Influence of Audio Biofeedback on Structural Properties of Postural Sway

1. Influence of Audio Biofeedback on Structural Properties of Postural Sway L. Chiari1, M. Dozza1,2, A. Cappello1, F.B. Horak2 OHSU 1Dipartimento di Elettronica, Informatica e Sistemistica Alma Mater Studiorum - Università di Bologna, Italia 2Neurological Sciences Institute OHSU, Beaverton (OR), USA

2. What is Audio Biofeedback (ABF)?

3. (van der Kooij, 2000)

4. BALANCE • Balance is the consequence of appropriate muscle activations processed by the brain fusion of sensory information Sensory Integration Internal Map SENSES BRAIN MUSCLES

5. BALANCE • Visual, Vestibular and Somatosensory information are the major cues used by the brain to perform balance • ABF may be used to involve more largely in the “game” the AUDITORY channel AUDITORY VISION Sensory Integration Internal Map VESTIBULAR SOMATOS. BRAIN MUSCLES

6. Diapason: a recent solution for ABF (Chiari et al., IEEE Trans Biomed Eng, submitted)

7. Sensor characteristics Release 1 - 2002 • The sensor used is able to provide the complete linear & angular kinematics of the trunk (3 accelerometers, 3 gyroscopes) • ABF in its present release uses only 2-D acceleration (AP and ML directions) (Giansanti et al., Proc. ISPG, Maastricht, 2001)

8. Diapason: the sonification procedure • 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 width) • Reference Region (RR) • represents the region for natural sway (±1 degree) • is processed using the subject’s height

9. Example of ABF signals

10. Remarks • ABF can provide similar information as one otolith: • If the trunk/head moves slowly, primarily gravitational information is provided • If the trunk/head moves quickly, primarily acceleration information is provided • Continuous ABF sound also provides trunk VELOCITY information (most critical)

11. Release 2 - 2003 • Subjects learn how to use Diapason in 1 minute • Subjective balance score (Schieppati et al., JNNP 1999) is lower also when ABF seems NOT actually helpful • It is small, light and comfortable to wear

12. Results: quiet standing • Improve balance (Sway Area decreases) • Increase control (Mean Velocity increases)

13. ABF can restore standing stability in patients WITH ABF NO ABF This subject can NOT stand on the foam with eyes closed. This subject can stand on the foam with eyes closed using ABF.

14. NO ABF ABF NO ABF ABF

15. Which is the origin of the changes observed in body sway trajectories? Cognitive Open-Loop ?? Feedback Sensory Tuning-fork Feed-forward

16. Some insights on the structural properties of body sway can be provided by Stabilogram Diffusion Analysis (Collins & De Luca, 1993)

17. Stochastic analysis: fractional Brownian motion (fBm) modeling through the Variance Analysis Method We model the output of the human postural control system as a system of (1-D and 2-D) bounded, correlated random walks. This is done by investigating the memory of the system through an analysis of the increments in displacement (x, y, r) Computed from the experimental data Estimated by LS techniques Vx(Dt) = <Dx2> - <Dx>2Dt2Hx Vy(Dt) = <Dy2> - <Dy>2Dt2Hy Vr(Dt) = <Dr2> - <Dr>2Dt2Hr For fBm the following correlation function holds (Feder, 1988): C = 2 (22Hj-1 - 1)

18. HjS > 0.5 j = x, y, r C > 0 (persistence)

19. HjL < 0.5 j = x, y, r C < 0 (anti-persistence)

20. Hence, COP fluctuations have a structure that is dependent upon the timescale of observation and not simply random (fractality). Moreover, AT LEAST TWO scaling laws are needed to accurately model the phenomenon in the range of interest (0.01- 10 s). - Scaling functions that describe how the values change with the resolution tells more about the data than the value of the measurement at any one resolution (in particular at the higher resolution as it is commonly done by the summary statistic scores, working with the original sampled time series). - Two modes of postural control take place over different timescales, associated with persistent and anti-persistent motion of the COP Collins & De Luca, 1993

21. HOW MANY SCALING REGIMES ? This mechanism of transition from persistent to anti-persistent behavior is a common property of many biological systems. How to model this ? We choose the easiest answer: correlation is assumed to change continuously H Chiari et al., Hum Mov Sci, 2000 Dt

22. Parameter K is the variance of the displacements for Dt=1 s, which is also proportional to the variance of the displacements for large time-lags (V(Dt)→4K as Dt→∞). Since V(Dtc)=K Dtc, K can be thought of as an estimate of the actual diffusion coefficient of the random process which is encountered by sampling the time series Dr at a sampling frequency 1/Dtc. • In fact, parameter Dtc is the midpoint of the sigmoid and represents the time-lag in which H = 0.5, corresponding to a purely random behavior. In this sense it is an estimate of the time-lag at which the real process switches from a persistent (positively correlated) to an antipersistent (negatively correlated) behavior.

23. K & Dtcallow to identify different postural strategies in control subjects - Post-adaptation to an incline (Chiari et al., 2001;) - Postural blindness (Chiari et al., 2000) K & Dtcdo correlate with several pathological conditions Central pathologies - Parkinson (Rocchi et al., 2000) - Multiple sclerosis (Chiari et al., in preparation) Peripheral pathologies - Peripheral Neuropathy (Lenzi et al., 1999) - Vestibular Loss (Kluzik et al., 2001)

24. Do structural properties of the postural sway change with ABF? If so, in which way this may help in the understanding of the mechanisms underlying ABF efficacy and improving the design of a rehabilitation strategy for balance disorders?

25. Results We present the results obtained from 9 healthy subjects in the condition with the least sensory cues (i.e. eyes closed on foam) that benefited the most from ABF * ** *p<0.05 **p<0.01 Both K and Dtc show a systematic reduction due to ABF Structural properties do change during biofeedback trials

27. Conclusions • ABF is comfortable and well accepted by the subjects • Subjects increase postural control using ABF (area decreases, mean velocity increases) • ABF may help people manage more easily inadequate surface somatosensory and visual information for postural control • ABF determines structural changes in the COP that may reflect a larger role for feedback (conscious?) control over feed-forward control of posture. • Future studies are needed to determine whether, with more practicing, subjects can use ABF without conscious control and hence how much this result is consistent over time.

28. Thank you for your attention Luigi Galvani, Guglielmo Marconi and Augusto Righi, Bononiensi

29. Work in Progress • Development of a portable wireless prosthesis for balance improvement • Use in clinical rehabilitation for subjects with balance deficits • Validation of ABF during dynamic tasks

30. 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)?