Sorry for the late update! I'll try to remember what happened in the first three days. Basically I tested the biopac with electrodes around my eyes. The signal is very clean, though really small. So I built a very simple amplifier and low-pass filter. The amplifier has a gain of 10 and the filter has cut-off frequency at 30Hz. It worked really well. See video below:
In this video the signal corresponded to the horizontal movement of the eye. The center electrode acts like the ground. The other two are Vin+ and Vin-. According to the EOG theory, Vin+ minus Vin- will be proportional to horizontal orientation of the eye.
Then I did a linearity test to see whether the signal is linearly dependent on eye movement. What I did was to stare at the red and blue circles from the left most one to the right most one, one by one. And then from right most one to left most one. Here's the circles I stared at and the signal I got:
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