As iris pigmentation decreases, corneal sensitivity increases, according to a newly published report. This inverse association is particularly notable in Caucasian subjects. The investigators looked at subjects of a variety of racial backgrounds and assessed their corneal sensitivity thresholds using a noncontact corneal esthesiometer. They also ranked the degree of iris pigmentation of each subject according to the Seddon method, using a set of graded photographs of iris pigmentation (grades 1 to 5, with 1 being a blue or gray iris and 5 being a particularly dark brown iris). The study supports previous research showing blue eyes have more sensitive corneas than brown eyes.

Although a moderate trend for increasing corneal sensitivity with decreasing iris pigmentation grade was seen among all ethnicities, the correlation was strongest within the Caucasian group—the only ethnicity with all iris pigmentation grades. However, the Caucasian group did not experience the highest levels of corneal sensitivities; Chinese participants had the lowest corneal sensitivity threshold.

The researchers speculate that melanin in the iris may correlate directly with the amount of neuromelanin in areas of the central nervous system and that differences in corneal sensitivity between light- and dark-eyed subjects arise from differences in central nervous system functions.