How can a light that burns so brightly (in your mind only) make your pupils contract? Through modulation of the pupillary response by the content of visual working memory, according to scientists at Oxford University, backed up by research in the Netherlands.

A study published in Proceedings of the National Academy of Sciences (PNAS) shows that simply thinking about bright light is enough to alter pupil size, even if there isn’t anything physical for the eyes to react to.

Dr Nahid Zokaei and her Oxford team repeatedly showed 22 men and women dark or light patches, each associated with a specific sound. When the patches disappeared after two seconds, participants had to picture the correct patch in their mind on hearing its corresponding sound.

Participants’ pupils dilated when thinking of the dark patch and contracted when they pictured the light one; the same results expected when physically seeing these.

“This seemingly small action could allow us to anticipate a change in brightness before it happens,” said Dr Sebastiaan Mathôt of Groningen University, who carried out a similar study that confirmed this finding.

The results showed our pupils could therefore constrict just before we turn on a light in a dark room to prepare us for the resulting glare, said Dr Mathôt.