Stages of Visual Processing
The neurons of the retina are arranged in 3 main layers separated by 2 intermediate layers whose main purpose is to make connections among the various neurons. The deepest layer of neurons processes the light first. These neurons are the photoreceptors, the only cells in the retina that can convert light into nerve impulses. The photoreceptor layer then transmits these impulses to the bipolar neurons in the second layer and on to the ganglion neurons in the third layer. It is only the axons of these ganglion neurons that exit the eye and carry the nerve impulses along the optic nerve to the first visual relay in the brain.
The retina contains two types of photoreceptors:rods and cones. Each type of photoreceptor is sensitive to different wavelengths of light. Rods are cells highly sensitive to light of many wavelengths. They require only a single photon to trigger a response. Incoming light triggers the rods to send signals to the brain. Basically rods allow us to see in black, white and shades of gray. Because the rods are very sensitive they help is see in dim light. Because rods are so sensitive, they will be responding as much as they can when the light is still quite dim, so they are completely useless in full daylight.
Cones are photoreceptor cells that allow is to see colors. Cones register either blue, green or red. Your retina organizes all colors with combinations of these three basic colors. Cones are less sensitive than rods so good lighting is required for color vision.
The macula is a small area of the retina that is crowded with cone cells.
Toward the center of the retina, there is a small indentation, or pit, called the fovea.
Photoreceptors are not evenly distributed across the retina. Because cone cells are densely packed here, we experience our sharpest vision and our best color perception when relatively bright light strikes the macula—and especially when light strikes the central foveal pit.
Because the macula covers only a small area of the retina, we must look directly at an object to experience sharp vision and good color perception. When you look straight at something, it is imaged on the fovea.
Poor Visual Acuity Away from the Fovea
As you focus your eyes on a target, you are essentially placing the target on your fovea. As your vision moves away from the center, your ability to distinguish detail decreases.
Close your left eye and focus on the plus sign at the center of the display above and move so that you’re about five inches away from the screen. Then, without moving your eyes, see if you can read the letters to the left or the right of the plus sign. The fact that you can see the plus sign clearly, but have increasing difficulty reading the letters to the left of the center of the display is an indication that you’re dependent on your fovea for clear vision. (Note, also, that colors are more difficult to distinguish as they appear in your peripheral vision.) Because they are larger, you may be able to read the letters to the right of the plus sign without moving your eye.
Only in the very center of the retina is there one photoreceptor to one ganglion cell, which accounts for the sharpest vision being there. As you get further from the center multiple photoreceptors feed into a single ganglion cell, so vision gets less sharp the further out from the macula you go. Rods are completely absent from the central fovea and are most densely packed some 12–15° into the periphery.
The peripheral areas of the retina are less densely packed with photoreceptors, and most of those receptors are rods.
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.
Close to the macula is a spot that is devoid of rods and cones, where the optic nerve enters the eye. This is an area called the optic disc.The million or so retinal nerve fibers (axons) from the retinal ganglion cells bundle together to form the optic nerve, which connects the retina and the brain.
The nerve fibers are each individually connected to the retina and bundled together behind the eye as the optic nerve which continues towards the brain. To get to the brain they must pass through the wall of eye (sclera), and they do so at the optic nerve head (or optic disc) through a sieve-like perforation called the lamina cribrosa.
Although the lamina cribrosa normally maintains pressure in the eye while providing a path out of the eye, the nerve connections and pathways of the optic nerve are very thin and exposed to potential harm. The nerve cells are long and extremely thin, about 1/2000 of an inch. As well, the retinal nerve fibers make a 90 degree turn as they exit the retina.
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. |
Diabetic retinopathy
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.
Epiretinal membrane
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).
Retinitis pigmentosa
This inherited retinal disorder primarily affects the rods. The name refers to the characteristic finding of pigmentary (color) changes that occur.
Glaucoma
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