In reply to Steve Tripp's compelling paper, I have no argument with his study or findings. Rather I find myself asking, "Why?"
If students really do learn better or retain more from reading the printed word, might it have something to do with the processes of encoding and decoding information?It seems to me that visual (pictorial) and auditory information are processed in more of an analog manner. The brain receives information that is processed in a such a way that it retains the "shape" of the information. If I study a photograph, I retain an image. If I learn a song, I can "hear" it playing in my head. However, when I read a book, I process symbolic information in the form of words. I then decode the meaning and in my mind I create something that is not at all like the printed page, but neither is it a Quicktime movie? I may remember a particularly striking passage verbatim, but most likely not.
Although the spoken language is itself an essentially symbolic form, it is not a new one, and our memory of the spoken word is, like the song, an analogous one. We "hear" the words in our heads as we heard them spoken.
I'm sure there must be research on this aspect of learning. It's not really my area.