In the past, that was a joke for me...
(2007 / Zhang) This system extracts both the shape and textural features of the tongue and uses them for recognition. The shape vector represents the shape features of the tongue, specifically its length, bend, thickness, and width, and the curvature of its tip. The texture codes represent the textural features of the central part of the tongue.
(2013 / Diwakar) The tongue is a unique organ in that it can be stuck out of mouth for inspection, in this act offering a proof of life, and yet it is otherwise well protected in the mouth and is difficult to forge. The tongue also presents both geometric shape information and physiological texture information which are potentially useful in identity verification applications.
(2017 / Jeddy) Three reference points were considered to determine the shape of the tongue. Results: The most common morphological feature on the dorsum of the tongue was the presence of central fissures. Multiple vertical fissures were observed in males whereas single vertical fissure was a common finding in females. (note: the study sample included twenty participants).