Human pupils physically shrink when looking at a "ghost image" in the mind, even if no actual light is hitting the eye.

Pupil diameter changes based on the brightness of internal afterimages that exist only in the brain. Most experts believed the pupillary light reflex was a purely mechanical response to physical photons entering the eye. Testing shows that the brain's internal representation of light is powerful enough to trigger the same physical constriction as the sun. This effect happens even when a person is just focusing their attention on a visual memory. It proves that the line between what we actually see and what we imagine is physically blurred within the nervous system. This means the brain treats mental imagery with the same physical weight as the real world.