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Rods and Cones

Rods and Cones

What is the retina? The theory and biology of trichromatic color vision.

The structure and complexity of the human eye is all for the purpose of providing the optimal conditions for image processing that allows us to perceive the movement, depth and colors of the surrounding world.

Retina

The way our eyes form an optical image is extremely complex and involves more than 200 million photoreceptors, conveniently named rods and cons due to their resemblance of these shapes. Of course, they alone are not alone responsible for our ability to see as the process involves many more pieces that together form the biomechanics of the eye.

The iris controls how much light enters the eye, followed by the lens that focuses the entered rays into an inverted and reversed image that is projected onto the back of the eye. In meantime, blood vessels in the eye provide the needed oxygen and nutrients to convert light into nerve signals as all processesses in the human body require energy.

After light has passed through the eye, it hits a thin layer of specialised nerve tissue called the retina and is the key step in our ability to see colors. There, the light rays are sorted out and interpreted in order to paint the whole picture of what was observed. Only after this complex initial processing is the information passed down to the brain via the optic nerve.

The retina can be described as an extremely dense field of light-sensitive cells, namely rods and cons. Both are responsible for our sight, however, each has its own very distinct function.

There are more than 100 million rods responsible for our ability to see in poorly lit environments by converting light into electrical signals. This is called scotopic vision. The photosensitive protein involved in this process is called rhodopsin and a single molecule of rhodopsin is so sensitive to light that it can detect even a single photon.

Furthermore, a single rod itself contains about 100 million rhodopsin molecules densely packed in disks and stacked on top of each other as a means to avoid mistakes.

When a rod absorbs a photon, it alters the shape of the rhodopsin molecule, which chemically triggers an electrical signal that is almost instantaneously processed by our brain. This is known as transduction of light.

Just as a light switch or any binary sensor, rods have only two states: either 1 or 0. Such simplicity allows rods to function extremely well in low light conditions. They are good at detecting movement and are heavily involved in our peripheral vision.

However, there is also a downside, rods simply detect light and cannot tell the difference between various wavelengths or in other words, they cannot distinguish between colors.

Here is where the few million cones in our eyes come into play as a way of evolution to show its talent for creative problem solving.

Essentially, both rods and cones are simple binary sensors. Light and darkness. On and Off. Ones and zeros, yet most of us can easily identify millions of distinct colors. These include all the in between hues, tints and shades of the visible spectrum.

Young–Helmholtz Theory

Sunlight is a vivid mixture of different wavelengths so soon enough it becomes apparent that our retina cannot possibly have a separate receptor for each one of the many colors we see.

To try and explain that in the early 1800s, Thomas Young suggested that there are only three types of photoreceptors with three specific types of spectral sensitivities.This theory was further developed by Hermann von Helmholtz who suggested that there are indeed three types of receptors but each was sensitive to a particular range of the visible light.

It is the combination of these three signals that is interpreted by our brain as colors. This became known as the Young–Helmholtz theory of trichromatic color vision.

Their assumptions were proven only a century later when the physiologist Gunnar Svaetichin for the first time ever demonstrated the existence of these three types of cone cells in 1956.

S cones that contain cyanolabe, most sensitive to short wavelengths of “blue-violet” at around 445 nm.

M cones that contain chlorolabe, most sensitive to medium wavelengths of “green” at around 540 nm.

L cones that contain the photopigment erythrolabe, most sensitive to long wavelengths of "greenish yellow" at around 565 nm.

In his renown experiment, Svaetichin developed a new methodology for the electrophysiological study of vision with the help of Ragnar Granit. Through the use of fine needle electrodes, they managed to register signals from the major nerve cells in the retina on their way to the brain via the optic nerve as means to understand how exactly image processing occurs.

Fovea

Nowadays, we know for a fact that in the average human eye there are about 6 to 7 million cones that are responsible for our color vision.

Fovea centralis is the tiny area in the center of the retina where most of these cones are densely packed, about 150,000 per square millimeter, but it is not unusual for some lucky individuals to reach even 300,000 per mm2.

Such density of cones in the fovea results in a high fidelity image and provides sharp and detailed central vision. When someone says “look” and we direct our eyes towards an object, this is what we mean.

The fovea has allowed us to see the world around us in detail throughout the ages. It has helped us spot prey or predators in our primal times and nowadays to read, drive or simply look at our phones.

To signify the importance of cones it must be mentioned that about half the nerve fibers in the optic nerve carry information solely from the fovea which is barely 2mm wide.

Vision is formed through the combined information on color and shapes acquired from the fovea where there are only cones and that of motion from the peripherally located rods.

Receptive field

Any image you see is meticulously constructed by your brain piece by piece. In a sense, reality is far from what we observe. Because human vision had evolved to help our ancestors survive in this complex world, its job was to work just well enough and not to portray reality as it is.

The world is in constant motion, shapes are changing, light flickers. One moment you see something and the next thing you know it is gone. The image of any object you see is never ideally projected onto the retina. The projection is blurry and approximate.

Rods and cones represent only a very thin layer from the retina and their signals are so simple and unreliable that it takes a great deal of processing to turn this raw data into somewhat digestible form. The rest of the retina consists of several layers of nerve cells which process raw visual data in three simple steps.

First of all, the rods and cones detect any changes from the incoming light as our vision relies on the fluctuations in light intensity, rather than the absence or presence of light. Even in the absence of light the photoreceptors maintain an “always-on” state and are always ready to fire a signal again.

Because light receptors need time to adjust to any sudden changes, this transition allows them to produce more nerve impulses and calculate the total duration of the stimuli (light). When the retina is exposed to light, both the rods and cones increase their firing rate allowing the signals to propagate through the farthest neurons, thus spreading the received information.

The second step is the transition of information from the rods and cones to the bipolar cell through the intermediate horizontal cells that act as transporters.

Each horizontal cell is responsible for a specific, local, spot of rods and cones. They summarise the signals from each spot as means to understand how bright or dark the spots are to one another.

If one horizontal cell is stimulated to be “bright” enough, it dims the signal of its neighbouring cells. By inhibiting surrounding cells the regions receiving light appear brighter and the surroundings appear darker. This process sharpens the edges and enhances the contrast.

Finally, the bipolar cells pass this information on contrast through the amacrine cells. The amacrine cells distribute the initial information from one bipolar cell to the ganglion cells who take on the most crucial part in forming our image processing.

There, the information from the millions of rods and cones is compared and evaluated. The ganglion cells add and subtract signals from the photoreceptors to define the “colors” you see.

These ranges do not actually correspond to the final variety of colors we perceive. In fact, several combinations of wavelengths can create the same perception of color.

Depending on the wavelength intensity of the initial signals that come from the cones, the ganglion cells “filter” this information to decide whether something is green/red or if it’s blue/yellow.

The green-red and yellow-blue filtering is the actual reason why something cannot be green and red at the same time nor can it be yellow and blue. These pairs simply fire mutually opposing signals to the ganglion cells.

After the visual data is processed by ganglion cells, it is sent to the visual cortex, located at the back of our brain, for the image to be reconstructed and “seen” by us.

The infrastructure of the eyes consists of more than a 100 million analog photoreceptors that convert light into electrical signals and are wired into just about a million ganglion nerve cells that transmit the final information about color and brightness for the brain to process and visualize.