Behind the Science: Fundamentals of the Retina
The retina is the light sensitive layer of tissue located at the back of the eye. It is responsible for translating an image into the electrical signals for the brain to process. There are 5 cell types in the retina responsible for processing and transferring this signal. Photoreceptors, Horizontal cells, Bipolar cells, Amacrine cells and Ganglion cells.
There two types of photoreceptors, the rods and the cones. The rods are more sensitive to wider ranges of wavelengths of light. Cones are sensitive to specific ranges of wavelengths, and the cones are responsible for color vision. For color vision the output of several cones are required to be compared.
The way you perceive colors is not unlike how a television or computer screen displays them. In the above image, several different configurations of RGB subpixels are shown. By varying the brightness of the individual subpixels, the image on a monitor can trick your eye into believing that it is seeing a wide range of colors. Just as there are red, green, and blue subpixel colors, there are also red, green, and blue photoreceptors in the retina. The different colored photoreceptors give the strongest signals when receiving photons with a color closer to the color of the photoreceptor. These signals are compared to each other to determine what color is being displayed.
Just as cones are compared to create the perception of color, many different rods are compared for more sensitive vision. While this comparison makes the eye more sensitive, it also reduces sharpness of the image produced.
Horizontal cells receive input from the photoreceptors, and output to bipolar cells. There are multiple different theories about the function of horizontal cells, but none of them have been conclusively proven.
Bipolar cells take the signal from photoreceptors and horizontal cells, and output it to amacrine cells and ganglion cells.
The bipolar cells make connections with either rods or cones, but never both. The different types of bipolar cells make connections with different photoreceptors. There are bipolar cells that make connections with only blue cones, only red cones, only green cones, and only rods.
Bipolar cells are also classified by which cells they form synapses on.
Amacrine cells compare signals from multiple bipolar cells. There are about 40 types of amacrine cells, each with their own function.
Sometimes they take the input from the bipolar cells and output in specified way to the ganglion cells. In the recent nature article published based on the data gathered from EyeWire, it was shown that through the inputs of different bipolar cells, starburst amacrine cells are able to detect the motion of an object and output a corresponding signal to the ganglion cells.
Ganglion cells are the final step in this process for the retina, before signals are sent to the visual cortex in the brain. Ganglion cells receive inputs from bipolar cells and amacrine cells.
Different types of ganglion cells may have different functions. Some may be responsible for comparing inputs from cone bipolar cells to create color vision, while others may be sensitive to a certain orientation of an object.
Looking for more information on the retina? Check out the wiki page!
Keep an eye on the blog for the next installment of Behind the Science.
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