Where's my nose? Why can't I see it?

Audit expert: Liu Dongbao

Chief Physician of Ophthalmology Department, Shenyang Red Cross Hospital

Can you see your nose? If you don't cross your eyes, you will find that you really can't see your nose.

But in fact, the nose is definitely within our field of vision. The reason why we ignore it is because our brain selectively ignores it, which is related to our "stereoscopic vision".

Human eye sight range source | Bing search

1

Your brain makes you ignore your nose

Stereoscopic vision is an advanced visual function that allows us to judge the distance, front and back position, height, depth, speed, etc. of objects in three-dimensional space.

When we look at something with both eyes at the same time, there is a parallax angle due to the distance between the two eyeballs. The images of objects on the retinas of the two eyes are similar but have certain differences, which will form binocular parallax , and then stereoscopic vision appears. This is the same as 3D movies. Many professions such as drivers, architects, modelers, electronic engineers, etc. require practitioners to have good stereoscopic vision.

The main defect of amblyopia is the lack of perfect stereoscopic vision, so it is difficult for amblyopic patients to perform various fine work, and the safety risks are far higher than those of normal people.

Human eyes image source | Bing search

There are many factors that affect stereoscopic vision, including vision, age, anisometropia, eye position, strabismus and amblyopia. The most important factor is vision. The worse the vision, the lower the level of stereoscopic vision . Stereoscopic vision can be classified in many ways according to different conditions: for example, it can be divided into fine stereoscopic vision and coarse stereoscopic vision according to the size of the parallax; it can be divided into crossed and non-crossed stereoscopic vision according to the direction of parallax separation; it can be divided into central eye and non-central eye stereoscopic vision according to the parallax processing mechanism; it can be divided into close-range stereoscopic vision and long-range stereoscopic vision according to the gaze distance, etc. It is worth noting that close-range stereoscopic vision refers to stereoscopic vision with an observation distance of 30 to 40 cm, while long-range stereoscopic vision refers to stereoscopic vision when the observation distance is greater than 6 meters.

In other words, stereoscopic vision is limited to a certain range. Beyond this range, we can only see roughly. The nose is too close to the eyes, just out of the range of stereoscopic vision. Theoretically, you can always see your nose, but because of the relationship of "stereoscopic vision", your brain will help you ignore it when processing visual information .

Stereoscopic Vision Poster Image Source | Bing Search

2

People are blind for 40 minutes a day

It's not just your nose, the brain also processes a lot of "blurry images" that you don't want to see. In addition, when awake, the eyes of humans and many animals often make rapid visual changes, which we often call scanning, to search for the desired target and image it on the fovea of ​​the retina.

Image source: Rhcastihos

From a physical point of view, during the scanning process, the world in our eyes should be blurry and shaky, but in fact, what we see is very clear. Obviously, it is our brain that actively and cleverly resolves this contradiction. While the brain receives stimulation, it is also filtering and processing information. In other words, when you are scanning, your brain can automatically help you filter out many blurry images , which is called "saccadic inhibition". If there is no saccadic inhibition to block a large amount of visual information, our brain will receive too much stimulation, causing dizziness, and there will be a phenomenon like "3D dizziness". In addition to scanning over a large range and changing the line of sight, saccadic inhibition will also occur in the "micro-eye movement" of the eyeball. When we look at an object, we often think that the eyeball is still. In fact, our eyeballs are constantly vibrating slightly to ensure a more detailed and comprehensive observation of the object, which is commonly known as "micro-eye movement".

Source: QbitAI

If you want to verify the existence of micro-eye movements, you can try staring at the picture below, and you will find that the image seems to move. Scientists have proved through experiments that micro-eye movements are accompanied by a brief period of visual inhibition , which can be said to be equivalent to blindness during this period of time.

Verify the source of the micro-eye animation | Tencent

But after that, vision will be restored immediately, and vision will be temporarily enhanced. According to scientists, each saccadic suppression lasts about 50 milliseconds . If you add up the time a person has saccadic suppression in a day, it will be nearly 40 minutes. It can be understood that you may be blind for 40 minutes a day.

3

Why does the brain actively help us filter images?

When we move our sight quickly, if our eyes do not block a lot of visual information, our brain nerves will receive excessive stimulation, causing dizziness . So the brain is worried that we can't bear it, so it turns on the automatic filtering function to present a clean and clear world to our eyes.

The well-known floaters are also related to this. Floaters are an eye disease. When people age or overuse their eyes, the vitreous in our eyeballs will naturally age and peel off, and vitreous floaters will appear in the eyeballs. When light enters the eye, the shadows of these floaters are projected onto the retina, forming the "floaters" seen by the patient. Patients with floaters sometimes feel that there are small things floating in front of their eyes, but they disappear after a while. This is not an illusion. In fact, the brain decides on its own so that you don't always see these small floaters . This is also a self-correction mechanism of the body.

4

The brain’s attention span is limited

In life, we tend to pay attention to some things and "ignore" others. In fact, this is the basic function of attention. Although the nose is always in front of our eyes, our main attention cannot always be on it, so the brain will ignore it and focus on what we need to pay attention to. When we look into the distance, the field of vision is quite wide, but our attention tends to be focused on a certain thing, blurring other scenes. This principle can be compared to the focus of a camera. When the focus is on a certain object, its background is selectively blurred.

From this point of view, the difficulty of making artificial eyes has increased a lot.