How important is seeing the face of the instructor? So, I am yet to make up my mind on which is a superior mode of teaching in flipped classroom videos:


Khan academy type model where you see slides or a digital drawing board and only listen to the instructor.
Or a video in which you see the instructor communicating with you. Maybe in one third of the video and the rest of the screen real estate is focused on graphics, video etc.


Let us assume that you have a superb communicator in both cases.

Which model is superior? Have there been any studies in this field?
I cannot cite studies, but extensive anecdotal observation (maybe 5,000 undergrad students, 500 grad students) suggests that about 1/4 of students do not need, or do not care, about the person who is the instructor. But a vast majority, perhaps 3/4, do care. There is a sense of needing/wanting reassurance from a person, and caring about the insights of a person. The lack of this need/want comes in (at least) two very opposite forms: obviously the oblivious/incompetent failure to see what's going on, but also the relatively-very-competent student who scarcely needs or wants reassurance and has better ways to spend their time. The middle ground seems not to exist.

So, operationally, for almost all purposes, the person who is the teacher matters, whether or not they are a truly wonderful teacher (as long as they're not completely inert as a human being). 

(The "funny case" is the few students, both at undergrad and graduate levels, who for some reason expect/demand the teacher to be an automaton, reciting a text, not looking at students, not caring about students, not contributing, not critiquing, not improving. Yet they come to class rather than read the book? Maybe in anticipation of "books on tape", to have the classic text read to them out loud, in a room with other people?)

(The aspect of trusting in the expertise of the instructor seems, sadly, less relevant than the mandated understanding of their "authority". Tsk.)