The big question is why does a hiccup cause the muscles of the iris to contract which opens the aperture of the pupil i.e. the eyes dilate? This normally happens when the surroundings become dark and the eye has to let in more light to see properly. Conversely, in bright light the pupil narrows down to a split as we all know. I am not going to find the answer on the internet partly because the hiccup itself is a bit of a mystery to people. But there is clearly a nerve connection perhaps via the brain between the hiccup and the nerve controlling the muscles of the cat’s iris. I have a theory: the hiccup is an action which takes muscle power and it is all-encompassing fleetingly. It takes a conscious muscular effort to contract the iris of the eye to create a smaller pupil size. The hiccup temporarily distracts the brain from contracting the muscles of the iris which means that the pupil dilates.
Cats' eyes dilate when they hiccup. pic.twitter.com/9eAQ5h06iQ
— Science is Amazing (@AMAZlNGSCIENCE) November 19, 2020
This is an embedded video and sometimes they stop working overtime. If that has happened I apologise but I have no control over it.
A nice aspect of the video is it shows us very clearly how the iris ‘stops down’ (in photographic terminology) to create a slit. This is particularly advantageous because it allows a cat to have better depth perception. It also allows a cat to protect the retina of the eye under bright conditions because the eyelid can pass down over the slit, like a blind going down a window. This is helpful because a cat’s eyes are particularly sensitive due to the reflective layer behind the retina which allows cats to see under dark conditions.
Some more articles about the cat’s eye