Research Summary
The pupillometry of the possible
Uncertainty is a natural part of our everyday life: we usually can’t fully understand the present or precisely predict the future. To manage this uncertainty, we imagine different possibilities, we contrast them and then make decisions. But when does the ability to imagine multiple possibilities emerge?

Let's take an example from children's everyday life: when playing hide and seek, they try to choose the best possible hiding place from multiple alternatives. But that's not the only factor that helps them win the game: the better they can predict the probability of the others' searching strategies, the more unexpected option they can choose and the more successful they will be at this game. To do that, they have to rely on their ability to imagine multiple possible scenarios. What are the origins of this ability?

Some studies – among others a few that we have conducted in the Cognitive Development Center – suggest that even 12 months old infants can represent more possibilities at once: for example, if they are shown that one of two mutually exclusive possibilities is eliminated, they expect the other alternative to be true. (You can read about this finding in more detail in our research summary titled Baby logic.) In contrast, older children can sometimes make striking mistakes when facing uncertainty. If we ask kids to catch a ball dropped into a forking tube, 4-year-olds prepare for both possibilities and cover both exits with their hands – making sure they succeed -, but 2-year-olds often cover only one exit (Redshaw and Suddendorf, 2016). This finding is sometimes explained by their lack of capacity to imagine multiple future possibilities at once, but perhaps they do have an intuitive understanding of possibilities, only they struggle to generate action plans considering every alternative outcome.  

These different studies bringing seemingly contradicting results have something in common: children were not simply required to represent multiple possibilities, but they also had to integrate them in further mental processes (for example, they had to make inferences or decisions based on them). CEU researchers Nicolò Cesana-Arlotti and Ernő Téglás developed a new method to be able to detect babies’ representations of possibilities that didn’t require any planning or decision from the infants. They based this method on the phenomenon that our pupils dilate when we make mental effort (e.g., try to remember an increasing number of things or make complex mental calculations). Their hypothesis was that if infants represent multiple possibilities at once, their pupil would dilate more than if they only represent one.

The authors presented simple videos to babies between 10 and 14 months of age, in which three characters appeared: a ball, an elephant and a doll. The three characters had something in common: the upper part of each figure was a red half circle, so when their lower half was covered, it wasn’t possible to tell them apart.One character ended up on one side of the screen while two on the other one, and they made different noises to draw the babies’ attention to their position, making it easier for them to remember that the elephant was on the left side and the doll and the ball on the right side, for example. Then an occluder hid the characters and one of them started moving towards the middle of the screen.The babies could not see which character was moving: they only saw a red half circle which was not enough to identify the object. However, if the character was coming from the left side, they could be sure that it can only be the elephant. Crucially, if the character came from the right side, it was not clear whether it was the ball or the doll. The researchers expected that if the babies hypothesize about this ambiguous object, and they represent both possibilities – the ball and the doll -, that requires more mental effort than only having to consider the one and only possibility in a given context (the elephant coming from the left side), which is reflected in a bigger pupil dilation. This is exactly what they found when testing 14-month-old infants, but the same pattern was not detected with 10-month-old babies.

This study didn’t only demonstrate that babies are able to represent multiple possibilities at once already at the beginning of their second year of life, before they even learn their mother tongue, but it also showed that pupillometry can be a useful method to study infants’ mental processes.


Original paper: Cesana-Arlotti N, Varga B, Téglás E. 2022 The pupillometry of the possible: an investigation of infants’ representation of alternative possibilities. Phil. Trans. R. Soc. B 377: 20210343. (

Cited: Redshaw J, Suddendorf T. 2016 Children’s and apes’ preparatory responses to two mutually exclusive possibilities. Curr. Biol. 26, 1758–1762. (doi: 10.1016/j.cub.2016.04.062)

Other Summaries