Mouse Grimace Scale
Dr Jeffrey Mogil and colleagues from the McGill University in Canada developed the Mouse Grimace Scale to recognize acute pain in mice from their facial expression (Langford et al. 2010). The Mouse Grimace Scale consists of five facial action units, i.e. orbital tightening, nose bulge, cheek bulge, ear position, and whisker change, which are scored on a scale from 0 to 2. The manual of the Mouse Grimace Scale including a description of the facial action units and an example for calculating Mouse Grimace Scale Scores can be found here.
To increase objectivity and reliability of Mouse Grimace Scale scores a lot of effort is made to standardize image or video generation (Ernst et al. 2019a, b) and to automatize facial expression analysis (Puig Puig 2018, Andresen et al. 2019).