Challenges
The frequency content of the EOG signal is very close to DC and hence the separation of DC drifts from the useful signal content is a difficult task. The shift in the DC level during EOG signal recording is much observed using any EOG measuring system. Measurements are affected when eye makes quick movements from one fixation point to another. The artifacts are caused by eyeball rotation and movement, eyelid movement, the EMG signal produced by the muscle of the eye, eye blinks, electrode placement, head movement, influence of luminance and others. Care should be taken to minimize all these artifacts. In the preprocessing, filter to reject out of band frequencies and amplifier to amplify the signals are required. Patient safety is an important issue. The designed amplifier should provide proper isolation of the subject from the hardware circuit.
Electrode Configuration
The literatures on the human eye describe the eye as a fixed dipole. Cornea is the positive pole and retina as the negative pole. This potential ranges from 10-100 microvolts. It was Emil du Bois-Reymond (1848) who observed the cornea as electrically positive with respect to retina. Horizontal EOG is measured as a voltage by means of electrodes strategically placed as close as possible to each eye (on canthi). Similarly, vertical EOG is measured as a voltage by means of electrodes placed just above and below the eye. The reference electrode is placed on the forehead. Solid gel electrodes were chosen over wet gel electrodes as a precautionary measure to prevent the gel from entering the eyes. Based on the half-cell potential Silver/Silver-Chloride electrodes were chosen. These electrodes have half-cell potential almost near to zero. Minimum half-cell potential causes the least amount of offset. The hydrogen and lead electrodes have very low half-cell potential, but since hydrogen being of gaseous nature and lead being hazardous to the health, they are not used. Hence considering an optimal solution for safety and precision, Silver (Ag)-Silver Chloride (AgCl) electrodes are used. Also these electrodes are the best compromise between cost and reliability. After cleaning the surface of the subject's skin, additionally an electrolytic gel is applied to the skin to reduce the skin impedance that is, for better contact and conductivity [3]. A total of 5 electrodes have been used, two for horizontal, two for vertical and one reference electrode placed on the forehead. The electrode placement is shown in figure 1.
Figure 1. Electrode Placement for EOG
Preprocessing -Amplification and filtering
The EOG signals have been obtained for the subject in vertical and horizontal channel. EOG has been recorded for several activities like right and left eye movements, by following a specific object, and also by reading a simple paragraph, keeping eyes closed, eyes open, up and down movements. Typically, the EOG signal has a frequency range between DC and 27 Hz and amplitude between 10 to 100μV. The EOG signals were pre-amplified and filtered initially, the noise including power line interference is suppressed through a 50 Hz notch filter. To further remove the noise, a band pass filter with a cut-off frequency of 0.05Hz and 30Hz (designed using low pass and high pass filters), and FIR filter with Bartlett window has been used for smoothing the waveform. The EOG signal, like most of the other bio-signals is affected by environmental conditions and biological artifacts. The design of the EOG amplifier needs proper attention. Therefore the primary design considerations that have been kept in mind during the design of the EOG bio potential amplifier are proper amplification, sufficient bandwidth, high input impedance, low noise, stability against temperature and voltage fluctuations, elimination of DC drifts and power-line interference.
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