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J Phys Ther Sci
J Phys Ther Sci
JPTS
Journal of Physical Therapy Science
0915-5287
2187-5626
The Society of Physical Therapy Science

2023-135
10.1589/jpts.36.161
Original Article
Sound source localization in blind soccer: differences between sighted and visually impaired players
Watanabe Masahiro PhD 1
1) Faculty of Medical and Health Science, Tsukuba International University: 6-8-33 Manabe, Tsuchiura-shi, Ibaraki 300-0051, Japan
*Corresponding author. Mashiro Watanabe (E-mail: m-watanabe@tius.ac.jp)
1 4 2024
4 2024
36 4 161166
09 11 2023
30 12 2023
2024©by the Society of Physical Therapy Science. Published by IPEC Inc.
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (by-nc-nd) License. (CC-BY-NC-ND 4.0: https://creativecommons.org/licenses/by-nc-nd/4.0/)
[Purpose] The sense of vision is omitted in blind soccer, and sound source localization to grasp the position of the ball is extremely important. The purpose of this study was to clarify whether there is a difference in ability in sound source localization in its approaching condition between visually impaired and sighted people, using the source actually used in blind soccer ball competitions. [Participants and Methods] Eighteen participants were divided into two groups; 10 sighted people and eight visually impaired people. The participants were asked to press a switch when a rolling blind soccer ball was sensed in any one of the four directions. We recorded time error as the difference between the time when the ball passed the optical sensor set under the participant’s feet and when the participant pressed the switch. [Results] The time error in response increased with the ball speed in all cases; however, its dependence on the ball speed was significantly different between the two groups. [Conclusion] The visually impaired participants made less time errors in response to the localization of the ball than the sighted participants, even when the ball speed increased. The results indicate that visually impaired people have better sound source localization ability than sighted people do.

Blind soccer
Sound source localization
Visually impaired
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pmcINTRODUCTION

Sports for disabled people, such as the Paralympics, play a major role in promoting our understanding of people with disabilities. According to the Nippon Foundation Paralympic Support Center Public Opinion Survey1), the Paralympic Games are well known both in Japan and abroad, and many people are “enhancing the environment and facilities for disabled people”. We hope that this will lead to “a promotion of understanding”. From these statements, it can be assumed that the Paralympic Games are highly likely to have a significant effect on the promotion of both the public awareness of and social participation with people with disabilities.

Blind soccer, which is a Paralympic event, is one of the few sports where both visually impaired and sighted people can participate. Blind soccer players attract spectators by performing brilliant dribbling and shooting, relying only on sound, and many people are interested in blind soccer in Japan. Therefore, it is one of the few sports involving people with disabilities that can make money as paid entertainment. Blind soccer can have a major impact on awareness regarding persons with disabilities and promotion of social participation of persons with disabilities. Furthermore, because it is included in Paralympic events, it also leads to financial independence of persons with disabilities.

Blind soccer is a five-player version of soccer in which visually impaired and sighted people play on the same field. The international competition rules require visually impaired people to be field players but do not exclude sighted people as field players. All the field players wear eye masks, and a ball with a bell is used. The field is the same size as that of futsal, and the game duration is 50 minutes. Compared to those of regular soccer, the field is smaller and the game time is shorter. The blindfolded field players use the sounds of the bell on the ball and calls from other players, coaches, and callers as cues while playing. For this reason, the ability of sound source localization for locating the position of the ball as well as physical agility and balance are extremely important to play. The ability to accurately perceive the constantly changing position of the ball during a match is a crucial skill for effective offense and defense. In other words, in blind soccer, the auditory source localization ability to track the moving ball significantly influences the outcome of the game, playing a pivotal role in determining victory or defeat.

From the sounds obtained from both ears, humans can obtain higher-order information through the central auditory pathway located higher than the superior olivary nucleus. From this higher-order information, humans can sense the direction and localize the sound source. There have been many studies on sound source localization that were conducted in a static space in which sound signals are presented using audio speakers or headphones2, 3). It should be noted that the presentation of sound using a pair of loudspeakers has a relatively small localization effect and a large variation, resulting in low accuracy2). And these did not addressed sound localization ability in source approaching condition. Therefore, we believe it is important to obtain measurements using an actual sound source and clarify the ability to track an approaching sound source.

It was reported that the ability of sound source localization of visually impaired people is superior to that of sighted people4), and researchers are developing pedestrian traffic lights with guidance functions based on sound source localization and acoustic displays used for PCs5, 6). In previous studies, differences in the ability of sound source localization between visually impaired and sighted people have been examined7, 8). However, experimental conditions have differed, and a definitive conclusion has not yet been reached.

In this study, the ability of sound source localization required for blind soccer was determined by using a blind soccer ball, the sound source of which is actually used in competition, to determine whether there is a difference in this ability between visually impaired and sighted people.

PARTICIPANTS AND METHODS

Eighteen participants were divided into two groups: 10 sighted people (age 21.3 ± 0.67 years, height 171.8 ± 5.4 cm) and 8 visually impaired people (7 people with low vision, 1 blind person (age 22.5 ± 4.5 years, height 170.4 ± 3.9 cm, visually impaired period 9.6 ± 5 years). Participants had no neurological or orthopedic disorders and no hearing problems. The sighted people had no experience with blind soccer, and the visually impaired people were blind soccer players (competition history: 2.8 ± 0.9 years). The visually impaired soccer players were active in the community. The participants signed consent forms after being explained about and understanding this study. This study was conducted with the approval of the Medical and Ethical Review Committee of the Medical and Ethical Review Board of the East-West Medical Integrated Medical Center, University of Tsukuba, University of Technology (No. 201705).

The experimental setup is illustrated in Fig. 1Fig. 1. Participant limb position and sensor position.

The blind soccer ball is rolled by the examiner using rails from a position 2 m away from the participant’s feet. The participant is seated in a chair and the height is set such that the ear canals were 120 mm above the floor. Sensor 1 is installed under the feet, and sensor 2 is installed 1 m in front of it. The ball speed is calculated for the time it takes the ball to travel between sensors 1 and 2.

. The participants sat in chairs, placed their chins on a table so that their ears were 120 mm above the floor, and wore eye masks. An optical sensor (sensor 1) was installed near the participant’s toes. Each participant pressed the switch at the moment when he or she thought that the rolling blind soccer ball had reached his or her toes. As shown in Fig. 2Fig. 2. The four directions in which the ball is rolled.

The blind soccer ball is rolled in four directions: Front, Back, Across the front, and Across the back. The ball is rotated six times using the rail at two different speeds in one direction (approximately 1.5 m/s × 3 times, approximately 2.8 m/s × 3 times). The examiner then rolls the ball a minimum of five times, at a random speed from among the following: minimum, slow, medium, fast, and high speeds (5 times between 1.0 and 9.0 m/s). In total, the ball is rolled 11 times.

, the experimental conditions of four rolling directions of the ball with respect to the participant, Front, Back, Across the front (right to left), and Across the back (right to left), were prepared.

The ball was rolled on artificial grass with a thickness of 5 mm and a length of 2 m. The experimental trials for each direction were conducted three times with ball speeds of approximately 1.4 m/s and 2.8 m/s using a sloped rail from a position 2 m away from the participants’ feet. Next, from the same place of the rail, an experimenter rolled the ball to each participant for 11 times in each direction with randomly selected ball speeds from the minimum, slow, medium, fast, and high speeds (1.0 to 9.0 m/s). A single experimenter rolled the ball for all participants. The sensor near the feet (sensor 1) was placed at the participant’s toe for the rolling directions of Front and Back, and at the middle point between the participant’s feet for the rolling directions of Across the front and Across the back. As shown in Fig. 1, another optical senser (sensor 2) was placed 1 m away from sensor 1 along the ball direction. The ball speed was calculated as the difference in the time points at which the ball passed through the two sensors.

The time error in response was calculated as the difference between the time point when the ball passed through the optical sensor under the foot and the time point when the participant pressed the hand switch.

The ball speed and the time error in response were used to compare the difference in ability between the two groups of sighted and visually impaired participants.

To compare the groups of sighted and the visually impaired participants, a multiple regression analysis was performed with the time error in response as the dependent variable and ball speed and group as the independent variables. For the independent variable group, values 0 and 1 were given for the visually impaired participants and sighted participants, respectively. The optimum model was then selected by the stepwise method. The priority was to model the effects of factors such as speed and participant. In addition, in order to investigate the relationship between the age of onset of visual impairment and the time error, the data distribution of time error was normalized using a log transformation and regression analysis was performed. IBM SPSS Statistics 25 (IBM Corp., Armonk, NY, USA) was used for statistical processing.

In the multiple regression analysis in this study, a linear model for the relationship of the ball speed S, the group G and their interaction SG with time error in response T was formulated as follows,

T=a_0+a_1 S+a_2 G+a_3 SG . (1)

Here, the first term represents an intercept, and the second term changes proportionally to the ball speed. The value of G was fixed as 0 or 1 for sighted or impaired groups, respectively. First, this model was applied to the data obtained in each condition in order to confirm absence of interaction between factors of ball speed and the groups, SG. For all conditions, a_3, the partial regression coefficient of SG did not indicate statistical significance (p>0.1). Then the model was refined by eliminating the final term from Eq. (1) and was applied to data in each condition in order to examine the statistical significance of the partial regression coefficients of Sand G, a_1 and a_2, respectively.

RESULTS

Time error in response increased along with speed in all cases; however, its dependence on speed was significantly different between the two groups. The partial regression coefficients for speed and the two groups in each direction condition were as follows, respectively: 0.06 (t=8.031, p<0.001) and −0.14 (t=−5.260, p<0.001) for Front; 0.05 (t=5.479, p<0.001) and −0.11 (t=−3.403, p<0.001) for Back; 0.04 (t=5.090, p<0.001) and −0.09 (t=−3.421, p<0.001) for Across the front; 0.06 (t=7.460, p<0.001) and −0.11(t=−3.726, p<0.001) for Across the back. These results were summarized in Table 1Table 1. Multiple regression analysis with time error of response, ball speed and group (n=18)

Variable	Front	Back	Across the front	Across the back	
	
B	SEB	β	t	B	SEB	β	t	B	SEB	β	t	B	SEB	β	t	
Ball speed	0.06	0.01	0.48*	8.031	0.05	0.01	0.36*	5.479	0.04	0.01	0.34*	5.090	0.06	0.01	0.46*	7.460	
(m/s)																	
Participant	−0.14	0.03	−0.31*	−5.260	−0.11	0.03	−0.22*	−3.403	−0.87	0.03	−0.23*	−3.421	−0.11	0.03	−0.23*	−3.726	
(sec)																	
*p<0.001.

. In the multiple regression for each direction condition, no significant interaction between the group and the ball speed was observed.

According to conventional physiological findings, the simple response time to an auditory stimulus has an error of 150 to 300 ms9, 10). In this study, a post-hoc test was used, and the sample size was confirmed using this value, but no issues were noted (allowable error 200 ms, confidence coefficient 95%, standard deviation in each direction).

The relationship between the age of blind onset and the time error was also investigated. As a result, the time error tended to increase as the age of blind onset was later (r=0.113, p=0.019, R2 0.013, SEE 6.15).

DISCUSSION

In this study, the sound source localization abilities of sighted and visually impaired people for blind soccer were compared. The results revealed that the time errors in response for visually impaired participants had lower values than those for the sighted participants, even when the ball was rolled faster.

Sound source localization is performed on the basis of the differences in reached time11), sound pressure level12) at left and right ears, and spectral characteristics of the sound13). Since the spectral characteristics of the sound are processed according to the transfer function in sound hearing, these characteristics becomes less effective for discriminating the position and direction of a sound source more than approximately 1 m away from the ear canal11). Therefore, when a blind soccer ball approaches a person whose vision is blocked, it is assumed that the time and sound pressure level differences are used for sound source localization.

A study on barn owls’ ability reported that listening experience in the early developmental stage has an important effect on the development of the ability of sound direction perception14). In studies on unilateral hearing loss in human newborn infants, sound stimulation to the normal ear resulted in changes in the function of the inferior colliculus neurons and compensatory changes in the auditory spatial map in the superior colliculus15, 16). Therefore, it is clear that auditory function is developed by sound experience and learning.

The above studies suggest that the development of the ability of sound source localization is influenced by the plasticity of the brain and listening experiences at a young age. In other words, it can be said that visually impaired people who rely only on hearing and tactile cues for external information have developed their sound source localization ability through the experience of learning auditory information from an early age.

In this study, when the visual information was blocked by the eye mask and only the sound of the ball was used, the responses of the visually impaired participants tended to be faster than those of the sighted participants in all directions. In addition, the difference in responses between the groups tended to increase with decreases in the ball speed. Notably, the visually impaired participants tended to respond to a low-speed ball just before the ball arrived at sensor 1.

All the visually impaired participants in this study were experienced blind soccer players. And from this study, sound source localization was considered to be also related to the onset time of the visually impaired. It was considered in the previous study that not only the sighted but also the visually impaired participants could improve their ability of sound source localization by repeated training. Also, from previous research, it has been found that the ability of sound source localization based on reflected sound and sound insulation can be acquired by sighted people, depending on the training17).

Previous studies reported that people who developed visual impairments early can accurately determine the location of various sounds18). The visually impaired participants in this study had lost their sight congenitally or by before adolescence. Therefore, the hearing of the visually impaired participants in this study may have been more developed than that of the sighted participants. This suggests that visually impaired people who have experience with blind soccer have better ability in sound source localization than that of sighted people due to their experience. And we thought that their early vision loss had an effect.

We believe that study’s groups, which consisted of sighted people with no prior experience with blind soccer and visually impaired people who had several years of competitive experience with blind soccer, will be a limitation of the study. This is because that the groups differed in two variables (vision ability and experience with blind soccer), and the study, therefore, has a potential confounding variable (experience with blind soccer) to which the differences in results could be attributed rather than the actual independent variable of interest (vision ability).

In addition, it was not clarified in this study whether the movement of the head affects the sound source localization of the object. Moreover, the difference between the auditory spatial map of the sighted participants and visually impaired participants is not clear. Therefore, in the future, it is necessary to reproduce a sound environment similar to that of blind soccer competition and to clarify the movement of the head in sound source localization and changes in the auditory spatial map.

In this study, we investigated the sound source localization of an object with no head movement and examined the sound source localization abilities of visually impaired people who had experience in blind soccer. Previous studies of sound source localization employed a method of turning the participant’s face toward the sound. This has been reported to result in neck movement limitation and eye movement compensation19, 20). Therefore, it is possible that the movement of the face may have an important influence on the ability of sound source localization.

We considered that visually impaired people had improved their abilities by repeated experience of sound source localization and appropriate learning. This is assumed to be the result of some changes in the brain due to experience and repeated learning. By clarifying these points in the future, methods can be developed to allow sighted people to improve their abilities of sound source localization.

In the future, it will be necessary to clarify the relationship between the ability of sound source localization and movement of the head by allowing free movement of the head while using an actual sound source. Furthermore, it is necessary to clarify the relationship between sound source localization ability and other physical abilities and develop training methods for improving sound source localization ability.

By clarifying these points, it is believed that participation in blind soccer, which is a Paralympic sport, will be improved and that it will evolve into a sport that will attract people all over the world. Additionally, by improving their ability of sound source localization, sighted people can play games in the same field as visually impaired people, leading to a respectful world. It is presumed that these improvements will lead to “enhancing the environment and facilities for disabled people” and “a promotion of understanding”.

Funding

This work was supported by the Japan Society for the Promotion of Science JSPS KAKENHI Grant Number JP17KO1731.

Conflict of interest

None.

I would like to thank T. Yamada for his advice from the beginning of the experiment. Finally, I am grateful to the referees for useful comments.
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