IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 53, NO. 3, MARCH 2006 559 Communications______________________________________________________________________ An Experimental Study to Investigate the Effects of a Motion Tracking Electromagnetic Sensor During EEG Data Acquisition Ali Bashashati*, Borna Noureddin, Rabab K. Ward, Peter D. Lawrence, and Gary E. Birch Abstract—A power spectral analysis study was conducted to investigate the effects of using an electromagnetic motion tracking sensor on an electroencephalogram (EEG) recording system. The results showed that the sensors do not generate any consistent frequency component(s) in the power spectrum of the EEG in the frequencies of interest (0.1–55 Hz). Index Terms—EEG, electroencephalography, motion tracking. I. INTRODUCTION There is evidence that when a person moves a part of his/her body, specific patterns are introduced in his/her ongoing electroencephalo- gram (EEG) signal [1]. Understanding the resulting changes in the EEG signal could lead to a better understanding of the underlying brain activity. Investigating such changes requires the simultaneous capture of these movements and the recording of the EEG signal. Some re- searchers use custom made devices such as a micro-switch (e.g., see [2]) or record the electromyogram (EMG) signal (e.g., see [3]) to de- tect the onset of the movement. The disadvantages of devices such as micro-switches are that they should be customized for each type of movement and they do not provide any knowledge of the dynamics of the movement such as speed and movement pattern of the moving part. Recording the EMG signal also has the latter disadvantage. By using an appropriate motion tracking method however, a complete pattern of the movement can be obtained. Subsequently, this information can be used to gain a better understanding of the relationship between the move- ment and the observed EEG patterns. This understanding can also be used to guide subjects to improve the quality of their executed move- ments during the experiment. The need for such a system arises in our present research in analyzing EEG signals which aims at building suc- cessful and efficient brain computer interfaces [4]. To analyze the EEG signals related to a specific movement, we need information about the movement, mainly its onset, duration, trajectory and speed. To be able to do this, we have to use a motion tracking system. As this device will be placed in our experimental environment, we must ensure that it does not introduce any artifacts in the EEG signal. Three other criteria for Manuscript received November 29, 2004; revised June 11, 2005. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada under Grant 90278-02 and Grant CBME-11R81758, in part by Canadian Institutes of Health Research under Grant MOP-62711, and in part by Precarn IRIS NCE. Asterisk indicates corresponding author. *A. Bashashati is with the Electrical and Computer Engineering Department, University of British Columbia, Vancouver, BC V6T 1Z4, Canada (e-mail: alibs@ece.ubc.ca). B. Noureddin, R. K. Ward, and P. D. Lawrence are with the Electrical and Computer Engineering Department, University of British Columbia, Vancouver, BC V6T 1Z4, Canada (e-mail: bornan@ece.ubc.ca; rababw@ece.ubc.ca; pe- terl@ece.ubc.ca). G. E. Birch is with the Neil Squire Society, Burnaby, BC V5M 3Z3, Canada and also with the Electrical and Computer Engineering Department, University of British Columbia, Vancouver, BC V6T 1Z4, Canada (e-mail: garyb@neil- squire.ca). Digital Object Identifier 10.1109/TBME.2005.869656 selecting the appropriate tracking device are accuracy, cost and versa- tility, versatility in the sense that the same device can be used to track the movement of different parts of the body. Currently, there are several commonly used technologies for tracking motion. These technologies are based on mechanical, electromagnetic, acoustic, optical, and inertial/magnetic principles [5]. The main dis- advantage of mechanical tracking systems is that the user’s range and type of the movement are constrained [5]. Optical and acoustic systems operate within specific ranges that are limited by their line-of-sight ob- servations. Electromagnetic tracking systems, on the other hand, have the versatility advantage and are not limited by the line-of-sight ob- servation. Such a system uses a “source” element (a transmitter) that radiates a magnetic field. A small sensor (receiver) that is placed on the body’s moving part reports its position with respect to the source [5]. The workspace of electromagnetic tracking systems is limited to a 6 m distance between the radiating source and the receiver. This limitation does not affect our experiments since the space in which movement is captured in our EEG recording environment is limited; actually the dis- tance between the source and the sensors is less than 1 m. Inertial/mag- netic sensors do not need an electromagnetic radiating source and they do not have space limitations. These sensors however are large and ex- pensive. Thus, a less expensive and smaller electromagnetic tracking technology is more suitable for our purposes. Previous work (e.g., see [6]) has demonstrated the application of electromagnetic sensors for locating electrodes before EEG recording. We were unable, however, to find previously reported simultaneous electromagnetic motion tracking and EEG recording. Since nonlinear effects in tissue or wiring may cause interference in the recorded EEG, we felt it prudent to examine the possibility of artifacts introduced by the use of electromagnetic sensors. To this end, we evaluate the Polhemus FASTRAK system, as this motion tracking device is readily available to us, relatively inexpensive and small in size. The goal of this study is to investigate the effects of using this motion tracking system on EEG signals. Specifically, we wish to test the two hypotheses: 1) the use of this system does not intro- duce any artifact with consistent frequency components to the power spectrum of the collected EEG signal in the frequency range of interest, and 2) if 1) does not hold, then any such artifacts can be removed by some practical means such as linear filtering. II. EXPERIMENTAL SYSTEM We wish to study whether the use of the electromagnetic motion tracking system affects amplifier calibration and frequency content of the recorded EEG signal. A. Electromagnetic Motion Tracking Sensor We use the Polhemus FASTRAK electromagnetic system for motion tracking. The Polhemus FASTRAK tracking system uses electromag- netic fields to determine the position and orientation of a small (2.83 cm width, 2.29 cm length, 1.52 cm height and 17 g weight) receiver (sensor) as it moves through space, and provides dynamic, real-time measurements of its position (X, Y, Z Cartesian coordinates) and its orientation (azimuth, elevation, and roll). The technology is based on generating near field, low frequency magnetic fields from a single as- sembly of three concentric and stationary antenna coils (transmitters) which are oriented perpendicular to one another and detecting the fields 0018-9294/$20.00 © 2006 IEEE