  Posturography is routinely used to qualitatively assess one of the cardinal
symptoms of Parkinson's disease -- postural instability. While most measures of
balance control are derived from displacement of the center of pressure there
is evidence that such control is more likely to be velocity-based. We performed
static posturographic tests (eyes open and eyes closed) during quiet standing
in narrow stance for n=30 patients with Parkinson's disease (PDP) in the ON
state and compared the results with those of n=30 age-matched senior controls
(HSC) and n=60 young controls (HYC). We used differentiator filters to generate
time series of low-frequency fluctuations of sway velocity and calculated their
Lempel-Ziv complexity (LZC). With eyes closed, the mediolateral LZC of HSC 0.21
(0.02) was significantly higher than those of HYC 0.19 (0.02) and PDP 0.18
(002). Thus aging and PD have opposite effects on mediolateral LZC which
strongly differentiates between HSC and PDP (92% sensitivity and 87%
specificity).
