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Our perception of a vibrant world is a remarkable feat of biological engineering that begins in the retina, a layer of cells at the back of the eye. This tissue contains two types of photoreceptor cells: rods and cones. Rods, of which there are over 100 million, are incredibly sensitive to low light but do not detect color, which is why we see in shades of gray in dim environments. In brighter conditions, our roughly six million cone cells take over, providing sharp, detailed vision and the rich tapestry of colors we see every day.
The vast majority of people have what is known as trichromatic vision, meaning they possess three distinct types of cone cells. These cones are each tuned to be most sensitive to the long (red), medium (green), or short (blue) wavelengths of light. An object's color is determined by the wavelengths of light it reflects, which in turn stimulate these cones to varying degrees. The brain then acts as a master interpreter, processing the complex combination of signals from all three cone types to distinguish an incredible spectrum of approximately 10 million unique hues.
Interestingly, a rare genetic variation found almost exclusively in women can lead to a condition called tetrachromacy. Due to genetics linked to the X chromosome, these individuals possess a fourth type of cone cell, a mutation that creates a new pigment sensitive to a wavelength between red and green. This additional cone has the potential to dramatically expand the range of perceived colors, allowing for the discrimination of up to 100 million shades that are indistinguishable to a trichromat. However, possessing the fourth cone type doesn't automatically grant this "super vision"; the brain must also be able to interpret the unique signals it receives, making true functional tetrachromacy exceedingly rare.