This article (19 page PDF) is a standard literature review, but it's the topic that is most interesting. "Smell is the greatest ally of memories: it allows us to travel through time and therefore ensures that the sense of smell is chosen as a privileged sense by memory." That may be true, but the research on it in relation to MOOCs is quite limited. Nonetheless, studies do exist! There's a full list at the end of the paper. Most of the research "zeroed in on student issues, for example, the student experience, social learning, commitment, self-controlled learning, inspiration, execution, and MOOC finish."