Variable physical functions in individuals in response to sensory stimuli

PURPOSE: Sensory stimulation affects physical functions; however, the type and range of physical function change remain unclear. This study aimed to evaluate the type and extent of changes in the simple physical functions resulting from exposure to color and taste stimuli. METHODS: Five basic colors (red, blue, yellow, green, and black) and foods representative of �ve basic tastes (sweet, sour, salty, bitter, and umami) were used as stimuli. Three different physical function tests, muscle strength (grip strength), �exibility (bending length), and balance (stabilometer trajectory area) were measured while wearing color-tinted goggles or after tasting food stimuli. Tests were repeated for six successive days in a controlled environment. RESULTS: Each stimulus had different effects on participants. For color stimulation, the median change ratio between the minimum and maximum effects was 5.68% (4.14%–8.07%) for muscle strength, 8.52% (5.11%– 13.39%) for �exibility, and 30.60% (26.81%–36.18%) for balance. The corresponding values in taste stimulation were 4.96% (3.67%–7.89%), 6.11% (4.37%–8.86%), and 28.92% (21.38%–34.01%). The rate of change in balance was the highest among the three physical tests and was signi�cantly different from the rate of change in the other physical functions (balance vs. muscle power, P < 0.001; balance vs. �exibility, P < 0.001). CONCLUSION: Color and taste stimuli have differential effects on physical function. The extent of physical change to stimuli differed among the participants, suggesting individual-level differences in sensitivity to stimuli. Sensory stimuli affect physical functions individually and affect balance more than any other physical function.


Introduction
Sensory stimulation is known to affect the brain, and changes in body function due to various types of sensory stimulation have progressively been reported.In particular, among the various sensory stimulation, color and taste have been widely reported.Color has been reported to play a role in increasing exercise intensity [1,2], affecting the competition outcomes of athletes [3,4].In this context, some reports also suggest a relationship between taste and physical performance, such as ergogenic effects [5], power output [6], and endurance performance [7,8].
It is strongly suggested that brain function involves a mechanism of bodily function change in response to sensory stimuli.The mechanism by which color stimuli affect the brain is directly quanti ed based on its maximum sensitivity at wavelengths of ~ 560 nm (red), ~ 530 nm (green), and ~ 420 nm (blue) [9].Color stimuli are transmitted via the optic nerve to the visual cortex (VC) [10] from photosensitive retinal ganglion cells [11].
Before reaching the VC, the stimuli pass through the hypothalamus, epithalamus, limbic system, and midbrain [12][13][14][15][16]. Regarding taste, oral receptors may affect the brain and in uence body functions [17], such as the activation of both insular and motor cortices, which subsequently excite the neuromuscular pathways [18], or the right insular cortex, which is linked to other parts of the brain that are involved in cardiovascular regulation during exercise [19].
Although understanding of the relationship between sensory stimuli and brain function is growing, only a few studies have reported the actual amount of change in basic body functions and the differences between them because the evaluation methods or the results of games might have been affected by various other physical functions.
Previous research on sensory stimulation has yielded variable results.Some studies have found that sensory stimulation is effective, while others have concluded that color and taste stimulation do not affect sports performance or outcomes [4,[20][21][22][23].These discrepancies in ndings may be due to individual susceptibility to sensory stimuli, determined by the perception of stimulus intensity, environmental factors (e.g., brightness and temperature), and physical conditions and emotional states [24][25][26][27][28][29].In past reports, a small number of measurements were performed for a small number of stimulus types, which is not su cient in terms of reproducibility given the various factors mentioned above.In this study, to increase the reliability and reproducibility of results, we evaluated the changes in physical function in response to sensory stimuli using a greater number of stimulus types and an increased number of tests.
Herein, we aimed to evaluate whether exposure to sensory stimuli contributed to change in the type and extent of physical function; we used basic physical function tests and examined the type and extent of differences between test types.To increase reproducibility, this study used more stimuli and tests than previous studies.

Ethics approval and informed consent
This study was approved by the Medical Ethics Committee of the National Center for Geriatrics and Gerontology, Obu, Japan (approval number: 1466), and was conducted in accordance with the tenets of the Declaration of Helsinki.Written informed consent was obtained from all study participants.Data were anonymized for analyses.

Study participants
This pilot study aimed to include members of the general healthy population; thus, we recruited participants who regularly attended a gym.We included individuals without any physical disorders aged between 20 and 70 years.
The exclusion criteria were as follows: 1) visual impairment that may affect color perception, 2) presence of complications or oral diseases that may affect taste perception, 3) presence of cognitive or motor impairment that may intervene with study procedures, and 4) allergies to any of the test foods used for taste stimulation.

Color
In this study, we used longer-wavelength colors (red and yellow), shorter-wavelength colors (green and blue), black, and transparent control as stimuli.Participants wore goggles tinted with each test color during analysis.
All measurements were performed in the same environment, which was an indoor gymnasium, to maintain constant color saturation and brightness representative of real-life conditions.Taste acid), salty (salt), bitter (coffee powder), umami (dashi stock, including glutamate), and control (potato starch) tastes.The participants consumed each type of granule independently.To reduce the impact of sample intake, we used familiar food for each taste stimulus.

Physical examinations
Three physical function measurements were selected because of their ease of evaluation, minimal mutual interference, and role in motor function.Muscle strength was measured twice in both arms using a grip dynamometer (Digital Grip Dynamometer; Takei Science Instruments Corp., Niigata, Japan), and the scores were averaged.We also assessed exibility by measuring the level of extension when each participant bent forward from a sitting position using a digital long-seat ante exion meter (Long-Seat Ante exion Meter; Takei Science Instruments Corp.).Finally, we evaluated balance using the stabilometer balance test, measuring the trajectory of the center of gravity (trunk movement during a wobble) while the participants stood on a stabilometer for 30 s with their eyes open (Gravicoder GW-10; Anima Corp., Tokyo, Japan).Higher values indicated better muscle strength and exibility, whereas lower values indicated less trajectory uctuation and suggested better balance.

Experimental schedule
Both color and taste experiments were performed over 6 days.Thethree physical function tests were performed separately for each color stimuli.In brief, the participants wore one of the six color-tinted goggles (red, yellow, green, blue, black, and transparent) during each test on the same day.Each participant repeated all tests on six separate days to eliminate the effects of fatigue and repetition.The participants began the tests 5 min after putting on the goggles.Between tests, they took a break of at least 10 min without any stimulation.The order of color stimuli was randomized, and the ordinal coe cient of color stimuli was the same (Fig. 1).
The three physical function tests were performed after the participants received taste stimuli.The participants were stimulated with all six tastes on the same day and performed physical examinations after each stimulus.Between each stimulus, the participants gargled with water and took a break of at least 5 min to neutralize the effects of the previous taste stimulus.Each participant repeated all tests on six separate days to eliminate the effects of fatigue and repetition.Again, the order of the stimuli was randomized, and the ordinal coe cient was the same as in color stimulation.

Statistical analyses
The average score of six tests obtained on six different days was de ned as the value of each physical examination for each taste stimulus.The best and worst scores for the physical tests among the six stimuli were de ned as the maximum and minimum scores, respectively.Data are presented as mean ± standard deviation.The change ratio was de ned as the change in the maximum effect compared with that in the minimum effect.In this study, only the degree of change, not the direction of change, was examined; thus, the absolute values were used for calculating the rate of change (change ratio = [{maximum score − minimum score} / minimum score] × 100).These data are presented as median (interquartile range [IQR]).To compare three or more groups, we adopted Friedman's test to evaluate signi cant differences.When a signi cant difference was observed, the Wilcoxon signed-rank test was performed to examine which combination of the three groups had a signi cant difference.Bonferroni corrections were applied to all multiple comparisons.Moreover, in the analysis of duplicates (muscle strength vs. exibility, exibility vs. balance, and balance vs. muscle strength), P-values of <0.05/3 were considered signi cant using the Bonferroni correction for the comparison of the last three groups (Fig. 2).Data management and statistical analyses were performed using IBM ® SPSS ® Statistics version 27.0 (IBM Corp., Armonk, NY, USA).

Data availability
Raw data were generated at the Innovation Centre for Translational Research, National Center for Geriatrics and Gerontology.Data generated during this study are available from the corresponding author upon reasonable request.

Study participants
A total of 41 participants were recruited, including 10 men and 31 women (mean age: 43.3 ± 10.1 years).Given the imbalanced sample composition, sex-based analyses, apart from overall analyses, were performed.

Comparisons of different stimuli
We examined the impact of each stimulus on physical examination outcomes (Table 1).We evaluated the percentage of each stimulus that resulted in the maximum and minimum scores for each participant (Table 2, 3).
Friedman's test revealed that no stimulus resulted in signi cantly better or worse physical evaluation outcomes.
In addition, sex-speci c analyses did not reveal any stimulus with apparently larger or smaller effects in men or women.

Comparisons between the maximum and minimum scores
We compared the maximum and minimum scores of all physical function tests.For color stimulations, the median (IQR) of the maximum and minimum scores were 1.36 kg (0.94-1.80) for grip strength, 3.08 cm (2.33-3.92)for exibility, and −1.34 cm 2 (−1.82 to 1.07) for balance.For taste stimulations, the results were 1.27 kg (1.03-1.67)for grip strength, 2.25 cm (1.88-3.42)for exibility, and −0.100 cm 2 (−1.44 to 0.53) for balance.
Figure 2 shows the overall change ratio in the maximum and minimum scores for each physical test.In the analysis of total samples, the rates of change in muscle strength, exibility, and balance scores upon color stimulation were 5.68% (4.14%-8.07%),8.52% (5.11%-13.39%),and 30.60% (26.81%-36.18%),respectively.Upon taste stimulation, the rates of change in muscle strength, exibility, and balance scores were 4.96% (3.67%-7.89%),6.11% (4.37%-8.86%),and 28.92% (21.38%-34.01%),respectively.One participant had a rate of change of ≥40% in the exibility tests upon taste stimulation.In the balance tests, there were six and ve cases with a rate of change of >40% upon color and taste stimulations, respectively.Friedman's test showed a signi cant difference in the rate of change among the three groups of physical functions (P < 0.001).Group comparison with Wilcoxon's rank-sum test showed a signi cant difference in the rate of change in balance scores compared with that in muscle strength scores (P < 0.001) and exibility scores (P < 0.001).This change was observed for both color and taste stimulations.

Discussion
In this study, we examined changes in grip strength, exibility, and balance in response to color and taste stimuli.
Sensory changes have frequently been of great interest in terms of their effect on daily activities, including physical function.Even professional athletes empirically use their preferred color or taste.However, the effects of these sensory changes on physical function are often not recognized beyond psychological and placebo effects.The impact of this study would be considered meaningful if the change in physical function was affected more than expected.Nevertheless, based on the results of this study, it would be relevant to analyze changes in brain function in response to these stimuli in future studies.
Studies on the effects of sensory stimuli require a large number of trials as they are affected by environmental conditions such as temperature, which in uence the reproducibility of ndings.Therefore, all participants were evaluated on six separate days for color-related effects and six separate days for taste-related effects.In addition, this study had a larger sample size (41 participants) and a greater number of variables tested (three physical function tests) than previous studies; it also involved a long assessment period (6 days) in the same environment.These factors improve the reproducibility of the study results and are the strengths of this study.
Accordingly, we aimed to evaluate 1) changes in physical function depending on each stimulus and 2) differences in the results of physical function assessments.

Change in physical functions depending on color and taste stimuli
In this study, no signi cant differences in physical function were observed upon any particular stimulus (Table 1).In addition, the percentage of stimulation with the best and worst results for each participant (Table 2 and 3) showed no signi cant difference.In the sex-speci c analysis, no signi cant stimulus was found for men or women.Based on these results, it can be inferred that the response of physical body functions to stimuli varies from individual to individual.Our results support the con icting ndings from previous studies that no speci c stimulus effects tend to occur [20,22,30].Regarding the cause of these results, we believe that different participants responded differently to stimuli and thus had varying reports.In addition, as shown in Table 2 and 3, the stimulants that affect the body the most and least are different for each individual; therefore, this study shows the possibility of the importance of providing personalized and appropriate stimuli to affect a change in an individual's physical functioning.Speci cally, these results suggest that while appropriate stimulation may be effective for improved physical function, inappropriate stimulation can impair physical function.In addition, in the present study, even in the same participant, there were different types of stimuli that performed well depending on the type of exercise; for example, the best color for grip strength was different from the best color for exibility in the same participant.This nding suggests a difference in the underlying mechanism.Future studies should involve brain imaging to elucidate these associations.

Extent of change in physical functions in response to color and taste stimuli
We next evaluated the extent of changes in physical function in response to various stimuli.As mentioned before, the cognitive process of interpretation of color and taste is becoming clearer.Exposure to the color increases blood ow in the prefrontal cortex [31], and brain connectivity changes in response to color stimulation [16,32,33].Exposure to taste activates the anterior cingulate cortex or insular cortex and ventral striatum via the gustatory cortex and orbitofrontal cortex, in uencing motor behavior [34,35].Although the mechanisms are gradually becoming clearer, there has been limited analysis of the actual changes; therefore, we examined differences in actual color and taste stimulation in this study.As shown in Figure 2, there were individual differences in the range of change; in all three tests, the range of change tended to be greater for color than for taste.There are no reports comparing the degree of change in physical function in response to taste and visual stimulation.To elucidate the causes of differences in the effects of color and taste stimuli on physical function, as in this study, further exploration of the changes in neural function in response to stimuli is an appropriate next step.In addition, regarding the differences in the physical assessment results, the balance outcome was signi cantly altered compared with the grip strength and exibility outcomes.Balance plays an essential role in various physical activities and sports, a point to which many different research and training theories have alluded.To the best of our knowledge, no study has examined the neural correlates of changes to balance in response to sensory stimuli.Further studies are required to elucidate the mechanisms through which sensory stimulation may affect balance.
Furthermore, the results of our study showed that participants' responsiveness to stimuli varied in the same participant as well, and in some cases, individuals were extremely responsive to the stimuli.For example, taste stimulus changed exibility by >40% in one participant, while similar ratios of change were observed in six cases by visual stimuli.Further, a >40% change ratio was seen in balance tests in ve cases upon taste stimuli.Another key to future research may be the "responders," those who are strongly responsive to stimuli.A more detailed analysis of these highly respondent participants, comparing them with people with weak responses, may contribute to elucidating the mechanism of changes in physical functions caused by stimuli.
Limitations study has some limitations.The present study only reported the range of change, and it cannot be concluded that the range of change is clinically signi cant.Therefore, it is crucial to analyze the actual neural network alterations using functional brain imaging, relating the ndings to the physical change evoked by the stimulation.
In this study, the number of male and female participants differed.Therefore, we conducted a separate analysis for each sex.The comparison between sexes is important as many previous reports indicate that the effects can differ based on sex [36][37][38][39].Since the imbalanced number of each sex in this study precluded proper comparisons between males and females, more participants in equal distribution of sex should be included and evaluated to analyze sex-related differences more accurately.
Another limitation is the measurement items.Three items were measured in this study because of their simplicity of evaluation and minimal mutual in uence.It is necessary to further evaluate the in uence of taste stimulation on various body functions in the future.
Regarding the study environment, all tests were performed under the same conditions in the same room, within an indoor sports gymnasium with constant light, and all participants wore the same type of goggles, which were designed to keep all elds of vision in the same color.However, color saturation and brightness were not measured using a colorimeter.Future studies should consider these limitations.For the evaluation of each color stimulation, the use of a device-independent measurement tool (e.g., spectrophotometer) is required; therefore, color speci cations and the use of standardized materials based on device-independent measurements are also inevitable for further reproducibility.
Additionally, the effects of taste samples may have not been representative of the taste effects.Coffee, for example, may affect body functions as a component rather than a taste sensation.There is also no consensus about the loading dose.Although the same dose was used for all participants in this study, it is necessary to evaluate differences in the sensitivity of individuals because taste preferences vary among individuals.Future studies should de ne taste components and sensitivity range, and perform analyses strati ed by sensitivity level.

Conclusion
In this study, no color or taste stimulus affected the participants' physical function more than any other stimuli of the same type.This nding suggests individual variation in the responsiveness to stimuli, suggesting that using sensory stimulation as a means to affect motor function should be personalized, as the choice of stimulus may improve or reduce outcome values.We also calculated the change ratio between maximum and minimum effects for each participant.The change ratio was larger for balance than for the other outcomes.
The mechanism underlying the change in physical function in response to the sensory stimulus is unclear.
Functional brain imaging, such as functional magnetic resonance imaging, is required to elucidate these mechanisms, including the differences between individuals with high and low responsiveness.These studies may provide insight into the effects of sensory stimuli on human body functions.

Figures
Figures

Table 1 .
Physical examination results for each stimulus

Table 2 .
Color: Maximum and minimum stimulation percentage for each participant in physical function tests.

Table Taste :
Maximum and minimum stimulation percentage for each participant in physical function tests.