Development of a Compact Wireless Laplacian Electrode Module for Electromyograms and Its Human Interface Applications

In this study, we developed a compact wireless Laplacian electrode module for electromyograms (EMGs). One of the advantages of the Laplacian electrode configuration is that EMGs obtained with it are expected to be sensitive to the firing of the muscle directly beneath the measurement site. The performance of the developed electrode module was investigated in two human interface applications: character-input interface and detection of finger movement during finger Braille typing. In the former application, the electrode module was combined with an EMG-mouse click converter circuit. In the latter, four electrode modules were used for detection of finger movements during finger Braille typing. Investigation on the character-input interface indicated that characters could be input stably by contraction of (a) the masseter, (b) trapezius, (c) anterior tibialis and (d) flexor carpi ulnaris muscles. This wide applicability is desirable when the interface is applied to persons with physical disabilities because the disability differs one to another. The investigation also demonstrated that the electrode module can work properly without any skin preparation. Finger movement detection experiments showed that each finger movement was more clearly detectable when comparing to EMGs recorded with conventional electrodes, suggesting that the Laplacian electrode module is more suitable for detecting the timing of finger movement during typing. This could be because the Laplacian configuration enables us to record EMGs just beneath the electrode. These results demonstrate the advantages of the Laplacian electrode module.


Introduction
Recent advances in information and communication technologies such as the Internet and e-mail have led to their world-wide use. As a consequence, the so-called "digital divide" between persons able to easily use these technologies and others who cannot, especially those with physical disabilities, has increased [1]. There are, therefore, urgent needs for developing interfaces that can help individuals with physical disabilities to operate pointing and/or character-input devices (see e.g., [2,3]). Because conventional human-computer interfaces work basically in response to physical motions (a good example for this is handwriting as input, see e.g., [4]), biosignals which eventually yield the motions can also be used for such interfaces [5]. In particular, electromyogram (EMG) signals have often been studied and actually used as control signals for artificial limb prostheses [6][7][8][9], robot hands [10], a manipulator [11] and a pointing device [12]. This may be because the EMG signal contains useful information about motion intent, muscle movement, muscle force, and muscle impedance.
The use of conventional interfaces employing wired electrodes and a wired amplifier is limited by the length of the connecting wires, and is restricted to persons who never suffer from involuntary limb actions. Therefore persons with physical disabilities such as cerebral palsy have little or no access to biosignal-based human-computer or human-machine interfaces. In addition, deaf-blind people cannot use a computer and accordingly, also have only little access to human interfaces. One method to communicate with those people is finger Braille, which uses tactile sensation. Some deaf-blind people can communicate with others through a finger Braille interpreter. Because finger Braille uses a code similar to Braille, it is relatively easy to develop an electro-mechanical device for finger Braille. Actually, there exist some studies aiming at developing a system which can automatically convert a text into tactile information and vice versa and function as interpreter [13,14]. To communicate more smoothly using such a system, understanding the prosody (rhythm and stress) of natural finger Braille is important.
In this paper, we have proposed a compact wireless Laplacian electrode module for EMG and apply it to character-input interface and evaluation of finger Braille typing. In our previous study, we proposed the Laplacian electrode configuration for EMG recording [15]. However, the developed system in [15] was not wireless, and it was not validated through any actual application. Although we confirmed synchronous firings with the Laplacian and conventional EMGs, we did not address the difference in characteristics as input for human interface. Our primary aim here was to demonstrate that the Laplacian EMG was better than the conventional EMG in actual human interface applications.
The Laplacian electrode configuration was easily implemented in the wireless module because the measuring electrodes can be accommodated together in a small area. In addition, as mentioned later, EMGs obtained with the Laplacian configuration were expected to be sensitive to the firing of the muscle directly beneath the measurement site. A character-input interface was developed using the electrode module with combination of an EMG-mouse click converter circuit and scanning cursor software. Investigations on the performance of the character-input interface were carried out in healthy male subjects. One of the purposes of this investigation was to verify the usability of the electrode module. In the application to finger Braille typing, four electrode modules were attached to an interpreter, two on each forearm, to detect finger movements during typing. One finger Braille interpreter participated in an experiment for finger movement detection. In this experiment, we also examined whether the four modules were able to work properly in parallel.

Theoretical Basis of Body Surface Laplacians
Use of the body surface Laplacian was first proposed by Hjorth for electroencephalogram (EEG) recording in 1975 [16], and was later applied to EMG derivation by Reucher et al. in 1987 [17,18], and to electrocardiogram (ECG) measurement by He and Cohen in 1992 [19]. While the use of the surface Laplacian has been growing for current source estimation in EEG and ECG measurements, it has not been widely used for EMG measurement, especially in relation to human-computer or human-machine interfaces.
Considering a local orthogonal coordinate system (x, y, z) with an origin at a point on the body surface where the z axis is orthogonal to the body surface, the Laplacian EMG, L S , is defined by applying a Laplacian operator to the body surface potential φ, as follows: If the body is assumed to be a linear, isotropic and piecewise homogeneous conductor, the gradient of the electrode potential φ is proportional to the electrical field E: On the other hand, Ohm's law requires:    where φ i represents the potential at one of the surrounding points, r is the radius of the circle, and n is the number of points surrounding the circle. In the bipolar scheme, the Laplacian potential can be expressed by: where the integral is taken around a circle of radius b [19].
We can implement the electrodes together in a small area with both configurations. This feature is preferable for the implementation of electrodes in a single module, and thus for fabrication of an active wireless device that functions not only as an electrode module but also as a device for amplifying, filtering and transmitting the detected signal. In the developed wireless Laplacian electrode module, we employed a configuration proposed by MacKay ( Figure 2) [22], where three positive electrodes were aligned on each apex of an equilateral triangle and one negative (source) electrode was placed at the center of the triangle. If the input impedance, Z, of the operational amplifier is far larger than the resistance, R, in Figure 2, then the current through Z is negligible. Then, we can express the total amount of current passing through the resistance according to Kirchhoff's law as follows: Therefore: Then v LS can be described using the gain A v of the amplifier by: Thus, the adopted configuration is a special case of the unipolar scheme when n equals 3 in Equation (7).

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A comm Tokyo, Japa communicat applications economic e regulation o reasons: (1) using FSK a  In order to convert the EMG signal into a click signal of a computer mouse, we assembled an EMG-mouse click converter circuit. Figure 4 shows its block diagram. First, the wireless transmitted EMG signal was fed to a voltage follower to prevent voltage loss and then high-pass filtered with a cut-off frequency of 5 Hz to reduce the baseline fluctuation. Next the filtered signal was rectified by a full-wave rectifier and converted to an integrated EMG (IEMG) signal. Finally the IEMG was compared with a threshold of a comparator and then transformed into a pulse signal when the IEMG exceeded the threshold. Threshold hysteresis was employed to prevent chattering. The threshold could be altered by a variable resistor in the comparator. Once the pulse signal was input to a relay circuit (G6C-211P-US, Omron, Kyoto, Japan), the two terminals of a mouse controller, which was connected to a personal computer via a universal serial bus (USB), were short-circuited and a click signal was generated by the controller. The controller was taken out from a commercially available mouse.

Character Input Software with Scanning Cursor
We modified a software program developed in [23,24] so that it assists disabled persons to input characters by contracting one of their muscles. Figure 5 shows examples of screens during the software execution. The user interface of the software was based on a standard matrix of Japanese "kana" characters and an active cursor. The standard matrix is familiar to the Japanese and used in many commercial communication aids in Japan. The active cursor scanned the characters horizontally or  As a benefit of the Laplacian electrode configuration, we expected it to reduce a risk of secondary disabling due to overwork of the muscle from which the EMG signal was derived for the interface. In order to evaluate the muscle activity required for character input, we compared the IEMGs with the Laplacian and conventional configurations. Eight male subjects without physical disabilities, aged between 21 and 29, participated in the experiment. The wireless module was attached to the flexor carpi ulnaris muscle of the right forearm. For the conventional EMG recording, two electrodes with lead wires were attached on both sides of the wireless module along the muscle, and a reference electrode was placed at the distal end of the radius of the same forearm. The Laplacian and conventional EMG signals were used as the input signal in Conditions 1 and 2, respectively. In both conditions, the subjects were instructed to input a sentence consisting of 115 characters using the system by flexing the forefinger or the middle finger in one trial. Five trials were conducted under each condition and the sequence of the conditions was randomized. The subjects were allowed sufficient practice before each trial. To evaluate the muscle activity during the use of the interface, the IEMG was calculated from the conventional EMG in both conditions so that the conditions except the electrode configuration were identical. The time constant was 0.016 s. The mean IEMGs in both conditions were compared statistically.

Detection of Finger Movement during Finger Braille
Understanding the prosody (rhythm and stress) of natural finger Braille is important to aid smooth communication using a computer-based finger Braille system. We, therefore, investigated whether prosodic information can be obtained from EMG recorded with the electrode module. In this experiment, we also aimed to examine whether multiple electrode modules were able to work properly in parallel.
One finger Braille interpreter, who has worked as an interpreter for more than 20 years, participated in an experiment for finger movement detection. He gave written informed consent. The interpreter translated three news articles of about 130 characters in Japanese (when read it took about 20 s) into finger Braille. The same translation was performed three times, and the average translation rate was 6.37 characters/s. As shown in Figure 6, four electrode modules were attached to the interpreter, two on each flexor carpi ulnaris muscle, to detect finger movements during finger Braille typing. No skin preparation was carried out for the electrode module. For a comparison, two conventional disposable electrodes (F-150S, Nihon Kohden, Tokyo, Japan) were also placed on the left flexor carpi ulnaris muscle. For the conventional electrodes, the earth electrode was placed on the left elbow, and a commercially available amplifier (EMG100C, Biopac Systems, Goleta, CA, USA) was used under the following conditions: the gain was 1,000 and the pass band was between 1 Hz and 5 kHz. Then the filtered EMG signal was rectified by a full-wave rectifier and converted to an IEMG signal. The time constant was 0.016 s. Based on the IEMG, the timing of finger Braille typing was determined with a threshold level: 10% and 20% of the maximum IEMG level. An error rate in detecting the typed Braille number was calculated to evaluate the performance. One of the purposes of this experiment was to demonstrate that multiple wireless modules could be used simultaneously.

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In additio wireless mo muscle cont he develop developed s affected part Figure  contractions. Approximately 80% of computer users, whose jobs involve repetitive wrist movements and awkward hand positions, exhibit musculoskeletal dysfunction [25,26]. Accordingly, users of EMG-based computer interfaces may also develop similar dysfunction due to repetitive contractions of a specific muscle for character input. Because the developed interface requires smaller muscle contractions, it is desirable to reduce a risk of secondary disabling due to overwork of the muscle from which the EMG signal was derived. This is another advantage of the Laplacian electrode module.

Finger Movement Detection during Finger Braille Typing
The finger Braille experiment demonstrated the proper functioning of multiple modules used simultaneously. A single receiver was able to receive signals from two transmitters simultaneously by using slightly different frequency. Accordingly, two receivers were used in this experiment. The experimental results indicate that the receivers were able to process the wireless transmitted EMG without any crosstalk even when used in parallel.
One advantage of measuring prosodic information using EMG is that the method can be used during actual communication by finger Braille. A pressure sensor can be used to detect the finger movement [27]. However, the pressure sensor can hardly measure the movement during the actual finger Braille communication because the pressure sensor is usually made of a hard material and thus, the finger movement of a person cannot be sensed by the other. Although the interpreter did not communicate with a deaf-blind in the experiment in this study, the EMG method will not interfere, in theory, with communication by finger Braille.

Conclusions
In this study, we have developed a compact wireless Laplacian electrode module for EMG. The performance of the developed electrode module was investigated in two human interface applications. First, the module was used as a part of a character-input interface system, which consisted of the module, an EMG-mouse click converter circuit and software. Then the module was used for detection of finger movements during finger Braille typing. The evaluation of the character-input interface indicated that characters could be input stably by contraction of (a) the masseter, (b) trapezius, (c) anterior tibialis and (d) flexor carpi ulnaris muscles. This wide applicability is desirable as a technical aid for persons with physical disabilities because the disability differs one to another. Experiments for finger movement detection showed that each finger movement was more clearly detectable when comparing to EMG recorded with conventional electrodes, suggesting that the Laplacian electrode module is more suitable for detecting the timing of finger movement during typing. These results demonstrated advantages of the Laplacian electrode module.