Table 1.

Action Units and Their Description During Pain Sensation

RGS action units	Description during pain
Orbital tightening	The area of the eye decreases from wide to squinted.
Nose and cheek flattening	The nose and cheek display a flatter and longer appearance.
Ear position	Ears appear curled or pointed and often appear spread out.
Whisker position	Whiskers appear tense and bunched together or angled down.

RGS, Rat Grimace Scale.