We have evolved to be able to look and listen simultaneously by means of two complementary mental routes.  They allow us to listen to words as we look at (or imagine) a scene.  Used intelligently in teaching, this dual ability can effectively double learners’ mental processing ability[1].

                   Click on ‘Language’ and ‘Images’ to discover more about each route

However, if we try to attend to two similar modes at once, such as reading text while listening to a lecturer, or alternatively looking at a picture at the same time as imagining a scene, the two similar forms interfere with each other.  The combination becomes too big and complex to be processed in one channel[2].

This is the reason that on-screen text and simultaneous speech is often worse than no visual aid at all; the method overloads the student’s language channel, and she/he is obliged to screen one out in order to let one be processed in the language channel.