Polysomnography Step 3: The Cyton Board

Obviously anybody who has a Cyton has at least done the introductory tutorial provided on the OpenBCI website, and this part doesn’t really require much explanation. There’s also a decent introduction on “The Autodidacts” website, demonstrating how to set up basic ECG using OpenBCI.  But again, for the sake of documenting the process, here is how I proceed.

The “front” of the Cyton consists of all the connection pins, in two sleek rows. These are all labeled from N1P, N2P, etc., where “N” means the bottom row and “P” means the top. On the far right are the SRB pins, and on the far left are the AGND and BIAS pins. I’ve no idea what the AGND pin is for (ground?), but be careful not to accidentally plug your Bias cable into it! As mentioned in the previous post, the Bias just functions to help eliminate noise, and is typically connected to a mastoid or earlobe or some other “neutral” central location. SRB, however, is pretty important. By default, the other pins will all be measured against this reference pin, which again typically goes to a mastoid or earlobe. The difference between the top and bottom SRB pins doesn’t much matter because I’ll only ever need to use the bottom pin. The difference has something to with being able to turn SRB on and off, so to speak. The bottom pin can be regulated – some channels can use it as a reference, others not. The top pin, however, cannot be “turned off” – if there is something connected to the top SRB, all other pins will use it as a reference no matter what, so no “bipolar” readings are possible (more below).

I start, for no particular reason, with EMG. The electrodes were placed an inch or two apart underneath the chin, and now each is plugged in to the first top and bottom pins (i.e. N1 an P1). In this configuration, I’m pretty sure the top serves as the reference for the bottom.

Exactly the same holds true for the EOG readings. I plug the cable leading from the center-eye position into the top 2 pin (P2), and the cable leading from the eye-corner into the bottom 2 pin (N2). I’m going to experiment with other configurations eventually, but that’s the setup for now.

Then the EEG electrodes can be connected to the bottom of the 3, 4 and 5 pins, all of which will be referencing the SRB (bottom) pin.

That’s it! Now, the MAJOR drawback of this setup, as I have been warned, is that it is prone to major disaster if, say, the SRB ear clip comes off. By contrast, the SpiSOP setup developed by Dr. Frederik D. Weber has a few backup plans, such that they can later on, after the fact, compare the electrode at O2 with one at Cz instead of A1 if they so desired. But because Dr. Weber and his pals are professional sleep scientists and I am not, I’m still a bit nervous about substituting a backup reference electrode in the post-recording software (EDF Browser)!!! I don’t think it is too difficult, but for the time being this seems simpler, and the risk of SRB failure isn’t too huge.

OK – everything is connected! Time to get recording!