What makes us see Jesus in a taco, or a human face on Mars?

Photographer Todd Terwilliger calls this picture "Skull Flower," for reasons that should be obvious. Its resemblance to a human cranium is, of course, purely coincidental — yet the urge for our minds to register this plant as a piece of human anatomy is all but impossible to resist. But why?

The one-word answer, as some of you may know, is "pareidolia." But here's what you don't know: scientists this week presented some of the most compelling evidence to date that this pscyhological phenomenon is mediated by a region of the brain known as the fusiform gyrus. How did they find this culprit? Simple: by jolting that part of the brain with electricity, and watching their test subject's perception of reality liquefy into mind-bending absurdity.

But we're getting ahead of ourselves. Those familiar with pareidolia can skip to the neurobiology and mind-melting below — but for the uninitiated, what is this "pareidolia," exactly?

Seeing What Isn't There

Basically, we humans have this niggling habit of extracting what we believe to be significant information from patently insignificant stimuli. It's where we get Jesus-toast, and why we find familiar objects in shapeshifting cumulus. Other examples abound. Here are a few:

Our Lady of Guadalupe, in a tree trunk

What makes us see Jesus in a taco, or a human face on Mars?

A face on the surface of Mars

What makes us see Jesus in a taco, or a human face on Mars?

An elephant — also on the surface of Mars

What makes us see Jesus in a taco, or a human face on Mars?

A drunk, belligerent octopus. Or a coat hook. No one knows for sure.

What makes us see Jesus in a taco, or a human face on Mars?

An apparition in plumes of volcanic ash

What makes us see Jesus in a taco, or a human face on Mars?

And, of course, skulls (not to mention other anatomical features) in flowers. Point being: our brains are wired-up in such a way that we often see things that aren't really there — but again: why? For pareidolia in general (see elephants, drunk octopuses, etc.), the answer is somewhat muddled; but on the subject of faces, specifically, scientists have some very strong leads.

Evolution & Neuroanatomy

The leading hypothesis is that our tendency to recognize faces in nonhuman objects is a highly evolved survival trait. As Carl Sagan wrote in The Demon-Haunted World:

As soon as the infant can see, it recognizes faces, and we now know that this skill is hardwired in our brains. Those infants who a million years ago were unable to recognize a face smiled back less, were less likely to win the hearts of their parents, and less likely to prosper. These days, nearly every infant is quick to identify a human face, and to respond with a goony grin.

This makes a lot of sense. Being able to not only spot a face, but decipher the emotional, social, or sexual cues signalled by that visage, could score you a meal, save your life, or land you a mate — all things that are crucial to the propagation of your genes.

What makes us see Jesus in a taco, or a human face on Mars?

Scientists have traced our ability to perceive faces to a few key regions of the brain. Functional MRI, positron emission tomography, and other brain-imaging studies, for example, have shown that the fusiform gyri and the inferior temporal gyri light up when a person is shown pictures of faces, or objects resembling faces. Damage to the fusiform area, shown here in red, is known to give rise to prosopagnosia, a neurological disorder (more commonly known as "face blindness") characterized by the inability to recognize faces.

We even have some idea of how our brain distinguishes actual faces from images that merely trigger a pareidolic reaction. In a study recounted in the January 2012 issue of Proceedings of the Royal Society B, researchers led by MIT neuroscientist Pawan Sinha presented test subjects with a variety of snapshots, ranging from photos of actual faces to images that look nothing like a human face. Wired's Mark Brown gives a tidy summary of their findings:

What makes us see Jesus in a taco, or a human face on Mars?

The neuroscientists found different activity patterns on each side of the brain. On the left, the activity patterns changed very gradually as images became more like faces and there was no clear distinction between faces and non-faces. The left side would flare if someone was looking at a human or an eerily face-like formation of rocks.

But on the right side, activation patterns in the fusiform gyrus were completely different between genuine human faces and face-like optical illusions. There was no fooling the right side of the brain, no matter how much they resembled a face.

The researchers could conclude that the left side of the brain ranks images on a scale of how face-like they are. The right side makes the categorical distinction between whether or not it's a human face.

The Mind-Melting!

In 2010, Stanford neuroscientists Kalanit Grill-Spector and Kevin Weiner discovered two nerve clusters in the fusiform gyrus — dubbed pFus and mFus — that respond more strongly to faces than they do to inanimate objects or other body parts. By building on these findings, they've offered some of the most compelling evidence to date of the fusiform's role in facial recognition.

In the latest issue of the Journal of Neuroscience, the researchers, with the assistance of neuroscientist Josef Parvizi, show that mild electrical stimulation of pFus and mFus caused a test subject's perception of faces to be instantly distorted. Just watch his reaction when the researchers turn on the juice:

For those tempted to skip the video: shame on you, this is interesting. Here are some actual quotes from the test subject:

  • "You just turned into somebody else. Your face metamorphosed."
  • "Your nose got saggy, went to the left. You almost looked like somebody I'd seen before, but somebody different. That was a trip."
  • "Only your face changed, everything else was the same."

Catch that? Tellingly, the test subject's perception of other body parts and inanimate objects was unaffected by the fusiform-targeted electrical stimulation.

Grill-Spektor and her colleagues say that findings such as these are likely to play a crucial role in understanding and treating disorders like prosopagnosia. Given the anatomical and functional parallels between face blindness and spotting faces in non-human objects, one has to imagine the same could be said for pareidolia.

The findings of Grill-Spektor, Parvizi, Weiner, and their colleagues are published in the latest issue of the Journal of Neuroscience and are open access (aka free!).

Top image used with permission from Todd Terwilliger; fusiform GIF via Wikimedia Commons; range of faces and face-like objects via Sinha et al.; fusiform reaction video via Grill-Spektor et al.; all other image sources linked to within the post.