Editing Facial expression (section)

== Creation ==

=== Facial muscles ===
[[File:Lars Jacob as Comedy & Tragedy 1972.JPG|thumb|An actor acting out ''Drama Masks'' ([[Thalia (Muse)|Thalia]] and [[Melpomene]]) in 1972]]
Facial expressions are vital to social communication between humans. They are caused by the movement of muscles that connect to the skin and [[fascia]] in the face. These muscles move the skin, creating lines and folds and causing the movement of facial features, such as the mouth and eyebrows. These muscles develop from the second [[pharyngeal arch]] in the embryo. The [[temporalis]], [[masseter]], and [[internal pterygoid muscle|internal]] and [[external pterygoid muscle]]s, which are mainly used for chewing, have a minor effect on expression as well. These muscles develop from the first pharyngeal arch.<ref name="Rinn">{{cite journal|last=Rinn|first=William E.|year=1984|title=The Neuropsychology of Facial Expression: A Review of the Neurological and Psychological Mechanisms for Producing Facial Expressions|journal=Psychological Bulletin|volume=95|issue=1|pages=52–77|doi=10.1037/0033-2909.95.1.52|pmid=6242437}}</ref>

=== Neuronal pathways ===
There are two brain pathways associated with facial expression; the first is voluntary expression. Voluntary expression travels from the [[primary motor cortex]] through the [[pyramidal tract]], specifically the [[corticobulbar tract|corticobulbar projections]]. The cortex is associated with display rules in emotion, which are social [[precepts]] that influence and modify expressions. Cortically related expressions are made consciously.<ref name="Rinn" />

The second type of expression is emotional. These expressions originate from the [[extrapyramidal motor system]], which involves subcortical nuclei. For this reason, genuine emotions are not associated with the cortex and are often displayed unconsciously. This is demonstrated in infants before the age of two; they display distress, disgust, interest, anger, contempt, surprise, and fear. Infants' displays of these emotions indicate that they are not cortically related. Similarly, blind children also display emotions, proving that they are subconscious rather than learned. Other subcortical facial expressions include the "knit brow" during concentration, raised eyebrows when listening attentively, and short "punctuation" expressions to add emphasis during speech. People can be unaware that they are producing these expressions.<ref name="Rinn" />

=== Asymmetries ===
The lower portions of the face are controlled by the opposite [[cerebral hemisphere]], causing asymmetric facial expression. Because the [[right hemisphere]] is more specialised for [[emotional expression]], emotions are more strongly expressed on the left side of the face,<ref>{{cite journal|author=Blonder LX, Bowers D, Heilman KM|year=1991|title=The role of right hemisphere in emotional communication|journal=Brain|volume=114|issue=3|pages=1115–1127|doi=10.1093/brain/114.3.1115|pmid=2065243}}</ref> particularly for [[negative affectivity|negative emotions]].<ref>{{cite journal|author=Ahern GL, Schwartz GE|year=1979|title=Differential lateralization for positive versus negative emotion|journal=Neuropsychologia|volume=17|issue=6|pages=693–698|doi=10.1016/0028-3932(79)90045-9|pmid=522984|s2cid=27771100}}</ref> [[Asymmetries]] in expression can be seen in chimeric faces (facial portraits made by combining the left and right halves of faces with different expressions)<ref>{{cite journal|author=Levy J, Heller W, Banich MT, Burton LA|year=1983|title=Asymmetry of Perception in Free Viewing of Chimeric Faces|journal=Brain and Cognition|volume=2|issue=4|pages=404–419|doi=10.1016/0278-2626(83)90021-0|pmid=6546034|s2cid=21456608}}</ref> and also in [[portraits]] which more often show the left, more emotional side of the face than the right.<ref>{{cite journal|author=Nicholls Mike, Clode D, Wood SJ and Wood AG|year=1999|title=Laterality of expression in portraiture: Putting your best cheek forward|journal=Proceedings of the Royal Society B|volume=266|issue=1428|pages=1517–1522|doi=10.1098/rspb.1999.0809|pmid=10467743|pmc=1690171}}</ref>