Visual Receptors and Transduction

Properties of Rod and Cone Photoreceptors

Rods and cones have complementary distribution in most species. In humans, cone spatial density is highest in the central area of the retina, known as the fovea. The human fovea is specialized for sharp vision. Indeed, whereas the fovea occupies only 2% of the total retinal area, it contains about 1/3 of all ganglion cells. As a result, our central vision provides us with more detail than our peripheral vision. A simple example of the different resolutions of our central and peripheral vision is our ability to count the number of fingers on our stretched hand when it is directly in front of our eyes but not when it is extended to our side. Cone density rapidly declines away from the center of the fovea and is low in the periphery of the retina. In contrast, the spatial density of rods in the center of the fovea is zero, but it is high in the periphery of the retina.

In addition to their complementary distribution, rods and cones exhibit complementary functional properties that expand the overall range of our visual perception. Rods are extremely sensitive to light. Using psychophysical experiments, Selig Hecht was able to demonstrate some 70 years ago that that the simultaneous activation of 7 rods was sufficient for light perception and concluded that a rod could be activated by a single photon. After the development of techniques for recording the electrical responses from individual photoreceptors in the 1970s, single photon responses from rods were indeed observed. Their high sensitivity makes rods perfectly suited for dim light conditions, such as between dusk and dawn, and our dim light vision is mediated exclusively by the rods. The high sensitivity of our rod vision is further enhanced by the convergence of their signals. Thus, between 15 and 50 rods send their signals to a single bipolar cell. The signals from multiple bipolar cells are then combined so that one ganglion cell sums the signals from hundreds of rods before sending it to the brain. The trade off of this convergence is the low spatial resolution of our rod vision.

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