vision and the somatosensory and vestibular sensory systems play an important role in 
 posture control1,2,3,4,5 .
vision , based on a change 
 in the information projected on the retina , guides the relationship between the environment 
 and the body .
plantar sensation provides information on the base of support and position of 
 the center of gravity .
vestibular sensation detects gravity and acceleration to provide 
 information about the position and movement of the head . if these sensory systems are 
 damaged due to brain diseases , failures in standing posture control are elicited , even 
 without motor paralysis .
pusher syndrome , wallenberg s syndrome , thalamic astasia , 
 unilateral spatial neglect , etc .
although there are previous studies about the 
 change in standing posture due to vibration stimulation and/or vestibular stimulation10,11,12 , there are few studies about the role of 
 visual intervention in standing posture .
prism glasses can bias a view on sagittal and/or 
 horizontal planes , but can not shift it on coronal plane13 , 14 .
although there is a 
 method of tilting seat surface during sitting , this method can not be an intervention of only 
 vision because it influences on somatosensory and vestibular sensory15 .
a tool to cause a lateral tilt of a view on the coronal 
 plane was only a large screen16 , 17 .
however , recently , an inexpensive 
 immersive head - mounted display ( hmd ) has been developed and applied in rehabilitation18 . in the present study
, we developed a 
 visual inclination system by using an hmd for investigating a visual intervention ( tilted 
 view ) on standing posture control .
eleven healthy male university students ( mean  standard deviation : age , 21.5  1.5  years ; 
 height , 170.9  6.1  cm ; weight , 64.4  8.0  kg ) participated in this experiment .
we excluded participants 
 who wore glasses on a daily basis and those with any orthopedic , neurological , ophthalmic or 
 otolaryngological disease .
the ethics committee of the international university of health 
 and welfare approved all study procedures ( no .
15-io-58 ) , which were consistent with the 
 principles of the declaration of helsinki . the authors obtained written informed consent 
 from all the subjects prior to their participation in the study .
ca , usa ) , a small stereo camera 
 ( ovrvision 1 , shinobiya inc .
, osaka , japan ) , and a laptop pc ( 15x8550-i7-vsb , unitcom , 
 osaka , japan ) were employed to constitute a visual inclination system for the experiment 
 ( fig .
( c ) an hmd shows the tilted 
 visual information to the wearer . ) .
the subjects were asked to maintain the standing posture twice for 5  s while 
 wearing the experimental system .
they were instructed to perform the experiment under two 
 visual conditions : normal view and 20 leftward inclined view conditions , in that order .
two 
 force plates were used to measure the vertical component of the floor reaction force of each 
 leg .
a three - dimensional motion analysis system was used to quantify the subjects body 
 movements ( i.e. inclination angles of the head ( h ) , trunk 
 ( t ) , and pelvis ( p ) in 
 absolute coordinates ) .
we adopted several angle definitions , h , 
 t , and p , which are described 
 in fig 2fig .
2.definition of the h , t , and 
 p . the black dots are reflective markers on top of 
 the head , bilateral acromions , anterior superior iliac spine , and posterior superior 
 iliac spine .
point b is the midpoint of points c and 
 d. points c and d are iliac 
 crests estimated from the position of the markers on the pelvis .
points 
 e and f are midpoints of the anterior superior 
 iliac spine and posterior superior iliac spine .
h is the 
 angle between the axis connecting from the top of the head to point a 
 and the vertical axis .
t is the angle between the axis 
 connecting from point a to b and the vertical axis . 

p is the angle between the axis connecting from 
 point e to f and the horizontal plane .. furthermore , we defined the relative head and trunk bending angles as the difference 
 between the inclination angles of the head and trunk , and those of the trunk and pelvis , 
 respectively .
we defined a leftward inclination angle as positive and a rightward 
 inclination angle as negative . visual inclination system .
definition of the h , t , and 
 p . the black dots are reflective markers on top of 
 the head , bilateral acromions , anterior superior iliac spine , and posterior superior 
 iliac spine .
point b is the midpoint of points c and 
 d. points c and d are iliac 
 crests estimated from the position of the markers on the pelvis .
points 
 e and f are midpoints of the anterior superior 
 iliac spine and posterior superior iliac spine .
h is the 
 angle between the axis connecting from the top of the head to point a 
 and the vertical axis .
t is the angle between the axis 
 connecting from point a to b and the vertical axis . 

p is the angle between the axis connecting from 
 point e to f and the horizontal plane .
a three - dimensional motion analysis system consisting of 10 infrared cameras ( vicon mx , 
 vicon , oxford , uk ) and two force plates ( amti , watertown , ma , usa ) was used to record 
 three - dimensional marker displacements and floor reaction force data at a sampling frequency 
 of 100  hz .
thirty - three reflective markers ( helen - hayes marker set ) were attached to each 
 subject . in the analysis , we used seven markers , which were attached to the top of the head , 
 the bilateral acromions , anterior superior iliac spine , and posterior superior iliac spine 
 of the participants .
a two - tailed paired t - test was used to assess individual differences between the inclined 
 view and normal view conditions .
the statistical analysis was conducted using the software package spss version 
 20 ( ibm inc .
the result of the paired t - test demonstrated that the vertical component of floor reaction 
 forces in both legs significantly changed in the inclined view condition .
the vertical 
 component of floor reaction forces decreased in the right leg ( 324.4  38.6 vs. 303.4  51.3 
 n , p=0.03 ) , whereas it increased in the left leg ( 322.3  53.3 vs. 345.9  52.2 n , 
 p=0.02 ) .
table 1table 1.the mean inclination angles of the head , trunk , and pelvis , and the relative 
 bending angle of the head and trunknormal view conditioninclined view conditionhead leftward inclination angle ( )0.2  2.31.7  3.7*trunk leftward inclination angle ( )0.8  0.82.0  0.9*pelvis leftward inclination angle ( )1.2  2.00.9  1.8neck leftward bending angle ( )1.0  2.70.3  3.3trunk leftward bending angle ( )0.4  1.91.1  2.1*values are expressed as a mean  standard deviation . * significant difference 
 ( p<0.05 ) between the normal view condition and the inclined view condition
. represents the mean inclination angles of the head , trunk , and pelvis , and the 
 relative bending angle of the head and trunk . in the comparison between the normal view and 
 inclined view conditions
, the paired t - test indicated that there was a significant increase 
 in the inclination angle of the head and trunk and the relative bending angle of the trunk , 
 but not in the inclination angle of the pelvis and the relative bending angle of the 
 head .
* significant difference 
 ( p<0.05 ) between the normal view condition and the inclined view condition .
in this study , we created a novel hmd system to alter inclined standing posture in a group 
 of male university students . by using this system , the head and trunk of participants 
 inclined leftward and the vertical component of the floor reaction force of the lower 
 extremities inclined leftward due to the view presented on the display .
these results are 
 identical to those of a previous experiment with a large - sized screen16 , 17 .
the present 
 study proves that it is possible to elicit a change of standing posture due to a visual 
 stimulus using an immersive hmd , and a large - scale apparatus is no longer necessary . as for the inclination angle of each body segment , even though the head and trunk angles 
 inclined toward the tilted direction , the angle of the pelvis did not incline .
the reason 
 why inclination in the pelvis did not occur is that it could not physically occur when the 
 participant stood with both legs extended .
therefore , inclination of the pelvis might occur 
 in dynamic movements such as walking .
as for the relative head and trunk bending angles , the effect of the tilted view was 
 confirmed only in the trunk .
the reason why the neck did not bend to the side is that a very 
 large number of muscle spindles were distributed in the neck muscles19 ; therefore , it seemed that these muscle spindles 
 compensated for the proprioceptive sensation .
the previous research about siting posture 
 using an electric balance board also showed lateral bending of trunk and no lateral bending 
 of the neck during tilting the seat surface15 .
these experimental results suggest that lateral bending of the 
 trunk is more available than the neck in the postural control using vision .
the inclination angle of the standing posture observed in the present study was smaller 
 than the tilt angle of view .
it seemed that the reason was compensation by the somatosensory 
 or the vestibular sensory systems .
somatosensory function decreases with age , and therefore 
 the elderly tend to rely on vision for postural control20 , 21 .
thus , it seems that 
 visual inclination has a large effect in the elderly .
in addition , if the presented tilted 
 view is combined with a vibration stimulus or vestibular stimulation , compensation due to 
 the somatosensory and the vestibular sensory systems will be difficult . therefore , the body 
 inclination effect might be enhanced .
when the tilt angle is too small , the 
 effect of inclining the body is low . on the other hand ,
even if the tilt angle is too large , 
 the effect of inclining the body becomes low too , because the subject is no longer trust the 
 view .
there is a previous study examining the effect of the standing posture when changing 
 the tilt angle of the view projected on a large screen17 .
the study compared three tilt angle conditions ( 5.1 , 9.1 and 
 20.1 ) and reported that 20.1 is the most effective .
it suggests that the tilt angle of 
 view in the present study is appropriate .
since the immersive hmd is wearable , unlike large screens or mirrors , its advantage is that 
 it continues to provide visual information to the wearer even when moving forward or 
 changing direction .
therefore , it is possible to use this system to reveal the effects of 
 the tilted view during walking or turning movements .
furthermore , we may be able to use the 
 visual inclination system for balance exercises to treat vertical misperception in brain 
 disease patients .
the balance exercises should be adjusted to the degree of difficulty for 
 maintaining balance for individuals22 . if 
 patients who can not maintain an upright posture because of severe impairment of vertical 
 perception , it may be easier for them to hold the upright posture by seeing a view that is 
 inclined opposite to the inclination of the vertical axis that the patient is aware of .
on 
 the other hand , there are patients with unstable gait due to the mild impairment of vertical 
 perception . for such patients , the balance exercises in normal visual conditions are less 
 effective because the degree of difficulty is low . thus , presenting an inclined view that 
 emphasizes the inclination of the vertical axis that the patients are aware of might be able 
 to increase the degree of difficulty of the balance exercises to an appropriate level . 

moreover , if the presented tilted view is combined with vibration and vestibular stimuli , 
 the training effect may be increased further10,11,12 .
the first is the possibility that the weight of the hmd 
 affected somatosensory perception , although it was truly a lightweight device .
use of a 
 lighter hmd may increase the effect of inclination in the standing posture .
the second is 
 that we did not strictly define the visual environment in the experimental room .
however , we 
 wanted to ensure that our results would not be affected by the presence or absence of 
 vertical products .
the third is that what we measured is only the effect of the presentation 
 of the leftward tilted view in order to avoid the burdens of the subjects .
inclination of standing posture due 
 to brain disease is likely to occur more toward the left than right6 .
therefore , the effect of the presentation of the rightward 
 tilted view may be less than that of the leftward tilted view .
the fourth is that we did not 
 measure the sensory modality that the subject was focused upon .
a method to determine the 
 sensory modality that the subject focused on should be developed .
the fifth limitation is 
 that we did not consider the effects of aging , since our study group consisted of young male 
 participants .
however , the current study revealed that presenting a tilted view using immersive hmd can 
 shift the relative trunk bending angle and center of gravity toward the same direction of 
 the tilted view .
the developed visual inclination system seems useful and it can be 
 applicable to various psychophysical experiments in future research .
 [ purpose ] the purpose of the present study is to clarify whether tilted scenery presented 
 through an immersive head - mounted display ( hmd ) causes the inclination of standing 
 posture . 
 [ subjects and methods ] eleven healthy young adult males who provided informed 
 consent participated in the experiment . 
 an immersive hmd and a stereo camera were employed 
 to develop a visual inclination system . 
 the subjects maintained a standing posture twice 
 for 5s each while wearing the visual inclination system . 
 they performed this task under 
 two conditions : normal view and 20 leftward tilted view . 
 a three - dimensional motion 
 analysis system was used to measure the subjects postures , and two force plates were used 
 to measure the vertical component of the floor reaction force of each leg . 
 [ results ] in 
 the 20 leftward tilted view , the head and trunk angles in the frontal plane were 
 similarly inclined toward the left , and the vertical component of the floor reaction force 
 increased in the left leg , whereas it decreased in the right leg . 
 [ conclusion ] when the 
 view in the immersive hmd was tilted , the participants trunk side bent toward the same 
 side as that of the view . 
 this visual inclination system seems to be a simple intervention 
 for changing standing posture . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION