Asked by: Andy McGeoch, by email
Floaters' are caused by tiny fragments of cell debris within the vitreous humour of the eye - that's the gelatinous substance between the retina and the lens. Despite the name, they do in fact sink slowly, so they tend to collect near the centre of your field of view when you lie on your back. In fact, it's only because they move that you can see them at all. If they were fixed, your brain would soon learn to tune them out completely. Floaters can be caused by eye trauma, age or pregnancy, but some are also present from birth and they are generally nothing to worry about unless they severely affect your vision.
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