Thank you Liske :)

I think you nailed it about the brain :D

During a while I had thought it would become some skull seen from the side, but I wanted to find something more subtle than "the mechanical thinking machine seen for a side" :D

About the eraser, to be honest, nothing really special : I just use it as if it was a white pen, drawing above dark areas, instead of using it as a correcting tool :)

The fact it erases the tint can reinforce the light effect by contrasting both against darkness and background color.

That could be a problem in soft distant lighting, but - I guess - can also give some striking 3D effect on foreground.

I still have to learn a lot in this area :)