Discrete Wavelet Analysis of center of pressure in patients with peripheral vestibular disorder

David Arriagada
Universidad Mayor

Rony Silvestre
Univeridad MAyor

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     Last modified: February 21, 2007

Abstract
Postural control is described as the result of the integration of extremely complex sensory systems (visual, vestibular and somatosensory systems). When the vestibular system is affected, the information is modified over the permanent postural accelerations, to which the man is submitted, a result of his interaction with gravity, needing more energy to keep his stance.

Objective: To describe , through an analysis time frequency (Discrete Wavelet Transform- DWT ) the behavior of the energy in the different frequency bands during the human bipedal stance in peripheral vestibular disorder patients.

Participants.
Twenty subjects were tested: 10 with peripheral vestibular disorder (29.6 years old, ?4.45), whose etiology was a mild traumatic brain, and 10 healthy control (28.8 years old ?3.76).

Setup and Protocol.
A posturografic platform was used to evaluate the displacement of the center of pressure (CoP). The subject tested stood erect, with their arms parallel to the axis of the body. The duration of the test was 60sec (30sec with eyes open/ 30sec with eyes closed).

Measurement and Analysis.
Anterior/posterior body sway was recorded. All signals were sampled at 40 Hz. The analysis was done with the DWT. The Daubouchies family of order 6 was used. The average energy for every frequency band was calculated between 4 and 0 Hz. Wilcoxon?s test was used to correlation the average values for each frequency bands in each test and for each group.

Results.
The significant numerical differences in both studied groups are much more noticeable in the low-frequency bands.
Open eyes: 1/16 Hz (p < 0.0005), 1/8 Hz(p <0.0009), 1/4Hz (p <0.0065).
Closed eyes: 1/16 Hz (p < 0.0015), 1/8 Hz (p <0.0019), 1/4Hz (p <0.0065),
1/2Hz (p < 0.0082).

Discussion.
The principal differences was in the low-frequency bands. This then implies that the visual system and the somatosensory system together are not capable of control completely the lowest frequencies in the postural control and when we eliminate the visual information the frequencies below 1/2Hz cannot be controlled fully.

Conclusions.
The vestibular system is fundamental in the control of the low frequencies bands in the postural control in peripheral vestibular patients.
The WDT analysis of the CoP could be a excellent tool in the support to diagnosis of the vestibular pathologies.
The use combined between CoP displacement and DWT, could be a good approach to understanding of some posture disorder.