I have an interesting article by H. Prechtl titled "The Directed Head Turning
Response and Allied Movements of the Human Baby" written in the 50's. He says
(from observations of 500 babies) "when the middle part of either the upper
or lower mouth was stimulated... that opening of the mouth occurred." "When
the lateral part of the upper or lower lip was touched the stimulated part of
the lip... pulled up or down respectively." He also says that sometimes when
the area above the middle part of the upper lip got stimulated the head was
thrown back, and sometimes when the lower part of the chin was stimulated the
baby threw the head forward and the chin dropped. He says the site of the
stimulation is more important than the intensity of the stimulation.
Hope this helps.

Fritzi Drosten RN IBCLC
Piedmont/Oakland Ca