I had been searching for a readiness potential for weeks, trying to sift through noise two orders of magnitude louder than the signal itself, with little success. This morning Greg, my research mentor, pointed out that since I’m using a bipolar electrode EEG, the op-amp is only magnifying the difference between the two leads over my secondary motor area, which happen to be just a few centimeters apart. The signal between the two electrodes is practically identical, which means I won’t be picking up much of anything. Instead, I want to amplify the difference between a lead over the SMA and another lead on some more neutral part of the head.
To do this, I’m placing one electrode over C3, a second electrode on the base of my occiput, and a reference electrode clipped to my left ear. Instead of buying medical grade EEG ear clips, I soldered two washers to a copper alligator clip.
Currently in MATLAB, I’m applying a 0.25 – 64 Hz bandpass Butterworth filter. This seems to be a good range for recognizing slow cortical potentials. I kept picking up EKG pulse artifacts when I used two grounds (one on my hand for EMG, one on my mastoid for EEG), so I’ve removed the EMG ground, which eliminated EKG as well as most cross-talk between the two channels.
By Patrick Glover