Degeneration of the Macula – When the retina dies

Degeneration of the Macula – When the retina dies

The macula, a small area of the retina, lies opposite the centre of the pupil, such that rays of light from objects that are looked at directly fall onto it. Two characteristics differentiate the macula from the rest of the retina. Firstly the photoreceptors – the light sensitive cells – within the macula are particularly densely packed. So one sees more sharply with the macula than with the surrounding retina. Put another way: we only see those things, which we focus on directly really sharply – by comparison the outside field of vision appears blurred.

Secondly, the macula and peripheral retina also differ as regards the photoreceptors, of which there are two types: the „rods“, which only respond to brightness and therefore detect the world in shades of grey, and the „cones“ responsible for colour vision. Three types of cones, sensitive to red, green and blue light respectively, create all the colours we see by mixing. Rods and cones are not evenly distributed within the retina. In the macula there are only cones, and so we see everything we focus on directly in colour. Outside the macula, the retina consists predominantly of rods. From the macula to the periphery of the retina the number of cones reduces rapidly. The impression that the whole field of vision is coloured is deceptive. This is easy to prove: the colour of an unknown object introduced slowly into the field of vision by another person can only be recognised when the object is relatively close to the centre of the visual field. The impression of a completely coloured visual field is created by head and eye movements bringing increasingly larger areas into contact with the colour-sensitive centre and our brain subsequently „colouring in“ the grey peripheral areas.

When the macula degenerates, the cones slowly die and cannot be replaced. As a result, the affected area of the retina ceases to perceive light – a „blind“ spot develops (not to be confused with the blind spot on that area of the retina at which the optical nerve leaves the eye). One of the symptoms of macular degeneration is over-sensitivity to light – just what Degas complained of. For him, bright light caused physical weakness that could last for days. Some people who suffer from a disease of the retina display similar symptoms. Just as in Degas‘ time, degenerative diseases of the retina cannot be treated and are one of the most frequent causes of blindness today.