I've made a couple additions to the Reference Bibliography, including this one:
Nakazawa, Jun. 2005. Development of Manga (Comic Book) Literacy in Children. In Shwalb, David, Jun Nakazawa, Barbara (Eds). Applied developmental psychology: Theory, practice, and research from Japan. Pp. 23-42
This English piece is a nice summary of the work of Japanese Psychologist Jun Nakazawa, as well as several other Japanese studies on "manga literacy." His various experiments cover a lot of ground, usually looking at students from 1st through 8th grade. Most all his findings show increased understandings with aging and expertise. I'll discuss only a few of the many studies in it.
The study I liked the most asked children to arrange randomly given four panels into a strip, finding that correct answers grew from fairly low for kindergarteners and 1st graders (5.2 and 6.6%) to high for 4th and 6th graders (around 80%). Another task on that test asked for students to fill in the blank of a missing panel, which no K/1st graders could get right with increasing percentages along older grades. Comparatively, adult college students were far better than the children.
He also has designed a "story comprehension" test to examine how fully they can recall plot aspects of a ten-page Doraemon manga. He showed again that the biggest growth came between 1st and 4th grades.
He also did some eye-tracking studies comparing the eye movements of an "expert" versus a "non-frequent" manga reader. The "non-expert" fixated far more on word balloons than images and had higher reading times. On the other hand, the "expert" reader made "fewer useless eye movements" that were smoother, in addition to a higher rate of skipping over more panels and balloons. However, the expert also had higher story comprehension recall than the non-expert, despite reading faster and skipping elements.
The second part of the paper looked a lot at the role of manga in education. One interesting finding showed that frequent reading of manga correlated to achievement in language arts (particularly sentence comprehension) and a liking of social sciences, though "not significantly with liking for art class." Several studies also indicated a higher comprehension for learning from manga than from pure textual "novelized" writing.
In all, the piece presents several very interesting findings related to children's (and some adults) understandings of manga, and it is a veritible treasure trove of citations and studies. It presents a "cognitive processing model" based on this work, though it's so general that it could apply to any type of media. Along those lines, it doesn't really break up understanding into any sort of "grammatical" components as I'd like to see, lumping in aspects of things together (like manga consisting of pictures, emblems, text, etc rather than breaking those things down). The best part of the paper is its overall picture: that the skills required to understand the "comic medium" are learned and increase over age and practice.