The Modality Principle states that we should use audio (narration) rather than on-screen text to describe graphics whenever possible. By presenting graphics and text at the same time, we overload the visual cognitive channel of our user and make it more difficult to process information.
Modality principle is a guideline rather than a rule, and has its limitations. “We recognize that in some cases it may not be practical to implement the modality principle because the creation of sound may involve technical demands that the learning environment cannot meet such (as bandwidth, sound cards, headsets) or may cause too much noise in the learning environment. Using sound also may add unreasonable expense or might make it more difficult to update rapidly changing information. We also recognize the use of the Modality Principle is limited to those situations in which the words and graphics are simultaneously presented, and thus does not apply when words are presented without any current picture or other visual input. Additionally there are times when the words should remain available to the learner for memory support – particularly when the words are technical, unfamiliar, not in the learner’s native language, or needed future reference.” Chapter 6, pages 119 -120, E-Learning and the Science of Instruction by Clark and Mayer. If the material is easy for the learner or the learner has control over the pacing of the material, the modality principle becomes less important.
There is strong evidence to support the Modality Principle. By using audio narration instead of text to explain graphics you are using two cognitive channels instead of one. You are using an auditory channel with spoken words, and a visual channel with a graphic. Your working memory has more exposure for deep learning in two ways: phonetic processing from your ears and visual processing from your eyes.