What’s behind a blush?

For Darwin, blushing was "the most peculiar and human of all expressions." Even the boldest of us will be familiar with the experience of blushing while feeling embarrassed, shy, shameful, or even proud. There is an ongoing debate, however, about what, exactly, triggers these flushes. Is it caused by an automatic, sudden surge of alertness when we are exposed to other people's judgement? Or, does it happen only when we reflect on how other people may be perceiving us — in his words, is it "the thinking of what others think of us, which excites a blush"?

To investigate which explanation might be right, Milica Nikolić at the University of Amsterdam and colleagues looked at what happened when 40 young participants watched videos of themselves, and other people, singing karaoke. When they signed up for the study, published recently in Proceedings of the Royal Society B, they didn't know that they'd be asked to sing. But, during their first visit to the lab, they were all recorded singing four songs, chosen specifically to be tricky to perform well. ('Hello' by Adele, 'Let It Go' from the movie Frozen, 'All I Want for Christmas Is You' by Mariah Carey, and 'All the Things She Said' by t.A.T.u.)

During a subsequent lab visit, these participants had their cheek temperature monitored and their brains fMRI scanned while they watched excerpts of their own singing, segments from fellow participants deemed by the team to be of a similar singing ability, and also a professional singer, whom they were told was a participant. (The team included a professional because they felt there was a risk that watching a fellow participant singing similarly badly might trigger empathic, or vicarious, embarrassment, which could affect the results.) They then also told the participants that videos of their singing were being watched by other participants in nearby scanners.

When the team analysed the cheek temperature data, they found that the participants blushed more when they watched themselves singing, compared with someone else. "This aligns with the notion that blushing typically occurs when the self is exposed," they write.

They also found that participants who blushed more while watching themselves sing had patterns of brain activity that suggested heightened emotional arousal and attention. Specifically, these people showed stronger activation in the cerebellum, a region long known to be involved in movement, but which recent research suggests also plays a crucial role in emotion processing. Their brain scans also suggested that they processed the visual information in their videos more deeply, and that they were more emotionally aroused by them. The researchers did not, however, find any differences in activity in regions of the brain linked to reflection.

Overall, the team writes, these results support the theory that blushing is triggered by a sudden surge of alertness when we are socially exposed — not, as Darwin suggested, by thinking about what others might be thinking of us. This was further bolstered by their analysis of data from two questionnaires that the participants also completed, which probed their levels of embarrassment while in the scanner and their levels of social anxiety, generally. If people who were more socially anxious or who felt particularly embarrassed blushed more, that could suggest a role for reflection on how we might be coming across to others in triggering blushing. However, the team did not find any such links.

Participants in this study were all young adult females, so perhaps people of different ages or sex may have responded differently. But the results do suggest that relatively basic brain processes may underpin blushing. This work therefore not only helps us to understand what causes blushing in adults, but also implies that it might be possible to monitor cheek temperature to explore self-awareness in babies and even other animals.

Read the paper in full:
Milica Nikolić, Simone di Plinio, Sauter, D., Keysers, C., & Gazzola, V. (2024). The blushing brain: neural substrates of cheek temperature increase in response to self-observation. Proceedings of the Royal Society B, 291(2027). https://doi.org/10.1098/rspb.2024.0958

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