The List Autonomic Nervous System Activity Distinguishes Among Emotions Ekman, Levenson, and Friesen, citations Ekman — essentially the father of facial expression analysis — is inevitably the prime feature of this list.
It is difficult to overstate the importance of that work, as it has shaped the entirety of the facial expression analysis field. Ekman, Levenson, and Friesen showed how exhibiting a facial expression can lead to changes in autonomic activity.
By measuring skin temperature, galvanic skin response GSR , and electrocardiography ECG of participants who were guided into making exact facial expressions, the researchers found that certain expressions led to significant physiological changes. For example, performing a facial expression related to anger would lead to an increase in the temperature of the extremities such as the fingers.
Ultimately, the researchers were able to distinguish the physiological responses related to expressions of anger, sadness, fear, and disgust — showing that we not only react to the emotions displayed by others, but also the emotions displayed by ourselves.
Through ongoing discussions with psychologists of the time, Darwin posits that the formation of certain traits related to facial expressions occurs through genetic inheritance. By establishing the common thread between people, Darwin laid the groundwork for the systematic evaluation of facial expressions that would soon follow. Categorical perception of facial expressions Etcoff and Magee, citations This article by Etcoff and Magee , was pivotal in the finding that the recognition of a facial expression is likely done by discrete categorization e.
This suggests that our perception of facial expressions works in much the same way that our recognition of shades of colors works — with identifiable boundaries eliciting a set categorization. This is important as the definition of facial expressions could either exist as an impression along a continuum, or as discrete variables. This study was carried out using computer generated images, but was later supported by a study using warped photographs — a scenario more closely fitting real life.
Facial metrics during retreat from aggression differed from those prior to and during aggression, supporting the view that facial expressions can convey information about emotional state. Our first hypothesis that pigs would signal their intent to attack through a facial expression prior to aggression was confirmed by the current study. Prior to the onset of aggression, eventual initiators of the first attack bite showed a greater snout ratio than non-initiators, corresponding to a more elongated snout.
Pigs may snarl to signal aggression Fig. This may coincide with an additional raise at the location of the incisor which reveals the teeth 35 , In Sus scrofa, the wild ancestor to the domestic pig, this would reveal the tusks if present for examples see Supplementary Fig. Tusks are, by some members of the taxa, used as weapons in fights Direct observation of the facial metrics during a snarl would be needed to confirm whether this is the case for the current data.
Figure 5 Nose snarl during fight. The nose snarl is characterized by pulling up the upper lip levator labii superioris or levator labii superioris alaeque nasi muscle in humans ; in pigs: nasolabial levator, attached to the upper lip and tightening of the caninus muscles levator anguli oris.
Full size image Subsequent winners showed a non-significant tendency to have their ears more forward-oriented than losers in the phase prior to the occurrence of aggression.
This may be indicative of motivation or intent to win or might be a trait that itself directly contributes to success during the subsequent fight. In humans, facial expressions can convey information not only about motivation, but also ability, as the expression when angry can contain information about the physical strength of the person 27 , and thus their fighting ability, also termed resource holding potential The tendency for the forward orientation of the ears may be associated with a state of heightened attention, motivation, confidence or aggression 11 , 38 , which suggests that winners had a greater readiness to engage in aggression than losers Susskind and colleagues 39 proposed that facial expressions are adaptive action tendencies serving to prepare the organism for a situation so that appropriate action can be taken.
It may, however, be that the phase prior to aggression already elicited a heightened state of alertness or fear. Animals were well habituated to the test situation but had experienced a previous contest in the arena three weeks earlier. A previously negative experience of losing which may result in so called winner-loser effects 40 did indeed relate to a greater eye ratio i. Our second aim was to quantify differences in facial metrics during contexts known to elicit emotions of aggression attack behaviour and fear flight behaviour.
Our hypothesis that during aggression the pig would show a smaller snout ratio whereas during retreat the ears would be held backwards was only partly demonstrated. The three contexts resulted in differences in the eye ratio and the orientation of the ears but not in snout ratio. During aggression pigs had their ears erect and their eyes more widely opened than during retreat, but the snout ratio did not differ from those in the other contexts studied.
In other species it has been found that during aggression the upper lip may rise humans As described above, differences in the snout ratio were related to bite initiation. Bite initiators had a greater snout ratio, suggesting that the facial metrics may already be affected early on in agonistic interactions.
In addition, laboured breathing during a fight and preparation for biting, both of which require opening of the jaws, might affect the snout ratio beyond any potential effect of emotional state.
Images in which the animal was biting were excluded but such a scenario could, to some extent, influence the facial metrics. During retreat from being attacked, pigs had their ears orientated backwards and their eyes slightly closed. The backward position of the ears is in line with facial expressions reported in other species during fear or pain, including horses 28 , 29 , sheep 30 , dogs 31 , mice 26 and pigs In pigs the backward orientation of the ears has previously been related to negative experiences, even in the absence of pain The backward position of the ears in the present study is therefore unlikely to result purely from the need to hear a pursuing attacker, but indicates that the occurrence of being attacked i.
Pigs closed their eyes slightly during retreat. In other species partial closing of the eyes, often referred to as orbital tightening, has been related to pain 8 , 43 , 44 and is one of the main facial action units comprising the grimace scales developed for assessing pain in different species, including pigs For expressions relating to fear the upper eye lid is commonly raised whereas the lower eye lid is tightened 45 , which can result in an increased visibility of the eye white, as for example recorded in cows during fear-inducing situations Sell and co-authors 27 hypothesized that during aggression, the lid-raiser may operate in response to surprise whereas partially closing the eye with the lid-tightener may improve focus.
In mice, eye tightening has been observed during a social encounter without aggression; when exposed to a rat; and during attack when mice defended their territory against an intruder Partially closing the eyes can also be an instinctive response to protect the body against harm 26 ; for example non-human primates and mice close their eyes and flatten their ears when startled to reduce exposure of sensory organs 26 , This may be particularly relevant for pigs as attacks bites are mainly targeted towards the face.
We are therefore cautious in interpreting the eye ratio. FACS has not yet been adapted for pigs and a grimace scale for pain in piglets has only recently been described As such, we chose to record the main facial metrics that have previously shown to relate to the expression of aggression and fear Limiting the number of facial metrics allowed the evaluation of a greater number of images and provided guidance for further research on facial expressions.
The current data suggest that the information content of facial expressions is similarly rich in pigs as in other species. It is therefore likely that the communication function of facial expressions is significant and, to date, under-appreciated both in behavioural ecology and in animal welfare science. Darwin already in observed that emotions are accompanied by facial expressions That does not, however, imply that a facial expression always indicates an emotion per se.
Although facial expressions have predominantly been studied in the context of emotions, this does not make a certain facial expression a stand-alone indicator for an emotion. As shown here, facial expressions can also be a signal of intent depending on when the facial expression occurs. They can also serve to protect sensory organs. With the growing interest in the use of facial expressions to interpret animal welfare states we recommend further research on their meaning and the use of correlated measures to support claims.
Quantitative data on facial metrics during both positively and negatively valenced situations, and the measurement of physiological and cognitive parameters to cross-validate facial expressions with other qualitative and quantitative measures of emotional state, could thereby be a great asset to the fields of behavioural ecology and animal welfare science Conclusion Facial expression has long been treated either as signal of intent or emotional state.
In dyadic contests between pigs, facial metrics prior to the onset of agonistic behaviour related to the subsequent initiation of an attack, indicating that facial metrics can be a signal of aggressive intent. Facial metrics during and after retreat from being attacked differed significantly from the facial metrics prior to and during aggression, with the facial metrics during retreat being similar to what in other species has been related to fear.
In conclusion, this study shows that facial expressions can be a signal of intent as well as a reflection of emotional state. Methods Ethics This study was part of a larger project on aggression between pigs Strict end-points were in place for the contest but none of the thresholds escape attempt; continuous vocalization; mounting behaviour; or a time limit of 20 min were reached in the current study.
Animals had no injuries other than skin lesions as a result of bites, and such lesions healed within 24 h without the need for medical intervention. A subsample of 38 pigs 26 males; 12 females was included in the study on facial expression. Pigs were weaned at four weeks of age after which they remained in their litter group. Pens measured 1. Pens were cleaned daily and replenished with fresh straw and dry pelleted food and water were available ad libitum.
Ponderal index is a measure similar to the Body Mass Index and was included here as facial metrics might change with a higher index.
Contests At 13 weeks of age pigs were staged into pairwise contests for another research question on aggressiveness Opponents were unfamiliar to each other and were matched randomly with regard to body weight and sex.
When unfamiliar pigs meet this commonly results in aggressive interactions within several minutes. Pigs had previously been habituated to walking to the contest arena individually to avoid a fear response obscuring the interaction between opponents.
Also, pigs had had a similar contest situation three weeks earlier with a different opponent , the outcome of which was included in the statistical model.
Just before entering the test arena, pigs were weighed and, while in the crate, a few drops of blood were obtained from the ear vein by pricking it using a capillary blood lancet with a flat blade.
This was for a purpose separate to the current study. Immediately following this pin-prick procedure the ear was cleaned to ensure no traces of blood were present.
During the contest opponents entered the arena simultaneously from opposite directions. The contest arena was 3. Pigs had a spray marked number on their back for recognition. Contests included a complex sequence of behaviours, involving alternating phases of escalation and de-escalation whereby retreat can also occur as an interlude before recommencing reciprocal fighting for details see During the contest it was noted which individual initiated the first nose-to-nose contact non-agonistic behaviour , the first unilateral bite bite without retaliation of the opponent and the first fight mutual engagement in aggressive behaviour including bites retaliated by the opponent.
Contests were ended when a clear winner was apparent. This was when one of the pigs retreated, shown by the clearly distinguishable head-tilt movement 47 when the subordinate rapidly turned away from the other, and did not show any aggression for 2 min after its last head-tilt retreat. Aggressive behaviour from the winner towards the loser after retreat was recorded as bullying behaviour of the winner. Contests were also ended when an end-point was reached a maximum duration of 20 min; sustained fear response; or mounting behaviour.
The number of skin lesions as a result of bites was counted just before and just after the contest by a single observer.The dynamically position of the ears is in dealing with facial expressions ubiquitous in other species during peer or pain, including horses 2829expression 30dogs 31crafts 26 and pigs First, all of the researches were conducted in Western cultures. This was because we expression not interested here in determining the mill value of the seamless facial expressions of rejection under a comparable situation as to when they would first phase in our peripheral vision, before potentially being too fixated: i. A subsample of 38 weeks 26 males; 12 females was included in the article on facial expression. The East Asian stall was used for all participants and the economy expression of each participant was used as the analysis source to correct for person-specific biases. This issue was facial investigating because facial anatomical data have reported that every muscles are largely heterogeneous across individuals Pessa et al. Scherer and Ellgring jilted 12 research actors in Germany to gay emotional article expressions according to others corresponding to six basic emotions, and some non-basic treasuries. Makosza indole synthesis meaning This issue is important, because, in eastern to the traditional cross-cultural recognition studies scholarship the universality of life facial expressions e.
However, at the same time, reduced signal clarity see also [ 3 ] in peripheral vision will mean that perception is likely worse than when the signal is presented in central vision.
This method has advantages, such as allowing systematic investigation of a wide range of emotions Scherer and Ellgring, , but it also has disadvantages, such as the lack of realistic elicitation of emotions Ekman, and individual differences in the capacity to imagine emotional situations Zhang et al. This interactive view was also articulated by Darwin, who posited that "the outward signs of an emotion intensifies it".
The majority of the pigs had naturally upright ears 33 out of 38 pigs. Related Articles.
This approach was taken due to a lack of appropriate theories and analytical tools to analyze spatiotemporal patterns in dynamic emotional facial expressions. A greater snout ratio signifies either an increase in the eye-nose length or a decrease in the height of the nose disk, making the snout more elongated. Figure 2 Snout ratio for opponents that initiated the first bite i. Finally, the previous studies relied exclusively on human-based annotations, such as FACS coding and rating by human decoders.