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Advanced Imaging Magazine

Updated: January 12th, 2011 09:49 AM CDT

Machine Vision and Your Face

The Facial Action Coding System is a comprehensive inventory of the muscles and their movements that form frowns, glares, grimaces, and smiles
Adapted from a montage created by Dr. P. Vanger, Center for Psychotherapy Research, Stuttgart, Germany
Figure 1. Humans create about 5,000 unique expressions; most seemingly transcend nationality, race, cultural and societal upbringing.
Patrick J. Lynch, North Haven, Conn.
Figure 2. Human faces contain between 40 and 90 distinct muscles. Anatomists deviate on an exact number because many muscles are extremely small and have overlapping fiber bundles.
Figure 3. A mosaic made up of hundreds of facial expressions, showing the rich reportoire of human expressions.
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By Lee J. Nelson
Contributing Editor

One of the many attributes that sets us apart from other primates, arguably, is exceedingly fine motor control—down to the level of individual muscles. An obvious selective advantage is our exquisitely precise discrimination of hands and fingers. We derive great benefit from learning, developing, and exercising such conscious and deliberate movement. What if that specificity also applied to instinctive, involuntary muscular motions?

Consider facial expressions. Human faces contain between 40 and 90 distinct muscles. (Anatomists deviate on an exact number because many muscles are extremely small and have overlapping fiber bundles.) While we cannot willingly innervate them all, we do create about 5,000 unique expressions; and, most seemingly transcend nationality, race, and cultural and societal upbringing. Even persons blind from birth exhibit those same features (although lacking minute gradations) which unconsciously form a standardized, nonverbal dictionary that can telegraph underlying emotion. We habitually draw impressions about friendliness, status, and trustworthiness and from the appearance of another’s face.

The flagship catalog of human facial expressions dates to 1976. That year, Dr. Paul Ekman (Langley Porter Psychiatric Hospital, University of California, San Francisco) and Dr. Wallace Friesen (Sanders Brown Center on Aging, University of Kentucky, Lexington) published the Facial Action Coding System (FACS), a scrupulously comprehensive inventory of the muscles and their movements that form frowns, glares, grimaces, and smiles.

Visually Distinguishable

To decipher those expressions, one first must understand how we create them. FACS categorizes every movement, independent of intent, as an Action Unit. There are 46—all anatomically separate and visually distinguishable. A smile, for example, is a complex endeavor. Dr. Ekman and colleagues identified 19 versions, each engaging a slightly different combination of muscles. A simple wink, the twitch of a single muscle, the obicularis oculi: that is Action Unit 46. Wrinkling the nose, Action Unit 9, involves two muscles, the levator labii superioris and the alaeque nasi.

Since its initial introduction by Ekman and Friesen, the FACS manual has been used to validate the physiological basis of emotion. FACS also enables discovery of new patterns related to emotive or situational states. Ekman and Dr. Richard Davidson (Laboratory for Affective Neuroscience, Waisman Laboratory for Brain Imaging and Behavior, University of Wisconsin, Madison), among others, find that smiles featuring both left and right orbicularis oculi (Action Unit 6), as well as the zygomatic major (Action Unit 12), correlate well with accounts of enjoyment and the associated regional brain activity, confirmed by EEG monitoring. A smile with only the zygomatic major (Action Unit 12) does not.

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