Your eyes are always shaking, but you just don’t know it

First, let’s experience a magical visual phenomenon discovered 200 years ago: please enlarge the picture below, then stare at the cross in the middle and keep your eyes still.

If nothing goes wrong, within a few seconds, you will feel that the rich colors around the cross have disappeared! It has turned into a grayish white.

If you feel that it is not enough, you can stare at the red dot in the picture below again and wait for a few seconds. This time you don’t need to enlarge the picture (but the experience will be better after enlarging it!)

Did you find that the light gray ring around the red dot in the center of the picture seems to have disappeared, as if it has merged into the white background.

So, are our eyes broken? What happened?

Don’t worry, your eyes are completely normal and healthy. These phenomena we see were actually first discovered by Swiss doctor Troxler in 1804. This phenomenon is also called the “Troxler effect”. It refers to when the visual center of the human eye is focused on a certain object, the static objects around this object will fade and disappear in the human eye.

In the late 1950s, Clarke, a scholar at the Imperial College of London, gave an explanation. He believed that this was due to the adaptability of the photoreceptor cells in the human eye that receive visual stimulation. Since stationary objects cannot continuously provide effective stimulation and information, the human eye’s visual perception of them quickly fades and turns gray.

At this time, a question is also on the paper. When we stare at the cross and the red dot, they are also stationary, why don’t they disappear? Have you guessed the answer?

Unable to perceive stationary objects

Scientists in this field like to use the visual system of frogs as an example. Frogs have a pair of big watery eyes, and you may have had the experience of staring at frogs. When you look at it quietly, it seems to look at you “without fear”. At this time, the frog is not afraid of you, but it can’t see you. In other words, they can’t see any stationary objects around them, even if it is food-such as a flying insect. Even if it is close in front of them, as long as it is still, the frog can’t see it. On the contrary, when the flying insect takes off, the frog can quickly detect it and attack it. If you move, it will immediately see the giant in front of you and jump away immediately.

This is because the frog’s visual system can only perceive the stimulation of moving objects and changing information. Some researchers believe that in a sense, all visual systems can only see moving objects, including human eyes. And this conclusion may be further extended, and other human senses are all perceiving changing information. Once the movement stops or the change disappears, people seem to lose their perception of it quickly. It’s like you don’t always know where your feet are and where your hands are. After entering the room for a while, you will ignore the strange smell inside. And after staying in the cool swimming pool for a while, you adapt. These are all the adaptability of human sensory neurons.

Back to the question in the first section, why don’t the stationary crosses and red dots disappear? The answer is that even if our eyes are staring straight and firmly at an object, our eyes are still moving, although we can’t detect it. And it is these eye movements-also called fixational eye movements-that keep our perception of the object from fading.

However, fixational eye movement occurs in a very small range, only concentrated in the central visual area (that is, the area where the human eye is directly looking), that is, the area where the cross and the red dot are located, and does not cover the peripheral visual area (Peripheral Vision) where the color and gray rings are located. However, even in the case of non-fixation, the human eye will have some small movements, such as spontaneous nystagmus, eye saccades, etc. During sleep, the eyes will also have some activities, and we are familiar with the rapid eye movement sleep stage.

Eyes are drifting

Fixational eye movement is a small-amplitude movement, which is mainly divided into three types, namely eye tremor (Tremor), drift (Drifts) and microsaccades (microsaccades). It is not easy to detect such a small movement of the human eye. In the experiment, the researchers first asked the participants with their heads fixed to continue to look at a small target. In addition, they also used very sensitive instruments to detect the activities of the participants’ eyes. Eye tremor cannot be studied by existing detection methods because of its extremely small amplitude and high frequency (40-100Hz). Drift is a continuous, seemingly irregular movement that occurs when the human eye is fixated. Microsaccades refer to very small, fast jumping movements that occur when the human eye is fixated, which is also the most studied type of fixation eye movement.

It can be imagined that when we are extremely focused on observing some details, our eyes are actually vibrating, jumping, and floating. As a paper published in Nature Reviews Neuroscience in 2004 said, there is an inherent paradox in our visual system – we must fix our sight to see the tiny details in the environment, but if our eyes are really completely fixed, the whole world will disappear from our field of vision. And these fixation eye movements determine that the visual cells in the eyes will not lose their activity when seeing a static picture, making people’s vision gray and blurry.

Thanks to the development of technology, researchers at the University of Bonn in Germany can now study the drift of the eyes. In a recent study published in eLife, they found that drift not only allows the human eye to accept new stimuli at any time, but is also likely to be a controlled, fast and precise movement. It can continuously bring the image of the object we are interested in on the retina to the most sensitive fovea of ​​the human eye, allowing people to have better vision.

Better vision

n bright light, every healthy person without visual impairment can usually reach a vision of 1.0, that is, they can distinguish the outline of about 1.75 mm at a distance of 6 meters. The E eye chart that we are very familiar with uses a similar method to detect human intelligence. The visual resolution of a person with a vision of 1.0 is about 1 arcmin (equal to 1/60 degree), which is an extremely small angle.

We can have such good vision thanks to the macula in the human retina and the fovea in the center of the macula. The center of the fovea is the area with the sharpest vision and the best color vision. This is an area with a diameter of about 0.35 mm, where the cones are extremely dense, with about 150,000 cones per square millimeter (equal to 16,997 cones per square degree). And there are almost no rods responsible for dark vision. There are no blood vessels here, which ensures that the cones can sense light without any dispersion. The macula is also the area where cones are most densely distributed. On the outer retina, the number of cones decreases rapidly, and there are more rods.

In a new study published in eLife, researchers from the University of Bonn in Germany used the adaptive optical scanning light ophthalmoscope (AOSLO) to measure for the first time the direct relationship between the density of cones in the participants’ fovea and the visual resolution threshold. In the experiment, they asked 16 healthy participants to perform letter discrimination tasks and measured their eye movements during this period. At the same time, they also conducted more detailed work to track the path of visual stimuli on the participants’ retinas to determine which cone receptors contributed to vision. However, the researchers also found an interesting phenomenon, that is, smaller drifts will increase visual acuity, but larger drifts will reduce the visual acuity of participants. They also found that this drift is not random. This process may occur because the nervous system of the eye fine-tunes the drift movement within a few hundred milliseconds, so that visual stimuli can continue to enter the fovea of ​​the eye, thereby enhancing retinal sampling and improving people’s visual acuity.

These 16 participants all had good vision, with cone density in their fovea reaching 10,692 to 16,997 cones per square degree. The higher the cone density, the smaller the distinguishable visual stimulus, which means the vision will be sharper. However, in the actual measurement, the researchers found an unexpected result. The actual vision of the participants was actually better than the vision results calculated based on the foveal cones, an increase of 18%.

So where does this extra vision come from? The researchers believe that this is related to the drift in fixation eye movement. During fixation eye movement, the eyes perform more continuous, seemingly random movements – drift. Imagine that when the eyes drift, although the movement is very small, it will pass through dozens to hundreds of cones in the fovea, and they will feel the change in brightness. And more changes will stimulate the cones to produce more neural activity. At this time, our visual system seems to be able to convert such a small drift – the brightness change felt by a single cone cell, into additional information, allowing us to see the position and shape of objects more clearly.

Not only can our eyes improve our vision through tiny movements, but some current projectors and cameras also increase the resolution of pictures and videos through jitter. Pixel jitter technology is to increase the resolution of images through tiny displacements, usually sub-pixel displacements. It’s like generating multiple image frames through tiny displacements (or “jitters”), and then superimposing these frames together to achieve higher resolution. The DMD chip used in projectors is a typical example.

Do you understand this more detailed world created by tiny jitters?