Stages of Visual Processing
Functions of the macula and the retinal periphery
The function of the retinal periphery is to detect changes and to initiate eye movement to bring the target of gaze into fixation (on the fovea).
The center of the retina’s function is to examine the area of interest and to recognize what it represents.
This means that at the retinal level the topography of functional losses, which was unimportant for optical problems, becomes very important. This topography is studied with visual field tests.
Potential problems in the retinal stage – and their solutions
Loss of central vision
Because macular diseases affect the the macula, which is responsible for central, fine-detail vision, they can have a devastating impact on the ability to enjoy activities of daily life, such as reading, driving, or even recognizing the face of a friend or family member. Central vision loss can significantly affect your orientation and mobility.
- “Orientation” refers to the ability to know where you are and where you want to go, whether you’re moving from one room to another or walking downtown for a shopping trip.
- “Mobility” refers to the ability to move safely, efficiently, and effectively from one place to another, such as being able to walk without tripping or falling, cross streets, and use public transportation.
Central loss can take many forms. Retinal changes usually affect some areas while in other areas function remains relatively good. When vision is lost in a limited area, we refer to that area as a scotoma or blind spot. While a scotoma may be an area of no sensitivity, it may also be a relative scotoma in which the sensitivity is reduced but not absent.
|Solution||With a relative scotoma, the cells in this areas can still contribute when given enough light. Consequently, increased illumination can be most helpful.|
A Central Scotoma is a scotoma that covers the original point of fixation (the fovea). The result is that when your eye detects an object in the periphery it elicits an eye movement to bring that object to the fovea, where the object disappears!
|Solution||In this situation the best way to see an object is to look past it, so that it can be seen beyond the edge of the scotoma. This is called eccentric viewing. Since the image now falls on a retinal area with a less dense receptor mosaic, the image will be less detailed; this effect can be countered by magnification.|
Effectively implementing eccentric viewing requires a recalibration of the oculo-motor system. There are no devices that can do this recalibration for you; it is accomplished by practice and training.
A para-central scotomata is an area of reduced or absent sensitivity outside the fovea.
Reduced sensitivity in macular disorders can result in lowered contrast sensitivity, either central or peripheral.
These effects can be mitigated by low vision rehabilitation and training. Taking action
Peripheral field loss
Peripheral field loss interferes with the detection of obstacles. Its primary functional effect is in orientation and mobility.
|Solution||To a limited degree compensation is possible with prisms (a technical solution) that deflect the image to a sighted part of the retina , but mostly compensation must come from training and practice, learning to make more frequent eye movements to the non-seeing area.When the loss is severe, scanning alone may not be sufficient and the use of other senses may be needed. This can include the use of touch, as in a long cane, and/or the use of hearing to listen to traffic.|
Diseases of the retina
There is a broad range of diseases in the retina.
Age-related Macular Degeneration (AMD)
During normal aging, yellowish deposits, called drusen, form under the retina. These can increase in size and number to the point that they interfere with proper functioning of the retina, damaging or killing the light-sensitive cells of the macula, resulting in blurring of central vision. In developed countries with an aging population AMD is the single most common cause of vision loss.
For those with AMD orientation and mobility performance may be adequate while reading will be severely affected because of their central loss.
Vision loss does not develop evenly in AMD and does not need to start in the very center. The result may be a central island of vision, surrounded by scotomatous areas. When this happens, one may be able to recognize a letter but not recognize a word. Thus reading acuity will be much worse than letter chart acuity. An example is shown in Fig. 7, where the word “wonderful” is projected on the retina; the small central island can only recognize one letter at a time.
|Solution||If you have AMD you may find, paradoxically, that reading small print is easier than reading headline print. In this situation, practice and training will again be more important than magnification. Even minification can sometimes be helpful to fit the word into the remaining central area. Sometimes, using a more eccentric area combined with higher magnification may be effective.|
This complication of diabetes can cause damage to the blood vessels of the retina. Diabetic retinopathy occurs in more than half of the people who develop diabetes. The longer someone has diabetes, and the less his or her blood sugars are controlled, the more likely the possibility that person will develop diabetic retinopathy.
Because diabetic retinopathy can affect the entire retina, it may result in central as well as peripheral vision loss. There may be scattered blind spots across the entire retina.
Also called a macular pucker, this formation of a semitransparent scar tissue on the surface of the retina can, with time, contract and result in distortion of the underlying retina and/or macular edema (swelling).
This inherited retinal disorder primarily affects the rods. The name refers to the characteristic finding of pigmentary (color) changes that occur.
Glaucoma is a group of disorders that damages the optic nerve in a particular way. Vision as we know it is the result of a continuous flow of signals through the optic nerve pathways. With the death of the ganglion nerve cells comes vision loss.
Glaucoma affects primarily peripheral vision; central vision is usually spared until late in the disease. Those with glaucoma may experience orientation and mobility problems. Since reading ability is maintained until late in the progression of the disease, glaucoma is often detected too late, so screening programs are very important.
For more on diseases and their impact on vision see Diseases and your vision