The uncanny valley is a hypothesis in the field of aesthetics which holds that when features look and move almost, but not exactly, like natural beings, it causes a response of revulsion among some observers. The "valley" refers to the dip in a graph of the comfort level of beings as subjects move toward a healthy, natural likeness described in a function of a subject's aesthetic acceptability. Examples can be found in the fields of robotics and 3D computer animation, among others.
The concept was identified by the robotics professor Masahiro Mori as Bukimi no Tani Genshō (不気味の谷現象) in 1970. The term "uncanny valley" first appeared in the 1978 book Robots: Fact, Fiction, and Prediction, written by Jasia Reichardt. The hypothesis has been linked to Ernst Jentsch's concept of the "uncanny" identified in a 1906 essay "On the Psychology of the Uncanny". Jentsch's conception was elaborated by Sigmund Freud in a 1919 essay entitled "The Uncanny" ("Das Unheimliche").
Mori's original hypothesis states that as the appearance of a robot is made more human, some observers' emotional response to the robot will become increasingly positive and empathic, until a point is reached beyond which the response quickly becomes that of strong revulsion. However, as the robot's appearance continues to become less distinguishable from that of a being, the emotional response becomes positive once again and approaches human-to-human empathy levels.
This area of repulsive response aroused by a robot with appearance and motion between a "barely human" and "fully human" entity is called the uncanny valley. The name captures the idea that an almost human-looking robot will seem overly "strange" to some human beings, will produce a feeling of uncanniness, and will thus fail to evoke the empathic response required for productive human-robot interaction.
A number of theories have been proposed to explain the cognitive mechanism underlying the phenomenon:
- Mate selection. Automatic, stimulus-driven appraisals of uncanny stimuli elicit aversion by activating an evolved cognitive mechanism for the avoidance of selecting mates with low fertility, poor hormonal health, or ineffective immune systems based on visible features of the face and body that are predictive of those traits.
- Mortality salience. Viewing an "uncanny" robot elicits an innate fear of death and culturally-supported defenses for coping with death’s inevitability.... [P]artially disassembled androids...play on subconscious fears of reduction, replacement, and annihilation: (1) A mechanism with a human facade and a mechanical interior plays on our subconscious fear that we are all just soulless machines. (2) Androids in various states of mutilation, decapitation, or disassembly are reminiscent of a battlefield after a conflict and, as such, serve as a reminder of our mortality. (3) Since most androids are copies of actual people, they are doppelgängers and may elicit a fear of being replaced, on the job, in a relationship, and so on. (4) The jerkiness of an android’s movements could be unsettling because it elicits a fear of losing bodily control."
- Pathogen avoidance. Uncanny stimuli may activate a cognitive mechanism that originally evolved to motivate the avoidance of potential sources of pathogens by eliciting a disgust response. "The more human an organism looks, the stronger the aversion to its defects, because (1) defects indicate disease, (2) more human-looking organisms are more closely related to human beings genetically, and (3) the probability of contracting disease-causing bacteria, viruses, and other parasites increases with genetic similarity." Thus, the visual anomalies of android robots and animated human characters have the same effect as those of corpses and visibly diseased individuals: the elicitation of alarm and revulsion.
- Sorites paradoxes. Stimuli with human and nonhuman traits undermine our sense of human identity by linking qualitatively different categories, human and nonhuman, by a quantitative metric, degree of human likeness.
- Violation of human norms. The uncanny valley may "be symptomatic of entities that elicit a model of a human other but do not measure up to it". If an entity looks sufficiently nonhuman, its human characteristics will be noticeable, generating empathy. However, if the entity looks almost human, it will elicit our model of a human other and its detailed normative expectations. The nonhuman characteristics will be noticeable, giving the human viewer a sense of strangeness. In other words, a robot stuck inside the uncanny valley is no longer being judged by the standards of a robot doing a passable job at pretending to be human, but is instead being judged by the standards of a human doing a terrible job at acting like a normal person. This has been linked to perceptual uncertainty and the theory of predictive coding.
- Religious definition of human identity. The existence of artificial but humanlike entities is viewed by some as a threat to the concept of human identity. An example can be found in the theoretical framework of psychiatrist Irvin Yalom. Yalom explains that humans construct psychological defenses in order to avoid existential anxiety stemming from death. One of these defenses is "specialness", the irrational belief that aging and death as central premises of life apply to all others but oneself. The experience of the very humanlike "living" robot can be so rich and compelling that it challenges humans' notions of "specialness" and existential defenses, eliciting existential anxiety. The creation of human-like, but soulless, beings is considered unwise; the golem in Judaism is a well-known example. Like anthropomorphic robots, a golem may be created with good intentions, but its absence of human empathy and spirit can lead to disaster.
- Conflicting perceptual cues. The negative affect associated with uncanny stimuli is produced by the activation of conflicting cognitive representations. Perceptual tension occurs when an individual perceives conflicting cues to category membership, such as when a humanoid figure moves like a robot, or has other visible robot features. This cognitive conflict is experienced as psychological discomfort (i.e., "eeriness"), much like the discomfort that is experienced with cognitive dissonance. Several studies support this possibility. Burleigh and colleagues demonstrated that faces at the midpoint between human and non-human stimuli produced a level of reported eeriness that diverged from an otherwise linear model relating human-likeness to affect. Yamada et al. found that cognitive difficulty was associated with negative affect at the mid-point of a morphed continuum (e.g., a series of stimuli morphing between a cartoon dog and a real dog). Ferrey et al. demonstrated that the mid-point between images on a continuum anchored by two stimulus categories produced a maximum of negative affect, and found this with both human and non-human entities. Schoenherr and Burleigh provide examples from history and culture that evidence an aversion to hybrid entities, such as the aversion to genetically modified organisms ("Frankenfoods") and transgender individuals. Finally, Moore developed a Bayesian mathematical model that provides a quantitative account of perceptual conflict. There has been some debate as to the precise mechanisms that are responsible. It has been argued that the effect is driven by categorization difficulty, perceptual mismatch, frequency-based sensitization, and inhibitory devaluation.
One study conducted in 2009 examined the evolutionary mechanism behind the aversion associated with the uncanny valley. A group of five monkeys were shown three images: two different 3D monkey faces (realistic, unrealistic), and a real photo of a monkey's face. The monkeys' eye-gaze was used as a proxy for preference or aversion. Since the realistic 3D monkey face was looked at less than either the real photo, or the unrealistic 3D monkey face, this was interpreted as an indication that the monkey participants found the realistic 3D face aversive, or otherwise preferred the other two images. As one would expect with the uncanny valley, more realism can lead to less positive reactions, and this study demonstrated that neither human-specific cognitive processes, nor human culture explain the uncanny valley. In other words, this aversive reaction to realism can be said to be evolutionary in origin.
As of 2011, researchers at University of California, San Diego and California Institute for Telecommunications and Information Technology are measuring human brain activations related to the uncanny valley. In one study using fMRI, a group of cognitive scientists and roboticists found the biggest differences in brain responses for uncanny robots in parietal cortex, on both sides of the brain, specifically in the areas that connect the part of the brain’s visual cortex that processes bodily movements with the section of the motor cortex thought to contain mirror neurons. The researchers say they saw, in essence, evidence of mismatch or perceptual conflict. The brain "lit up" when the human-like appearance of the android and its robotic motion "didn’t compute". Ayşe Pınar Saygın, an assistant professor from UCSD, says "The brain doesn’t seem selectively tuned to either biological appearance or biological motion per se. What it seems to be doing is looking for its expectations to be met – for appearance and motion to be congruent."
Viewer perception of facial expression and speech and the uncanny valley in realistic, human-like characters intended for video games and film is being investigated by Tinwell et al., 2011. Consideration is also given by Tinwell et al. (2010) as to how the uncanny may be exaggerated for antipathetic characters in survival horror games. Building on the body of work already undertaken in android science, this research intends to build a conceptual framework of the uncanny valley using 3D characters generated in a real-time gaming engine. The goal is to analyze how cross-modal factors of facial expression and speech can exaggerate the uncanny. Tinwell et al., 2011 have also introduced the notion of an unscalable uncanny wall that suggests that a viewer’s discernment for detecting imperfections in realism will keep pace with new technologies in simulating realism. A summary of Dr Angela Tinwell's research on the Uncanny Valley, psychological reasons behind the Uncanny Valley and how designers may overcome the uncanny in human-like virtual characters is provided in her book, The Uncanny Valley in Games and Animation by CRC Press.
In computer animation
A number of films that use computer-generated imagery to show characters have been described by reviewers as giving a feeling of revulsion or "creepiness" as a result of the characters looking too realistic. Examples include:
- According to roboticist Dario Floreano, the animated baby in Pixar's groundbreaking 1988 short film Tin Toy provoked negative audience reactions, which first led the film industry to take the concept of the uncanny valley seriously.
- Several reviewers of the 2004 animated film The Polar Express called its animation eerie. CNN.com reviewer Paul Clinton wrote, "Those human characters in the film come across as downright... well, creepy. So The Polar Express is at best disconcerting, and at worst, a wee bit horrifying."  The term "eerie" was used by reviewers Kurt Loder and Manohla Dargis, among others. Newsday reviewer John Anderson called the film's characters "creepy" and "dead-eyed", and wrote that "The Polar Express is a zombie train."  Animation director Ward Jenkins wrote an online analysis describing how changes to the Polar Express characters' appearance, especially to their eyes and eyebrows, could have avoided what he considered a feeling of deadness in their faces.
- In a review of the 2007 animated film Beowulf, New York Times technology writer David Gallagher wrote that the film failed the uncanny valley test, stating that the film's villain, the monster Grendel, was "only slightly scarier" than the "closeups of our hero Beowulf’s face... allowing viewers to admire every hair in his 3-D digital stubble." 
- In the 2010 film The Last Airbender, the character Appa, the flying bison, has been called "uncanny". Geekosystem's Susana Polo found the character "really quite creepy", noting "that prey animals (like bison) have eyes on the sides of their heads, and so moving them to the front without changing rest of the facial structure tips us right into the uncanny valley".
- The 2011 film Mars Needs Moms was widely criticized for being creepy and unnatural because of its style of animation. The film was the second biggest box office bomb in history, which may have been due in part to audience revulsion. (Mars Needs Moms was produced by Robert Zemeckis's production company, ImageMovers, which had previously produced The Polar Express and Beowulf.)
By contrast, at least one film, the 2011 The Adventures of Tintin: The Secret of the Unicorn, was praised by reviewers for avoiding the uncanny valley despite its animated characters' realism. Critic Dana Stevens wrote, "With the possible exception of the title character, the animated cast of Tintin narrowly escapes entrapment in the so-called 'uncanny valley.'"  Wired Magazine editor Kevin Kelly wrote of the film, "we have passed beyond the uncanny valley into the plains of hyperreality." 
A number of design principles have been proposed for avoiding the uncanny valley:
- Design elements should match in human realism. A robot may look uncanny when human and nonhuman elements are mixed. For example, both a robot with a synthetic voice or a human being with a human voice have been found to be less eerie than a robot with a human voice or a human being with a synthetic voice. For a robot to give a more positive impression, its degree of human realism in appearance should also match its degree of human realism in behavior. If an animated character looks more human than its movement, this gives a negative impression. Human neuroimaging studies also indicate matching appearance and motion kinematics are important.
- Reducing conflict and uncertainty by matching appearance, behavior, and ability. In terms of performance, if a robot looks too appliance-like, people will expect little from it; if it looks too human, people will expect too much from it. A highly human-like appearance leads to an expectation that certain behaviors will be present, such as humanlike motion dynamics. This likely operates at a sub-conscious level and may have a biological basis. Neuroscientists have noted "when the brain's expectations are not met, the brain...generates a 'prediction error'. As human-like artificial agents become more commonplace, perhaps our perceptual systems will be re-tuned to accommodate these new social partners. Or perhaps, we will decide "it is not a good idea to make [robots] so clearly in our image after all."
- Human facial proportions and photorealistic texture should only be used together. A photorealistic human texture demands human facial proportions, or the computer generated character can fall into the uncanny valley. Abnormal facial proportions, including those typically used by artists to enhance attractiveness (e.g., larger eyes), can look eerie with a photorealistic human texture. Avoiding a photorealistic texture can permit more leeway.
A number of criticisms have been raised concerning whether the uncanny valley exists as a unified phenomenon amenable to scientific scrutiny:
- Good design can lift human-looking entities out of the valley. David Hanson has criticized Mori's hypothesis that entities approaching human appearance will necessarily be evaluated negatively. He has shown that the uncanny valley that Karl MacDorman and Hiroshi Ishiguro generated – by having participants rate photographs that morphed from humanoid robots to android robots to human beings – could be flattened out by adding neotenous, cartoonish features to the entities that had formerly fallen into the valley.
- The uncanny appears at any degree of human likeness. Hanson has also pointed out that uncanny entities may appear anywhere in a spectrum ranging from the abstract (e.g., MIT's robot Lazlo) to the perfectly human (e.g., cosmetically atypical people). Capgras syndrome is a relatively rare condition in which the sufferer believes that people (or, in some cases, things) have been replaced with duplicates. These duplicates are rationally accepted to be identical in physical properties, but the irrational belief is held that the "true" entity has been replaced with something else. Some sufferers of Capgras syndrome claim that the duplicate is a robot. Ellis and Lewis argue that the syndrome arises from an intact system for overt recognition coupled with a damaged system for covert recognition, which leads to conflict over an individual being identifiable but not familiar in any emotional sense. This supports the view that the uncanny valley could arise due to issues of categorical perception that are particular to the manner in which the brain processes information.
- The uncanny valley is a heterogeneous group of phenomena. Phenomena labeled as being in the uncanny valley can be diverse, involve different sense modalities, and have multiple, possibly overlapping causes, which can range from evolved or learned circuits for early face perception to culturally-shared psychological constructs. People's cultural backgrounds may have a considerable influence on how androids are perceived with respect to the uncanny valley.
- The uncanny valley may be generational. Younger generations, more used to CGI, robots, and such, may be less likely to be affected by this hypothesized issue.
An effect similar to the uncanny valley was noted by Charles Darwin in 1839:
The expression of this [Trigonocephalus] snake’s face was hideous and fierce; the pupil consisted of a vertical slit in a mottled and coppery iris; the jaws were broad at the base, and the nose terminated in a triangular projection. I do not think I ever saw anything more ugly, excepting, perhaps, some of the vampire bats. I imagine this repulsive aspect originates from the features being placed in positions, with respect to each other, somewhat proportional to the human face; and thus we obtain a scale of hideousness.
A similar "uncanny valley" effect could, according to the ethical-futurist writer Jamais Cascio, show up when humans begin modifying themselves with transhuman enhancements (cf. body modification), which aim to improve the abilities of the human body beyond what would normally be possible, be it eyesight, muscle strength, or cognition. So long as these enhancements remain within a perceived norm of human behavior, a negative reaction is unlikely, but once individuals supplant normal human variety, revulsion can be expected. However, according to this theory, once such technologies gain further distance from human norms, "transhuman" individuals would cease to be judged on human levels and instead be regarded as separate entities altogether (this point is what has been dubbed "posthuman"), and it is here that acceptance would rise once again out of the uncanny valley. Another example comes from "pageant retouching" photos, especially of children, which some find disturbingly doll-like.
Use in the media
In the 2008 30 Rock episode "Succession", Frank Rossitano explains the uncanny valley concept, using a graph and Star Wars examples, to try to convince Tracy Jordan that his dream of creating a pornographic video game is impossible. He also references the computer-animated film The Polar Express.
The 1977 Doctor Who serial "The Robots of Death" describes a mental illness called "Grimwade's Syndrome" or "robophobia": a condition where the lack of body language from humanoid robots provokes in certain people the feeling that they are "surrounded by walking, talking dead men."
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Mori's influence on the world of robotics is immeasurable. His classic hypothesis, "The Uncanny Valley," published in 1970, is still a key work defining robotic design.
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- Your Brain on Androids UCSD news release about human brain and the uncanny valley.
- Massimo Negrotti Study on the reality of artificial objects.
- Held in Tsukuba, Japan, near Tokyo on December 5, 2005:
- The Uncanny Valley - Dave Bryant
- Almost too human and lifelike for comfort - research journal for an uncanny valley PhD project
- Relation between motion and appearance is communication between androids and humans
- Wired article: "Why is this man smiling?", June 2002.
- The Uncanny valley - a visual explanation of the hypothesis with the application in gaming.