The eye is filled with a gel substance (called the vitreous humour) through which light passes to the retina (which is situated at the back of the eye). The retina translates the information received and sends it to the brain. If the vitreous humour contains an opacity, then the light path is interrupted, a shadow is cast over the retina and this is interpreted as a dark speck or floater.
In a similar way, when the vitreous gel in the eye gradually becomes more watery with age, some gel may be left in a streaky suspension. The eye will interpret these streaks as a cobweb type of image. Others have described these images as threads, clumps or spots.
Why does the eye not see specks and streaks all the time?
Because normally the vitreous humour is still, any specks or streaks are continually still, and consequently the brain will ignore or "tune out" such images. This is a phenomenon called neural adaptation. As the eye has adapted to see moving things, then the brain assumes the eye sees nothing.
Any movement of opacities in the internal gel will of course generate an image because the status quo has changed. so when the head or the eyes move, then the gel moves with it as do any specks or streaks. What are seen are not optical illusions but what are called "entopic phenomena".
So the eye does not necessarily record if anything is in the liquid, but only if it changes its position. (This effect is similar to the brain normally not hearing a clock ticking because it tunes out the continual sound, but hears it when it actually stops, in other words when the sound changes.)
Should you be concerned if you have floaters?
No, not usually, but if the floaters begin to appear against more backgrounds and not just light or plain coloured ones, then it's time to get things checked out as this could indicate early signs of a retinal tear, break or detachment.
Similarly, should sparkling lights similar to a camera flash appear, or there is a sudden appearance of new floaters, then an optometrist or ophthalmologist should be consulted as soon as possible and a dilated pupil examination performed.
The sudden appearance of multiple floaters could mean the retina is pulling away from the back of the eye (which is called a posterior vitreous detachment) and again immediate review is required.
Treatment of floaters is not usually undertaken as they become less obvious with time, however particularly annoying floaters can be removed via a vitrectomy whereby the vitreous humour is replaced with a clear saline liquid. This is a very specialised procedure with some associated risks.
Young people suffering from myopia (nearsightedness) where distant objects are incorrectly focussed by the eye are prone to developing floaters, as are people who have had cataract surgery.
The main cause of floaters is due to age related changes that occur to the vitreous humour (the liquid gel inside the eye) This benign deterioration of the vitreous is a process known as vitreous syneresis.
Author: Stuart Macfarlane