Blind spots or 'scotoma' are small areas where no vision is present.

One of these occurs naturally in all people because the light-sensitive layer - the retina - lining the back of the eye is not continuous. There's a gap in the retina where the optic nerve, which takes the visual information to the brain, leaves the eye. We're not usually aware of this blind spot because our brains 'ignore' this small patch of missing information and 'fill in' our view.

However in some eye diseases, the retina is damaged and new 'holes' appear within the field of vision, but these may not at first be noticed. After a certain amount of damage, the brain is unable to compensate for the lack of information and a person will then become aware of a gap in their vision.