You might know that the size of the pupils in our eyes changes depending on how well lit our environment is, but there’s more to the story: Scientists have now discovered that the pupil also shifts in size depending on how many objects we’re observing.

The more objects in a scene, the bigger the pupil grows, as if to better accommodate everything that it has to look at. This “perceived numerosity” is a simple and automatic reflex, the new research shows.

In a new study, researchers observed the pupil sizes of 16 participants while they looked at pictures of dots. In some of the pictures, the dots were linked together in dumbbell shapes – creating the illusion that there were fewer objects – and pupil size then shrank.