The Issue of Clarity versus Color
Although surprising, the primary reason for these differing types of photoreceptors is not to perceive colors, but is to accurately see objects, more specifically spatial acuity. Think of a caveman trying to find an apple surrounded by rocks. Although the differing colors would be considered the telltale sign of which is which, detecting the shape and seeing the object clearly, whether close or far away requires spatial acuity. The physics of the retina (the location and spacing of of different photoreceptors) allows us to see objects clearly, and our ability to see colors came as an result of adhering to these necessary physics.I have condensed and added my own insight to an excellent blog post on Visual Acuity, from the blog WebVision, in order to help me explain this intriguing system.
Why not have more Cones for more color?
Based off of what I explained in Part 1, You might ask: Why not have more Cones types to see more colors? The reason is this magical idea of visual or spatial acuity, and the Physics of our retina.
Snellen letters are used to measure visual acuity in target recognition tasks. Each piece of the letter (stroke width and gap width) subtends 1/5th of the overall height, and the entire letter corresponds to 5 minutes of arc. A person with 20/20 vision should be able to read the letter E at 20 feet (6 meteres), where one of the strokes of the letter subtends one minute of arc at the eye. Visual acuity in Snellen notation is given by the relation: Acuity = D’/D where D’ is the viewing distance, and D is the distance at which each letter subtends 5 minutes of arc. Target resolution, on the other hand, has a threshold defined as the smallest angular size at which subjects can discriminate the spacing between the critical elements of a stimulus patternlike a grating.
Limitations of the Retina
Our ability to detect fine details depends on two factors, an optics system that combats aberrations, and the spacing and size of our receptors. The optics of our eyes cause distortions, where an object is imaged on the back of the retina as a distribution or point spread function.
The optics of the eye combats two necessary aberrations that would inhibit spatial acuity. First, shorter wavelengths of light refract more than longer ones, meaning that Blue wavelengths would be brought into focus infront of red or green ones in the Retina. The eye corrects for this chromatic aberration by placing Blue cones further out in the periphery of the retina, so they refract to the same nodal point as the Red and Green light wavelengths. As a result, Blue cones have larger receptive fields than Red and Green Cones, creating a sort of blue blur. Second to combat is spherical aberration, which occurs from wavelengths of light at the periphery of the lens refracting more than those going through the center. The eye compensates for this by allowing the pupil muscles to contract and dilate, subsequently increasing and decreasing the size of the lens, in order to block unnecessary peripheral rays that would blur the images of varied distances.
Receptor Spacing and Diffraction
Raleigh’s criterion is used to calculate the resolution of the eye for stimuli that are degraded by these optics. Two points or lines are resolved (are clear) if the peak of the point spread function lies on the first trough of the other point spread function, or when as=1.22L/d, where L is the wavelength of light, and d is the size of the pupil. This means, that two objects must be separated by the width of their point spread functions in order to be seen as two separate objects, if the width is any less, they are seen as a uniform object.
The spacing and number of Cone receptors therefore affects the quality of the image perceived. Helmholtz proposed that a grating would be resolved if there is a row of unstimulated cone in between rows of stimulated cones (Peters, 2003). In laymans terms, this means that if each individual receptor were to pick up an individual pixel of visual info, the spacing between adjacent receptors must be small enough and the number of cones large enough (at least in the foveal area) to not accidentally skip over a pixel, which would cause a blur or distortion of the image (ex. seeing the white separate from the black lines in the grating). Cone spacing at the fovea is approximately 2.5 um (Curcio, 1990), allowing for a maximum of 60 cycles per degree, directly hindering our visual acuity.
Summing it All Up
Due to the concave shape of the retina, the properties of refraction and diffraction, and the spacing of receptors throughout the eye, there is no possible way to incorporate more colors into our visual palette without hurting our spatial acuity. Creating a different cone type that was tuned to a larger or shorter wavelength, would require an increase in the spacing between adjacent receptors of similar types, a decrease in the size of the receptors, or a decrease in the number of receptors for each respective wavelength. Decreasing the size of the receptors is not logical, and a decrease in the number or increase in the spacing would both directly affect acuity. With regard to optics of the eye, the lens would have to refract even more than it does already or pupil size would be too small or too large to clearly pick up any image.
For these reasons, we aere left with only Red, Green and Blue Cones. Because wavelengths for Red and Green only vary by 30nms-compared to Blue wavelengths-we are able to have three types of cones, instead of just two. Therefore, in a sense, our ability to evolve towards only trichromatic color vision is a quite ingenious solution to a rather complex problem: color versus clarity.