preliminaries
facial action coding system
duchenne was an anatomist who first studied human facial muscles. he had experimented with electricity through the electrodes to estimate the muscles' working. however, the problem is some human is muscles overlap when an electrical stimulus to the muscles. it may be leased to a bundle of muscles moving along later. ekman and friesen developed a system called FACS
this system records facial motion observed by eye and called the motion action unit as shown in table 1 each action unit can work together more than two muscles such as frown caused by the interaction of muscle 3 parts, depressor glabellae (frown), depressor supercilii (wrapped in strips), and corrugator supercilii (caused wrinkles on the forehead)