Lipopolysaccharide-Induced Changes in Facial Expression and Orofacial Movement in Mice

Sickness behavior is defined by a coordinated set of behavioral changes that emerge during infection and other inflammatory states. A major challenge for assessment, however, is the broad suppression of behavioral output, which hampers the dissociation of specific affective states from generalized motor inhibition. In this study, we applied facial expression analysis to characterize the sickness state in lipopolysaccharide (LPS)-injected male mice, leveraging the well-established capacity of rodents to express affective states through facial action units. LPS induced changes in both movement-related and grimace-like facial features. Inter-feature correlation analysis revealed that grimace-like features, including orbital tightening and nose bulging, showed LPS-specific temporal correlations in a subset of time bins. Furthermore, grimace-like readouts showed greater inter-individual variability than movement-related measures. Together, these findings support the viability of facial expression analysis as a noninvasive readout for behavioral states in inflammatory models that induce hypoactivity.