According to scientists from the University of Glasgow’s Institute of Neuroscience and Psychology, there are only four basic emotions that we all experience.

A commonly-held belief, first proposed by Dr Paul Ekman, posits there are six basic emotions – happiness, sadness, fear, anger, surprise and disgust, which are universally recognized and easily interpreted through specific facial expressions.

But the Glasgow team challenges this view. The researchers claim that while the facial expression signals of happiness and sadness are clearly distinct across time, fear and surprise share a common signal – the wide open eyes – early in the signaling dynamics. Similarly, anger and disgust share the wrinkled nose. It is these early signals that could represent more basic danger signals. Later in the signaling dynamics, facial expressions transmit signals that distinguish all six ‘classic’ facial expressions of emotion.

“Our results are consistent with evolutionary predictions, where signals are designed by both biological and social evolutionary pressures to optimize their function,” said Dr Rachael Jack, who is the first author of a paper published in the journal Current Biology.

“First, early danger signals confer the best advantages to others by enabling the fastest escape. Secondly, physiological advantages for the expresser – the wrinkled nose prevents inspiration of potentially harmful particles, whereas widened eyes increases intake of visual information useful for escape – are enhanced when the face movements are made early.”

“What our research shows is that not all facial muscles appear simultaneously during facial expressions, but rather develop over time supporting a hierarchical biologically-basic to socially-specific information over time.”

Dr Jack with colleagues used special techniques and software called Generative Face Grammar platform to synthesize all facial expressions.

The platform uses cameras to capture a three-dimensional image of faces of individuals specially trained to be able to activate all 42 individual facial muscles independently. From this a computer can then generate specific or random facial expressions on a 3D model based on the activation of different Actions Units or groups of units to mimic all facial expressions.

By asking volunteers to observe the realistic model as it pulled various expressions – thereby providing a true four-dimensional experience – and state which emotion was being expressed the researchers are able to see which specific Action Units observers associate with particular emotions.

It was through this method they found that the signals for fear/surprise and anger/disgust were confused at the early stage of transmission and only became clearer later when other Action Units were activated.

“Our research questions the notion that human emotion communication comprises six basic, psychologically irreducible categories. Instead we suggest there are four basic expressions of emotion. We show that ‘basic’ facial expression signals are perceptually segmented across time and follow an evolving hierarchy of signals over time – from the biologically-rooted basic signals to more complex socially-specific signals,” Dr Jack said.

“Over time, and as humans migrated across the globe, socio-ecological diversity probably further specialized once-common facial expressions, altering the number, variety and form of signals across cultures.”

The team is planning to develop their study by looking at facial expressions of different cultures, including East Asian populations whom they have already ascertained interpret some of the six classical emotions differently – placing more emphasis on eye signals than mouth movements compared to Westerners.

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Rachael E. Jack et al. 2014. Dynamic Facial Expressions of Emotion Transmit an Evolving Hierarchy of Signals over Time. Current Biology, vol. 24, no. 2, pp. 187-192; doi: 10.1016/j.cub.2013.11.064