Pupillary Responses to Faces Are Modulated by Familiarity and Rewarding Context

Every day we see dozens of faces and we are experts in their processing. Faces carry a lot of information, one of which is feedback and reward for our actions. For example, when we do something and our friend smiles in response, it’s rewarding. On the other side, sometimes we see people smile, but this smile is not a response to our actions. If smiling faces are per se rewarding, we should feel rewarded in both situations. If, however, the rewarding value of faces depends on our actions, the smile is only rewarding in the first situation. Thus, in this study we compared how people process smiling faces when they serve as feedback and when they simply appear on the screen. Further, faces differ in how familiar (known, recognisable) and socially relevant (personally important) they are. We hypothesised that more familiar and relevant faces would also be more rewarding (when providing feedback). We found that 1) familiarity plays a larger role than social relevance when processing rewarding smiling faces, and that 2) smiling faces are rewards only when they are delivered in response to some actions, and not when we passively watch them on a screen.

Image by Lenka Fortelna from Pixabay 

You can find the original article here (open access, ENG).


How faces are familiar, relevant, and rewarding

Every day we see dozens of faces and each of them carries multiple information: about who the other person is, how they feel, what they are thinking about, how they react to us. Since faces are such crucial social signals, our brains must process them fast and accurately. Indeed, the human brain has specialised in face processing over thousands of years of evolution to make us face experts.

Another very important role of faces of others is that they often offer feedback for our actions. Imagine a child showing the mum a new drawing. The mum will likely smile in response and praise the child for the efforts. That smile tells the child that the mum approves their actions. Because getting positive feedback and mum’s smile is typically rewarding, the child will likely engage in similar actions in the future to again make the mum smile. Thus, the mum has rewarded the child with a smile. Indeed, there are many examples in the scientific literature showing that the brain’s reward network (a group of structures in the brain which are responsible for recognising rewarding stimuli and processing them) is activated when we observe smiling faces and faces of our close ones.

However, not all the faces (and their owners) are equally important and known for us. Some of them are familiar: we have already seen them before and we often know them very well, like those of our parents, friends, colleagues from work, or even a shop-assistant in the local grocery store. Some of those people are especially socially relevant for us, like family members, partners, and friends. Some are less relevant, but still familiar, like a TV news presenter. Finally, some are unfamiliar and irrelevant, like strangers we pass on a street. Thus, faces we encounter every day differ in their levels of familiarity and social relevance. But do familiarity and social relevance change how rewarding we find these faces?

What we did in the study

To see whether familiarity and social relevance influence reward values of faces differently, we designed a study in which we asked participants to perform two tasks in which they could see pictures of smiling faces. The persons depicted in the pictures were more or less familiar and more or less socially relevant. The most unfamiliar and irrelevant ones were strangers: persons the participants had never seen before. Others were more familiar but hardly socially relevant: celebrities (actresses and singers). Finally, some of the faces were those of the experimenters: during the study, they became familiar and, to some degree, socially relevant.

There were two tasks: an active one and a passive one. In the passive task, participants were asked to just observe a screen, on which we displayed the smiling faces. They did not have to do anything to make the faces appear. In the active task, participants played a repeat-a-pattern game: they saw a sequence of coloured buttons appear of the screen, and they were asked to repeat it on a gamepad. If they were successful (and only then), they would see one of the smiling faces as feedback. Thus, in the two tasks participants saw the same faces, either carrying feedback (in the active task), or not (in the passive task).

In both tasks, we recorded the pupil sizes of the participants in response to the faces. The pupils dilate and constrict with cognitive and emotional processes and had been previously used as indicators of reward processing.

What we expected

We predicted that the pupil would be more dilated (larger) when seeing more familiar and socially relevant faces. Also, we expected to see this only (or more) in the active task, and not in the passive one. Our reasoning was that if the pupil sizes indicate reward processing, the faces would only be a reward when participants do something and receive feedback for it. On the other hand, when they passively see smiling faces, these faces would be positive stimuli, but not rewards.

What we found

We found that the pupil was influenced by familiarity, but not by social relevance. Against our predictions, the pupil was more constricted (not more dilated) in response to more familiar faces. Although the direction was surprising (smaller instead of larger pupil sizes), it shows that familiarity modulates reward processing. As we expected, these differences were only shown in the active and not in the passive task.

What we take from that

There are two take-home messages from this study. First, familiarity of faces plays a bigger role in processing of social rewards than does social relevance. In other words, our brains and bodies process smiles of familiar and unfamiliar persons differently. Second, the feedback context (active vs. passive task) is crucial in reward processing. When we simply see people smile, but this smile is not a response to our actions, it is not considered a reward for us. On the other hand, when we perform an action and get a smile carrying (positive) feedback for this action, this smile is considered a reward.

You can find the original article here (open access).

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