VR in Movement disorders
The objective of this module is the development and use of virtual environments to study motion in some diseases of the basal ganglia (particularly Parkinson's disease).
Description of the problem
The term "basal ganglia" is used to describe five closely related nuclei: caudate, putamen, globus pallidus, subthalamic nucleus and sustantia nigra. The basal ganglia receive no direct sensory inputs and, like the cerebellum, send no direct motor output to spinal cord. However, there is no doubt that these structures are involved in the control of movement. All diseases of the basal ganglia in man have some disorder of movement as their primary symptom. Common to all these conditions are either an excess of (abnormal) involuntary movements or a lack of spontaneous movements. In addition, there may be changes in muscle tone and defects of postural reflexes.
Some brief definitions may be useful. Hyperkinesia refers to an excess of movement; akinesia or hypokinesia to a lack of spontaneous movement. Akinesia may also be used to refer to a lack of normal associated movements such as swinging of the arms when walking. In contrast to these terms which describe the amount of movement, bradykinesia defines the speed of movement: bradykinetic movements are slow (whether or not there is an excess, or a lack of them).
1. Rationale of the system
2. Description of the VR experimental system
Many papers (T.C. Britton et al., 1994; Thompson et al., 1988; Berardelli et al., 1986) have described the kinematics characteristics and the electromyographic patterns of movements, for example of the wrist, in patient with essential tremor, coexistence of bradykinesia and chorea in Huntington's disease and Parkinson's disease. These studies don't take into consideration the relationship that exists between the visual sensory information and control of movement being performed. The aim of these proposal is the development and use of immersive virtual environments to study the aforementioned relationship in people affected by diseases of the basal ganglia particularly Parkinson's disease.
In order to analyze this relationship we propose to consider a simple touch task, studied under four different conditions. The conditions are:
The task under examination can consist in the subject touching a lamina equipped with strain gauge with the tip of his/her index finger.
The advantages of the use of virtual environments over traditional methodologies can basically be summarized as follows: possibility of making infinite variations (which are also easily quantifiable) on experimental protocol, accurate quantification (stimulus, duration, kinematics characteristics of movement etc...), flexibility, conscious and unconscious involvement of patient.
TEST STATION. Only movements of flexion-extension of the index finger of the index finger of the hand are examined. The subjects are seated on a chair situated in front of the test station. The forearm is placed on a support specially studied to prevent fatigue and to ensure that the position of the metacarpophalangeal articulation of the index finger of the hand is fixed and precisely determined. The only movement allowed is the flexion-extension of the finger in a horizontal plane parallel with the support. By bending his/her finger in the above mentioned plane the patient touches the lamina. the radial distance between the position of the metacarpophalangeal articulation and the lamina can be regulated according to the length of the phalanxes of the patient's finger. The angular distance between the position of the lamina and the initial position of the finger, is also adjustable, the aim being to identify different test conditions.
CHARACTERISTICS OF THE EXOSKELETON. The exoskeleton will be a mechanical device aimed at measuring the angular displacements of the proximal and distal phalanxes. The exoskeleton will be based on the following cinematic model.
CINEMATIC MODEL OF THE FINGER. We approximate the metacarpophalangeal articulation and the interphalangeal articulations to revolute joints whose axes are perpendicular to the support plane. Furthermore we decide to make the distal phalanx rigidly solid with the media phalanx thus preventing relative movements. The cinematic model of the finger of the hand is therefore mathematically approximated by two elementary joints connected by two linear segments (B. Bucholz et al., 1992; T.H. Speeter, 1992).
CHARACTERISTICS OF THE VIRTUAL ENVIRONMENT. The virtual world will reproduce the test station and the hand of the subject placed on the support. Tracking the finger with the exoskeleton, the movement of the subject will be reproduced in the virtual world. The characteristics of the movement in the virtual world can be manipulated during the execution of the test, adding visual cues to help the subject in doing a correct movement or altering the representation of the movement in order to create a conflict between the perceived visual space and the perceived proprioceptive space.
SIGNALS TO BE ACQUIRED. The signals that we must acquire during the execution of each test are: 1) Angular displacements of the proximal and media phalanx starting from the initial position corresponding to a completely extended finger (The angular speed of the two phalanxes can be determinated by differentiating the angular displacements). 2) Force exercised on the lamina. 3) Superficial electromyogram of the extensor and flexor muscles of the index finger.
The acquisition of these signals require the following equipment: a) Surface electromyograph. b) Exoskeleton to measure the angular displacements of the proximal and distal phalanxes. c) Lamina fitted out with strain gauge. d) An analogue bandwidth filter. e) An analogue amplifier. f) A data acquisition card which provides an Analog/Digital conversion sector with al least the following characteristics: 12 bit resolution, maximum sampling frequency 50 khz, XT bus (ISA and EISA compatible). For instance we can use the National Instruments PC-LPM-16 data acquisition card.
The sampling frequency that we can use must be GE that 2 Khz. Acquisition begins 1 second before the acoustic (or visual) start signal; the signals are saved on a virtual disk and the copied onto a file associated with the subject only if the test deemed significant by the doctor. In this case the signals are processed automatically in order to identify points of significant importance. the coordinates of these points are saved on a file and can subsequently be modified by the doctor during the graphs analysis phase.
DEFINITION OF SIGNIFICANT POINTS. The significant points determined automatically must be: 1) Touching of the lamina. 2) Point indicating absolute maximum of the force signal. 3) Start of .movement of proximal and media phalanxes . 4) Points indicating absolute maximum of the angular speed of the proximal and media phalanxes. 5) Points at which the angular speed of the proximal and media phalanxes becomes null. 6) Onset points of the electromyographic burst of the flexor and extensor muscles. 7) Points indicating absolute maximum for the electromyographic signals of the flexor and extensor muscles.
Another parameter that must be evaluated is the number of movements used by the subject to reach the target.
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