Stage 07 · in depth
There is a hole in your vision. Here it is.
About 15 degrees to the side of wherever you are looking, in each eye, there is a patch of visual field roughly ten full moons across that receives nothing at all. It has been there your whole life. In the next minute you are going to find it, and then watch your own brain paint over it.
Why the hole is there
The retina is assembled with its wiring in front of its sensors. Light crosses the ganglion cells, their axons and two synaptic layers before it reaches a photoreceptor at all. That arrangement leaves one problem: about 1.2 million axons are now sitting on the inner surface of the eye, and every one of them has to get out.
They converge on a single point and leave together through a sieve of collagen called the lamina cribrosa. At that point there is no room for photoreceptors, and there are none. The result is the optic disc, and its projection into the visual field is the physiological blind spot: about five degrees wide, seven degrees tall, and permanent.
It sits on the temporal side of each eye's field, which takes one step of reasoning. The disc lies nasal to the fovea on the retina, and the eye's optics invert the image, so a nasal position on the retina maps to a temporal position in the field. The right eye's blind spot is therefore to the right, and the left eye's to the left. That is why the demonstration below mirrors when you switch eyes.
The part that matters
You do not perceive a hole. You do not perceive a grey patch, or a blur, or a sense that something is missing. The visual system completes the region using whatever surrounds it and presents the result with exactly the same confidence as the parts you are genuinely seeing. It never flags the difference. Whatever else you take from this atlas, take that.
Find it
Most blind spot demonstrations online place the target at a fixed number of pixels from the fixation mark, which puts it at an unknown angle on an unknown screen and works by luck. This one measures your screen against a bank card first, then tells you the viewing distance that makes the separation come out at the right angle.
Step 1 · Measure your screen
Hold a bank card against the screen.
The blind spot sits at a fixed angle from where you are looking, about 15 degrees. An angle is a size divided by a distance, so this only works if the drawing below is a known physical size. Every bank and ID card in the world is 85.6 mm wide, which makes one a ruler. Drag the slider until the outline matches a card laid flat on the screen.
1 MM = 3.78 PX
Step 2 · Pick an eye and a background
The right eye's blind spot lies to the right of where it is looking, so the target goes on the right and you fixate the cross on the left.
Step 3 · Cover your left eye
- Cover your left eye with your hand. Do not squint it shut.
- Look straight at the cross with your right eye, and keep looking at it. If your gaze slides toward the target, this will not work.
- Sit about 34 cm (13 in) from the screen, then move slowly forward and back through that distance.
- Move slowly toward and away from the screen. At one distance the dot vanishes completely.
Separation
90 mm
15 degrees at the stated distance
Viewing distance
34 cm
Derived from the separation that fitted your screen
Target
9.4 mm
Inside a blind spot roughly 5.5 by 7.5 degrees
When it works
The dot has landed on your optic disc, where there are no photoreceptors. You are not seeing a grey patch or a smudge. There is simply nothing there, and nothing is exactly what it looks like.
What this does not mean
- Finding it means nothing is wrong
- Every healthy eye has this blind spot, in the same place, for the same reason. It is the optic disc, where the axons leave. You have had it your whole life.
- Not finding it means the geometry is off
- Almost always the viewing distance, the fixation drifting toward the target, or the covered eye not being fully covered. Move slowly, keep your gaze locked on the cross, and try again. It is not a result.
- This is a demonstration, not a test
- It measures nothing and screens for nothing. If you want to monitor your central vision on a schedule, the Amsler grid is the tool for that, and it is free on this site.
- A blank area anywhere else is different
- The physiological blind spot sits about fifteen degrees to the side. A missing, dim or distorted patch near the centre of vision, or one that is new, is not this and is worth an appointment rather than an experiment.
Why this is more than a party trick
Filling-in is not a special mechanism for the optic disc. It is how the visual system handles missing information generally, and it does not distinguish between a gap that has always been there and one that appeared last month.
That has a direct consequence. Field loss from glaucoma, from retinal disease, or from damage further along the pathway can be substantial before anyone notices it, because the same machinery that hides the blind spot hides the new gap too. People routinely discover advanced field loss during a routine examination, having reported nothing at all.
This is the entire argument for having your eyes examined on a schedule rather than when something feels wrong. Your perception is not a reliable monitor of your visual field, and you have just proved that on yourself.
Common questions
Why does everyone have a blind spot?
Because the retina is built with its wiring in front of its sensors, every axon has to leave the eye through one opening. That opening is the optic disc, and there is no room for photoreceptors in it. It is a structural consequence of the retina's layout rather than a defect, and every healthy human eye has one.
How big is the blind spot?
Roughly five degrees wide and seven degrees tall, centred about fifteen degrees from fixation on the temporal side of each eye's visual field. That is around ten full moons across, sitting permanently in each eye.
Why can I not see the blind spot normally?
Two reasons, and only the second is interesting. The eyes cover each other, since the two blind spots are on opposite sides. But it stays invisible with one eye closed, because the visual system fills the region in with whatever surrounds it and gives the result the same confidence as genuinely seen parts of the field.
The test did not work. Is something wrong?
Almost certainly not. The usual causes are the viewing distance being off, the gaze drifting toward the target instead of holding on the cross, or the covered eye not being fully covered. Move slowly forward and back through the stated distance while keeping your eyes locked on the cross.
Does this test for eye disease?
No. It demonstrates a piece of normal anatomy and screens for nothing. Because filling-in conceals gaps in general, real field loss can also go unnoticed for a long time, which is the argument for examination on a schedule rather than waiting for symptoms.
What is the difference between the blind spot and a scotoma?
A scotoma is any blind area in the visual field. The blind spot is the physiological one that everyone has, in the same place, for a structural reason. A scotoma appearing anywhere else, especially near the centre of vision or newly, is not this and is worth an appointment.
Sources
- 01Ramachandran VS, Gregory RL. Perceptual filling in of artificially induced scotomas in human vision. Nature. 1991;350(6320):699-702.
- 02Jonas JB, Schmidt AM, Müller-Bergh JA, Schlotzer-Schrehardt UM, Naumann GO. Human optic nerve fiber count and optic disc size. Invest Ophthalmol Vis Sci. 1992;33(6):2012-2018.
- 03Kolb H, Fernandez E, Nelson R, eds. Webvision: The Organization of the Retina and Visual System. University of Utah Health Sciences Center. NCBI Bookshelf NBK11530.
- 04Kandel ER, Koester JD, Mack SH, Siegelbaum SA, eds. Principles of Neural Science. 6th ed. McGraw-Hill; 2021. Part V, Perception.