Photoreceptors (cones) in the retina allow us to see. The more tightly packed they are in the retina, the better the visual acuity. The maximum density of photoreceptors are present in the center of macula in an area called 'Foveola'. If the foveola is normal, 20/20 vision is possible. The visual potential rapidly falls as one moves away from foveola. At the outside edge of 'fovea' the potential for vision is about 20/50. This level of vision is inadequate to read a newspaper print at 25 cm.
When we fix our gaze on any object, it is the foveola that points in that direction (central fixation) and since the foveola is capable of 20/20 vision, we are able to see the fine details of the object or read newsprint comfortably. No other area of the retina can match the foveola when it comes to visual acuity. If this important area of the retina gets damaged as in macular degeneration, then the ability to see fine detail is seriously affected. If one is forced to use another area on the retina (outside the damaged fovea) to enable central vision, the ability to see details is reduced (eccentric fixation or viewing).


Normal VisionEarly vision defectMagnifier & enhanced contrast help

In early stages of macular degeneration the sensitivity of the retina in the foveal area is reduced. The fovea still has functioning photoreceptors, but the number of functioning photoreceptors is less than normal. The density of functioning photoreceptors is reduced. This means that the fovea no longer has the ability to see 20/20. Therefore, although one can still see centrally, to be able to read the print will have to be larger (i.e. 20/40 or 20/50 level) and with more contrast. There are many visual aids (optical & electronic) that allow for magnification and contrast enhancement (guides, magnifiers, CCTV, telescopes, guidelines for reading aids).

Advanced vision defectMagnifier aloneEccentric Viewing + Magnifier
(central vision is lost)(still cannot read)(can read now)

If macular degeneration completely destroys the fovea, then the only way a person can see is by using the retina outside the fovea. However, at this extrafoveal retinal area, the photoreceptors are not tightly packed and therefore the potential to form vision is limited. At the outside edge of fovea 20/20 vision is not possible and at best one can only hope to see 20/50. Therefore to see or read anything, the print would have to be increased to 20/50 level - something that can be achieved using magnifiers. However, magnifiers alone will not solve the problem. One has to be trained to learn to read from the extrafoveal 'healthy' retina. This is termed as eccentric fixation & viewing. The extrafoveal retina is the preferred retinal locus (PRL) is now the new center of the visual field. The non-seeing area (scotoma) due to macular degeneration is shifted away from the center of the visual field. The resolution of the PRL is insufficient to read newspaper print. However, if the text is magnified, reading ability is regained This is the basis for the use of magnifying visual aids.

Full Text of Journal Review Article: Functional and cortical adaptations to central vision loss.

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