Vitreous floaters are tiny deposits of various shape, consistency, refractive index, and motility floating about in the eye's vitreous humour, which is normally transparent. Floaters may be of embryonic origin or acquired due to degenerative changes of the vitreous humour or retina. The perception of floaters is known as myodesopsia, or less commonly as myiodeopsia. Floaters are visible because of the shadows they cast on the retina or their refraction of the light that passes through them, and can appear alone or together with several others in one's field of vision. They may appear as spots, threads, or fragments of cobwebs, which float slowly before the sufferer's eyes. Since these objects exist within the eye itself, they are not optical illusions but are entoptic phenomena.
One specific type of floater is either called Muscae volitantes, or mouches volantes (from the French), which consist of small spots. These are present in most people's eyes and are attributed to minute remnants of embryonic structures in the vitreous humour. Floaters usually follow the rapid motions of the eye, while drifting slowly within the fluid. When they are first noticed, the natural reaction is to attempt to look directly at them. However, attempting to shift one's gaze toward them can be difficult since floaters follow the motion of the eye, remaining to the side of the direction of gaze. Floaters are, in fact, visible only because they do not remain perfectly fixed within the eye.
Vitreous floaters are able to catch and refract light in ways that somewhat blur vision temporarily until the floater moves to a different area. Often they trick the sufferer into thinking they see something out of the corner of their eye that really is not there. Most sufferers, with time, learn to ignore their floaters. For people with severe floaters it is nearly impossible to completely ignore the large masses that constantly stay within almost direct view. Some sufferers have noted a decrease in ability to concentrate while reading, watching television, walking outdoors, and driving, especially when tired.