What does moving the electrodes from the scalp to the surface of the brain gain? · Grey Matter

It removes the skull, the layer that most attenuates and blurs the signal.

What does moving the electrodes from the scalp to the surface of the brain gain?

It removes the skull, the layer that most attenuates and blurs the signal. Electrodes on the cortex record from a patch millimetres across instead of centimetres, see amplitudes several times larger, pick up far fewer muscle and eye artefacts, and keep the high-frequency activity (above about 70 Hz) that tracks local firing and that the skull filters out. The price is surgery and limited coverage, since a grid only sees the cortex it lies on, and it still records populations of neurons, never single cells.