Underwater Pupillary Reflex
Moken Children on the West Coast of Thailand
If you practiced, you could see clearly underwater without goggles. It wouldn’t take you very long.
Like the Bedouin of the Arabian desert, a handful of Austroindonesian tribes have drifted amongst the Mergui Archipelago on the west coast of Thailand, hunting and living at sea for the last thousand years. Out of efficiency these tribes use nets to fish, however like most aquatic mammals, they find themselves with an appetite for mollusks and sea cucumbers, for which they freedive regularly. Their hunting style and physiology have adapted to their environment, including the ability to flip a fundamental reflex in their eyes to see clearly underwater.
Your pupils dilate in a dark environment or if you take a stimulant (caffeine, MDMA, cocaine). In the dark, this is to allow as much light as is available into your eye. After taking a stimulant it is because dilation is a sympathetic response (fight or flight – see to survive). In contrast, your pupils constrict in light or if you take a depressant (alcohol, benzodiazepines, opioids). In the light it is because you don’t need excess light to see better. Following a depressant, constriction is a parasympathetic response (rest and digest). If you are a photographer, you recognize that pupil constriction is a lot like narrowing aperture.
Austroindonesian tribes constrict their eyes to the anatomical limit in the dark underwater environment which is the exact opposite of what I just described. It is remarkable because this reversal occurs despite the sympathetic-parasympathetic foundation.
How does this happen?
Light travels straighter in air than in water. As light travels through water it bounces all over the place compared to air. If an object is far away, light has more potential to scatter.
The nerds in white coats use “refractive index” to say the same thing: air has a refractive index of 1 while water is 1.3. The curvature of our cornea and thickness of our lens orient scattered light straight back to our retina, this corrective effect is known as refractive power. Underwater, or to compensate in certain ocular diseases, our pupils engage in refractive correction by constricting. By constricting, they block very scattered light allowing selection only for the light that is head on. This allows you to see more distant objects that otherwise had the potential for more light scattering. In other words, pupil constriction increases your depth of field.
Top Image - Mask is filled with air, pupils are dilated. Middle Image – Mask is flooded with water, pupils are constricted. Bottom Image – Child from Moken tribe, pupil constricted to anatomical limit.
In the picture above you can see how the eyes constricted to compensate for the increased scattering of light when the mask is flooded with water. This diver reports, he can “see slightly better, but realistically still can’t see shit”.
The tribes of Austroindonesia constrict their pupils to the anatomical limit and can actually see clearly a few feet in front of them. This is not enough to shoot a fast moving fish at distance, but provides a totally different experience when digging for clams out in the open, collecting shellfish and sea cucumber. I wonder how they would fair hole hunting because despite the close range you tend to be in a lower light environment.
Researcher Anna Gilsen discovered this phenomenon and took it one step further, training European children on holiday in Thailand. After 11 training session without goggles over the course of 1 month, the European were able to constrict their eyes underwater to the limit of human ability like the tribal kids. This skill lasted for 8 months following training (Gislen et. al. 2003).
I am disappointed that tourists can train themselves at this skill. I like to wish there was some physiological reward to abstaining from a modern metastatic society. The scales don’t seem balanced and my excuse to drop everything and spearfish for the rest of my days remains unvalidated. But alas, go ahead and train yourself in this if you are interested, I will. It may be useful in a dive emergency if your mask is flooded.