Periodic spikes in recordings
Hello,
I'm a beginner when it comes to EEG. So I need some advice on the following problem. I get regular spikes when recording and I don't know what could be causing it.
I also tried to display the signal in the matlab according to Irene Vigué-Guix's instructions. ("https://irenevigueguix.wordpress.com/2016/04/22/loading-openbci-datasets-in-eeglab/")
The attached picture shows the filtered record(left) and also the raw data(right).
I found that the cable leading to the BIAS pin was faulty and after replacement the signal improved overall. However, in reviewing the recording, I still seem to be getting some periodic spikes.
Does anyone know what could be causing these spikes?
I would appreciate any help or suggestions. Thank you in advance for your replies.
Comments
Hi Sourevoj,
You have not mentioned what electrodes you are using and their scalp locations. Also include if this is paste, gel, dry, etc.
In EEG there are many sources of 'artifacts'. Common sources include eye blinks, eye movement, jaw and facial movement, head movement, etc. Typically electrodes closer to the source have stronger response; so eye artifacts appear stronger in frontal areas.
Your final time series has NO artifacts, and amplitudes are only a few uV.
William
Hi William
I made my own headband using gold electrodes with gel. I have the placement on Fp1 and Fp2, with a reference electrode between them. Then I use an ear clip for the BIAS pin.
I am posting a link to a short recording where the measurements were taken with eyes closed and in a relaxed state. Could you possibly advise me if this recording is ok ?
[https://drive.google.com/file/d/1bgE7Y9D1rz4ecbD6eSLMA_jT6mJk32DF/view?usp=sharing]
Thank you very much for your reply.
As mentioned, the forehead 10-20 sites will have the WORST potential for eye artifacts.
Placing your reference so close to the measured sites, means that your signal amplitude will be very low and contain more noise. Why not use one ear clip for reference and one for bias/ground?
I wanted to build an alternative to the commercial Zmax device from Hypnodyne, which has electrodes only on the forehead.
The purpose was to perform sleep studies with this device and I was concerned that the clips on the ears might interfere.
I tried putting ear clips on the reference and Bias/ground as you advised. Since I only have two channels from my headband, I wanted to turn the other channels off. But as soon as I turned them off I got some noise induced. Could you advise me what could have caused this?


re: EEG for sleep recording and gel or paste
You cannot use water based electrolytes like gel or paste for long term recordings. As these dry out in a couple hours and are no longer effective. You might consider using the dry flat electrodes in the shop. Perhaps in an array of four across the entire forehead. The outer most pair could be your reference / bias. And the inner pair your Fp1 Fp2. Or vice versa.
https://shop.openbci.com/products/openbci-eeg-headband-kit
re: turning off channels
It used to be that clicking on the colored channel number in the Time Series graph would turn the channel off. That no longer is the case? You are turning them off in the channel settings page?
When did you purchase this Cyton? Was it purchased from the OpenBCI website or somewhere else?
William
If I want to turn it off, I usually press the channel number in the time series. I assumed that when the channels are off, it doesn't pick up any noise. Or does it not have to?
I did another measurement where I also tried the dry electrodes that I purchased from you along with the Cyton board in 2019. But I kept getting weird spikes, maybe it's just an artifact from blinking. However, the FFT graph looked different than the other blinks. Could you tell me if it was a blink or not? I know tha this one was a blink.


But I'm not sure about this one.