Why Is There a Hole in Your Vision You Never See? The Brain Fills It In
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Every eye has a patch with no light-detecting cells at all, where the nerve leaves for the brain. Nobody notices it, and what the brain does about it is stranger than simply ignoring it.
Why the gap exists
The vertebrate retina is built with its light-detecting cells at the back, behind the layers of nerve cells that process their output, so light passes through the wiring before reaching the detectors. That wiring must get out of the eye somehow, and it does so by gathering into a bundle that passes through the retina at one point and exits as the optic nerve. At that point there is no room for detectors, so a patch of roughly one and a half millimetres across has none, corresponding to a region several degrees wide in the visual field, which is large enough to hide several full moons side by side.
Why nobody notices
Several things conceal it and they operate together:
- •The two eyes have their gaps in different places, so each covers the other
- •Constant small eye movements sweep the gap across the scene
- •It sits away from the centre, where attention rarely rests
- •There is no dark patch to notice, since nothing reports darkness
- •The brain fills the region with whatever surrounds it
- •Closing one eye and staying still is required to find it at all
What filling in actually does
The brain does not merely ignore the region, it actively constructs content for it, which has been demonstrated with experiments that are easy to replicate. A line passing across the gap is perceived as continuous. A patterned background is perceived as continuing through it, with the correct texture and colour. If the surrounding area is changed, the perceived content of the gap changes to match. That construction is not a reconstruction of anything real, since nothing is being detected there, and it is a reasonable guess presented to the viewer with the same confidence as genuine information, which is the point that makes it philosophically interesting.
When it stops being harmless
The natural gap is one thing and an enlarged or additional one is quite another, which is why measuring it is a routine clinical test. Raised pressure inside the skull swells the head of the optic nerve and enlarges the gap measurably, which can be detected before a patient reports anything. Glaucoma damages nerve fibres and produces additional blind regions that also go unnoticed for a long time, because the same filling in that hides the natural gap hides the new ones, which is why the disease is frequently advanced before it is noticed. Mapping the visual field by asking a patient to report a light at many positions detects both, and the pattern of loss identifies where the damage is.
Why the eye is built backwards
Putting the wiring in front of the detectors and punching a hole through the retina to get it out looks like poor design, and it is frequently cited as an example of evolution working with what it has rather than starting fresh. Octopus and squid eyes, which evolved independently, have the detectors at the front and the wiring behind, and consequently have no gap at all. The vertebrate arrangement is not as bad as it first appears, since specialised cells guide light through the overlying layers efficiently and the arrangement places the detectors next to the tissue that supplies them with oxygen and recycles their pigment, which they need in large quantities. The gap remains an unforced cost of the layout.
The takeaway
Nerve fibres leaving the eye pass through the retina at one point, leaving a patch several degrees wide with no light detectors, large enough to hide several moons. The other eye covers it, small eye movements sweep it about, and the brain constructs plausible content for it from the surroundings, presenting a guess with the same confidence as real information. Octopus eyes are wired the other way and have no gap.