A new study published
The act of looking back and forth from the subject to the drawing involves the coordination of perceptual, cognitive, and muscular skills. You have to see a shape, then remember it briefly, and finally translate your understanding of it into hand movements.
The main focus of this study is how the saccadic eye movements of an artist engaged in the task of drawing differ from the eye movements of a person who is free-viewing a subject without such a task in mind.
It turns out that the kind of looking we do while drawing is quite different from normal free-viewing:
1. The saccadic leaps are slower
2. The eyes tend to follow contours more
3. They move in saccades of shorter distance
4. And they fixate longer on individual details, rather than skipping around the whole scene.
No huge surprise there, I suppose, especially if you give someone a task of copying a curving line.
I would also be interested to see someone examine how artists use peripheral vision, squinting, blurring of the visual field, seeking alignments, and other specialized skills to shift attention from small details to the "big picture." These are skills that beginning artists take a while to master.
Later in the article, the authors note that there has been a lot of debate about what drives saccadic eye movements, not only in a specialized task like drawing, but in normal viewing. Are our eye movements passively driven by features in the scene, or are they actively controlled by the conscious attention? I would suspect that it's a combination of the two, and that artists in the act of drawing are much more on the "active control" end of the spectrum.
Visuomotor characterization of eye movements in a drawing task by Ruben Coen-Cagli, Paolo Coraggio, Paolo Napoletano, Odelia Schwartz, Mario Ferraro, and Giuseppe Boccignone.
I'd like to thank the authors for making their study available for free, and I'd also like to thank Paul Foxton for sharing it with me.