Photoreceptor density

The density of photoreceptors across the retina varies greatly, as plotted in Figure 5.5. The most interesting region is the fovea, which has the greatest concentration of photoreceptors. The innermost part of the fovea has a diameter of only $0.5$mm or an angular range of $\pm 0.85$ degrees, and contains almost entirely cones. This implies that the eye must be pointed straight at a target to perceive a sharp, colored image. The entire fovea has diameter $1.5$mm ($\pm 2.6$ degrees angular range), with the outer ring having a dominant concentration of rods. Rays that enter the cornea from the sides land on parts of the retina with lower rod density and very low cone density. This corresponds to the case of peripheral vision. We are much better at detecting movement in our periphery, but cannot distinguish colors effectively. Peripheral movement detection may have helped our ancestors from being eaten by predators. Finally, the most intriguing part of the plot is the blind spot, where there are no photoreceptors. This is due to our retinas being inside-out and having no other way to route the neural signals to the brain; see Section 5.2.

Figure 5.6: An experiment that reveals your blind spot. Close your right eye and look directly at the ``X''. Vary the distance of the paper (or screen) from your eye. Over some range, the dot should appear to vanish. You can carry this experiment one step further by writing an ``X'' and dot on a textured surface, such as graph paper. In that case, the dot disappears and you might notice the surface texture perfectly repeating in the place where the dot once existed. This is caused by your brain filling in the expected texture over the blind spot!

The photoreceptor densities shown in Figure 5.5 leave us with a conundrum. With 20/20 vision, we perceive the world as if our eyes are capturing a sharp, colorful image over a huge angular range. This seems impossible, however, because we can only sense sharp, colored images in a narrow range. Furthermore, the blind spot should place a black hole in our image. Surprisingly, our perceptual processes produce an illusion that a complete image is being captured. This is accomplished by filling in the missing details using contextual information, which is described in Section 5.2, and by frequent eye movements, the subject of Section 5.3. If you are still not convinced that your brain is fooling you into seeing a complete image, then try the blind spot experiment shown in Figure 5.6.