Kevin M. Hoffman, A List Apart :
In a meeting, absorbing something seen and absorbing something heard require different parts of the brain. Those two parts can work together to improve retention (the quantity and accuracy of information in our brain) or compete to reduce retention. Nowhere is this better illustrated than in the research of Richard E. Meyer, where he has found that “people learn better from words and pictures than from words alone, but not all graphics are created equal(ly).” When what you hear and what you see compete, it creates cognitive dissonance. Listening to someone speaking while reading the same words on a screen actually decreases the ability to commit something to memory. People who are subjected to presentation slides filled with speaking points face this challenge. But listening to someone while looking at a complementary photograph or drawing increases the likelihood of committing something to working memory.