Unit of Gain Matrix (Lead Field)

Hi all

I have read about units of Lead Field matrix generated in Brainstorm, but still i have doubts abouts this.

I have generated my lead Fields from Freesurfer surfaces, i got the maximum and the minimum this is 4000 and -2000 approximately, but i don`t know what is the unit. My goal is to know what is the unit of every one of the points in the Gain Matrix grid. Cuz after i want to generate my own J to simulate my eeg from my model.

EEG (uv) =Gain Matrix * J

Cheers

Two types of answers:
http://neuroimage.usc.edu/forums/showthread.php?1837-EEG-units
http://neuroimage.usc.edu/forums/showthread.php?2010-Leadfield-Matrix-units