Why is it that when you shine light in only one eye, they both still constrict t!

Best Answer - Chosen by Voters

What pixilated_elf said is true. Problem is, it doesn't answer your question. You have asked WHY there is a consensual response. This is why. The temporal (left) half of the retina in the left eye sends its information to the left brain while the nasal (left) half of the retina in the right eye does the same. Visa versa, the temporal (right) half of the retina in the right eye sends its information to the right brain while the nasal (right) half of the retina in the right eye does the same. SO, each eye SPLITS its information and send half to one side of the brain and half to the other side. This creates and response from the brain such that when light is shined into the left eye it is actually sent to BOTH sides of the brain and visa versa for the other eye. Therefore, since each side of the brain is stimulated whether the light is shined in the right or the left eye the brain sends a response signal to both eyes at the same time. Thereby constricting both pupils. There ya go.

Me. I am an optometric physician.

If there is nothing wrong with a person, when you shine a light in one eye the other pupil will constrict. It's called the consensual response. When the pupil constricts in the eye that you shine the light into, it is called the direct response.