Rapid facial processing – identifying and studying a human face quickly – is a core part of human socialisation, says David Alais, a researcher at the University of Sydney and lead author on a new study published in Proceedings of the Royal Society B.

“We are a highly evolved social species, and therefore rapidly detecting and recognising faces is incredibly important,” he says. “We have a whole brain area dedicated to face processing.

Faces convey meaning and emotion in ways that help us interact with one another and understand each others’ motivations, which is especially handy for detecting danger or prospecting for mates.

However, our brains erroneously perceive faces and expressions in day-to-day objects – a  process known as face pareidolia – because brains have a shortcut to facial recognition that identifies the common structure of two eyes over a nose and mouth.

“The way the brain detects faces is to use a quick and dirty method to make sure it detects them fast,” says Alais. “So the thing with pareidolia images is they satisfy that basic global structure – two eyes, a nose and a mouth – and so trigger that rapid response.”

The research team, from the University of Sydney, wanted to understand whether the brain identified the error, or processed the object as a face. They examined 17 university students across two experiments, showing them 40 images of real faces and 40 images of inanimate objects with strong pareidolia.

Each image was displayed for 250 milliseconds, then rated by the participant for emotional expression. Each participants’ ratings were averaged into a mean estimate of the image’s expression, and the results showed that variability in rated expressions was the same between the human and non-human faces. Further, the rating of the perceived expression of each face – whether the face was real or not – was skewed towards the rating of the expression of the previous face.

This is known as positive serial dependence, but it disappears for human faces if they are rotated, so its occurrence between the real and illusory faces suggests that pareidolia engages the same mechanisms in the brain as the recognition of actual human faces. The authors conclude that expression processing is a broader process than once thought, and is not tightly linked to human facial features.

“Clearly, the negative consequences of mistaking an object for a face are probably much less than the consequences of missing a face, because it might be an enemy with aggressive intent,” says Alais.

He adds that it’s particularly interesting that the brain does not correct these errors. “You might think that the slower cognitive processes come in and you realise it’s actually not a face.

“And yet somehow, you keep perceiving it as a face and you process it for its emotional content. So you end up with that weird experience where you know it’s an object and yet you keep seeing a face.”

Alais attributes this overpowering perception to the key importance of social interaction and facial recognition to our species’ survival: “We’re highly evolved socially; we can’t afford not to look at faces."

Image: Getty Images