ISSN 2321-1822
Original Article
EFFECTS OF VIEW I N G DISTAN CE AN D HEAD FLEXI ON ON
POSTURAL CONTROL DURING ONE AND TWO-LEGGED STANCE
Osamu Aoki
* 1, Yoshitaka Otani
2, Shinichiro M orishita
3, Kazuhisa Domen
3.
* 1 Facult y of Rehabilitat ion, Shijonawate Gakuen Universit y, 5-11-10 Houjo, Dait o, Osaka, Japan.
2 Facult y of Rehabilitat ion, Kobe Internat ional Universit y, 9-1-6 Kouyocho-naka, Higashinada, Kobe,
Hyogo, Japan.
3 Depart ment of Physical M edicine and Rehabilitat ion, Hyogo College of M edicine, 1-1 M
ukogawa-cho, Nishinomiya, Hyogo, Japan.
Background: Short view ing distances and head flexion decrease and increase post ural sw ay, respect ively. Few st udies have exam ined t he effect s of t hese fact ors during one-legged st ance or volunt ary body leaning w it hin t he base of support , bot h of w hich oft en occur in daily lif e. The purpose of t his st udy w as t o exam ine t he eff ect of view ing dist ance and head flexion in sever al post ur al cont rol condit ions.
M aterials and M ethods: Fift een healt hy young subject s part icipat ed in t his st udy, and cent er of pressure (CoP) displacem ent w as m easured in five condit ions (gazing 600 cm f orw ard, 150 cm forw ard, dow nw ard, forw ard w it h eyes closed, and dow nw ard w it h eyes closed) during t w o- and one-legged st ances and volunt ary body leaning. M easurem ent s included t hat t he root m ean square (RM S) of t he ant eropost erior (A-P) and mediolat eral (M -L) direct ions during t w o- and one-legged st ance, and m axim um A-P and M -L dist ances during volunt ary body leaning.
Results: Our result s show ed t hat t he M -L RM S of 150 cm w as less t han t hat of 600 cm during one-legged st ance (p = 0.01). M oreover, t he A-P and M -L RM S values of dow nw ard gazing were low er t han t hose of 600 cm (A-P RM S: p = 0.003 and M -L RM S: p = 0.002). The M -L dist ances of 150 cm and dow nw ard w ere larger t han t hat of 600 cm (p = 0.002 and p < 0.001, r espect ively).
Conclusions: Our findings suggest t hat t he effect of view ing dist ance was m ore evident during one-legged st ance and volunt ary body leaning.
KEY W ORDS: Balance, Vision, Stance, Head f lexion, Post ural sw ay.
ABSTRACT
INTRODUCTION
Address for correspondence: Dr. Osamu Aoki, Facult y of Rehabilitat ion, Shijonaw ate Gakuen Universit y, 5-11-10 Houjo, Dait o, Osaka 574-0011, Japan. Tel.: +81-72-863-5043,
Fax: +81 72 863 5022, E-M ail: aoki-o@umin.ac.jp
International Journal of Physiotherapy and Research, Int J Physiother Res 2015, Vol 3(5):1215-20. ISSN 2321-1822 DOI: ht t p:/ / dx.doi.org/10.16965/ ijpr.2015.179
Quick Response code
Access this Article online
International Journal of Physiotherapy and Research
ISSN 2321- 1822
w w w.ijm hr.org/ ijpr.htm l
DOI: 10.16965/ ijpr.2015.179
Received: 12-08-2015 Peer Review : 12-08-2015 Revised: None
Accept ed: 03-09-2015 Published (O): 11-10-2015 Published (P): 11-10-2015
The i nt egr at i on o r i n t er act i o n o f v i sual , vest ibular, and propriocept ive informat ion is necessary for successful balance cont rol in a st an di ng p osit io n [ 1] . In par t icul ar, vi su al informat ion is crit ical for balance cont rol, and a num ber of st udies have dem onst rat ed t hat
ISSN 2321-1822 sho r t co n si d er i n g t h e d i st an ces t yp i cal l y encount ered in dai ly living. Test ed view ing dist ances have ranged from 10 t o 100 cm [7] or 40 t o 200 cm [4, 5]. In fact , only Bles et al. [2] assessed long view ing dist ances (from 0.5 t o 200 m), but t hey only included six subject s. In t his st udy, w e tested w het her relat ively long view ing dist ances (from 150 t o 600 cm, w hich w e oft en encount ered inside rooms or buildings) affect post ural sway.
The effect of head ext ension/ flexion on post ural sw ay has also been evaluat ed and w as show n t o influence visual and vest ibular informat ion [2, 8-11]. In healthy elderly subject s, head flexion increased post ural sw ay w hen t heir eyes w ere opened [8, 10] and closed [ 10]. It has been hypot hesized t hat t he vest ibular informat ion disrupt ion during head flexion led t o post ural inst abilit y in elderly subj ect s. That is, head flexion changed t he ut ricular ot olit h posit ion beyon d t heir w o r ki ng ran ge [ 9] . We w ere int erest ed in exam ining t he ef f ect of head flexion on post ural sw ay because head flexion commonly occurs during st anding and w alking. The effect of head extension/ flexion and view ing dist ance on post ural sw ay are m ore obvious w hen subject s st and on a foam rubber t han on a firm plat form [2, 10]. In daily act ivit ies, w e oft en adopt a one-legged st ance, such as w hen w e put on socks. One-legged st ance is a more unst able condit ion t han t w o-legged st ance. M oreover, t his condit ion is reproducible and more sensit ive for visual informat ion needed t o maintain balance [12]. We t herefore post ulated t hat t he effect s of view ing dist ance and head flexion on post ural sw ay w ould also becom e more obvious during one-legged st ance.
Functionally, balance is divided int o t hree levels: post ure maint enance during st anding, cont rol of t he cent er of mass (CoM ) during act ivit ies such as t urning and reaching, and t he maint enance of CoM w it hin t he area over t he base of support in response t o a dest abilizing force [13]. Wit h regard t o t he second aspect of post ural cont rol, i t i s a m o r e chal l en gi n g co nd i t io n t h an one-legged stance, and it has been report ed t hat d yn am ic p ost u r o gr ap h y can p r edi ct t h e li kelihood of falling in elder ly people [ 14] . Although this study assessed a young population, know ledge of t he effect s of view ing dist ance
and head posit ion on a dynam ic balance t o cont rol t he CoM w it hin t he base of support during leaning could be useful for prevent ing falls.
The purposes of t his st udy w ere t o examine t he effect s of view ing dist ance and head posit ion on post ural sw ay and t o det erm ine w het her these effects were more pronounced during one-legged stance by healt hy young subject s. We also examined the effect of view ing dist ance and head posit ion w hile t he subject s leaned w it hin t heir base of support .
M ATERIALS AND M ETHODS
Sub j ect s: Fi f t een h eal t hy yo u n g su b j ect s part icipat ed in t his st udy (age [mean ± SD]: 26.3 ± 4.0 years, BM I: 20.2 ± 1.6 kg/ m2, visual acuit y:
1.1 ± 0.4, m ale [n]: 7). All subject s w orked at t he hospit al w here t his st udy w as conduct ed. The exclusion crit eria w ere a hist ory of vert igo or dizziness, vest ibular neurit is, or neurological disorders and ort hopedic diseases of t he neck, t runk, or low er limbs.
Ethics: This st udy w as approved by t he et hics committ ee of t he hospit al. All subject s provided w rit t en informed consent prior t o part icipat ion.
2.3 Procedure
A force plat e equipped w it h a dat a processor (Anim a Co. Lt d., Tokyo, Japan) w as used for m easu r in g cen t er o f p r essu r e (Co P) displacement , and t he sampling rat e w as set at 50 Hz. M easurem ent s w ere m ade in a large rehabilit at ion room wit h moderat e light and low noise levels. The cumulat ive CoP displacement w as m easured in five condit ions (Fig. 1a–e) during t w o- and one-legged stance (Fig. 1A– B). The maximum CoP displacement dist ance w as m easu r ed in t he sam e f i v e co n di t i on s (Fig. 1a–e) during t w o-legged stance (Fig. 1C).
ISSN 2321-1822 (closed-downward condition). The t wo- and one-l egged st an ces w er e as f o one-lone-l o w s d u r i n g cumulat ive CoP displacement measurement : in t w o-legged st ance, t he subject s st ood barefoot on the force plat e w it h t heir arms alongside their t runk and w it h t heir feet t oget her. In t he one-legged st ance, t he subject s st ood only on t heir right leg because t hey report ed no effect for leg preference [12, 15]. In bot h st ances, subject s w ere inst ruct ed t o st and as st ill as possible, and t h eir cu m ul at i ve CoP d isp lacem ent s w er e r ecor ded fo r 30 seco nd s. To m easur e t h e maximum CoP displacement dist ance, subject s w ere required t o m axim ally lean t heir body forw ard, backward, right ward, and then left ward w ithout any part of t heir feet off t he floor during t w o-legged st ance [ 13, 16] . Subj ect s w er e required t o maintain for 10 seconds each leaning posit ion.
St ances w ere performed in t he follow ing order: t w o-legged, one-legged, and t w o-legged w it h leaning. Tw o t rials in each of t he five condit ions were performed randomly, and the subjects were allow ed t o rest for 30 s bet w een condit ions. The mean of t w o t rials w as used for dat a analysis.
Fig. 1: Schem at ic representat ion of t he m easurem ent condit ions.
M easurem ent s w ere conduct ed in five condit ions (a t o e) during t hree st ances (A t o C).
A: Tw o-legged stance. B: One-legged stance.
C: Voluntar y body leaning in t he a: 600-cm , b: 150-cm , c: dow nw ar d, d: closed-f orw ar d, and e: closed-dow nw ard condit ions.
u sed as p ar am et er s t o cap t u r e t h e Co P fluct uat ion charact erist ics (A-P RM S and M -L RM S, respect ively) [2, 8, 16]. The maximum CoP displacem ent dist ances in t he A-P and M -L direct ions w ere calculat ed from t he cent er of 10-s CoP data at each leaning position (betw een f o r w ar d an d b ack w ar d , A-P di st an ce an d right w ard and left w ard, M -L dist ance).
Statistical Analysis: To evaluate t he effect of view ing dist ance, t he A-P and M -L RM S values w ere compared bet w een t he 600- and 150-cm condit ions for t w o- and one-legged stances. To evaluat e t he effect of dow nw ard gazing, t he A-P and M -L RM S v al ues w er e com p ar ed bet w een t he 150-cm and dow nw ard condit ions in each stance. To assess t he combined effect of view ing dist ance and dow nw ard gazing, t he A-P and M -L RM S v al ues w er e com p ar ed bet w een t he 600-cm and dow nw ard condit ions in each stance. Finally, t o evaluate t he effect of head flexion w ithout vision, the A-P and M -L RM S values w ere com pared bet w een t he closed-forw ard and closed-dow nw ard condit ions.
The evaluat ion of A-P and M -L dist ances had t he sam e aim as t hose of A-P and M -L RM S v al u es, t h e co m par iso ns w er e co nd u ct ed bet w een t he same condit ions as described for t he comparisons of A-P and M -L RM S above.
Pair ed t -t est s w er e used f or com par isons. St at ist ical signif icance w as set t o 0.05, and Holm’s met hod [14] was used t o adjust t he p values t o cont rol t he family w ise error rat e. All st at ist ical procedures w ere conduct ed using JM P 10.0.2 soft w are (SAS Inst it ute, Cary, NC, USA).
RESULTS
M easurem ents: During t w o- and one-legged st ances ant eropost erior (A-P) and mediolat eral (M -L) direct ions of root mean square (RM S) were
ISSN 2321-1822
Fig. 2: Ant eropost er ior and m ediolat eral RM S values for each condit ion during t w o- and one-legged stance.
Bars and w hiskers represent t he m ean values and standard deviat ions, respect ively.
An ast er isk (* ) denot es st at ist ical significance bet w een condit ions (significance in t he pair ed t -t est w as accept ed at a correct ed P value of 0.05 w it h Holm ’s m et hod).
A-P and M -L distances: The M -L dist ance of t he 150-cm condit ion w as higher t han t hat of t he 600-cm condit ion (p = 0.002). In evaluat ing t he com bined ef fect of view ing dist ance and dow nw ard gazing, t he M -L dist ance for t he dow nw ard condit ion w as higher t han t hat of t he 600-cm condit ion (p < 0.001). For head flexion w it hout vision, t he A-P dist ance for t he closed-dow nw ard condit ion w as great er t han t hat of t he closed-forw ard condit ion, but t his difference did not reach t he stat ist ical significance (p = 0.051, Table 1).
Table 1: Ant eropost erior and m ediolat eral distances bet w een condit ions.
Values are t he m ean ± SD of A-P and M -L distance of each condit ion.
An ast er isk (* ) denot es st at ist ical significance bet w een condit ions (significance in t he paired t -t est w as accept ed at a correct ed p value of 0.05 w it h Holm ’s m et hod).
DISCUSSION
t h e t w o-legged st ance w er e i ncr eased byapproximat ely 130 t o 170% and 190 t o 260% from t hose in the one-legged st ance w it h vision, and by approximat ely 200 t o 290% and 380 t o 400% from t hose w it hout vision, respect ively (Fig. 2).
For one-legged st ance, w e found t hat t he M -L RM S for t he 150-cm condit ion w as low er t han t hat for t he 600-cm condit ion. This result is
A-P and M -L RM S: We observed differences for view ing dist ances only w hen subject s adopt ed a on e-legged st an ce. Th is i s p r esu m ab l y b ecau se t w o-l egged st an ce w as n ot a challenging condit ion for healt hy young subject s, so t he effect s of view ing dist ance w ere not emphasized in t he t w o-legged st ance [2, 10]. Indeed, t he mean A-P and M -L RM S values in
p = 0.003, t = -3.62; M -L RM S: 4.8 ± 1.1 mm and 5.9 ± 1.4 mm, p = 0.002, t = -3.69). No significant differences w ere observed for t w o- and one-legged st ances for t he effect of head flexion w it hout vision (Fig. 2).
p value t value p value t value
6 00 cm vs. 1 50 cm 127.4 ± 26.3 133.4 ± 23.0 0.105 -1.73 192.4 ± 39.6 211.1 ± 41.8 0.002* -3.75
15 0 cm vs. dow nw ard 133.4 ± 23.0 131.2 ± 27.1 0.574 0.58 211.1 ± 41.8 205.4 ± 38.5 0.262 1.17
60 0 cm vs. dow nw ard 127.4 ± 26.3 131.2 ± 27.1 0.181 -1.41 192.4 ± 39.6 205.4 ± 38.5 < 0.001* -6.07
c-forw ard vs. c-dow nw ard 118.5 ± 32.6 122.1 ± 29.7 0.051 -2.13 188.4 ± 48.2 192.5 ± 49.9 0.251 -1.2
ISSN 2321-1822
condit ion w as significant ly low er t han t hat of t he 600-cm condit ion, w hich is consist ent w it h our findings from t he RM S comparison. Le and Kapoula [5] report ed t hat t he effect of view ing dist ance w as more evident in A-P t han M -L body sw ay because A-P body sw ay caused angular size variat ion on t he ret ina. In cont rast , Guerraz et al. [3] report ed t hat t he effect of view ing dist ance w as more evident in t he M -L direct ion, and t hey post ulat ed t hat t his w as due t o ext ra-ocular muscle propriocept ion [21] and a sense of m ot ion parallax. Furt herm ore, ot hers have reported t hat t his effect can be observed in bot h t he A-P and M -L direct ions [2, 4, 21]. We found t hat w hen subject s volunt arily moved in A-P and M -L direct ions, the sense of mot ion parallax w as more obvious during M -L movement (i.e., M -L movement yielded ret inal slip but A-P movement only generated vergence). We post ulated t hat mot ion parallax w as more evident for det ect ing M -L movement ; t herefore, t he increment in t he M -L dist ance w as observed for t he view ing dist ance effect .
The M -L dist ance of t he dow nw ard condit ion w as larger t han t hat of t he 600-cm condit ion. M oreover, t he M -L distance was not significantly different bet w een t he dow nw ard and 150-cm condit ions. These result s might be due t o t he view ing dist ance effect as descr ibed above. How ever, t he A-P dist ance was not different betw een the 600-cm and dow nw ard condit ions. Buckley et al. [10] report ed t hat the A-P direct ion of CoP w ould be moved forward by head flexion. We post ulated t hat t his change in CoP locat ion i nh ib it ed vo lu nt ar y m ov em en t i n t h e A-P direct ion (especially backw ard movement ), and t hat t his result ed in no change in A-P dist ance, even if t here w as a view ing dist ance effect .
Our r esult s indicat e t hat view ing dist ance improved bot h post ural cont rols during one-legged stance and volunt ary body leaning w it hin t he subject s’ bases of support . These findings suggest t hat opt imizing view ing dist ance could h el p pr ev en t f al l s. Th e do w n w ar d gazin g condit ion also improved post ural cont rol abilit y compared t o t he far-forw ard gazing condit ion, w hich indicat es t hat t his im provem ent w as sol ely du e t o t he view ing d ist ance ef f ect . M oreover, t his improvement during dow nward gazing might only be observed in young people consist ent w it h findings report ed in a previous
st udy in w hich t he effect of view ing dist ance w as only observed in t he M -L direct ion of CoP sw ay [17]. Our result s also show ed t hat t he A-P and M -L RM S values of t he dow nw ard condit ion w ere low er t han t hose of t he 600-cm condit ion, but no difference w as observed bet w een t he dow nward and 150-cm condit ions. These result s indicat e t hat t he low post ur al sw ay in t he dow nw ard condit ion might be at t ribut ed t o t he view ing dist ance effect . Because t he dow nw ard and 150-cm condit ions had sim ilar view ing dist ances when consider t o t he subject s’ height s (150–175 cm), t here seemed t o be differences bet w een condit ions due t o head flexion. Our f i ndi ng of a dif f er ence i n A-P RM S v alu es bet w een t he dow nw ard and 600-cm condit ions might support our assumpt ion of a predominant view ing dist ance effect. The decreased A-P RM S in t he dow n w ar d con dit i on seem ed t o be explained by t he finding t hat t he det ect ion t hreshold during A-P sw ay w as low er t han t hat in t he f orw ard condit ion (i.e., up-dow n eye movement during dow nw ard gazing could more easily det ect subt le sw ay t han vergence during forw ard gazing) [7, 8].
M ost st udies t hat have examined t he effect s of head flexion on post ural sw ay report ed t hat t he post ural sw ay w as increased [2, 8, 10], w hich is in cont rast w it h our result s. It w as hypot hesized t h at t hi s bal ance lo ss occu r r ed b ecau se disr upt ed vest ibular inform at ion w as used. Alt hough t he vest ibular syst em is considered t o w ork predominant ly under conflict ing sensory condit ions [9, 18, 19], Kogler et al. [20] report ed t hat t he balance score for young subject s w as n o t si gn i f ican t l y al t er ed f o r sev er al neck posit ions (i.e., neut ral, forw ard, right , and left ). We found t hat t he RM S values for t he closed-forw ard and closed-dow nw ard condit ions w ere not different . We post ulated t hat t he condit ions in t his st udy did not creat e a sensory-conflict ing sit uat ion for our young subjects; t herefore, t hey did not use t he disrupt ed vest ibular informat ion for post ural cont rol.
ISSN 2321-1822 because t hey t end t o have minimal degenera-t ion of sensory modalidegenera-t ies such as vision, propr-iocept ion, and t he vest ibular syst em. Furt her st udies of frail elderly or physically im paired populations were needed t o clarify these effect s.
CONCLUSION
In conclusion, t he result s of t he present st udy indicat e t hat t her e is an ef f ect of view ing dist ance on post ural cont rol during one-legged st ance and volunt ary body leaning in young subj ect s, but t his eff ect w as not observed during t w o-legged stance. We did not note any ef f ect of h ead f lexion, w hich d et er ior at es post ural cont rol abilit y, in t his st udy.
ACKNOW LEDGEM ENT
The aut hors are grateful t o Naoki Yoshida at t he Inst it ut e of Rehabilit at ion science for t he helpful suggest ions, and also Tom ohiro Hirot a at t he Hyogo prefect ure r ehabilit at ion hospit al at Nishi-Harima for helping t he dat a acquisit ion.
Conflicts of interest: None
REFERENCES
[8] . Aoki O, Ot ani Y, M or ishita S, Dom en K. Influence of gaze dist ance and dow nw ard gazing on post ural sw ay in hem iplegic st r oke pat ient s. Experim ent al brain research. 2014;232(2):535-43. doi:10.1007/ s00221-013-3762-3.
[ 9] . Br andt Th, Kr af czyk S, M alsbenden I. Post ur al im balance w it h head ext ension. im provem ent by t raining as a m odel for ataxia t herapy. Annals New Yor k Academy of Sciences. 1981;374:636-49. [10]. Buckley JG, Anand V, Scally A, Elliott DB. Does head
ext ension and flexion incr ease post ural inst abilit y in elderly subject s w hen visual inf orm at ion is kept co n st an t ? Gai t & p o st u r e. 2005;2 1(1):59-64. doi:10.1016/ j.gait post .2003.11.005.
[11]. Jackson RT, Epst ein CM . Effect of head ext ension on equilibr ium in norm al subject s. Ann Ot ol Rhinol Lar yngol. 1991;100:63-7.
[12]. Goldie PA EO, Bach TM . St eadiness in one-legged st ance: developm ent of a r eliable force-plat f orm t est i n g p r o ced u r e. Ar ch Ph y s M e d Reh ab i l . 1992;73:348-54.
[13]. King M B, Judge JO, Wolfson L. Funct ional base of support decreases w it h age. Journal of geront ology. 1994;49(6):M 258-M 63.
[14]. Girardi M KH, Am in M , Hughes LF. Predict ing fall risks in an elder ly populat ion: Com put er dynam ic post urography versus elect ronyst agm ography t est result s. Lar yngoscope. 2001;111(9):1528-32. [15] . Goldie PA, Bach TM , Evans OM . Force plat form
m easu r e s f o r e val u at i n g p o st u r al co n t r o l : reliabi lit y and validit y. Arch Phys M ed Rehabi l. 1989;70(7):510-7.
[ 16] . Ow ings TM , Pavol M J, Foley KT, Gr abiner M D. M easures of post r ual st abilit y are not pr edict ors of r ecover y fr om lar ge post r ual dist ur bances in healt hy older adult s. J Am Geriat r Soc. 2000;48:42-50.
[17]. Fox CR. Som e visual influences on hum an post ural equilibr ium binocular versus m onocular f ixat ion. Per cept ion & Psychophysics. 1990;47(5):409-22. [18]. Black FO, W all III C, Nashner LM . Effect s of visual
and support surface orient at ion ref erences upon post ural cont rol in vest ibular deficient subject s. Act a Ot olaryngol. 1983;95:199-210.
[ 19] . Nashner LM BF, Wall C. Adapt at ion t o alt ered su p po r t and vi sual co nd i t i o ns d u r in g st an ce: Pat i en t s w i t h vest i b u l ar d ef i ci t s. J Neu r o sci . 1982;2(5):536-44.
[20]. Kogler A, Lindf ors J, Odkvist LM , Ledin T. Post ural st abilit y using different neck posit ions in nor m al su bject s and pat ient s w it h neck t r aum a. Act a Ot olar yngol. 2000;120:151-5.
[ 21] . Paulus W SA, Kraf czyk, Br andt TS. Dif fer ent ial ef-fect s of r et inal t arget displacem ent , changing size and changing diparit y in t he cont rol of ant erior / post erior and lat eral body sw ay. Exper im ent al brain research. 1989;78:243-52.
[1] . Berg K. Balance and it s m easure in t he elderly: a review. Physiot herapy canada. 1989;41(5):240-6. [2]. Bles W, Kapt eyn TS, Brandt T, Arnold F. The
mechan-i sm o f p h y smechan-i o l o gmechan-i cal h e mechan-i gh t v e r t mechan-i go . II . Post urography. Acta Ot olar yngol. 1980;89:534-40. [3] . Guerraz M , Sakellari V, Burchill P, Bronst ein AM . Inf luence of m o t ion par all ax in t he cont r ol of sp o n t an eo u s b o d y sw ay. Exp er i m en t al b r ai n r ese ar ch . 2 0 0 0; 1 31 ( 2 ): 2 4 4- 5 2. d o i : 1 0. 1 0 07/ s002219900307.
[4]. Kapoula Z, Le TT. Effect s of dist ance and gaze posit -ion on post ural st abilit y in young and old subject s. Experim ent al brain research. 2006;173(3):438-45. doi:10.1007/ s00221-006-0382-1.
[5]. Le TT, Kapoula Z. Distance im pairs postural st abilit y only under binocular view ing. Vision resear ch. 2 00 6 ; 46 ( 2 1) : 3 58 6 -9 3 . d o i :1 0 .1 0 1 6/ j .vi sr es. 2006.06.018.
[ 6] . M or aes R LA, Bar ela JA. M onocular vision and increased dist ance reducing t he effect s of visual m anipulat ion on body sw ay. Neuroscience lett ers. 2 0 0 9 ; 4 6 0 ( 3 ) : 2 0 9 - 1 3 . d o i : 1 0 . 1 0 1 6/ j . n e u l e t .2009.05.078.
[7]. Paulus WM , St raube A, Brandt Th. Visual st abilizat ion of post ure. Physiological st im ulus charact erist ics and clinical aspect s. Brain : a jour nal of neurology. 1984;107:1143-63.
