'Filling in' (visual phenomenon)

It's been known for centuries that human eyes (and all other vertebrate eyes) have a so-called Blind Spot. The spot is located where the optic nerve connects to the retina* - there are no light-perception cells in this area. Around 5° (a bit less than 1%) of the visual field of each eye is profoundly blind.

However, the blind spot seems to present little or no restriction to vision - because the brain 'fills-in' the missing detail. How the visual cortex accurately simulates the missing data for colour, brightness, texture etc etc is unknown.

It has been claimed that the brain uses data from the other eye to compensate - but, if you close one eye, there is normally no sensation of a blind spot in the visual field of the one that's open.

The 'filling in' phenomenon is so seamless that a science-based demonstration of the blind spot's existence can be quite disturbing. See Blind Spot at Wikipedia.

The filling-in phenomenon has been researched for many years, using 'recordings' from the retinal cells etc, and also with fMRI.

This research is still at an early stage, and many questions remain to be answered about the neural mechanisms of filling-in. Understanding the details of these mechanisms is important, because it might provide answers as to where and how subjective visual experience emerges, a fundamental question about visual perception."

Source : Nature Reviews, Neuroscience, volume 7, pages 220–231 ^{[ paywalled ]}

* Editor's Note : The way that the optic nerve connects from the 'wrong' side of the retina in all vertebrates is often cited as an example of how evolution can permanently build-in 'mistakes'. In the eye of the octopus - which has many similarities with vertebrate eyes (iris / lens / retina etc.) but co-evolved completely separately - the nerves connect from behind the retina, thus octopuses have no blind spot.