MEASUREMENT OF TREMOR USING DIGITIZING TABLETS Guilherme A. A. Miotto, Adriano O. Andrade, Alcimar B. Soares Federal University of Uberlândia, Faculty of Electrical Engineering, Uberlândia – Minas Gerais guilherme.miotto@gmail.com Abstract – Tremor is the most common movement disorder. The assessment of tremor is in general carried out in a very subjective way. This paper aims to review and explain the use of digitizing graphic tablets for a precise measure of tremor. First it is explained why a accurate measure of tremor is important. Then some topics are discussed: (I) the influence of tablet specifications, (II) the acquisition and pre-processing of the data, (III) the reasons for using specific drawing tasks, and finally (IV) how to quantify the tremor. 1 Keywords – Digitizing graphic tablet; Drawing; Movement disorders; Spectral analysis; Spirographs; Tremor. I. INTRODUCTION Tremor, a rhythmic, involuntary oscillatory movement of body parts, is the most common movement disorder [1]. A common method employed for evaluation of patients with pathological tremor is the execution of drawing tasks. In this exam patients are instructed to make some drawings (i.e. Archimedes spirals, squares) and the result is subjectively rated using clinical scales simply by looking at the drawing. Though very simple, this approach fails in providing precise measures of tremor amplitude and frequency, which are of great interest for the clinician. This precision can be easily achieved using a digitalizing tablet for acquisition of the drawings. A digitizing tablet (Fig. 1) is a very simple mechanism that keeps track of the position of a pen. The data provided by the tablet can be stored in an ordinary computer and it is suitable for most signal analysis techniques. Fig. 1. A digitizing tablet. Another common way for precise detection of tremor is by means of accelerometry, however, digitalizing tablets has been revealing itself as a better method for clinical environments due to its simplicity and low cost [2]. A. The importance of frequency-amplitude information More than 10 distinct types of pathological tremor have been identified [3] and each one of them has different frequency-amplitude characteristics. For example, Spieker [4] found the frequency bandwidth for three pathological tremors: • Cerebelar Tremor: 1.5 - 4 Hz • Essential Tremor: 4 - 12 Hz • Parkinsonian Tremor: 3 - 8 Hz So, if a clinician has access to the frequency information of the patient’s tremor, he/she would have one additional tool for improving his/her diagnosis just by looking at the typical bandwidths of tremor syndromes. In addition to the bandwidths, each one of tremor pathologies has its own way of altering amplitude-frequency characteristic during the progress of the disease. Monitoring this kind of information is of great importance in the treatment of tremor pathologies. II. THE USE OF TABLETS FOR TREMOR DETECTION Following are some important considerations that are necessary to carry out a tremor measurement by means of a digitizing tremor. A. Tablet especifications A digitizing tablet has under its writing surface a set of parallel horizontal wires covered by a set of vertical parallel wires. The pen emits an electromagnetic field that excites the wires in the neighborhood permitting the tablet to calculate the pen position. The larger the number of wires per unit of area is, the larger the resolution of the tablet is. The resolution of the tablet is important to decide whether it is capable of acquiring a specific kind of tremor. For example, if it is desirable to detect physiologic tremor (amplitude below 0.5 mm [5]) with a resolution of 0.01 mm the tablet must have at least 100 wires per mm, which in commercial values is equivalent to 2520 Lines per Inch (LPI). Tablets with 3048 LPI can be easily found commercially. Actually, a tablet that is capable of detecting physiologic tremor can detect any other kind of tremor because the physiologic tremor is the one that presents the smallest amplitude. Another desirable specification for digitizing tablets that will be used to detect tremor is the sensitivity of the pressure. Modern tablets can measure how much force is applied on the pen while a person is drawing. This feature is important