BALANCE • Balance is the consequence of an appropriate muscles activation processed by the brain fusion of sensory information Sensory Internal SENSES Integration Map BRAIN MUSCLES BALANCE • Vision, Vestibular and Somatosensory information are used by the brain to perform balance • ABF adds AUDITORY AUDITORY channel to provide trunk movement information VISION VESTIBULAR Sensory Internal Integration Map SOMATOS. BRAIN MUSCLES ABF components Head-phones audio amplifier Sensory Unit Laptop with DAQ board force plate • Sensory Unit: provides accelerations trunk information • Laptop with DAQ board: acquires the accelerations information and generates the audio feedback signals • Amplifier & Headphones: make audible the audio feedback signals • Force plate: is NOT part of the system, has been used to acquire COP data for ABF validation analysis Sensor characteristics Accelerometric sensors Amplifier and low-pass filter • The sensor used is able to provide the full complete kinematics of the trunk (3 accelerometers, 3 gyroscopes) • ABF uses only 2-D acceleration (AP and ML directions) ABF movement representation Safety Region (SR) • represents the limit of stability • is the region in which the COM projection is inside the subject’s support base • the support base is processed on anthropometric parameters (feet length and wideness) Referencing Region (RR) • represents the region for natural sway (1 degree) • is processed using the subject’s height ABF sound generation functions A Volume B Frequency AP direction: - Frequency INcreases moving forward (B) - Frequency DEcreases moving backward (B) AP acceleration AP acceleration - Volume increase going far from vertical position (A) C Volume Balance D ML direction: - Left/right ear volume channel increase moving left/right (CD) - Left/right ear volume channel ML acceleration ML acceleration decrease moving right/left (CD) ABF instructions for the subject - Balance the sound between the two audio channels AP - Keep the lower volume (=400Hz sound wave) ML Example of ABF signals ABF practical considerations • ABF can provide similar information as one otolith: – If the trunk/head moves slowly, primarily gravitational info is provided – If the trunk/head moves quickly, primarily acceleration information is provide • Continuous ABF sound also provides trunk VELOCITY information (most critical) ABF control interface • Subject’s anthropometric data • Trial condition • Control ABF variable • Input frequency • Output frequency • Calibration and trial durations • ABF Direction • Velocity information • Threshold controller COP & Trunk acceleration are highly correlated 0.1 A AP A ML r = 0.88 0.08 0.06 r = 0.90 ] 2 [m/s 0.04 ML ,A AP 0.06 A 0.02 0.04 0.02 0.1 0.08 0 0 0.06 0.04 -0.02 0.02 0 COP -0.04 ML -0.02 -0.02 COP AP -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 COP , COP [m] AP ML Correlation between COP and trunk acceleration: - ML direction r: 0.88 - AP direction r: 0.90 ABF is EASY • Subjects learn to use ABF in 1 minute • Personal balance score is higher with ABF also when it is NOT really helpful • It is really easy and comfortable to wear ABF effects on standing • Improve balance (Sway Area decrease) • Increase control (Mean Velocity increase) ABF effect on CONTROL SBJs with eyes closed and foam under the feet In this particular Mean Velocity condition the effects of using ABF are magnified since the sources AP ML AP ML of information (senses) are more limited 5 subjects: age: 30, 23- 33 (yrs), weight: 62, 58- Root Mean 78 (kg), height: 166, Square Sway Area 160-179 (cm). distance ABF information is SPECIFIC AP and ML feedback ABF only for AP direction Mean Velocity AP ML 10 % parameters difference with ABF AP ML 5 AP ML ML 0 AP -5 Providing ABF only -10 -15 in AP direction we affect mainly AP -20 sway (RMSAP) and -25 AP control (MVAP) -30 -35 Root Mean Square distance With PRACTICE sbjs improves their skill to use ABF Sway Area decrease with practicing Within three days the subject became so skillful that he could stand on the 600 500 Threshold foam with eyes closed [mm 2] 400 maintaining his movement 300 inside the referencing 200 region i.e. not receiving 100 any additional information 0 from ABF 1 2 3 days Bilateral Vestibular Loss Subjects 9 Subjects. Age: 55,38-73 Weight: 71,51-115 Height: 171,160-193 ABF reduces VESTIBULAR LOSS subjects’ Sway Area 95 % confidence ellipse (Sway Area) Vestibular Loss -10 Subjects reduce -20 sway more than -30 control subject -40 when standing on foam with eyes closed Control Vestibular % Reduction Sway Area in Vestibular Loss subjects using ABF This subject was able to perform the trials ONLY with the help of ABF This subject wasn't able to perform this condition both with and without ABF. This subject fell twice without ABF but never fell during the trials using ABF Bilateral Vestibular Loss subject 9 NO ABF WITH ABF This subject can NOT stand on the foam with eyes closed. This subject can stand on the foam with eyes closed using ABF. Time spent inside the Referencing Region increases using ABF % difference using ABF 500 400 300 200 100 0 Control Vestibular ABF Tuning Fork effect • Platform rotation: 4 6 degrees, 1degree/s COM [degree] • BVL subject 2 0 PRE ABF -2 WITH ABF -4 POST ABF Plat. Rotation -6 4 8 12 16 Time [s] Rambling & Trembling Analysis Control 25 • BVL subjects RMS reduction using ABF Vestibular improve 20 performance by reducing both rambling and 15 trembling RMS 10 • CTRL subjects improve performance 5 mainly by [mm] reducing rambling RMS COP Rambling Trembling RMS RMS RMS Effect of adding each sensory channel on Sway Area 8000 • Adding ABF Sway Area [mm2] difference 6000 information decreases sway area 4000 • Adding vision, somatosensory or vestibular information 2000 decreases Sway Area more than adding ABF Sensory channels ABF interacts similarly with all sensory channels 4000 Sway Area [mm2] difference Some subjects 3000 improve more than others with 2000 ABF when 0 another sense is available Sensory channels ABF controls subjects’ position Sound dynamic displacement Subject COP • A sinusoidal function was added to the 7 acceleration fed back by ABF 6 • The subject tried to Displacement [cm] 5 keep constant the ABF tone following the sine 4 function 3 • The trial was performed 2 with different sine wave frequencies (.05, .1, .2, 1 .4, .6, .8, 1.2) in the AP and in the ML direction 0 20 40 60 80 100 Time [s] Slow frequencies are easier to follow • The gain was largest at the lowest frequencies and AP ABF decreased with Normalized Averaged Gain ML ABF increasing frequency • At the lowest frequencies (0.05Hz and 1Hz), subjects were unaware that the sound induced them to sway. •AP and ML sway induced different movement strategies. 0.05 0.1 0.2 0.4 0.6 0.8 1.2 Frequencies [Hz] Conclusions • ABF reduces sway • ABF is comfortable and easy to understand for subjects • Subjects increase postural control using ABF • ABF information is specific and simple for the subjects to follow Future of ABF system • Development of a portable wireless prosthesis for balance improvement • Use in clinical rehabilitation for subjects with balance deficits • Validation of ABF during dynamic tasks 1st Open question: What’s the best information we should provide with ABF? • Up to now we investigated the effect of providing trunk acceleration information • Also, ABF using CoP displacement was tested obtaining analogous results to trunk acceleration • Feedback of CoM displacement was less effective perhaps because it added a 30 msec delay 2nd Open question: Where is the auditory information actually fused with the other sensory channels? • ABF adds an external information closed loop for sensory control • Vision, vestibular and somatosensory information are fused by the brain to perform balance. Is ABF part of this elaboration? Does ABF require a different (voluntary) muscle activation strategy? 3rd Open question: Can use of ABF become more automatic with practice? • We have shown that practicing with ABF increases subject’s balance performance • Vestibular loss subjects have difficulties using ABF when they are already controlling balance using a voluntary strategy i.e. concentrating specifically on the other senses (Divided Attention problem). Can use of ABF become more automatic (less voluntary)? 4th Open question: What is the real effect of the foam? How do subjects adjust their strategy with foam under the feet? • We used the foam to simulate the lack of proprioceptive information but it also affect coordination • Foam provided reaction forces different from the those expected by the subject familiar with firm surface. • Subjects automatically, over a long period (days), learn how to remain stable on the foam and improve their ability to balance on the foam.
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