From my very short learning of sketching, I had a feeling that there's a precursor step before the pencil touches the paper: a mental rasterization process. Our brain wants to see things in a symbolic form, a dog, a yard, a tree. To draw, however, we must force our brain to see the imagery data: the highlights, the shadows, the gradients.
My drawing education was only a few months long, so I could be drawing a far-fetched conclusion here. I'd love to hear what experienced artists think of their own mental process before actually putting down the image.