16.1 Physiology

There are three basic components of human vision: the point spread of the lens, the lateral inhibition of the retina, and the saccation of the eye. Each component is described by a MTF, which acts on the irradiance. The physiology of the retina includes rods, cones, horizontal cells, bipolar cells, amacrine cells, and ganglion cells.

The human eye evolved in numerous steps. The eye began as a simple photoreceptor that detected light and shadow. Then, an eye cup created a camera with a finite angular field of view. A retina created an image. A fixed lens sharpened the image. A variable iris controlled brightness. And lastly, a dynamic lens controlled focus. Even Charles Darwin confessed to such a progression by natural selection as "absurd." However, Darwin did believe that incremental improvements could define a progression by natural selection. The intermediate steps are present within vertebrates of the current day.

The fovea comprises tightly packed cones with very few rods. The absence of blood vessels and other cells creates a pit that defines the spatial limit of the fovea. The foveola (the center of the fovea) comprises red, green, and blue cones without any rods. The annular portion beyond the foveola contains a few rods but is still largely clear of blood vessels, bipolar cells, amacrine cells, and ganglion. The current model employs an average cone diameter at 3.8 um. At this diameter, in a hexagonal packing scheme, the foveola contains 5000 cones within a 200-μm diameter, while the fovea contains 20,000 photoreceptors within a 400-μm diameter. Within the fovea, there is a single incremental ganglion per photoreceptor. Beyond the fovea, rods are more common.

The photoreceptors connect to bipolar cells that feed into ganglia. An incremental bipolar-to-ganglion path responds to an increment of light. Sjostrand et al. calculated three ganglia per foveal cone from experimental quantification of both. Thus, a single cone within the fovea connects to three ganglia: one incremental ganglion, one decremental ganglion, and a third type, such as a parasol ganglion. An incremental bipolar-to-ganglion path responds to an increment of light.