Yawns should be identified holistically according to the System for Coding Perinatal Behavior (SCPB), which is based in part on selected facial Action Units (AUs) from the anatomically based Baby FACS: Facial Action Coding System for Infants and Young Children and previous studies in the literature. For reliability assessment purposes, once an event is identified as a yawn based on this description, the onset should be scored at the first frame where mouth opening is visible.
Yawning is defined in the SCPB as a stereotyped behavior characterized by a slow mouth opening with deep inspiration, followed by a brief apnea and a short expiration and mouth closing, typically accompanied by limb stretching. The expansion of the pharynx can quadruple its diameter, while the larynx opens up with maximal abduction of the vocal cords. One of the characteristic features of yawning is its timing, consisting in a progressive acceleration, followed by an abrupt deceleration in the intensity of the facial muscle Action Units (AUs) involved, designated by numeric codes and verbal labels. Yawning usually emerges from a relaxed face, initially involving mouth opening (AUs 25, 26, 27) and eyes closing (AU 43E), followed by upper eyelid drooping (AU 43A-D), flattened tongue on the bottom of the mouth (AU 75) and usually swallowing (AU 80). During the plateau brow knitting (AU 3), brow knotting (AU 4), nose wrinkling (AU 9), lateral lip stretching (AU 20), nostril dilatation (AU 38) and head tilting back (AU 53) also typically occur.