Extent of cortical generators visible on the scalp: Effect of a subdural grid

- Subdural grids attenuate the scalp potential of generators located under them.
- Attenuation of grids and amplification of skull holes do not cancel each other.
- Minimum cortical extent of scalp activity may be smaller than previously reported.

The effect of the non-conducting substrate of a subdural grid on the scalp electric potential distribution is studied through simulations. Using a detailed head model and the finite element method we show that the governing physics equations predict an important attenuation in the scalp potential for generators located under the grid, and an amplification for generators located under holes in the skull filled with conductive media. These effects are spatially localized and do not cancel each other. A 4 × 8 cm grid can produce attenuations of 2 to 3 times, and an 8 × 8 cm grid attenuation of up to 8 times. As a consequence, when there is no subdural grid, generators of 4 to 8 cm2 produce scalp potentials of the same maximum amplitude as generators of 10 to 20 cm2 under the center of a subdural grid. This means that the minimum cortical extents necessary to produce visible scalp activity determined from simultaneous scalp and subdural recordings can be overestimations.