The role of temporal inversion in the perception of realistic and morphed dynamic transitions between facial...

Recent studies suggest that video recordings of human facial expressions are perceived differently than linear morphing between the first and last frames of these records. Also, observers can differentiate dynamic expressions presented in normal versus time-reversed frame orders. To date, the simultaneous influence of dynamics (natural or linear) and timeline (normal or reversed) has not yet been tested on a wide range of dynamic emotional expressions and the transitions between them. We compared the perception of dynamic transitions between basic emotions in realistic (human-posed) and artificial (linearly morphed) stimuli which were presented in reversed or non-reversed order. The nonlinearity of realistic stimuli was demonstrated by automated facial structure analysis. The results of the behavioral study revealed that the recognition of emotions in time-reversed stimuli significantly differed from recognition of the normally presented ones, and this difference was substantially higher for videos of a dynamic human face than for linear morphs. Emotions displayed at the end of the transitions were recognized better than the first-frame emotions in all types of stimuli except in the time-reversed videos, which showed a similar recognition rate for both the starting and ending emotions. Our findings suggest that nonlinearity, which is present in a realistic facial display but absent in linear morphing, is an important cue for emotion perception, and that unnatural perceptual conditions (inversion in time) make the recognition of emotions more difficult. These results confirm the ability of the human visual system to use subtle dynamic cues on an interlocutor's face, and reveal its sensitivity to the timeline organization of the displayed emotions.