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You may have once seen a giant face in the clouds. Perhaps it took you aback, amused you, or maybe it prompted an “uncanny valley” kind of sensation—realness, but with a lingering unease.

In any case, it’s not a modern phenomenon. It’s thought that a similar experience was shared by an early hominid approximately 3 million years ago. Researchers say a rock that bore resemblance to a face was carried, over some four kilometers from where it was probably found, to an Australopithecine home. Known as the Makapansgat pebble, it was found in 1925 in a South-African cave, in what may well have been a camp or dwelling. Paleontologists can’t say with certainty what the hominid saw in the rock, but clearly our proclivity for symbolic thought is rooted deep in our ancient past. And as scientists continue to discover, symbolic thought can provide a unique window on how our brains work and how our senses can go remarkably askew.

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In the mid-19th century scholars came up with a word for seeing images of objects where none exist—pareidolia. Its etymology involves the Greek words “para,” meaning in this case “abnormal,” and “eidolon,” spirit-image. The Makapansgat pebble exemplifies visual pareidolia—Jesus on toast or the face on Mars. Experiences of visual pareidolia are not uncommon. It happens when we perceive visual patterns (data) in randomness (noise) and is the basis for the perception of stellar constellations and images in the famous Rorschach inkblot test.

Pareidolia is not, however, confined to a single sensory modality. There is another form—auditory pareidolia. Concise definitions of the phenomenon remain elusive, but in clinical circles it’s usually defined relative to the more common visual form—the perception of patterns in randomness where none exist, but via an auditory mode.

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“I thought I was going crazy. When my air conditioner is on, I wake up and hear light conversations.”

It’s often colloquially described byway of exemplary mishearings. Some reported cases have stirred controversy. Take Fisher Price’s “Little Mommy Real Baby Cuddle’n Coo” doll. Presumably with no mischievous intentions, the toy company made the doll giggle, babble and coo, as babies do. But within weeks after its release in 2008, one parent misheard the doll’s babbling sound as saying “Islam is the light.” When a news network caught wind of this, a cascade of other reports ensued inciting more controversy and ultimately, the “Little Mommy Real Baby Cuddle’n Coo” was pulled from the shelves of major stores.

Notably it was an election year, and associations between Islam and terrorism were widespread, suggesting that such interpretations were not entirely random. It seems that the doll’s meaningless babble somehow evoked these mishearings in a somewhat Freudian manner—an “auditory Rorschach.”

Is this a joke?: When Kevin, in “Home Alone,” descends into his basement, the rumblings of the radiator sound like a demon’s voice.William Wilkinson / YouTube / 20th Century Fox
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Neil Bauman is an audiologist who runs a center in Pennsylvania called The Hearing Loss Help Center. He’s created a discussion forum for those experiencing a wide range of anomalous auditory perceptions including auditory pareidolia. Commenters detail their experiences, often believing they are symptomatic of mental illness. For example, one commenter writes: “I thought I was going crazy. When my air conditioner is on, I wake up and hear light conversations. I would go to the window to see if anyone was outside, or I would turn the air conditioner off [and] it would stop. Sometimes it sounds like a radio.”

Another, more at-ease commenter, writes about her similar experience of hearing voices from the sound of central air control: “I would hear faint voices—whispering, conversing, singing, or chanting! It sounded like a crowded room, full of people at a party in a distant room somewhere in the building. After a while I came to enjoy the sound, as they seemed to be enjoying themselves at the ‘party,’ and it helped lull me to sleep at night.”

These examples of auditory pareidolia might make you wonder what the sonic characteristics of natural speech are, and moreover, how the sound of an air conditioner—or any other noise-making object—could possibly resemble speech. At one point scientists were generally in agreement that the waveform characteristics essential for perceiving speech sounds were largely understood. That changed when a single study showed that speech could be perceived in sounds that did not contain any of these acoustic properties.

If you have an experience of auditory pareidolia, how concerned should you be?

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The study, titled “Speech Perception Without Traditional Speech Cues,” was published in Science in 1981 and involved a new synthetic speech stimulus called Sine Wave Speech (SWS). SWS is composed entirely of sine waves in different frequency bands and contains none of the frequency patterns thought to signal natural speech. Hearing it evokes an uncanny valley feeling, yet it is intelligible and listeners are surprisingly accurate when guessing its content. It consistently intrigues me (and my cat) as I listen to the examples provided by Yale University’s Haskins Laboratories, where it was originally developed.

I recently spoke with the lead author of that 1981 Science study, Robert Remez, now at Columbia University, about auditory pareidolia, asking him why we recognize sounds from noises—the sound of an air conditioner, for example—that we know are clearly not vocal, and don’t sonically resemble speech, as speech. He suggested that there must be some kind of sensory activity “spill-over from parts of my auditory system that recognize the air conditioner, or the shower or the sound of the kettle, into areas of my nervous system that are responsible for processing speech and language.” He went on, “And that spillover doesn’t require similarity. It just requires some kind of shared activity.”

But much of our perception of the world also results from the interaction between what we might expect to happen in a given situation, given our beliefs, and the processing of incoming sensory information, said Chris Frith, a neuroscientist and Royal Society Fellow, from University College London. Since there is no actual verbal content to the sound of the air conditioner, our “top-down” unconscious expectations may be given free rein to color the sensory input. “Since our past experience, and the most complex auditory analysis we do, is mostly concerned with listening to other people, I suspect this is why the patterns we perceive in noise are mostly speech,” he said. “Of course, when the signal is noise, top-down processes will dominate our perception.”

The “reality” our minds impose on random noise, influenced by our idiosyncratic beliefs and predispositions, can also be harmful. David Smailes, currently a researcher at Sunderland University, studied cognitive and emotional aspects of psychotic-like behavior and then began research into auditory hallucinations, as part of a collective research group called Hearing the Voice. I asked Smailes about the distinction between perceiving voices while knowing they are not present, and believing they are present and real.

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Imagine a person, he said, who hears the phrase, “Let’s get him,” in the rumble of a conversation in a local bar. If he’s a skittish sort, “that person is likely to believe that the voice he heard was the voice of another person, that it’s an indicator of a threat, and that he needs to leave the bar. His belief that the voices were ‘real’ is less about the initial perceptual experience of the voice and more about his beliefs about the world more generally, cognitive biases, and his reaction to the voice.”

Although technically it describes a mishearing of a rumble of conversation, this example qualifies as a hallucination. In fact, all experiences of auditory pareidolia are a form of hallucination—the sensory perception of something not present. So if you have an experience of auditory pareidolia, how concerned should you be?

The difference between what normal people and patients hear, Remez explained, is that normal people realize a strange sound is a hallucination, while patients perceive it as real and intended. “It’s the difference between thinking X is like Y and believing X is Y,” he said. “Honestly, I wish we knew more about that distinction—the distinction between resemblance and identification.”

As scientists work to sort out that distinction, remain vigilant about what you hear. The fragility of symbolic thought is such that wires get crossed in your brain. If you hear a symphony in Manhattan traffic, thank your auditory pareidolia for relieving your stress. But if you are convinced a toy doll is reciting a religious message, you probably have cause for concern.

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Philip Jaekl is a freelance writer interested in cognitive neuroscience. He’s held postdoctoral research positions, investigating auditory-visual sensory integration, at Pompeu Fabra University in Barcelona and at the University of Rochester in New York State, where he currently resides.

*Acknowledgement: The author thanks Joe Banks for his valuable input. Joe has overseen an ongoing art and research project called Rorschach Audio for over 20 years. The project is directly connected with the phenomenon of auditory pareidolia.

WATCH: The cognitive scientist and music composer Jonathan Berger on hearing meaning in meaningless noise.

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The lead image is courtesy of MiGrü via Flickr.

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