the trunk midline is the reference for orientation of the body in space and is the  point  
 of comparison for the synthesis of all tactile , kinesthetic , and pressure - related 
 information from the two sides of the body1 .
thus , the trunk midline can be considered the axis of symmetry 
 around which the body organizes motor behavior2 .
the corpus callosum is the principal interhemispheric commissure , 
 and its functions include the exchange of information between the two central hemispheres 
 and integration of the inputs that reach one or both of them3 .
all somatosensory callosal connections appear to provide a common anatomical substrate : 
 this phenomenon is known as midline fusion .
the body regions that are represented in the callosally 
 connected zones of the first somatosensory area ( si ) were identified as the midline of the 
 somatosensory space4 .
the subjective 
 vertical midline of the body is the result of multisensory integration of vestibular , 
 proprioceptive , visual , and tactile afferences5 , 6 .
meilinger and colleagues hypothesized that 
 every alteration of these afferences produces an abnormal representation of the body system , 
 which receives incoherent information , and answers in a reflex and primitive manner7 .
the integration of spatial information that is perceived8 , 9 from different viewpoints 
 is a common yet largely unexamined cognitive ability10 , 11 .
locomotion is supported 
 by additional content , which is closely related to behaviors , such as modulation of the gait 
 cycle , from vision control12,13,14 . as logan and 

colleagues have pointed out ,  frequency response functions between visual scene motion 
 ( input ) and trunk kinematics ( output ) change very little or not at all with gains in trunk 
 orientation in the standing posture and under walking conditions15 , 16 .
there is evidence of a correlation between heading and the rate at which strategic 
 modifications in trunk yaw decrease , because adaptive recalibration of locomotor trajectory 
 using optic flow stimuli depends on the rate at which the kinematic variability that is 
 associated with strategic control is reduced15 .
however , logan and colleagues consider that the increased gain 
 reflects a decline in stability , due to a change in the control problem from standing to 
 locomotion . keeping the body upright with the use of vision during walking
when vision is deprived , subjects operate not according to a pre - established pattern of 
 action but in relation to its internal representation of the previously seen target . 

visual feedback allows 
 a subject to have the same gait patterns in various surroundings , whereas visual deprivation 
 has environment - specific effects on gait dynamic stability16 .
the significance of visual feedback underscores its importance in pathological conditions , 
 such as patients with post stroke hemiplegia who might be unable to adapt to changing visual 
 or surface conditions17 .
iosa and colleagues reported that the surrounding environment influences the performance of 
 subjects who are asked to walk toward a memorized target .
when fewer external cues were 
 available , participants relied more on information on body mechanics and body feedback to 
 complete the task accurately .
conversely , in a small indoor environment that was rich in 
 environmental cues , subjects perceived the target as a finish line that was not to be 
 overshot18 .
mohapatra and colleagues indicated that under blindfolded conditions , anticipatory postural 
 adjustments are not generated .
they proposed that the increased emg activity in leg and 
 trunk muscles after a perturbation , indicating lower postural stability , reflects a lack of 
 anticipatory postural adjustments ( and relative compensatory postural adjustments ) when 
 vision is unavailable19 .
other research 
 suggests that the priority of whole - trunk control in the mediolateral direction is higher 
 than in other directions and is linked to attention , whereas whole - trunk control in the 
 vertical rotation and anteroposterior directions is passively regulated and requires minimal 
 attentional control20 .
as recently demonstrated in chronic pain conditions , alterations in somatosensory 
 perception can change the body s sense of posture21 .
the possibility of training a subject to understand his position 
 with respect to the gravitational axis through a specific cutaneous training program opens 
 new avenues in body posture perception and action22 .
also , the maintenance of chronic low back pain is linked to a 
 disorder of altered perception of the trunk23 .
perceptual surfaces ( pss ) reduce static and dynamic balance 
 impairments , even in patients with parkinson disease ( pd)24 .
the function of the trunk is crucial in pd : other studies have 
 shown how vibrotactile biofeedback of the trunk improves postural stability in pd25 . thus , multiple sensory stimuli contribute to conscious awareness of the body , but the 
 manner in which the central nervous system constructs and updates the body schema after 
 injury or on visual deprivation is unknown .
for example , 
 pereira and colleagues demonstrated that local muscle vibration does not improve the 
 sit - to - walk performance of healthy young adults26 .
the purpose of this study was to determine the effects of tactile and proprioceptive 
 sensory stimulation of the back on perception of the body midline and on the capacity to 
 walk toward a memorized distance without vision or trunk stability in healthy subjects .
we hypothesized that training individuals perceptive capacity ( pressure and somesthetic 
 sensation ) by providing dedicated instruction that is based on cognitive exercises that are 
 performed using a specific rehabilitative tool would modify their estimation of walking 
 distance and upright gait stability .
ten healthy subjects ( mean age 31.90  2.47  years , mean weight 64  11.4  kg , and mean 
 height 170.4  7.56  cm ) were recruited and matched with 10 healthy control subjects ( mean 
 age 30.5  2.8  years , mean weight 65  10.20  kg , and mean height 169.5  8.40  cm ) .
the 
 subjects gave their informed consent to participation in the study , and approval was 
 obtained from the local ethics committee of the s. lucia foundation ( n ce - prog 266 - 09 ) .
the 
 subjects were asked to perform an experimental indoor test wearing comfortable shoes that 
 they usually wore , not specialty shoes , such as boots , ballet shoes , high heels , and 
 flip - flops .
we examined the subjects abilities to perceive and estimate the length that they were 
 required to walk over a given distance in an indoor environment .
the subjects had to walk 
 toward a memorized target 3 m , 6 m , or 10  m from a reference position ( 0 m ) in a 20 by 
 5  m laboratory , without obstacles on a linear trajectory in the middle of the 
 laboratory , with their eyes closed and prompted by an acoustic signal to begin walking .
the environment was 
 quiet , with good natural lighting , and there were no other items or furniture that could 
 serve as external references for the subject .
similar to the experiment that was conducted 
 by iosa and colleagues27 , before starting 
 the test , the investigator positioned the subject at 0  m and showed him the distances that 
 were set at 3 m , 6 m , and 10  m to facilitate memorization of the trajectories that the 
 subject had to cover blindfolded .
the target was a person who stood on 1 of 
 3 strips that were placed on the ground 3 m , 6 m , and 10  m from the starting line .
the 
 subjects were asked to memorize the position of the target , fixing it for several seconds ; 
 blindfold themselves ; and , after an acoustic signal , walk to the target .
participants were 
 asked to stop walking when they believed that they had reached the target .
the experimenter promptly warned a subject if he was going to 
 hit a wall .
the sequence of the 3 tasks ( 1 for each distance ) was randomized among the 
 subjects . at the end of each test when a subject stopped at one of the targets ,
a tape measure was 
 used to determine the distance that was traveled and the distance that remained to complete 
 the task . after each sequence , the subject , still blindfolded , was returned to the starting 
 line by the investigator , preventing him from knowing whether an error had been committed to 
 avoid learning opportunities .
no side effects were recorded during the test , and the investigator never had to stop a 
 subject .
the entire study group was tested before starting treatment with pss ( = t0 ) and 
 after 10 treatment sessions with pss (= tend ) .
five randomly chosen subjects repeated the 
 test immediately after the first treatment session of sensory - motor evaluation with pss (= 
 t1 ) to exclude the possible influence of learning of the task . to complete the test ,
an accelerometer was fixed at the level of the l2/l3 vertebrae with 
 an elastic band ( freesense , sensorize , sampling frequency 100  hz , weight 94  g ) . 

accelerometry is a technique that generates data on the dynamic stability of gait with 
 regard to movements of the trunk during walking .
the accelerometer provided acceleration data of the trunk along the 3 body axes 
 ( anteroposterior , laterolateral , and craniocaudal ) to assess upright gait stability .
upright gait stability has been defined as the capacity to minimize upper body 
 accelerations18 .
upper body 
 accelerations were analyzed after subtracting their mean values and low - pass filtering at 
 20  hz , and summarized in terms of acceleration root mean square ( arms ) , which is a measure 
 of acceleration dispersion ( coinciding with the standard deviation due to subtraction of the 
 mean signal ) .
we computed the arms for each body axis , to obtain information on upright gait 
 instability18 .
the arms was computed 
 for each of the 3 acceleration measures along the 3 body axes and averaged over the 3 values 
 of 3 consecutive steps in the central section of the walking pathway28 .
the number of steps that were performed was computed as the number of ap - acceleration 
 negative peaks29 , and the average step 
 length was calculated as the ratio between the distance that was walked and the number of 
 steps ( step length refers to the distance between 2 successive placements of the 2 feet , 
 whereas stride length refers to the distance between 2 successive placements of the same 
 foot , formed by 2 steps ) . the perceptual surfaces protocol ( ps )
stimulation of awareness of 
 the trunk midline is effected using a specific tool , called super ( perceptual surfaces , ps ) , 
 patented in 1997 by ennio spadini ( reference 01291920 , rome , italy ) .
super is a therapeutic 
 system that is based on the interaction between a subject s back and a support surface 
 comprised of small latex cones of various dimensions ( height : 38 cm ; base diameter : 24 cm ) 
 and rigidity ( 20 to 60% ) .
these cones are applied with their inferior bases on a rigid wood 
 surface through elastic strips , and typically , over 100 cones are used for each session . 

subjects were asked to lay supine on the surface that was formed by the smoothed apex of 
 these cones , creating reaction forces to the patient s weight , generated by interaction with 
 the cones .
the base conformation of the pss comprises blue cones that represent the anteroposterior 
 trunk midline ( 60% rigidity ) , yellow cones that represent the paravertebral , and the 
 remaining areas that are symmetrically adjacent on either side of the midline ( 40% rigidity ) 
 ( fig.1fig .
1.surfaces for perceptive rehabilitation in the basic configuration and below an 
 examples of cones with varying dimensions . ) .
we chose this conformation on the basis of the following principles : to provide 
 greater stimulation to the skin of the posterior midline of the trunk ( blue cones with 
 greater rigidity than the yellow ones ) , and to provide symmetrical stimulation to other 
 areas of the body .
surfaces for perceptive rehabilitation in the basic configuration and below an 
 examples of cones with varying dimensions .
subjects were asked to lie supine on these cones , with their knees and hips flexed , so that 
 their weight was supported by many reaction force vectors , 1 for each cone .
these forces 
 generate high pressure in the small areas of contact , resulting in intensive perceptive 
 stimuli .
the base conformation allowed us to determine spatial alterations in the back 
 ( subjects with antalgic postures and scoliosis ) , based on the distribution of pressures on 
 the subjects backs and their perception of back pressures ( i.e. , sensory - motor evaluation 
 in the first session ) .
subjects were asked to relax and find the most comfortable position , breathing normally . 

subjects had to recognize the areas of support , indicating the surface of the body that was 
 in contact with a particular area , describing and counting the number of cones , checking the 
 distribution of the load on the bed and correcting it , and paying attention to posture .
the 
 subjects reported how they perceived and felt the cones , particularly if the load was 
 distributed uniformly and symmetrically with respect to the trunk midline .
after the evaluation session , each subject underwent 10 sessions in 2 weeks ( 5 days per 
 week ) .
subjects performed 45 minutes of a cognitive - perceptive task ( divided into 
 perceptual - motor and active phases ) to improve their perception of the trunk and , in 
 particular , their midline .
the perceptual phase helps a subject become aware of the position of his body segments with 
 respect to the various cones .
the perceptual - motor phase is characterized by growing 
 awareness of the trunk midline .
diaphragmatic breathing , associated with retroversion of the 
 pelvis in the expiratory phase , allows the curves of the spine to flatten and the muscles to 
 stretch to increase the support surface .
the actual exercise comprises a perceptual task 
 that increases in difficulty , asking the patient to perceive the elasticities and heights of 
 the surfaces .
the active phase involves movement of the arms and legs , displacement , and 
 weight control of the trunk and pelvis .
the base conformation can be modified during the 
 session to improve contact between the trunk and the surfaces of the cones . at the end of each session ,
the experimenter examined the interaction between the skin on 
 the back and the surfaces that relieved the hyperemic area on the patient s back . in subsequent sessions , cones with varying elasticities were positioned by the therapist to 
 improve the symmetry of contact between the surface and the patient s back , considering the 
 hyperemia in the previous session .
the experimenter measured perception through the 
 symmetry , quality , and uniformity of the supports .
empirically , in the static posture after sessions with the pss , the perceptive capacity of 
 the trunk was assessed using quality representation and symmetry , with respect to the line 
 of the spinous processes , of the hyperemic areas that were created by the support with the 
 pss . 
quality  was defined as the magnitude of the hyperemic area that is left by the cones 
 on the skin and signs of pressure from the cones that remain on the skin of the trunk .
the aim was to improve the subjects abilities to recognize , perceive , and discriminate the 
 trunk midline and enhance their sensory experience .
in the control group , subjects were asked to lay supine with the knees and hip flexed . 

subjects were asked to relax and find the most comfortable position , breathing normally . 

lumbar exercises were performed : lumbar bridging , lumbar pelvic tilt and hamstrings and 
 piriformis supine stretching .
all measures were continuous and distributed normally ( lilliefors test ) ; thus , parametric 
 statistical methods were employed .
to compare the results between pre- and post - treatment performances , 
 repeated - measures analysis of variance was performed using time ( pre- vs post - treatment or 
 first vs second assessment ) and distance ( 3 m , 6 m , or 10 m ) as the main factors , and axis 
 ( ap , ll , or cc ) as an added factor when acceleration was analyzed .
before treatment , the subjects displayed the greatest errors in walking 10 m , followed by 
 walking 6  m and 3  m. this error was always negative : subjects tended to walk less distance 
 than required . by analysis of variance , there was a significant effect of time ( pre- vs post- treatment , 
 f=5.968 , p=0.037 , observed power 59% ) and distance ( f=8.055 , p=0.012 , observed power 91% ) 
 but not of their interaction ( f=2.977 , p=0.108 , observed power 36% ) .
the reduced error was 
 related to a longer distance being walked , which was founded on longer steps ( in mean + 5% ) 
 and more steps performed ( in mean one more ) .
although step length and number of steps did 
 not improve significantly over time ( analysis of variance , p>0.05 for both ) , their 
 combination significantly improved the subjects performances .
there were no significant changes in the error in walking distance estimates among the 
 subjects who repeated the test immediately after the first treatment session ( effect of 
 trial : f=0.374 , p=0.574 ) ; the absence of visual and verbal feedback did not allow the 
 subjects to correct their mistakes .
non - significant differences in these errors were 
 observed at tend in those who undertook the test for the second time at 
 t1 or at tend . despite the general increase in speed and acceleration after treatment ,
their changes were 
 not statistically significant ( table 
 1table 1.mean and standard deviation of the gait parametersparameterstime3 m6 m10 merrors ( m)t00.150.110.700.291.570.45tend0.050.110.130.180.330.29ws ( m / s)t00.840.200.820.130.680.18tend0.890.200.880.150.720.15step length ( m)t00.430.090.510.090.520.09tend0.460.100.530.090.550.05number of stepst071102162tend71112171arms - ap ( m / s)t01.160.351.230.321.180.27tend1.200.331.350.421.330.29arms - ll ( m / s)t01.150.431.140.301.090.22tend1.170.391.320.501.260.31arms - cc ( m / s)t01.400.561.600.511.470.37tend1.400.401.730.521.750.43 ) .
time did not affect self - selected walking velocities pre- and post - treatment ( f=2.924 , 
 p=0.121 ) .
subjects walked slower over the 10-m distance than over the other 2 distances 
 ( factor distance : f=15.734 , p<0.001 ) , without any significant interaction with time 
 ( f=0.067 , p=0.935 ) ( table 1 ) .
similarly , by 
 analysis of variance of arms values along the 3 body axes ( table 1 ) , there were no significant effects of time , pre- versus 
 post - treatment ( f=2.091 , p=0.182 ) , or distance ( f=2.454 , p=0.114 ) ; only axis had a 
 significant effect ( f=11.689 , p=0.001 ) , with greater acceleration along the cc axis .
the 
 interactions of time and distance ( f=1.963 , p=0.169 ) and between time and axis ( f=0.141 , 
 p=0.869 ) were not statistically significant .
the purpose of this study was to determine whether a training program that is based on 
 tactile and proprioceptive sensory stimulation of the trunk , performed using perceptive 
 surfaces , modified the locomotor body schema and improved estimation of walking distance and 
 upright gait stability .
after treatment with perceptive surfaces , the distances that were traversed were closer to 
 the target than those before treatment , because the subjects increased their awareness 
 regarding perception and estimation of the walking body .
there was a non - significant increase 
 in trunk acceleration , which were likely related to the increase in gait speed30 , after training31 .
the concept of the trunk having a fundamental dynamic function during walking is a recent 
 model . until the 1990s
, the trunk was considered to be a static passenger unit of a 
 locomotor apparatus that was located primarily at the lower limb level34 .
our study shows that tactile and proprioceptive stimulus 
 of the trunk , with a new rehabilitation tool , can influence the behavioral pattern of 
 locomotion .
in particular , the most important result of our study was that stimulation of the trunk 
 improved the ability of the locomotor body schema to walk toward a memorized target without 
 visual support .
this result is in conflict with the older model , in which the lower limbs 
 are a locomotor unit and the upper part of the body is merely a passive passenger5 .
our findings are consistent with a recent 
 suggestion that the upper body has an active function during walking32 .
thus , our results suggest that the locomotor body schema 
 includes the upper body not just the lower limbs . in this model ,
the cns integrates many 
 sensory inputs visual , vestibular , cutaneous , gravito - inertial , and proprioceptive inputs to 
 compute spatial and body coordinates during a walking task , assigning a different weight to 
 each input , depending on the environment and the constraints in which the movement is 
 performed33,34,35 .
shenton suggested that 
 proprioceptive inflow is an important sensory input in the online representation of the body 
 in space36 , and that the processing of 
 proprioceptive information is context - dependent37 .
the presence of a body schema has been well described , as has the function of the location 
 of tactile stimuli on body surfaces in promoting a body schema during the spatial 
 representation of the body ( position ) and walking ( locomotor body schema)38 .
the perception of tactile stimuli and proprioception is critical for conveying information 
 about the relative positions of body parts when assuming a posture or during walking .
the 
 decrease in perception , for example , during pain conditions could drastically change the 
 body schema39 , 40 .
two studies of patients with chronic and non - specific low back 
 pain reported that training with perceptual surfaces ( pss ) , targeting back midline 
 perception , reduces pain41 and improves 
 postural control42 .
how the body schema is generated in the cns is unclear , but we know that the body schema 
 and sensory information interact ; thus , their conflict might generate pain , as shown in the 
 experiment conducted by mccabe and colleagues using healthy volunteers43 . furthermore , the position of the body ( sitting or 
 standing ) might alter the accuracy of the imagination of movements ( i.e. , mental simulation 
 of an action without its actual execution ) such as walking44 .
trunk biomechanics and thus trunk movement perception and action are fundamental for 
 establishing self - body representation ( position ) , and during body movement , efficiency of 
 the locomotor body schema37 .
our proposed exercise , the use of the trunk midline , and tactile and proprioceptive 
 stimulation of the trunk enhance the locomotor body schema by generating more accurate 
 information regarding trunk perception , the trunk - body midline , and gravity perception , and 
 the interaction between trunk - body midline perception and space during a specific task .
this 
 enhancement in the locomotor body schema is a likely reason why the subjects displayed fewer 
 errors in estimating walking distances after the intervention . that the subjects showed no 
 improvement in estimating errors in a second evaluation ( after the first session ) indicates 
 that the learning effect was negligible during repetition of the task
thus , the improvements are likely attributable to the increase in movement perception 
 during walking ( locomotor body schema ) while blindfolded .
the blindfolded walking test is a complex task in which perception of the distance that has 
 been walked should match the actual distance that is covered without visual correction .
as 
 reported by schmidt , better trunk proprioception improves the efficiency of walking while 
 blindfolded .
schmidt and colleagues applied vibratory stimuli if the trunk muscle spindle 
 noted changes in the trajectory direction during walking , even in the presence of 
 vision45,46,47,48 .
overstimulation of perceptual and proprioceptive trunk sensory information , as occurs 
 during trunk muscle vibration , might disrupt the steering of locomotion49 .
this was likely due to enhanced trunk proprioception , which was the target of 
 our training program .
further studies should confirm these preliminary results and our 
 hypothesis with more rigorous experiments and a larger sample .
our study highlights the importance of trunk perception in computing egocentric space 
 information during body locomotion .
an example of the relative dependence of postural 
 control on visual , vestibular , and somatosensory inputs was discussed by horak and nashner , 
 who measured changes in postural sway in the standing position .
they proposed that the 
 apparent displacement of visual information and postural vertical input was the result of a 
 recalculation of the central orientation of the gravity vector , based on a model in which 
 the central nervous system extracts and interprets afferent information to construct a 
 picture of reality ( motor imagery)50 . before treatment , subjects made errors that were proportional to the distance that was to 
 be traveled .
the mistakes were always 
 negative : subjects tended to choose a shorter distance than the distance required .
this 
 pattern might be due to fear of hitting a wall or the ground , resulting from decreased body 
 perception during walking ( the locomotor body schema ) .
this hypothesis is supported by the 
 finding that before treatment , subjects walked with shorter steps , which is typical of 
 people who are fearful of falling51 , and 
 of those with reduced somatosensory information , as might occur during indoor blindfolded 
 walking29 .
our results reinforce the hypothesis that working on the representation of the midline and 
 motor imagery of the trunk creates a bridge between perception and movement , which can give 
 rise to new functional strategies52 . 

blindfolded subjects evaluate movements better , because the motor patterns that are learned 
 for ambulation , compared with specific tasks , can be affected by and depend on visual 
 representations of the same image and are related to motor imagery , which is linked to 
 somesthetic perception . our results also show that after training , upright gait stability was unchanged , likely 
 because upright gait stability is modified in pathological walking pattern conditions to a 
 greater extent than in healthy subjects30 .
but , this hypothesis should be confirmed with an ad hoc sample with pathological conditions 
 and age - matched healthy subjects .
this study had several limitations , such as the absence of a control group ; however , the 
 repetition of the test immediately after the first treatment session should eliminate any 
 suspicion of a bias linked to the learning effect .
furthermore , our small sample size did 
 not allow us to examine the differences between genders29 , 53 , only within - subject 
 comparisons were made pre- and post - treatment .
we did not plan to measure lateral errors , 
 but we noted lateral deviations during the tests .
also , we did not measure joint kinematics 
 during walking , losing important information , especially with regard to the function of the 
 hips . finally , a potential bias of our test was the learning effect .
although we did not 
 find statistically significant differences in the performance of subjects who were tested 
 and immediately retested , the absence of significant differences could be due to the reduced 
 sample size ( 5 subjects ) with which this sub - analysis was performed .
nevertheless , the 
 values of their performance during the retest were similar to those that were recorded in 
 the first test , suggesting the absence of any effect .
the potential bias of learning should 
 be taken into account , and the interpretation of our results should be made with 
 caution . in light of our results ,
future studies in this field should be performed with larger 
 sample sizes to determine whether perceptive rehabilitation , integrated into traditional 
 rehabilitation programs , improves locomotor body awareness and enhances stability and 
 walking performance in pathological conditions ( e.g. , stroke , ataxia , cerebral palsy , 
 juvenile idiopathic scoliosis , and low back pain ) . in conclusion , this pilot study 
 demonstrated that perceptive stimulation of the trunk midline improves the estimation of 
 walking distance , implicating the upper body in the locomotor body schema .
 [ purpose ] recently , there has been growing interest in the somatosensory system , but 
 little data exist on the interaction between dynamic postural control and the 
 somatosensory system . the purpose of this study was to determine whether a training 
 program , based on tactile and proprioceptive sensory stimulation of the trunk with the use 
 of perceptual surfaces , improved the estimation of walking distance by healthy subjects , 
 the ability to walk toward a memorized distance without vision , and whether it increases 
 upright gait stability . 
 [ subjects and methods ] ten healthy subjects with a mean age of 
 31.9  2.5  years were enrolled and participated in 10 daily sessions of perceptive 
 training using perceptual surfaces , for 45 minutes each session . 
 an experimental indoor 
 test measured the subjects ability to perceive walking distances to a memorized target in 
 an indoor environment . 
 [ results ] after treatment , the distances that were traversed were 
 closer to the target than before treatment . 
 trunk acceleration did not differ 
 significantly between pre- and post - training and did not increase significantly after 
 training . 
 [ conclusion ] treatment with perceptual surfaces stimulating the trunk midline 
 improves the estimation of walking distance and modifies proprioceptive gait patterns , 
 allowing various corrective strategies to be implemented during ambulation . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION