What was your reference and what is the time scale?

Left and right earlobe for each hemisphere. The time scale is 30mm/s and amp is 5mm/50 microV.

I meant the distance between purple lines correspond to how much time?

The scale is 1 sec.

I thought that the frequent artifact was due to your electrode being close to the vein and picking up the heartbeat. However, the frequency is too low. Really, I don't know. Regarding your rare artifact it looks like an eye artifact. Usually it would be stronger at the anterior sites and weaker at the posterior sites. But sometimes with the transitory increase in the impedance at the reference the ground signal may be reflected at the posterior electrodes, especially if your ground electrode is close to forehead.

Thanks Dan Nemrodov for your explanation..

Regarding the rare artifact it wasn't eye artifact because I have recorded eye artifact as you said it is stronger and reducing gradually from anterior to posterior sites.

The artefact on the first jpeg is really strange, there is no gradient with the surrounding electrodes and it seems that the frequency is not stable (the intervals betweens 2 artefacts is not egual). So the questions I have are what kind of electrodes are you using (cap?)? and how cables are secured? have you this artefact only with one subject or is it recurrent?

The goal of my questions is to determine wether or not the origin is physiological, linked to the activity of the subject or due to your setup.

best

I have recorded many subjects with the same setup but this is first time i get this type of artifact . I don't think there is any problem with electrodes or cables. It is appear for only one subject and for one time only (disappeared in the second run for same subject).

Both are artefacts of moving electrods. Move a little bit and attach betters the electrods

Very simple: A square wave pulse convolved with a digital filter. You can reproduce that artifact by creating your own square wave pulse and then applying the same filter you used for the data.

The hard question is where that square wave pulse came from. Because it's on only one electrode, it is likely that there is some kind of problem with that electrode.

Probably electrode popping artifact.

Both behave as artefacts from moving electrodes -agree with the responses of Lilia Curzi-Dascalova and Eleftherios Papathanasiou. Most often due to to bad contact with the scalp, with sudden change in impedance. Next time you can try to reproduce it by touching or briskly moving the electrode. Good luck !

There are several articles which shows pictures of artifacts. Let me know if you need them. I don't have them right now, but can search

I don't see the montage labels, that would be helpful.

Electrode pop, a sudden change in the electrical potential between electrode and scalp gives rise to the pop