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Think of the "ticker tape" at the bottom of ESPN or CNN, people read this information and it stays with them longer than the hosts that they tune out. The "facts" at the bottom people read and hear it in their heads which is held onto possibly longer than someone elses voice on a video screen. That would be an interesting study.
You could also look into Khan's black screen approach where he only uses that backdrop. He is humorous and captivating like in his Ted.com talk, so I extract more by listening to his voice and seeing the examples he is drawing up. Your videos with students (assumed) add another element of stimulus that might take away from their efficency.