MaqFacs: Upper Facial Movements
AU1+AU2, AU41
As in humans, upper facial movements are quite easy to identify in the Rhesus macaque. Here is a description of the main upper face movements, AU1: inner brow raiser, AU2: outer brow raiser, and AU41: glabella lowerer. You will note that some of these are quite different from what has been described for humans. For example, there is little evidence that AU1 and AU2 move independently in Rhesus monkeys (or in chimps). Moreover, there is little anatomical evidence for a depressor supercilli muscle in the rhesus monkey. In humans, brow lowering (AU4) is attributed to 3 muscles, depressor supercilli, procerus and corrugator supercilli. Because the depressor is not readily identifiable, and because the rhesus monkey brow lowering movement does not appear to corrugate, but rather depress centrally between the brows, we have focused our identification of the brow lowerer movement to one main muscle, the procerus. This is given AU41 in the human FACS and we are keeping consistent with this notation.
Brow raiser (AU1 + AU2)
Similar to the FACS, brow raising in rhesus occurs through contraction of the frontalis muscle. The two main sections are medial/inner (AU1) and lateral/outer (AU2). We have not seen convincing evidence for the unilateral movement of these muscles.
The width of the brow region involved typically extends from pupil to pupil and not beyond the corners of the eyes. Movement of the lateral portion is not as notable. This may be due to the lateral portion of the muscle contracting to its full extent, or that the movement of the lateral sections are modified by stronger medial sections.
If an indivudal has a prominent glabella in the neutral state, the brow may appear to form a 'V' shape at its midpoint, so that the frontalis contraction changes the entire shape of the brow.
Appearance changes due to AU1+AU2
1) Raises the brow up and lifts the hairline. Due to the heavy brow ridge in monkeys, movement may be constricted to a minor rolling movement over the brow.
2) Reveals greater surface area in the underbrow region, which may be lighter in color as it has more skin than darker colored hair.
3) When extreme, the brow profile flattens and widens vertically, reducing or smoothing the sharpness of the vertical nose ridges between the eyes.
4) Changes the curvature of the brow line which is often 'v' shaped in the neutral position (indenting inward) and creates a smooth arching brow instead.
5) Sometimes, movement can be seen higher up on the medial forehead, including evidence of bulging beheath the hair. This may be due to a stronger medial versus lateral contraction of the frontalis, as noted above, causing the medial area to appear thicker.
Click here to see a video example of AU1+2
Click here to see a video example of AU1+3 showing appearance change #5
Click on the picture below for larger size
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Glabella lowerer (AU41)
The human FACS provides individual action unit codes for the 3 separate muscles typically implicated in the AU4-brow lowerer: AU41-glabella lowerer (Glabella), AU42-inner eyebrow lowerer (Depressor supercilli) and AU44-eyebrow gatherer (Corrugator supercilli). In most cases, it is difficult, at best, to differentiate the contribution of each individual muscle in lower the brow. Thus, brow lowering is coded with an AU4.
In rhesus monkeys, however, both the corrugator supercilli and procerus appear present on dissection, while evidence for the depressor supercilli is less clear (Burrows et al., in prep). Stimulating the corrugator causes very slight corrugation of the brow in addition to brow lowering (Waller et al., 2008). There does not, however, appear to be spontaneous movement of the corrugator (AU44) in natural behavior. In contrast, stimulation of the procerus caused a strong brow lowering movement and this (AU41) is clearly identifiable in natural behavior. Thus, brow lowering in the rhesus macaque is referred to as AU41 (glabella lowerer).
Appearance changes due to AU41
1) Pulls brow downward
2) Reduces the visibility of the underbrow region
3) Changes the brow curvature by lowering the glabella, sometimes resulting in a distinct 'V' shape, or amplifying the sharpness of the 'V' if already presen in the neutral state, at the midpoint of the brow.
4) Sometimes, this appears to push the very medial portion of the glabella forward and may also result in a bulge beneath the hair being seen in the medial forehead region. This is often difficult to see.
Click here to see a video example of AU41
Click on the picture below for larger size
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For more information on the development of the
Macaque Facial Action Coding System contact Dr. Lisa Parr (parr@rmy.emory.edu).