On Progress Bars and Sanity

I found this TED video featuring Daniel Engber very interesting and it got me wondering about the implications of progress bars on e-learning. I know it is often suggested to include a progress bar as a way to keep the learner informed of where they are in the course, and what's to come (more like breadcrumbs) but I wonder if any studies have been done on how the inclusion of a progress bar impacts the learners motivation and satisfaction.  Would different types of progress bars or other progress mechanisms impact learning?

It’s food for thought as an instructional designer. The easy route is often the built-in progress bar on a course player (thinking Articulate Storyline), but maybe custom designed progress bars could increase learner engagement and motivation. On the flip side, can a progress bar be a de-motivator if a student feels the course itself is not engaging? Will knowing they are only 3% into a course they are not enjoying have a negative impact?