Monocular Fixation
When we view an object directly, so that it appears to be more distinct than surrounding objects, we are said to “fix” or “fixate” it.
As the fovea is normally the most sensitive part of the retina, affording by far the most distinct vision, “fixation,” in the great majority of cases, is so performed that the image of the object that is “fixated” falls upon the fovea of the eye that is “fixing.” This is known as central or muscular “fixation.”
When central vision is absent, however, the patient is compelled to see with a portion of the retina outside of the fovea. The eye must then be so directed as to cause the image of the object to fall on this outlying portion of the retina. This is termed “eccentric fixation,” and usually denotes that vision is exceptionally poor.
The ability to “fix” is apparently acquired in early infancy by constant practice in looking at objects. Any marked interference with vision, particularly with central vision—present at birth or soon thereafter—will tend to prevent the acquisition of this ability, and in extreme cases the eye does not learn to “fix” at all, but aimlessly wanders in all directions.