each elapsed time point, as for the α-wave activity. The results revealed that, compared with during the cognitive (a) Mean scores for "comfort -discomfort" (b) Mean scores for "arousal -sedation"

Cerebral Blood Flow in Prefrontal Cortex
(c) Mean scores for "feeling of relaxation" (d) Mean scores for "fatigue state" load task, the thermal stimulation alone, "music + thermal", "movie + thermal", and "odor + thermal" stimulation conditions induced significantly lower oxy-Hb levels in all channels during stimulus presentation (p < 0.05).
Regarding the "illumination + thermal" condition, oxy-Hb was lower than during the cognitive load task (p < 0.05), except for channel 1. The "vibration + thermal" condition also induced lower oxy-Hb compared with during the cognitive load task (p < 0.05), except in channels 4 and 5.
Additionally, for these two stimuli, the decrease in cerebral blood flow was smaller in all channels compared with the other stimuli. The "music + thermal" stimulation condition significantly reduced oxy-Hb compared with the other stimulation condition at the 3 -4 minute time periods (immediately after stimulus presentation) in channels 3 and 4 (p < 0.05). Regarding the "movie + thermal" and "odor + thermal" conditions, oxy-Hb levels were significantly lower than for thermal stimulation alone at 6 -7 minutes in channels 3 and 4 (p < 0.05). These results indicate that speed differed depending on the type of stimulus. Thus, the "music + thermal" condition appeared to quickly reduce cerebral blood flow, and the "movie + thermal" and "odor + thermal" conditions further reduced cerebral blood flow more than the thermal stimulation alone condition; however, the responses were relatively slow.

DISCUSSION
For all stimuli, α-wave activity increased and cerebral blood flow in prefrontal cortex decreased during stimulation compared with during a cognitive load task. In this section, we consider the results of physiological response measurement and sensory test for each stimulus presentation modality. Additionally, we discuss the combined effects of the simultaneous presentation of two stimulus types to enhance feelings of relaxation.

Movie + thermal stimulation
The results revealed no significant differences in subjective evaluation scores between the "movie + thermal" stimulation condition and the thermal stimulation alone condition. The level of EEG α-wave activity was relatively low compared with the other (e) 5ch stimulation conditions, likely because of α-attenuation. However, because cerebral blood flow was substantially reduced in these conditions compared with the other stimulation conditions, it is possible that the simultaneous presentation of "thermal + movie" stimuli induced a feeling of relaxation.

Music + thermal stimulation
The "music + thermal" stimulation was evaluated as the most relaxing condition in sensory tests. Decreased cerebral blood flow and increased EEG α-wave activity were confirmed, and these results are consistent with the subjective evaluation in the current experiment and previous studies [9,11] involving the presentation of classical music. Additionally, immediately after the stimulus presentation (3 -4 minutes), cerebral blood flow in the center of the frontal lobe (channels 3 and 4) was significantly lower compared with the other stimulation conditions. The results did not reveal any significant differences between the thermal stimulation alone condition in α-wave activity or cerebral blood flow, except at 3 -4 minutes for oxy-Hb levels at channels 3 and 4. The above results suggest that the "music + thermal" stimulation can induce a feeling of relaxation in a short period of time.

Illumination + thermal stimulation
The results revealed no significant differences between sensory test scores for the "illumination + thermal" stimulation condition and the other stimulation conditions. However, some participants reported that they felt strange when the illumination changed from white to blue. We speculate that discomfort in response to blue illumination and α-attenuation may have caused a slow increase in α-wave activity and a slow decrease in cerebral blood flow. 4. Vibration + thermal stimulation "Vibration + thermal" stimulation was evaluated as slightly less relaxing than the other conditions in sensory test. EEG α-wave activity increased with a peak at 4 -6 minutes. However, the decrease in cerebral blood flow was small, and there were no significant differences between channels 4 and 5 during the cognitive load task. Some participants reported that they felt strange when the vibration was presented, as in the vibration presentation. We attribute the small reduction in cerebral blood flow to this subjective impression. From the findings discussed above, we speculate that the vibration stimulus induced a feeling of relaxation by being presented simultaneously with thermal stimuli, but not when the vibration stimulus was presented alone.

Odor + thermal stimulation
For each evaluation term of the sensory test, the "odor + thermal" stimulation did not exhibit a significant difference from other stimuli. EEG α-wave activity increased with a peak at 5 -6 minutes, and cerebral blood flow decreased, with a minimum value at 6 -7 minutes. Because the orbitofrontal cortex (OFC) of the frontal lobe responds to the inhalation of odors, we speculated that cerebral blood flow decreased because of the presentation of a relaxing odor. These trends in physiological responses are consistent with previous studies [18,19] of the effects of relaxing odor presentation. Therefore, we conclude that "odor + thermal" stimulation induced a feeling of relaxation. Additionally, the response speed appeared to be slower than for the other stimulation conditions. The physiological responses in each stimulation condition were consistent with the subjective evaluation and physiological responses when presenting a stimulus to one sensory modality. Therefore, we were unable to verify the effects of combining two stimulus types. Because physiological responses were not measured when each stimulus was presented alone, we were unable to verify whether presentation of two stimuli increased or decreased α-wave activity and cerebral blood flow. However, an increase in α-wave activity and a decrease in cerebral blood flow were confirmed during stimulus presentation, indicating that the activity of the whole brain and activity in the prefrontal cortex were suppressed. This result implies that there was a relaxation effect. Based on the above findings, we conclude that presenting thermal stimulation to the soles of the feet has a substantial effect on the feeling of relaxation. Additionally, the feeling of relaxation induced by thermal stimulation does not appear to be impaired by the simultaneous presentation of other stimulus types.

CONCLUSION
In the current study, we examined differences in physiological responses to comfortable thermal stimulation of the soles of the feet when presenting other sensory stimuli and determined whether these combinations of sensory integration can lead to different psychological and physiological states. Here, underwater movie footage, classical music, light blue illumination, vibrations to the trunk, and the smell of grapefruit oil were presented simultaneously with a thermal stimulus. Additionally, we measured EEG and cerebral blood flow in prefrontal cortex. The results revealed an increase in EEG α-wave activity and a decrease in cerebral blood flow in all stimulus presentation conditions. This result indicates that all stimuli induced a feeling of relaxation. Additionally, the results revealed differences in the speed of physiological responses depending on the type of stimulation. However, we were unable to verify any significant differences in physiological and psychological responses because of the large effect of thermal stimulation on the feeling of relaxation.
We plan to measure physiological responses in the autonomic nervous system in future research, to clarify the combined effects of multiple stimulus types by measuring whole-body physiological responses (autonomic nervous system and central nervous system).