I believe it was an observation that lead him to look at the role of the skin as the mechanism to control speech. Tomatis finally came to the belief that ultimately the stutterer does not hear or listen to themselves, and this is because he is incapable of auditory awareness on the level of bone conduction. This doesn't mean that the individual cannot hear through bone conduction, it more that they don't process through bone conduction. When we speak the sound comes out, with the higher sounds thrown out in a straight line, and lower sounds spread out everywhere. At the same time the skill vibrates and transmits sound information directly in the ear and down the spinal column directly into the whole body.
Great singers control their voices through bone conduction.
So these four factors became very important in Tomatis' theories regarding the treatment of listening disorders. By the 1950's he had identified these following principals. The voice can only produce what the ear can hear The leading ear has a role in the establishment of audio-vocal control. The body has an important role in self-listening. Bone conduction plays an important role in self-listening.