BioSemi.nl

Hi,

The "10-20" locations of several electrode sites seem to be way off. For instance, temporal electrodes T7 and T8 should be 10% up from the preauricular points (10-20 standard), but in reality they are typically 15% or so. The problem cannot be corrected by increasing the tension on the straps. For instance, on a typical subject with a distance of 40 cm (large/medium cap) between the left and right preauricular points, the distance between T7 from the left preauricular point is about 5.6 cm (instead of 4.0 cm). Same for T8. We tried both chin and chest straps, but there is no way that the cap can be stretched and "pulled down" to get these sites to where they should be.

Thus, it seems that these caps are "pseudo-10-20", and probably they should not be called 10-20 to avoid confusion and inconsistencies in the literature. I hope I am not missing something obvious here. More importantly, where can we get caps that conform to the 10-20 standard? Thanks.

Giorgio

Edit: see below comments

Giorgio, you are using a headcap which is too small.
If I understand correctly, since your subject has 40 cm distance between the 2 preauricular points, the 2 cap holders (T7 and T8) right above the 2 preauricular points should be at 4 cm distance of these 2 preauricular points, but you measured the distance to be 5.6 cm. You say you can not tighten the Velcro chin straps enough to make this distance equal to 4 cm.

I just tried a Medium/Large headcap on a typical male subject with a preauricular distance of 37 cm.

When I adjust the 7" chin strap at the most tight position, pulling the cap downwards as much as possible, the distance between T7 or T8 and its adjacent preauricular point can be made well less then 3.7 cm.

When I adjust the 7" chin strap at the most loose position, lifting the cap as much as possible, the distance between T7 or T8 and its adjacent preauricular point can be made well more then 3.7 cm.

In other words: A medium/Large sized cap used on my typical subject with 37 cm preauricular distance can be adjusted enough to match the 10-20 system.

You actually say the answer yourself: the cap can not be stretched enough to reach the points it should reach. The cap size you are using is too small.

40 cm preauricular distance is not typical. It is Large, maybe XL.

Thanks for your comments, Robbie. You are right about the 40 cm, the typical distance is indeed 37 cm (silly glitch with the tape measure), and the cap is a mid/large that otherwise fits very well on subjects with that head size (head circ. is within the range recommended for that cap). Yet, T7 and T8 are way dorsal, at least on this cap, and there is no way of pulling them down with any reasonable tension on the straps (the chin strap is already uncomfortable with mild tension). I'll examine some of the other caps we have. I wonder if some batches may have had electrode sites placed at incorrect locations.
Just to compare, what is the interelectrode distance on your caps (assuming they are medium/large ones; red/blue)? Say, cz-c1,c1-c3,c3-c5, and c5-t7? I assume it should be around 3.7 cm, if it conforms to the 10-20 system... Thanks.

G

Giorgio, I just tried a Large headcap on a subject with 40 cm preauricular distance. I must admit that I also found that T7 and T8 are located a bit too high with respect to the preauricular points. When I tried a Large/XL headcap on this subject, the location was about right.
I would recommend you to use a larger headcap (Large/XL), and maybe use a chin strip from a Medium (or smaller) sized headcap. Those straps are shorter (5" instead of 7"), and are able to pull the headcap downwards a bit more.