Head movement of negation

[Klimis Antzakas]

Dear all,
I am a student at City University of London and I am doing a study about
negation in Greek Sign Language (I am Greek).
Negation is expressed by Deaf people in Greece, as in other Deaf
communities, by head movements. Apart from expressing with a headshake
which is a common head movement of negation for many Deaf communities,
Deaf Greeks express negation with a backward tilt of the head. This is
also a common gesture for the Greek hearing community.
I found it difficult to find references or bibliography on that gesture
concerning Deaf or hearing communities. I would appreciate any
references on this subject.
Does anybody know the e-mail of Kendon Adam? I think he is working in
University of Pennsylvania.
Thank you in advance.