The uncanny valley is a hypothesis regarding the field of robotics. The theory holds that when robots and other facsimiles of humans look and act almost like actual humans, it causes a response of revulsion among human observers.
The ‘valley’ in question is a dip in a proposed graph of the positivity of human reaction as a function of a robot’s lifelikeness. People are not as affected in an emotional way by an object if it is easy to tell it is not human. After a certain point, they start to feel emotionally about it, but feel bad emotions because it is so nonhuman. As it gets closer to looking human, they start to feel more positive emotions towards it.
The term was coined by roboticist Masahiro Mori in 1970, and has been linked psychologist Ernst Jentsch’s 1906 essay, ‘On the Psychology of the Uncanny.’ Jentsch’s conception is famously elaborated upon by Sigmund Freud in a 1919 essay titled ‘The Uncanny.’ A similar problem exists in realistic 3D computer animation, such as with the film ‘The Polar Express.’
A number of theories have been proposed to explain the cognitive mechanism underlying the phenomenon. The mate selection theory proposes that 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.
The ‘mortality salience’ theory argues that viewing an ‘uncanny’ robot elicits an innate fear of death and culturally-supported defenses for coping with death’s inevitability. ‘Partially 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.’
The ‘pathogen avoidance’ theory argues posits that 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.
According to the ‘sorites paradox’ (e.g. how many grains of sand must be removed from a ‘heap’ for it to no longer be a ‘heap’) 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.
Another theory is based on 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.
Finally, the uncanny valley may be a product of a 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, as constructed in the West and the Middle East. This is particularly the case with the Abrahamic religions (Christianity, Islam, and Judaism), which emphasize human uniqueness. 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.
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 testing 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 (neurons also known as ‘monkey-see, monkey-do 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 tuned to care about 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 analyzing how cross-modal factors of facial expression, and speech may 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 number of design principles have been proposed for avoiding the uncanny valley. For example, 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.
Another tool is 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.’
Lastly, 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. For example, 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.
Also, 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 social brain processes information.
Another criticism is that 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.
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 (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.
The Daily Omnivore 
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