Figure 1. Time in smiling and cry-faces as a proportion of time in each episode of the Face-to-Face/Still-Face (FFSF).

Error bars indicate standard errors of the mean. The images are of smiles and of cry-faces without eye constriction. The images are from a six-month-old in the FFSF in the current study. Written informed consent, as outlined in the PLOS consent form, was obtained for publication of these images.