If the cones were distributed evenly among the rods across the retina, their average distance would be relatively large, and thus the acuity-the ability to detect fine spatial patterns-would be relatively poor for daytime colour vision. Therefore, the cones are concentrated in the centre of the retina, in a circular area called macula lutea. Within this area, there is a depression called the fovea, that consists almost entirely of cones, and it is this area of high acuity, which extends over a visual angle of approximately 2° , that humans use to make their detailed observations of the surrounding world.
The remaining part of the retina offers peripheral vision, which is characterized by being of only 15-50% of the acuity of the fovea (Jacob 1995)-partly because there is only one efferent optical nerve for approximately every 300 rods-and less colour-sensitive, but the structure of the peripheral part of the retina is more reactive to flashing objects and sudden movements (Andreasen & Asmild 1995).
A visual angle of 2° is slightly less than the width of one's thumb held out at arm's length, or approximately the width of an average word held at normal reading distance. Thus, detailed observations of a reasonable part of the surrounding world requires moving the eye (and head and body) to successively focus different parts of the ambient light array on the fovea, thus foveating various regions of the observed scene.
Figure 5: The human eye; the nose is to the left of the depicted eye. Note the three sets of muscles for horizontal, vertical and rolling movement.
To this end, the eyeball is controlled by three sets of muscles: one set for horizontal eye movement, another for vertical movement, and a third for rotational (rolling) movement, i.e. movement around an axis in the direction of sight (see figure 5).
