Common examples are perceived images of animals, faces, or objects in cloud formations, the man in the moon, the moon rabbit, hidden messages within recorded music played in reverse or at higher- or lower-than-normal speeds, and hearing indistinct voices in random noise such as that produced by air conditioners or fans.
The word derives from the Greek words para (παρά, "beside, alongside, instead [of]"—in this context meaning something faulty or wrong) and the noun eidōlon (εἴδωλον "image, form, shape"—the diminutive of eidos).
Pareidolia can cause people to interpret random images, or patterns of light and shadow, as faces. A 2009 magnetoencephalography study found that objects perceived as faces evoke an early (165 ms) activation of the fusiform face area at a time and location similar to that evoked by faces — whereas other common objects do not evoke such activation. This activation is similar to a slightly faster time (130 ms) that is seen for images of real faces. The authors suggest that face perception evoked by face-like objects is a relatively early process, and not a late cognitive reinterpretation phenomenon. An fMRI study in 2011 similarly showed that repeated presentation of novel visual shapes that were interpreted as meaningful led to decreased fMRI responses for real objects. These results indicate that the interpretation of ambiguous stimuli depends upon processes similar to those elicited by known objects.
These studies help to explain why people identify a few circles and a line as a "face" so quickly and without hesitation. Cognitive processes are activated by the "face-like" object, which alert the observer to both the emotional state and identity of the subject – even before the conscious mind begins to process – or even receive – the information. The "stick figure face", despite its simplicity, conveys mood information (in this case, disappointment or mild unhappiness). It would be just as simple to draw a stick figure face that would be perceived (by most people) as hostile and aggressive. This robust and subtle capability is hypothesized to be the result of eons of natural selection favoring people most able to quickly identify the mental state, for example, of threatening people, thus providing the individual an opportunity to flee or attack pre-emptively. In other words, processing this information subcortically (and therefore subconsciously) – before it is passed on to the rest of the brain for detailed processing – accelerates judgment and decision making when a fast reaction is needed. This ability, though highly specialized for the processing and recognition of human emotions, also functions to determine the demeanor of wildlife.
Pareidolia can be considered a subcategory of Apophenia.
Rocks may come to mimic recognizable forms through the random processes of formation, weathering and erosion. Most often, the size scale of the rock is larger than the object it resembles, such as a cliff profile resembling a human face. Well-meaning people with a new interest in fossils can pick up chert nodules, concretions or pebbles resembling bones, skulls, turtle shells, dinosaur eggs, etc., in both size and shape.
In the late 1970s and early 1980s, Japanese researcher Chonosuke Okamura self-published a series of reports titled Original Report of the Okamura Fossil Laboratory, in which he described tiny inclusions in polished limestone from the Silurian period (425 mya) as being preserved fossil remains of tiny humans, gorillas, dogs, dragons, dinosaurs and other organisms, all of them only millimeters long, leading him to claim, "There have been no changes in the bodies of mankind since the Silurian period... except for a growth in stature from 3.5 mm to 1,700 mm." Okamura's research earned him an Ig Nobel Prize (a parody of the Nobel Prizes) in biodiversity in 1996.
The Rorschach inkblot test uses pareidolia in an attempt to gain insight into a person's mental state. The Rorschach is a projective test, as it intentionally elicits the thoughts or feelings of respondents that are "projected" onto the ambiguous inkblot images. Projection in this instance is a form of "directed pareidolia".
In his notebooks, Leonardo da Vinci wrote of pareidolia as a device for painters, writing, "If you look at any walls spotted with various stains or with a mixture of different kinds of stones, if you are about to invent some scene you will be able to see in it a resemblance to various different landscapes adorned with mountains, rivers, rocks, trees, plains, wide valleys, and various groups of hills. You will also be able to see divers combats and figures in quick movement, and strange expressions of faces, and outlandish costumes, and an infinite number of things which you can then reduce into separate and well conceived forms."
There have been many instances of perceptions of religious imagery and themes, especially the faces of religious figures, in ordinary phenomena. Many involve images of Jesus, the Virgin Mary, the word Allah, or other religious phenomena: in September 2007 in Singapore, for example, a callus on a tree resembled a monkey, leading believers to pay homage to the "Monkey god" (either Sun Wukong or Hanuman) in the monkey tree phenomenon.
Publicity surrounding sightings of religious figures and other surprising images in ordinary objects has spawned a market for such items on online auctions like eBay. One famous instance was a grilled cheese sandwich with the face of the Virgin Mary.
Pareidolia also arises in computer vision, specifically in image recognition programs, which can spuriously detect features. In the case of an artificial neural network, higher-level features correspond to more recognizable features, and enhancing these features brings out what the computer sees. These reflect the training set of images that the network has "seen" previously.
Striking visuals can be produced in this way, notably in the DeepDream software, which falsely detects and then exaggerates features such as eyes and faces in any image.
In 1971 Konstantīns Raudive wrote Breakthrough, detailing what he believed was the discovery of electronic voice phenomenon (EVP). EVP has been described as auditory pareidolia.Allegations of backmasking in popular music, in which a listener claims a message has been recorded backward onto a track meant to be played forward, have also been described as auditory pareidolia. In 1995, the psychologist Diana Deutsch invented an algorithm for producing phantom words and phrases with the sounds coming from two stereo loudspeakers, with one to the listener’s left and the other to his right. Each loudspeaker produces a phrase consisting of two words or syllables. The same sequence is presented repeatedly through both loudspeakers; however, they are offset in time so that one when the first sound (word or syllable) is coming from the speaker on the left, the second sound is coming from the speaker on the right, and vice versa. After listening for a while, phantom words and phrases suddenly emerge, and these often appear to reflect what is on the listener’s mind, and they transform perceptually into different words and phrases as the sequence continues. 
Various European ancient divination practices involved the interpretation of shadows cast by objects. For example, in molybdomancy, a random shape produced by pouring molten tin into cold water is interpreted by the shadow it casts in candlelight.
A shadow person (also known as a shadow figure, shadow being or black mass) is often attributed to pareidolia. It is the perception of a patch of shadow as a living, humanoid figure, particularly as interpreted by believers in the paranormal or supernatural as the presence of a spirit or other entity.
The Romanian Sphinx in Bucegi Mountains
Human face on Pedra da Gavea in Rio de Janeiro, Brazil, known as the "Head of the Emperor"
“Elephant Rock” on Heimaey, Iceland
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|Wikimedia Commons has media related to Pareidolias.|
|Look up pareidolia in Wiktionary, the free dictionary.|
- Cloud pareidolia 33 examples of meteorological pareidolia.
- Skepdic.com Skeptic's Dictionary definition of pareidolia
- The Stone Face: Fragments of An Earlier World
- Feb. 13, 2007, article in The New York Times about cognitive science of face recognition
- Joe Bello art Example of contemporary pareidolic art
- Snopes.com Faces of 'Satan' seen in World Trade Center smoke
- NPR: Teapot on billboard looks like Hitler
- Google faces