Imagine you’re in a dark room with a single window. You have a photocell (a device that converts light into electricity) in one hand and a computer chip in another, and wires connecting them together. Logic states you position the photocell near the window and make sure the connecting wires do not block light from falling on the photocell. Simple right? Why then, did evolution ‘think’ differently and construct our photocells - the mammalian retina - with layers of neuronal wiring in front of our photoreceptors?
As our eyes are constructed, photons pass through the cornea, lens and transparent vitreous humor before they finally reach the retina. The retina itself has layers, and photons must penetrate all of them before they finally strike our eye’s photoreceptors at the very back.
The Retina’s Layers
First light must pass through the nerve ganglion layer – the ‘wiring system’ whose axons ultimately converge to form the optic nerve. The ganglion layer is followed by three more: the amacrine, bipolar and horizontal cell layers through which the photons must pass before they arrive at their target photoreceptor cells – the rods and cones. These cells do more than just transmit signals upward to the nerve ganglion layer. They are responsible for decoding motion (for example, downward motion) and time-based information (speed of the downward motion) from the photoreceptor; much of the signal that process form, movement and color take place within these very layers. Once they reach here, the signals are passed back up toward the ganglion nerves and on to our visual cortex.
Behind the photoreceptors, a dark pigmented retinal pigment epithelium layer exists, which absorbs stray photons and prevents them from reflecting back to the rods and cones, ensuring an efficient system.
So why this arrangement?
It’s All About Energy
Our photoreceptor cells require an enormous amount of energy to function.
Light-responsive molecules are constantly being used, metabolized and renewed; therefore, they need constant readily available energy. In fact, they require more oxygen, glucose, retinol (a form of vitamin A), water and other nutrients than almost any other part of the body. To position a circulatory system able to meet these demands anywhere but immediately behind and available to the photoreceptors would interfere with incoming light far more than the existing layers do.
Let’s return to our electrical photocell analogy. If the photocell needed to be continuously refurbished to keep working, and doing so required all the manufacturing equipment used to originally produce it, you would want to keep that equipment behind the photocells - not in front of them where it would certainly block much of the light coming through the window.
Voila. Stay close to the hand that feeds you. It seems evolution got it right after all.
Kandel, Schwartz, Jessel, Principles of Neural Science. Chapter 24 (2001)
Bergman, J., Journal of the American Scientific Affiliation. 52(1):18-30 (2000)
Strauss, O., American Physiological Society. The Retinal Pigment Epithelium in Visual Function (2013)
Kolb, H., Webvision: The Organization of the Retina and Visual System (2011)