Two researchable questions: do talking heads and moving hands improve learning?

I would like to see studies testing the hypotheses that seeing a talking head or a hand writing on the screen improves student comprehension and retention.

I've developed a collection of modules for my digital literacy course. Each module contains an annotated PowerPoint presentation and other material. As I find time, I record videos of the PowerPoint presentations, using the annotation as a script.

To conserve bandwidth and storage space, I only use still images of the PowerPoint slides in my videos. There is no video of me speaking. I reasoned that my students knew what I looked like, and viewers who were not in my class could see what I looked like in a photo.

But, I notice that Google (left), Coursera (right) and others include video of the speaker's face during their presentations.

Is there evidence that talking heads improve comprehension and retention?

We see a similar contrast in the screencast videos produced by Udacity (left) and the Khan Academy (right}. Udacity videos are screencasts with superimposed video showing the teacher's hand and pen while drawing and writing on the screen (with noticeable parallax). Khan produces screencasts in which only a cursor is shown.

Does seeing the hand and pen improve comprehension and retention?

(If these research questions sound interesting, see a proposal for a related study of the effect on comprehension and retention of varying playback speed and bullet point images).