Rods and Cones

The Photoreceptors in The Retina

There are two types of photoreceptors in the retina, namely, rods and cones.

In fact, millions and millions of photoreceptors are there in the retina that make the bulk of it.

These photoreceptors turn the light rays falling on them into electrical impulses, that then travel along the optic nerve to the brain where they are turned back into visual images again.

Photoreceptors are of two types:

• Cones: There are approximately 6 millions of them in the retina.

  They are mostly concentrated in the macula that gives the eye its central vision.

  They are most densely packed inside the fovea that is the very central portion of the macula.

  Cones are the cells that perceive and recognize colors, but for this they need a bright light falling on them.

  That is why we can't recognize colors under low light conditions although shapes are still clear to the eyes!

• Rods: Rods are much more in number counting to approximately 125 millions, spread all over the peripheral retina!

  These rods function best under low light conditions and hence, are responsible for the peripheral and the night vision.

There are three types of cones - the red cones making the bulk of color perception to the tune of 64%, the green cones making it to 32%, and the blue cones limited to only the rest 2% left; but these blue ones are the most sensitive ones, and unlike red and green, lie outside the fovea.

Color blindness is a malfunction of the retina. Retina converts light energy into electrical energy that is then transmitted to the brain through the optic nerve. This conversion is accomplished by two types of photoreceptor cells in the retina: rods and cones.

Now the cones are the color encoders. Each one of them contains visual pigments sensitive to one of the three, color wavelengths - red, green, and blue. All the colors can be synthesized by mixing three of them.

If any one of them is absent or doesn't function properly in the cone cells of a person, it results in color vision deficiency. Red - green deficiency is common; blue deficiency is rare.

Common Types of Color Vision Deficiency:

• Protanomaly (1% male population) - Characterized by 'red weakness'. They see less of red, hence will tend to miss the reddishness of purple and mistake it for blue, for example.

• Deuteranomaly (5% male population) - This is the opposite of protanomaly and is characterized by its 'green weakness'. This man too will have difficulty perceiving purple and mistake it to be reddish.

• Dicromasy (2% male population) - They see no perceptible difference between red, orange, yellow, and green. They are all same for them.

• Protanopia (1% male population) - The brightness of red, orange, and yellow dims down a lot. Even the red traffic lights may appear to be extinguished!

• Deuteranopia (1% male population) - Same problem but with less of dimming.

Let's try to get a feel of how these different types perceive the colorful world that this ours is!

Normal Tricolor Vision

Monocolor Vision

Red Defective Tricolor Vision

Green Defective Tricolor Vision

Blue Defective Tricolor Vision

Red Absent Bicolor Vision

Green Absent Bicolor Vision

Blue Absent Bicolor Vision

Okay, the cones are less light sensitive than the rods, but they are responsible for color vision as well as the highest resolution vision!

The acutest vision comes from focusing the light on the fovea - this is what is called the central fixation of the eye or third eye opening!

Now the rods, employing a sensitive photo-pigment called rhodopsin, are more light sensitive as well as more in number, although not color-sensitive.

Hence they provide a much better peripheral vision - better known as scotopic vision - required for tracking motion in wide-angled field of vision.

On the other hand, the cones provide a much better visual acuity when directly looking at the point of visual interest - better known as perfect central vision, requiring central fixation of the eye or third eye opening for the purpose of aquiring a flaw-less vision!

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