The concave rear wall of an eyeball’s interior is called the retina. There is a pinpoint area of the retina called the foveola that is significantly responsible for visual acuity, the ability to sharply distinguish color and detail. In human eyes, it is densely packed with four very specialized cells, the smallest self-contained units of life. Each discharges an electrical signal when exposed to a particular color of light. The reason why the human eye must move to track, and read, this text is so that each successive letter comes to sharp focus onto this tiny spot.
The entire retina is covered with light-sensitive cells. Its outer edges are mainly cells called rods which register only light versus dark, and interpret movement. Progressively toward the center of the retina are cells of a different type called cones which register colored light. Off-center, all the nerves attached to the retina’s millions of cells converge to create a “blind spot.” Almost exactly center is a small oval area about 0.2 inches (5mm) in diameter called the macula.
Near the center of the macula is an indentation called the fovea where cone cells are concentrated. Finally, in its center is the foveola. In the average human, it is only about 0.008 to 0.014 inches (0.2 - 0.35 mm) in diameter. Measured horizontally across a human’s entire field of vision, it captures an arc of only 1.2 degrees.
The foveola contains cone cells exclusively. Humans have three types of cone cells, but some animals’ eyes may have four, to enable a different range of sight. There is some overlap between the three types, but they essentially differ in their sensitivity to either red, green or blue light. From a combination of whether these three colors are detected or not by their respective cone cells, the human visual system is able to interpret the entire color spectrum.
The structure of the cone cells within the foveola is slightly different from normal because they are compacted so densely here. Their prime importance to human vision is indicated by the fact that all the nerves originating from the foveola are bundled into one large nerve that takes a more direct path to the brain, differently from the main optic nerve. There is one other type of cell in the foveola called Müller cells of special interest to medical scientists. They are glial cells — cells which provide structural support, protection and nutrients to companion nerve cells, such as rods and cones. Some preliminary research suggests that Müller cells are capable of morphing into, and thus regenerating, other types of eye cells.