This is a very basic science paper, but those of you who know children with difficulty perceiving body boundaries (usually in the context of general sensory processing problems) it may be interesting. The scientists here were studying an area of brain that seems to be rich in polysensory neurons - that is, neurons that respond to different types of sensory input. The idea is that maybe these neurons serve the purpose of forming the "protective bubble" a sense of space around ourselves that protect us from being injured by others.
When these polysensory neurons were stimulated in the monkey, aversive reactions, like pulling away and grimacing were seen.
The location of polysensory neurons in humans is an area that can be affected by mild birth stress, prematurity, or autism...so what we may be seeing with some young children's extreme fight or flight behaviors, is an abnormal triggering of a hypersensitive escape reaction.
Basic Science Paper - Personal Space and Aversive Reactions in the Monkey pdf