Children with severe brain damage often are unable to accurately control their eye muscles. The eyeballs appear to move independently of each other and to float randomly about. The children also seem to have little or no proprioceptive awareness of the position of their eyeballs, and making eye contact is all but impossible. The degree of eye control appears to be related to the overall body tone of the child at any particular time.
It is extremely difficult for these children to point their eyes at any object of regard, and certainly they cannot sustain fixation long enough for any near point activities. Also, typical of this type of physical impairment, children seem to process at distance better than at near point, and they navigate through the environment even without stable oculomotor control. Head and body must be stabilized before there is any hope of stabilizing the eye muscles. Sunlight, surprise, and wind can trigger reflex eye and lid movements (a startle reflex is possible).
A full range of oculomotor impairments (insufficiencies) parallel the physical anomalies associated with brain damage; generally the less severe the physical manifestations of the brain damage, the less severe are the problems with eye muscle control (with individual exceptions). Binocularity is rare or fleeting. Accommodation (of the lens) and convergence (of the eyeballs inward- toward the nose) are rarely normal so that a clearly focused and steady nearpoint image is hard to hold and/or impossible to sustain. The ability to smoothy track moving objects is usually not possible.
What do these children see? Is the world swimming around as their eyes float? How can both eyes see at the same time when they are not looking in the same place? Can these children see clearly when they look or is the image blurred? Can they learn using eyes so impaired? We can begin to comprehend the answer to these questions if we understand that there are two vision systems (and lots of subsystems), and that vision processing is a whole brain task. Below is an organizational outline for discussing the functional expectations we can expect from this population of children. Also, refer to a separate section of this e-book to read about the two vision systems.
1. There are two vision systems, the central system and the peripheral system
2. Each of these separate vision processing networks has an independent (but interactive/linked) navigation control system
3. The central system sees in color, it freezes motion (tracks and scans), it has a normal acuity of 20/20
4. The central system is the "what is it (or who is it)" vision system. It is related to naming, language, and higher brain concepts.
5. The peripheral system sees in black and white, it is very motion sensitive, it has a normal acuity of 20/200
6. The peripheral system is the eyes navigation vision. It answers the question "where am I" (or, where is something)
7. Vision is a "whole brain" process. There are more than 30 "neural centers" for vision, all connected, and located in every part of the human brain
8. The central vision system goes from the occipital lobe to the frontal cortex via the temporal lobes (where the language centers are located)
9. The peripheral vision system goes from the occipital lobe to the frontal cortex via the posterior lobes (where the spatial centers are located)
Given the functional problems characteristic of this population, studnets should receive the label of "visually impaired" and be served (at least in a consultative fashion) by school vision specialists
We do not understand the anatomy and physiology of vision, so we can not say with total certainty that there are two major distinct vision systems. However, studies of neural pathways and studies of the functional areas of the brain, strongly suggest that two separate visual processing operations exist. Many explanations become clearer when we use the language of the two systems. These two neural pathways are called the Central Visual Processing system, and the Peripheral Visual Processing system.
The best way to see the distinction between the two processing centers is to compare what they do functionally. Below, we will take a look at different visual tasks and compare the two systems.