1. An Experiment: Making a Circle Vanish by Looking at a Cross
Draw a cross and a circle side by side on white paper. Make the circle about 1 cm in diameter, with a gap of about 8 cm between it and the cross. Close your left eye and stare only at the cross with your right eye. Hold the paper about 30 cm from your face, then slowly move it closer or farther away. The key is not to look at the circle. Keep your gaze fixed on the cross, keeping the circle at the edge of your vision.
At a certain distance, the circle will suddenly vanish. If you move the paper slightly, it reappears. The distance at which it disappears varies from person to person and depends on the size of the marks and the gap between them, so 30 cm is just a rough guide. If you cannot find it, try moving the paper more slowly to search for the spot. The 8 cm gap is calculated from the position of the blind spot (about 15 degrees from the center of the field of view), which will be explained later. At a distance of 30 cm, a 15-degree offset corresponds to about 8 cm to the side, according to geometric calculation.
2. The Retina's Area That Cannot Detect Light
The inner wall of the eye has a thin membrane called the retina, lined with photoreceptor cells that detect light. Signals received by these cells travel through the optic nerve to the brain. The circle disappears because there are no photoreceptor cells where the optic nerve exits the eye. If light hits this area, no reaction occurs. This area is called the blind spot [1]. An article from the University of Tokyo also explains that the retinal area corresponding to the blind spot lacks photoreceptor cells because it is a pathway for blood vessels and the optic nerve [2].
According to one encyclopedia, the blind spot is an oval shape about 7 degrees vertically and 5 degrees horizontally in the field of view, located about 15 degrees toward the ear side from the center, and slightly downward [1]. However, these numbers vary slightly depending on the source, and there are individual differences. So why do we not see a black hole where the circle disappeared? It is thought that the brain fills in the missing part based on information surrounding the blind spot. In research introduced by the University of Tokyo, when a donut-shaped light was shone around the blind spot, the area inside the blind spot, which received no light, felt bright [2].
Another reason we do not notice this in daily life is that the blind spots of the left and right eyes are in different parts of the field of view, so one eye can compensate for the gap in the other [1]. But who was the first person to confirm this "invisible place" through an experiment?
3. How Edme Mariotte Demonstrated the Blind Spot
The French scientist Edme Mariotte clearly demonstrated this phenomenon through an experiment. It is reported that he published his findings in "New Discoveries on Vision" in 1668 [3]. At that time, it is said that there was a view that the place where the optic nerve enters the eye should be the clearest part of vision [4]. However, Mariotte found that nothing was visible there.
His method is very similar to the home test described above. He placed two white marks on a dark background, covered one eye, and stared at the central mark with the other eye. When he moved back about 9 feet (about 2.7 m), the other mark disappeared. If he moved his head or eyes slightly, the mark returned [3].
Regarding this result, the anatomist Pecquet, who was studying the eye at the same time, objected. In 1669, an experiment was conducted at the Academy, and opinions were divided. Mariotte himself believed that the center of vision was not the retina but the choroid, a membrane outside the retina, but this idea was later rejected [3]. Nevertheless, the tools were just white marks and a dark background. Because anyone could try the same experiment, it is thought that the existence of an invisible place inside the eye became accepted as a verifiable fact [4]. So, why does the place that should be invisible not look black?
4. Why the Brain Fills In the Gap with Surrounding Patterns
In 1991, Richard Gregory and V.S. Ramachandran reported an experiment in Nature magazine. They showed that the brain fills in gaps even in places that are not blind spots [5]. They placed a uniform gray square in a flickering background and observed the effect when viewed from about 6 degrees away. The square disappeared and was filled by the flickering background; the average time across 40 experiments was about 5 seconds [5]. The "about 5 seconds" is an average under experimental conditions, so it does not disappear in the same number of seconds for everyone.
The same paper also includes experiments on the blind spot itself. A red ring covering the blind spot appeared as a red disk. When concentric circles were added around the ring, the blind spot appeared to be filled by the rings [5]. This suggests that the blind spot is easier to fill if there are patterns connecting on both sides. Researchers disagree on exactly where and how the brain fills in the blind spot. What is known is that "surrounding information is used."
You can try this yourself. Draw a circle on striped paper and do the same as in Section 1. You might see the stripes continuing into the place where the circle disappeared. This allows you to observe how the blind spot area is filled.
5. Why Octopus and Squid Eyes Have No Blind Spot While Human Eyes Do
The blind spot is a feature common to the eyes of vertebrates (animals with backbones), not just humans. However, the eyes of cephalopods like octopuses and squids are said to have no such blind spot. This is because the optic nerve approaches from behind the retina, so no hole pierces through the retina [6].
Human eyes have nerves exiting through the retina, so photoreceptor cells cannot be placed at the exit. Even for the same "eye," the presence or absence of an invisible place is affected by the direction of the wiring. However, it is premature to say that human eyes are flawed because of this. The brain compensates for the invisible place, so we rarely have trouble in daily life.
6. Testing the Blind Spot with Both Eyes and a Striped Background
What you can do today: Try with both eyes and compare the distance at which the mark disappears. Next, draw a circle on striped paper and do the same. Compare whether the stripes appear to continue into the place where the circle disappeared with the result on plain colored paper. If you compare with family members, treat it as a game to see that the disappearance distance varies from person to person, rather than a competition of who is better.
If you want to read more: The University of Tokyo website has an article introducing research using the blind spot. It is available in Japanese.