These are several articles I found particularly interesting looking at the understanding of thought bubbles, and using them to help autistic children.
Wellman, H. M., Hollander, M., & Schult, C. A. (1996). Young Children’s Understanding of Thought Bubbles and Thoughts. Child Development, 67, 768-788.
Several experiments on 3 and 4 year olds show that thought bubbles are understood at fast rates by both age groups as depicting thoughts. This is interesting, because at similar ages, other visual conventions such as speed lines are acquired over age (I'll post on these articles next maybe).
This contrasts with the findings in the Yannicopoulou study, which showed that preliterate children could not distinguish speech from thought balloons – which was not tested in these experiments. They suggest that this shows evidence for “something recognizable from our everyday understanding or experience of thoughts themselves” tapping into "theory of mind" knowledge...
Wellman, H. M., Baron-Cohen, S., Caswell, R., Gomez, J. C., Swettenham, J., Toye, E., et al. (2002). Thought-bubbles help children with autism acquire an alternative to a theory of mind. Autism, 6(4), 343-363.
Children with autism have specific difficulties understanding complex mental states in other people like thought, belief, and false belief and their effects on behavior (what are known in psychology as "Theory of mind").
These children benefit from focused teaching about thoughts, where beliefs are likened to photographs-in-the-head. Here two studies, one with seven participants and one with 10, tested a picture-in-the-head strategy for dealing with thoughts and behaviour by teaching children with autism about cartoon thought bubbles as a device for representing such mental states.
This device led children with autism to pass not only false belief tests, but also related theory of mind tests. These results confirm earlier findings of the efficacy of picture-in-the-head teaching about mental states, but go further in showing that thought-bubble training more easily extends to children’s understanding of thoughts (not just behaviour) and to enhanced performance on several transfer tasks. Thought-bubbles provide a theoretically interesting as well as especially easy and effective teaching technique.