In a unique experiment, mice received infrared vision

Special nanoparticles (shown in white) cling to rods (left) and cones (right) in mouse photoreceptors.

By injecting nanoparticles into the eyes of mice, the scientists allowed them to see near infrared light - electromagnetic radiation, usually not visible to rodents (or humans). A unique breakthrough, which is even more unusual when it is understood - such a technique can be used on a person.
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A research team led by Tian Xue of the University of Science and Technology of China and Gang Khan of the University of Massachusetts Medical School changed the vision of mice so that they could see near infrared light (NIR), while retaining their natural ability to see normal light. This was done by injecting special nanoparticles into their eyes. The effect lasted for about 10 weeks and without any serious side effects.

A series of tests showed that mice actually saw infrared light, and not some other things. Scientists say that the human eye is not too different from the eyes of mice, which leads to a fantastic prospect of applying this technique to humans.

People and mice are able to see only a narrow portion of the electromagnetic spectrum, indicated by a rainbow stripe. Other animals, such as birds or bees, are able to see ultraviolet, and snakes are infrared.

People, like mice, are able to see only a narrow portion of the electromagnetic spectrum . The spectrum of wavelengths invisible to humans is huge, we see nothing beyond the limits of the so-called visible spectrum (wavelengths of 380 - 740 nanometers). Infrared radiation exists in the form of longer waves, from 800 nm to millimeter.

Objects in the world, whether people or a hot plate of soup or something cold, like an ice cube, emit infrared radiation. Mammals, such as humans and mice, cannot see NIR, but we have the technology, namely night goggles or thermal vision glasses, which can transform this invisible spectrum into light that we can see. The new technique used in mice does something similar, but instead of relying on wearable technology, scientists used a biological solution.

To allow mice to see outside the normal visible spectrum, Thian and Ganges developed special nanoparticles that increase the frequency of radiation and are able to function in the already existing rodent eye structures. Liquid droplets containing tiny particles were injected directly into their eyes, in which they fitted snugly to photoreceptor cells with the help of special anchors. Photoreceptor cells - rods and cones - usually absorb the wavelengths of visible light, which the brain interprets as sight. In the experiment, injected nanoparticles transformed NIR into a visible wave, which the mouse brain could perceive as visual information (in this example, they considered NIR as green light). Nanoparticles have been in the eyes for two months, allowing mice to see both NIR and visible light with minimal side effects.



Graphic representation of the process of vision. When infrared light (red) penetrates the photoreceptor cell (light green circle), the nanoparticles (pink circles) transform NIR into visible green light.

Nanoparticles on photoreceptor cells served as an infrared light converter. Infrared waves were captured in the retina by nanoparticles, which then emitted them as shorter waves of visible light. Thus, rods and cones that absorb shorter wavelengths were able to receive this signal and then send the transformed information to the visual cortex. In particular, the injected particles absorbed NIR with a length of about 980 nm and transformed it into light of length 535 nm. Mice perceived infrared light as green. The result was similar to the observation of NIR in night vision goggles, except that the mice could also maintain their normal perception of visible light. As already mentioned, the effect was temporary, about a few weeks, in some mice the cornea was clouded, which quickly cleared up.

To prove that the method really works, Tian and Ganges conducted a series of tests and experiments.

For example, the pupils of mice decreased when exposed to NIR, whereas the pupils of mice did not do this without injection. And when exposed exclusively to NIR, measurements of the electrical activity of the brain in mice injected with nanoparticles showed that the eyes and visual cortex function as in the presence of visible light.

Behavioral tests also showed that the technique works. Mice placed in a Y-shaped maze were taught to recognize the location of the hidden shelter platform pointed to by the NIR. During the tests, injected mice constantly found a platform, and mice without injections swam around the maze. Another test included a box with two compartments: one completely without light, and another illuminated NIR. Mice, like night creatures, are made to darkness. In tests, mice injected with nanoparticles spent more time in the compartment without light, and mice without injections showed no preference.

“These extensive experiments leave no doubt that mice injected with nanoparticles sensitive to infrared radiation acquire the ability to see infrared light and receive visual information,” said Vladimir Kefalov, a professor of ophthalmology and visual science at the University of Washington in St. Louis.

In a press release, Tian pointed out that nanoparticles clung to rods and cones, and were activated by near infrared light, so "we believe that this technology will work in human eyes, not only as supervision, but also for therapeutic purposes." In an interview with Cell, he clarified by saying:

Unlike mice, humans and other primates have a retinal structure called fovea, which provides high-definition central vision. In human fovea, cone density is much greater than that of rods; while in the retina of the mouse the number of sticks is greater. Since cones have different sensitivity to the spectrum and intensity compared to rods, we may need to fine-tune the emission spectrum of UCNP for more effective activation of cones of the desired type in humans.

As Tian said, in order for this technology to work for a person, it needs to be changed, but new experiments show that its change is possible. Kefalov said that the potential of applying such a concept to a person is real and exciting, but he warned that we still have a long way to go.

"The authors showed that a single injection of nanoparticles does not adversely affect the mouse's retina," said Kefalov. "However, it is not yet clear whether practical infrared vision will require repeated injections and, if so, whether chronic infrared vision will not affect the structure and function of our eyes."

The ability to see infrared light seems fantastic, but it would certainly be a useful sign. We could see a lot of things beyond our usual visual spectrum — and we would have a built-in night vision system. As Tyan explained to Cell:

Scientists are trying to develop a new technology that allows you to use the ability beyond our natural capabilities. Visible light, which can be perceived by a person’s natural vision, occupies a very small part of the electromagnetic spectrum. Electromagnetic waves longer or shorter than visible light carry much more information. Depending on the material, the object may also have different absorption and reflection in the near IR. We cannot detect this information with the naked eye.

Another interesting feature of this potential improvement is that the person does not need to wear bulky and energy-intensive equipment, such as night vision goggles. And technology does not require any genetic manipulation. Most likely, the military will be interested in this work.

Sydney, Diong Jin of the School of Mathematics and Technology at the University of Technology, described the new work as “very innovative and inspiring”. Dayong said that, to his knowledge, "this work is the first example of implantable and" wearable "optical nanodevices." He said that it is important that mice do not have inflammation or cell death, but it is possible that some cells absorb nanoparticles, a prospect that “deserves more careful study.”

Similarly, Kefalov was impressed with the study, stating that “the authors did an amazingly good job, describing the effect of injecting infrared-sensitive nanoparticles on the visual function of mice,” adding that this “innovative work demonstrates an original and powerful method of enhancing the ability of the visual system to detect light boundaries of the natural visible spectrum. " He considers it “amazing” that nanoparticles most likely do not interfere with the normal function of photoreceptors in visible light.

Regarding whether this technique can be used to correct visual impairment, such as color blindness, is less clear, he said.

“Since the reception is based on the ability of photoreceptors to detect and amplify light signals, using it to treat impairments of photoreceptor function will require the development of new stages, in addition to converting light beyond the visible spectrum,” said Kefalov.

Looking to the future, Tian and Ganges would like to improve the technique with the help of nanoparticles on an organic basis, consisting of substances approved by the FDA, which can lead to even brighter infrared vision. They would also like to customize the technique so that it is closer to human biology. Optimistically assessing the direction of technology, Tian and Ganges have already filed a patent application for their work.

I am already presenting television commercials: “Ask your doctor if the near-infrared spectrum suits you.”

www.cell.com/cell/fulltext/S0092-8674(19)30101-1