Blue eyes are better adapted to dim light compared to brown eyes
British researchers conducted an experiment involving volunteers of European descent and found that blue-eyed people see better than brown-eyed people in low light conditions.
The iris is an opaque ring-shaped membrane that regulates the size of the pupil (the hole in its center) and therefore the amount of light reaching the retina. Anatomically, it is the anterior part of the vasculature of the eye and can contain up to five layers of cells. Among these are melanocytes, whose melanosomes produce and store the light-absorbing pigment melanin. The color of the iris depends on the number of pigment granules, but not on the number of melanocytes and the size of melanosomes (they are approximately the same in all people). Pigmented irises appear brown because of the high content of brownish-black eumelanin and reddish-yellow pheomelanin. Depigmented irises contain little melanin and appear blue-blue due to Tyndale scattering of short-wavelength light by the collagen fibrils of the stroma. The level of pigmentation is not related to pupil size.
Eye color variations are seen almost exclusively in people of European descent. Iris depigmentation is most common in the northernmost latitudes of the continent, which were part of the Eurasian tundra belt about 10,000 years ago. It is caused by a point mutation rs12913832 of the HERC2 ubiquitinligase gene, which causes decreased activity of the promoter of the melanocyte-specific transport protein OCA2. Despite evidence of positive selection for this trait in the European population, it is still not known exactly why it emerged and became fixed - the available data and hypotheses are conflicting.
Faith Erin Cain and Kyoko Yamaguchi of Liverpool John Moores University hypothesized that one of the drivers of positive selection for iris depigmentation may have been the ability to see better in low light conditions. To test this hypothesis, they invited 40 volunteers of European descent aged between 18 and 30 with blue or brown eyes who were intolerant of laser vision correction to participate in the study. Their irises were photographed while facing a white wall with the iPhone XR's main camera with flash from a distance of six centimeters. At specific six points of the rainbow, the color was objectively determined with Photoshop's "dropper" tool using the RGB system.
During the experiment, each participant stood three meters away from a wall with a screen on which an unlit code (five randomly selected capital letters in black Calibri 190 size font horizontally on an A4 sheet of dark gray RGB: 51,51,51,51) was fixed. At a distance of 120 centimeters from this wall, a cardboard box measuring 29.5×25.0×18.0 centimeters with 25 holes in the lid stood between it and the volunteer. Inside it were 120 LEDs connected in series to a wire with a power of 3.6 watts each. The illumination was increased in steps, starting from total darkness, by removing the next LED from the box through the hole in the lid. Code light levels at each step were measured with a standard digital luxmeter.
Before starting the test, participants were given 30 seconds of adaptation in complete darkness, then the lights in the room were turned on for 30 seconds (at which time the researcher pulled another LED from the box), after which the participant was given 30 seconds to read the code at the new light level and the correct response was recorded. The lights were then turned on again for 30 seconds, the next LED was prepared, and the cycle was repeated. Statistical processing of the results was carried out by nonparametric tests, the use of glasses or contact lenses was taken into account as an independent variable.
It was found that the mean illuminance at which brown-eyed participants distinguished the code was 1.76 levels (0.12 lux) higher than the mean of blue-eyed participants and 1.13 levels (0.08 lux) higher than the generalized mean. There was a small but significant difference between the mean illuminance levels in brown-eyed (12 LEDs / 0.82 lux) and blue-eyed (10.24 LEDs / 0.70 lux) volunteers according to the Mann-Whitney U-criterion (U = 107.5; p = 0.046), even more pronounced when two statistical outliers were excluded (U = 81.5; p = 0.012). Wearing glasses or contact lenses did not significantly affect the results.
The authors of the paper suggest that the better ability to see in low light may be due to an increase in the amount of diffuse light in the depigmented iris, which would be disadvantageous in high light levels typical of low latitudes. That said, the study was pilot in nature and did not test the relationship of twilight visual acuity to objectively measured amounts of melanin in the iris, nor did it take into account many associated factors, they note.