Notes

Phototransduction in Rods and Cones The retina has a very broad dynamic range, it functions under very varied illumination conditions.

• Rods: Vision in low light (e.g., night)
• Cones: Vision in stronger light (e.g., day). The graph below represents how changes the membrane potential to changes in light intensity shed on the photoreceptors.

Distribution of Rods and Cones: A View from the Side A different distribution of cones and rods is present on the retina. The cones work in day light and are capable of color vision, while the rods are saturated during day-time, but they provide uncolored night vision.

The figure above shows the distribution of rods and cones in the retina. We can notice that the fovea is rod-free and has a very high density of cones. The density of cones falls rapidly to a constant level at about 10-15 degrees from the fovea. Notice the blind spot which has no receptors. At about 15-20 degrees from the fovea, the density of the rods reaches a maximum. The color vision and resolution become quite poor in the periphery.

Response of a Cone to Light of Two Different Wavelengths In the picture below, light is flashed through one photoreceptor and changes in membrane potential are observed. What we can observe is that this cone responds differently to different wavelengths of photon light. Hence, the response of cones is wavelength-dependent. Indeed, there exist 3 types of cones:

• short-wavelength sensitive (blue cones)
• middle-wavelength sensitive (green cones)
• long-wavelength sensitive (red cones)

and only 1 type of rods. These different cones allow us to process different wavelengths and interpret them as colors. This concept is shown in the following figure, where the Principle of Univariance is explained.