Face pareidolia is the perception of illusory faces in objects. There are many examples online, and you have probably seen examples in real life in your cup of coffee, or in a tree, or a piece of fruit. You can watch an academic talk on the science of face pareidolia on YouTube, or read this article in Mental Floss for an overview.

The human brain has a network of regions specialized for face perception. This TED talk by Prof. Nancy Kanwisher at MIT describes the discovery of face-selective brain regions and explains how scientists use functional brain imaging to study human cognition in real-time. 

How are illusory faces processed by our brain? 

A study by Dr Susan Wardle and colleagues from the National Institutes of Health in the journal Nature Communications used functional magnetic resonance imaging to scan the brains of people while they looked at pictures of human faces, illusory faces in objects, and ordinary objects without a face. The same regions of the brain that responded when people looked at a human face also responded when they saw an illusory face in an object. This included brain regions known as the fusiform face area and the occipital face area. To find out how quickly our brains can resolve the mistake of face perception when we see an illusory face, the researchers repeated the experiment using a second brain imaging method with better temporal resolution known as magnetoencephalography. This revealed that it takes only a quarter of a second for the brain to realize its mistake and represent examples of face pareidolia more like objects than faces. 

What would happen if we stimulated the regions of the brain that are associated with face perception? 

In a study published in the journal PNAS, researchers directly stimulated the fusiform face area in the brain of a patient who had electrodes implanted for epilepsy treatment and found this caused him to misperceive facial features on ordinary objects. This suggests a causal link between activity in the face regions in the brain and the perception of illusory faces in objects.

More than just a face

One surprising feature of face pareidolia is that we see much more than just a face— many of these illusory faces appear to have a specific gender, age, or facial expression. A recent study published in the journal PNAS found that people are more likely to perceive illusory faces as male than female, by as much as 4:1. This male bias occurred for both men and women, and suggests that we are more likely to see faces as male unless there is extra information (e.g. hair, makeup, eyelashes) to indicate that a face is female. Think of LEGO characters and emojis — the female versions usually have extra features added to the simpler male version of the face. You can read articles about this study in Forbes and Science News.

Are humans the only species to see illusory faces in objects?

This article in New Scientist describes the study led by Dr. Jessica Taubert which discovered that monkeys see faces in objects by measuring their eye movements, published in the journal Current Biology. This was an important finding because it revealed that face pareidolia is a fundamental aspect of how the primate brain processes faces, rather than something unique about the human brain.