The second paragraph of this Guardian article mentions, and links to, but doesn't expound on the biophysical pupillary response to mental visual imagery.
https://www.vice.com/en/article/qjbq8w/how-pupil-size-can-re...
If someone with normal mental visualization ability pictures a "bright" scene, the pupils contract just as they would in response to a bright visual stimulation. Aphants' pupils show no physical response to this test.