Just about everybody sees “floaters” especially when looking at a very light background. Floaters are specks or dark areas that the eye “sees” but that are non-existent in the real world the eyes are viewing.
The eye has a vitreous gel inside it through which light passes from the cornea to the retina at the back, which translates the information in image form to send to the brain. If the internal gel has a speck of some kind in it, then the light path is interrupted, a shadow is cast over the retina and interprets it as a dark speck.
Summarising, the eye is unable to distinguish whether something is inside or outside of it since the gel is clear and is not expected to contain any debris – the eye always takes the option of assuming what it sees is outside the eye.
In a similar way, when the vitreous gel in the eye slowly becomes watery with age some gel may be left in a streaky suspension – the eye will interpret these streaks as a cobweb type of image. Other images seen have been described as cobwebs, threads, clumps and spots.
Why does the eye not see specks and streaks all the time?
Because normally the internal liquid is still, and consequently any specks or streaks are continually still, the eye will reject or “tune out” any image they cause due to a phenomenon called neural adaptation, so the brain assumes the eye sees nothing – the eye has adapted to see moving things.
Any movement of something in the internal eye liquid will of course generate an image because the status quo has changed. What are seen are not optical illusions but what are called medically “eutopic 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 with more backgrounds and not just light ones, then it’s time to get things checked out as this could indicate early signs of a detached retina.
Similarly should sparkling lights appear to be seen, or a quite large single floater, then an optometrist or ophthalmologist should be consulted as soon as possible.
The sudden appearance of multiple floaters could mean the retina is pulling away from the back of the eye – and again immediate attention is required.
Treatment of floaters is not usually undertaken as they fade, however serious cases could mean considering replacing the gel with a clear saline liquid, though this is a very specialist procedure undertaken by very few practitioners.
Young people suffering from myopia (nearsightedness), when distant objects are incorrectly focussed by the eye, are prone to “seeing” floaters, as are people who have had cataract surgery.
The main cause is due to the age related physical change that happens as the vitreous humour – the liquid gel inside the eye – deteriorates.