Harp, S. F. & Mayer, R. E. (1997). The role of interest in learning from scientific text and illustrations: On the distinction between emotional interest and cognitive interest. Journal of Educational Psychology, 89(1), 92-102.

A textbook lesson may be made more interesting by promoting emotional interest through adding entertaining text and illustrations or by promoting cognitive interest through adding signals for structural understanding such as summary illustrations with captions. In Experiment 1, skilled readers who read summary text and illustrations about the process of lightning performed worse on retention of important information and on transfer when entertaining text, illustrations, or both were added. In Experiment 2, skilled readers rated entertaining text and illustrations relatively high in emotional interest and low in cognitive interest and rated summary illustrations and text relatively low in emotional interest and high in cognitive interest. The results suggest benefits of cognitive interest over emotional interest for helping students learn scientific explanations.