Superior Underwater Vision in Humans

Sammanfattning: Amphibious vision is accomplished in many animals by special optical and physiological adaptations in the eyes. However, human eyes are considered to be adapted to a life in air since our curved outer cornea – which is used to refract more than two-thirds of the incoming light on land – becomes useless underwater. This thesis produces evidence that Moken children, a population of sea nomads (or sea-gypsies) from Southeast Asia, have developed an underwater visual acuity that is more than twice as good as that of European children. They achieve this by heavy accommodation and concurrent pupil constriction underwater, a reaction absent in European children. This skill is most likely learned since this thesis also shows that European children can be trained to achieve the same level of acuity underwater as the Moken children. European children learned to control their accommodation in only 4-6 months, depending on the individual. Experiments were also made to determine whether the physiological diving response is involved in the process of pupil constriction underwater, but the results were negative. The Moken children constrict their pupils even when diving with goggles, a reaction also present in trained European children. In untrained European children however, this pupil constriction is absent. It is possible that training affects the connection between the accommodative response and pupil constriction so that it becomes a conditioned response elicited when the children immerse their heads in water. Furthermore, optical theory is used to predict underwater acuity in humans. Considering that the normal ranges of contrast sensitivity vary a great deal between individuals, the experimental values of underwater acuity are well within the range of values predicted theoretically.