This increased movement of the vitreous in old age can tug on a strand which is adhering to the vitreous. This can lead to a hole in the retina and a retinal detachment, although this is rare. The symptoms of retinal detachment are a shower of floaters, persistent flashing lights or a shadow or curtain coming across the vision. The Western Eye Hospital London a. If they tend to stay in one position when you keep your eye still, they are probably muscae volitantes-the shadows of small opacities in the vitreous body.
Most people have muscae, and their number tends to increase with age.
However, if the spots move in a jerky fashion even when you keep your eyes still, and tend to follow fixed trajectories, you're seeing blood corpuscles that are being pushed by your pulse through the capillaries that supply the retinal receptors. This implies that the capillaries are in front of the retina and, improbable as it might seem, this is the case. We don't see the capillaries themselves because any pattern that is stabilised on the retina fades within a few seconds as a result of the adaptation of the receptors and associated neural wiring. Normally, an object that we are looking at fixedly does not disappear because our eyes constantly make small movements. However, there are various optical methods to stabilise images on the retina, the most astounding of which is a large contact lens carrying a tiny slide projector.