Walking There Enviromental Influence on Walking Distance Estimation

8/9/2019 Walking There Enviromental Influence on Walking Distance Estimation

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Behavioural Brain Research 226 (2012) 124132

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Behavioural Brain Research

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Research report

Walking there: Environmental inuence on walking-distance estimation

M. Iosa , A. Fusco, G. Morone, S. PaolucciClinical Laboratory of Experimental Neurorehabilitation, Fondazione Santa Lucia I.R.C.C.S., Rome, Italy

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Article history:Received 3 July 2011Received in revised form 30 August 2011Accepted 4 September 2011Available online 12 September 2011

Keywords:Path length estimationTarget-directed walkingLocomotor body schemaGaitDistance perceptionEnvironmental cues

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In a dark environment, when vision is excluded, humans are usually able to walk towards a target theposition of which was previously memorized. Changes in spatio-temporal gait parameters, the presenceof obstacles on the ground or pathway tilt can affect their performances. The aim of this study was

to

investigate

the

inuence

of

the

environment

on this

ability.

We

have

enrolled

sixty

healthy

subjects,separately tested in a small indoor and in an outdoor open-eld environment. In experiment 1, signicantdifferences were found between 15 indoor and 15 outdoor blindfolded walkers. According to previousstudies, the distances walked outdoors were not signicantly different from the three-tested targetsdistances (3 m, 6 m and 10m). Conversely, a systematic and signicant undershooting was observed forblindfolded indoor walkers for all the three distances (errors: 0.34, 0.73 and 1.99 m, respectively).This indoor undershooting was found related to shorter steps not compensated byany increment of thestep number. In experiment 2, also the perception of the indoor distance resulted underestimated inother two tested groups of 15 subjects each. But the perceived distance resulted poorly correlated withmotor performances (R = 0.23, p = 0.410). In spite of the fact that the errors were consistent among trials,when indoor walkers could not access to environmental acoustic features, their performance resultedhighlyvariable among subjects, but it improved, on average. Atthe light of these results, the environmentseems acting as a selective tuning between different strategies.

2011 Elsevier B.V. All rights reserved.

1. Introduction

Slowly, he opened the door and walked into the dark room . Peo-ple are usually accurate in walking without vision to previouslyseen targets, such as above described for Father Frollo, the famouscharacter of Victor Hugos The Hunchback of Notre Dame.

Many studies have investigated this ability that requires theminimization of the distance between the body position, contin-uously updated during walking, and the memorized position of apreviously seentarget.Visionimprovesbody stabilityduringstand-ing and locomotion, and it drives gaitcycle modulation, navigation,and obstacle avoidance [13] . However, when there are no visualinformation,other senses could be used, such as acoustic feedback,tactile exploration or internally generated self-motion signals fordetermining own current position and orientation [1,15] . Accurateperformances of target-directed blind walking were recorded fordistances out to 24m under open eld conditions [21] , even forpassive translations [9,15] .

Abbreviations: WS, walking speed; WD, walked distance; RD, real distance; N steps,number of steps; SL,step length; SF,step frequency; RMS, root mean square;HR, harmonic ratio; AP, antero-posterior; LL, latero-lateral; CC, cranio-caudal. Corresponding author. Tel.: +39 06 51501005; fax: +39 06 51501004.

E-mail address: [email protected] (M. Iosa).

This shows that efferent, proprioceptive and vestibular infor-mation about locomotion could be closely calibrated to visuallyperceived path, especially for athletes [3] and young subjects [14] .

To explain the ability to estimate the distance walked whileblindfolded, it has been hypothesized the existence of an inter-nal model, called locomotor body schema [7] . This should combinethe internalized knowledge of body segment lengths with the per-ceived exo-extensions of lower limb joints during the gait cycle,allowing humans to estimate the length of their steps. In fact, thesteplengthis theresult of a complexrelationship betweenhip, kneeand ankle angular positions and thigh, shank and foot lengths. Andthe walked distance is the result of the combination of the steplength and the number of performed steps.

However, this target-directed blind walking ability was foundaltered if subjects were asked to walk out of self-selected condi-tions.Whensubjectswereaskedto walk at slower/faster speeds [3]or with shorter/longer steps [15] , they tended to over/undershootthe target. On the other hand, faster self-selected speed after manytask repetitions were observed associated with overshooting [19] ,while undershootingwas observed in subjects walking on stilts [7] .

From a perceptive point of view, our capacity to judge thedistanceof a targetfromus canbe affectedby thesurrounding envi-ronment [12] . For example, this distance appears greater when itis near the end of a hallway than when farther from its end [20] , orwhen the pathway is ascending and hence requiring more effort to

0166-4328/$ seefrontmatter 2011 Elsevier B.V. All rights reserved.

doi: 10.1016/j.bbr.2011.09.007
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126 M. Iosa et al. / Behavioural Brain Research 226 (2012) 124132

Fig. 1. Thetwo experimental environments:indoor (above) and outdoor (below). The two grounds weresimilar in dimensions (indoor: 18 m 4 m; outdoor: 18.4m 4.4m)and paved by tiles similar in dimensions (indoor: 0.44 m 0.44m; outdoor: 0.40 m 0.40m), colours (light grey forboth of them), and material.The indoor environment is a long hall with two windows on the short sides, and doors on lateral long walls (kept closed during experiments). Distance between posteriorwall andstarting line was 1.5m, whereas thedistance between theline of 10m and frontal wall was 6.5m.The outdoor environment is a little paved place in the park of our hospital, encompassed by lawn. Its global dimensions are 18.4m 8.4m, however, a large strip (0.4m)formed by marble slab (different in colour from the surrounding tiles) divided it into a side large 3.6m and another one, in which experiments were conducted, of 4.4m.Distance between posteriorborder and starting line was2 m, and between the lineof 10m and anteriorborderwas 6.4m. Themost close obstacleswere trees,distant morethan 20m further the lineof 10m along the walking line and morethan8 m onthe left side of subjects.

walkers (for 3 m, 6m and 10m, respectively). Higher errors werehence observed indoors and higher inter-subjects variability out-doors. In fact, the standard deviations observed for the errorsperformed outdoors were generally higher than the ones observedindoors. This difference in terms of variance resulted signicantfor the distance of 3 m as revealed by Levenes homogeneity tests( p = 0.026, Table 1 ).

A repeated measure ANOVA showed that errors were sig-nicantly affected by the environment (between subject factor,main means: 1.02 1.04m indoor vs 0.03 1.11m outdoor, p < 0.001), by the distance (within subject factor: p = 0.001), and bytheinteractionofthesetwo factors( p = 0.003,as detailedin Table1 ).In fact, the undershooting was higher indoor and for longer dis-tances, as shown in Fig.3 . When the errors were evaluatedin terms

Fig. 2. Antero-posterior accelerometric signal for a male subject enrolled in the experiment 2 during the rst trial of 6 m test performed indoor. The methods to compute

gait

spatio-temporal parameters from this signal andmeasured walked distance (WD) are reported forthis exemplicative case.


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M. Iosa et al. / Behavioural Brain Research 226 (2012) 124132 127

Fig. 3. Mean andstandarddeviationof theerrorrecordedfor indoor(black circles)and outdoor walkers (greycircles) during blindfoldedwalking of experiment1. Theerror wasdenedas thedifferencesbetween walked distance(WD)and realdistanceto walk (RD), in respect of the three tested distances. Circles are joined by a 2ndorder polynomial t.Stars indicated a p < 0.008 (=0.05/6 for Bonferronis correctionon six comparisons) obtained by one-sample two-tail t -tests performed to assessthe difference from an ideal error of 0.

of percentage of the distance to walk, they resulted affected onlyby the environment.

The environment did not signicantly affect the step numbers,the time spent walking and the step frequency. Conversely, therecorded walking speed and step length were signicantly higherfor outdoor walkers than for indoor ones (see Table 1 and Fig. 4).Post hoc analyses ( p-threshold set at 0.025 for Bonferronis cor-rection) revealed that these differences resulted signicant for thedistances of 10m (walking speed: p < 0.001; step length: p = 0.004)and 6 m ( p = 0.022; p = 0.007), but not for that of 3 m ( p = 0.057; p = 0.175). Different mean velocities were also observed among thethree distances (effect of distance: p < 0.001), with the slowest onerecorded for the distance of 6 m both indoors and outdoors, as evi-

dent in Fig. 4 . This gure shows that for both indoor and outdoorsubjects, higher walking speed and step length were associated tolonger walked distances.

3.2. Effects of environment and vision on 10 m walking

During open eyes walking towards a visible target (a task per-formed after blindfolded walking), the analyses (repeated measureanalyses of variance reported in Table 2 , relevant post hoc anal-yses in Table 3 ) showed the absence of signicant differencesbetween indoor and outdoor walkers in terms of spatio-temporal

gait parameters, upper body stability and gait rhythmicity. Con-versely, when visual information was removed, signicantly lowervalues of spatio-temporal parameters were observed in indoorwalkers.

The indoor walkers performed errors signicantly correlatedwith their walking speed ( R=0.55, p = 0.034): slowerwas their gait,shorter was the walked distance, undershooting the target. On theother hand, theoutdoorerrors resultedsignicantlycorrelatedonlywith the numberof steps( R= 0.63, p = 0.013), and notwith the otherparameters, such as the walking speed ( R= 0.44, p = 0.104).

Lower upper body accelerations along the three body axes wererecorded for blindfolded indoor walkers, related to their reducedWS, in respect of blindfolded outdoor walkers. In terms of gaitrhythmicity, it is noteworthy the signicant interaction of environ-ment andvisionon the HR inLL direction. Duringopeneyeswalkingit was higher for indoor than outdoor walkers (see Tables 2 and 3 ),the opposite occurred in blindfolded condition, with all the threeHR values quite higher outdoors than indoors.

Neither thenumber of steps northe time spent walkingresultedsignicantly different between indoor and outdoor walkers (seeTable 2 ). For the distance of 10m, indoor walkers performed anumber of steps not signicantly different in closed and open eyesconditions (post hoc analysis: 15.7 1.4 vs 16.4 2.4, p = 0.215).Conversely, participants took a longer amount of time to walk tothe target while blindfolded than when walking under visual con-trol (post hoc analysis: 11.1 1.5s vs 9.1 0.9s, p < 0.001). For thesame distance, outdoor walkers implied more steps and more timewhen blindfolded than when supported by vision (closed vs openeyes: number of steps =17.1 2.6 steps vs 15.1 1.6 steps withopen eyes, p = 0.020; time= 10.6 2.5s vs 8.5 1.1s, p = 0.007).

4. Discussion of experiment 1

Indoor and outdoor walkers did not show any differences whenwalking for 10m under visual control. Conversely, many differ-ences were recorded when they were asked to walk blindfolded.Outdoor walkers were, in mean, accurate into walking to a mem-orized target, in accordance with previous studies [3,5] . On thecontrary, indoor walkers systematically undershot the target. Theeasiest explication of this behaviour could be the fear to hit oneof the indoor walls. Furthermore, we tested nave subjects with-out any experience of blindfoldedwalking in the two experimentalenvironments [3] . It has been shown that the given practice, some-times at great length before testing, could imply learning or areduction of fear on the subjects during the recorded performance[19] .

The closeness of indoor lateral walls and the fear to hit one of them could imply a shorter distance walked by our blindfolded

Table 1Effects of environment and distance on subjects performances and gait spatio-temporal parameters.

Tests on 3m, 6m,10mblindfolded

Environment(indoor vs outdoor)

Distance(3m;6 m; 10m)

InteractionEnvironmentDistance

Leuvenes test p-value

F p F p F p 3 m 6 m 10 m

Performance Error [m] 15.69


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Fig. 4. Correlations between walked distance (WD) and (A) walking speed (WS, above) and (B) step length (SL, below) during blindfolded walking indoors (black markers)and outdoors (grey markers) for thethree possible distances: 3 m (squares), 6 m (empty circles) and 10m (lled circles) in theexperiment 1. Regression lines were showedin gure foreach oneof thesix conditions: indoors (black lines)and outdoors (grey lines)per 3 distances. Thevalueof Pearsonscorrelation coefcient R wasshownfor each

correlation. The correlation was signicant ( p 0.51.

Table 2Effects of environment and vision support on subjects performances, gait spatio-temporal parameters, and upper body accelerations.

Tests on 10 m Environment(indoor vs outdoor)

Vision(eyes open vsblindfolded)

InteractionEnvironmentVision

Leuvenes test p-value

F p F p F p Eyes open Eyes closed

Performance Error 14.05 0.001 19.98


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Table 3Parametervaluesand post hoc analyses for thedistance of 10m with eyes open and closed condition.

Tests on 10 m Eyes open walking Blindfolded walking

Indoor Outdoor p Indoor Outdoor p

Performance Error [m] 1.99 1.03 0.17 1.56 0.001Time [s] 9.07 0.87 8.55 1.11 11.13 1.53 10.64 2.52 N steps 15.73 1.44 15.13 1.55 16.40 2.41 17.07 2.63

Spatio-temporal

parameters

SF [1/s] 1.74 0.12 1.78 0.13 0.383 1.48 0.12 1.63 0.21 0.005

SL [m] 0.64 0.06 0.67 0.07 0.262 0.49 0.07 0.58 0.08 0.004WS [m/s] 1.11 0.11 1.19 0.17 0.157 0.73 0.11 0.95 0.18


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Table 4Repeatability of subjectsperformances and gait spatio-temporal parameters.

Repeatability Tests on6 m blindfolded

ICC(2,1)

Indoor Outdoor

Performance Error 0.731 0.720Time 0.728 0.379N steps 0.776 0.619

Spatio-temporalparameters

SF 0.472 0.727SL 0.703 0.691WS 0.756 0.609

Two way random intra-class correlation coefcient assessed for the three trials of 6 m testin experiment2 on mainspatio-temporal gaitparameters: error, timespentto complete thetask walking, numberof performedsteps( N steps), step frequency(SF), step length (SL), walking speed (WS). In bold ICC(2,1) 0.7.

Table 5Repeatability of subjects errors.

Environment Signsof motor error Experiment 1 Experiment2 Total

Indoor 12 9 21 + 2 2 4 + + 1 2 3+ + + 0 2 2

Outdoor 5 6 11 + 3 4 7

+ + 2 2 4+ + + 5 3 8

The number of subjects who performed one of the reported series of motor errorare tabled. This series was related to the motor errors performed during the threedistances of the experiment 1 and the three trials of 6m test of the experiment 2.Positive (+)and negative ( ) signscorrespond to over-and under-shooting, respec-tively. The series of signs in table do not correspond to the temporally sequence of trials, but only to thenumber of occurrences.

outdoors), but also the step length (0.53 0.09m vs 0.58 0.08m)and the walking speed (0.76 0.13m/s vs 0.88 0.17m/s). Con-versely, the acoustic feedback directly affected the time spentto complete the task (10.88 2.06 s with environmental acousticfeedback vs 12.46 2.81s with white noise), the step frequency(1.56 0.19steps/s vs 1.41 0.20steps/s) and the harmonic ratioalong cranio-caudal axis (8.52 3.69 vs 6.50 2.51; F [1,56] = 6.16, p = 0.016). Also the interaction between environment and acousticfeatures resulted signicantlyaffecting error, step length andwalk-ingspeed.It was dueto the fact that,in absence ofacousticfeedback,only the performances of indoor walkers changed (post hoc anal-ysis: p = 0.004), and not that of outdoor walkers ( p = 0.817). In fact,theunderestimationobservedin theexperiment 1 ( 1.99 1.03m)wassignicantly reducedin thesubjectshearingwhitenoise during

experiment 2. However, the motor response variability increasedamong subjects when both visualand acoustic clues were excluded(mean response: 0.31 1.83m).

Finally, it is noteworthy that the numberof performedstepswasthe only parameter not signicantly affected neither by the envi-ronment(indoors: 17.0 3.7steps; outdoors:17.10 2.8steps) norby acoustic features (environmental acoustic feedback: 17.4 3.9steps; white noise: 16.7 2.5 steps).

6. Discussion of experiment 2

It is known that people are usually more accurate when askedto walk blindfolded to a memorized target than when asked toverbally judge the distance between them and a target [28] . Fur-thermore, the accuracy in distance perception was already foundreduced in an indoor smallenvironment, despite its richnessof spa-tial cues [12] . Similarly, we found a general association betweenunderestimation and undershooting indoors. However, even if poorer, verbal accuracy was expected to be correlated with motorresponses [18] , but neither an agreement (i.e. similar percentagevalues for verbal and motor errors) nor a signicant correlation(i.e. a linear relationship) between them were observed in ourresults.

Could be the underestimation of the target distance the reasonfor the indoor undershooting? This hypothesis does not explain themain reduction of undershooting when indoor acoustic informa-tion was eliminated. In this condition, the inter-subjects variabilityhighly increased, but on the average the performance surpris-ingly improved (whereas the mean error for outdoor walkers didnot signicantly change). Furthermore, the deprivation of environ-mental acoustic feedback altered the behaviour of subjects. Whenindoor walkers were allowed to hear environmental sounds, theyundershot the target for all the three different distances of exper-iment 1 as well as for all the three trials of the 6m-distance inexperiment 2 (probably subjects needed more than 5 repetitionsto improve their performance [19] ). But when acoustic informa-tion was removed (for the distance of 10m) the undershootingwas substantially reduced. It is noteworthy that the lack of acous-tic cues mainly affected the parameters related to the timing of locomotion, such as the step frequency and the gait rhythmicity[8] .

It could be possible that the surrounding acoustic features of the indoor environment could act as distracters, but in that casetheir elimination was expected to reduce the performance vari-ability andnot increasing it. Moreover, the positive role of auditorycues for enhancing visual and motor performances [17,26] or forguidancein blindness [2,25] hasbeenalreadydocumented.Anotherpossibleexplanation is thatsoundsreectedfrom nearbywalls may

Table 6Effects of environment and its acoustic cues on subjects performances and gait spatio-temporal parameters.

Tests on 10 m blindfolded Environment(indoor vs outdoor)

Acoustic features(environmental vswhite noise)

InteractionEnvironmentAcoustic features

F p F p F p

Performance Error 5.22 0.026 3.79 0.057 5.23 0.026Time 0.33 0.569 5.99 0.018 0.04 0.851N steps 0.14 0.908 0.55 0.463 0.14 0.511

Spatio-temporalparameters

SF 2.36 0.130 9.36 0.003 2.85 0.097SL 4.81 0.032 2.49 0.120 4.14 0.047WS 9.27 0.004 1.28 0.263 8.08 0.006

Results of repeated measures ANOVA performedon allthe 60 participantson thedata recorded during the10 m test performedblindfoldedto investigate themain effects of the environment ( F [1,56], p), acoustic features ( F [1,56], p) and their interaction ( F [1,56], p). The measured parameters were: error, time spent walking to complete thetask,number of performed steps ( N steps), step frequency (SF), step length (SL), walking speed (WS). In bold the p-values


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enhance the fear to hit one of them, bringing towards a reductionof step length and walked distance. Further studies, involving awiderrange of distancesto walk andmorespecictests areneeded.These should allow a deeper investigation on the role of the acous-tic cues on the capacity of subjects to walk blindfolded towards amemorized target. Also they should clarify if the reduced under-shooting was a consequence of a better assessment of the walkeddistance or was more related to the adoption of a different motorstrategy.

Interestingly, the number of performed steps was similar foreach specic distance for all the investigated conditions. Althoughthe differences between explicit awareness and actions werealready highlighted [29] , it was hypothesised that the perceptionof a distance could be affected by the intended action to traverseit, and not solely by optical and oculomotor information related tothe geometry of the environment [24,30] .

7. General discussion

7.1. Different performances in different environments

The aim of this study was to investigate if the surrounding envi-ronment could affect the capacity to walk towards a memorizedtarget without the visual support. The most striking result wasthat although healthy subjects were accurate in an open outdooreld, as it could be expected [3,5,15] , they undershot the targetin an indoor small environment. It was a surprising result becausethe elimination of some environmental cues increased the perfor-mance variability among subjects, but, on the average, it improvedtheir performance.

Infact,in anindoor room, rich of informative cues related tospa-tial layout, such as doors and windows, subjects underestimatedthe targetdistance and undershot it walking blindfolded. On thecontrary, in an outdoor environment, poor of informative cues,their performances were, on the average, more accurate, even if more variable among subjects (as shown by higher variances andlower values of intra-class correlation coefcient). Analogously,when indoor walkers could not access to environmental acousticcues, their performances became more accurate, but more variableamong subjects.

Gross errors were recorded in the judgement of a distance interms of conventionally used metrics (m). However, verbal andmotor performances were not signicantly correlated. And theunderestimationof theindoordistancecould notexplainthe reduc-tion of undershooting in absence of acoustic cues. Even if the roleplayed by distance estimation needs further studies, our resultsseem in agreement with the idea that people were not able todescribe the environment scaling it in arbitrary, unspecied units(even if commonly used, as meters). Buttheycouldbe able to trans-form it into units related to intended actions, such as the number

of steps needed to travel a seen distance [30] . So, the memorizeddistance can be evaluated in terms of number of steps needed toachieve the target.

However, when the subjects were blindfolded, they performedshorter steps than those performed with open eyes (especiallyindoors). It implied that the number of steps needed to beincreased, as outdoor walkers did. Conversely, indoor walkers didnot change their number of steps in open vs closed eyes con-dition, in spite of their step length reduction when blindfolded.Moreover, the number of steps was the only parameter that didnot vary indoors between different conditions (with and withoutvisual feedback and/or with and without acoustic feedback). Con-versely, the other gait spatial parameters seemed to be affectedby spatial layout features and the temporal ones by acoustic

features.

At this point, two issues need to be addressed: (1) the goodperformance outdoors and (2) the undershooting indoors.

7.2. The good performance outdoors

In accordance withmany previous studies, theaverage responseresulted accuratewhenthe taskwas performedoutdoors [3,14,15] .It implies that humans can be able to plan a proper motor strat-

egy in an environment poor of informative cues to walk towardsa target even without visual control. Under less informative con-ditions (such as outdoor open eld or in absence of acousticfeedback), subjects probably need to exploit only the internal-ized information to decide when they should stop themselves,comparing the memorized distance and the self-evaluated walkeddistance. The step length can be estimated by the locomotor bodyschema [7] , combining the internalized knowledge of body seg-ment lengths with the continuously updated feedback about jointangular range of motions provided by proprioceptive signals com-ing from mechanoreceptors in joints, skin, and muscles [10] . Butwhy did subjects seem unable to estimate or to take into accounttheir self-chosen step length reduction indoors?

7.3. The undershooting indoors

The undershooting of our indoor walkers is probably the mostsurprising result of this study. Especially because the indoorenvironment was expected to be more similar to the common con-ditions in which subjects could experience theblind walkingduringreal life.

The step length recorded indoors, when subjects walked blind-folded, was signicantly lower than the one outdoors. This steplengthreduction could only be related to a generally reductionintoexo-extensionrange of motion [7] , thatcanreduce proprioceptivesignals because they areweaker in themid-range of motionthan inits extremities [10,27] . However, if the locomotor bodyschema wasused also indoors, the reduced signals should imply higher stan-dard deviations, and not the indoor systematic error. In addition,we observed that for outdoor walkers standard deviations werehigher than for indoor ones.

It should be noted that people usually walk without vision sup-port only in small indoor environments. In these environments thetarget could be a door, a light switch, a wardrobe or a bed. It meansthat the target is like a nish-line to approach without crossingit: the possibility of undershooting a closed door is less dangerousthan overshooting it. In these environments, subjects could alsouse a tactile exploration, but this kind of explorative strategy, evenif safer, is more time consuming. On the other hand, in real sit-uations, subjects could combine exploitation and exploration [5] .They could walk forward exploiting their memorized information,but in a conservative manner to avoid possible hitting with the tar-

get, and then shifting to a feedback based strategy, using tactileor even acoustic exploration once in proximity of the target. Thishypothesis obviously needs further investigations, but our resultsseemed to support it. Our subjects resulted able to compute thecorrect number of steps needed to achieve the target indoors, butthey performed shorter steps, that could be a safe strategy to avoidovershooting and to reduce the effects of an eventual impact. Thisbehaviour could lead to a less accurate but more reliable motorresponse. In fact, it does not need an online continuous updating of body position based on the estimation of step length and a conse-quent adaptation of the step number. Also Dominici et al. [7] f oundan undershooting of the target when subjects walked on stilts,and authors hypothesized that their subjects probably planned areduced number of expected (butnot real)longersteps.Thisplan-

ning might avoid the need of an online use of the locomotor body


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schema during the task, reserving more attention to the externalfeedbacks than to the internal computations.

Subjects could prefer this kind of cautious strategy under quitedangerous conditions, such as walking indoors or on stilts. Con-versely, when environmental cues are not available, subjects couldneed to shift towards a predictive and less conservative strategy.

7.4. Conclusions

Although further studies are needed to verify the abovehypotheses, they are in line with previous studies about trade-off between exploration needing sensory feedbacks and exploitationbased on internal representation of external world. The environ-ment can affect this trade-off, implying a selective tuning betweendifferent strategies [4,22] .

Even though, further studies are needed to deeply investi-gate the role of different environmental characteristics, our resultsclearly showed that the surrounding environment inuenced theperformance of subjects asked to walk towards a memorized tar-get. When less cues were available, participants seem to rely moreon information related to body mechanics and body feedbacks toaccurately complete the task. Conversely, in a small indoor envi-ronment rich of environmental cues, subjects seem to perceive thetarget as a nish-line to not overshoot.

Acknowledgements

This study was supported by the Italian Ministry of Health andby our Foundation. We are grateful to Chiara Felici for her supportand English editing of this manuscript.

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