The Sciences

After 'Superpower' Nano Injection, Mice Can See Infrared

Humans remain blissfully unaware of how much beauty lies beyond the limited portion of the universe's electromagnetic spectrum that they can see. It needn't always be this way.

Sometime in the near future, a soldier is on a mission to track a missing child in dense jungle. She searches all day in trying conditions. As night approaches, she will be forced to give up on the search because she can’t see in the dark.

Thankfully, she has access to new technology that can enhance her vision in such conditions. As the Sun sets, she fetches a small bottle from her gear and applies a few drops of a fluid into her eyes. The fluid contains bio-engineered nanoparticles that seep into her retina and attach themselves to the rod cells, which sense brightness. These nanoparticles change how the rod cells work, making them receptive to infrared light. Now, she can see in the dark, and she can continue her search.

This might sound far-fetched – but Chinese and American scientists already have the conceptual proof of such technology. It’s only a matter of time before it’s available to ‘enhance’ humans. Their study, in which they injected nanoparticles into mice’s retina and modified their photoreceptors, was published on February 28.

The dark side

What we perceive with our visual system is only a small part of the electromagnetic radiation in the universe. Our eyes are sensitive to radiation between the wavelengths of 400 nm and 700 nm.

Other animals can see more. In the early 19th century, an English scientist named John Lubbock cleverly demonstrated that ants see ultraviolet light. He knew that ants avoided light when carrying their their young ones. So when Lubbock shone light through a prism, splitting it into its consequent wavelengths, he observed that the ants didn’t avoid red light, ergo they couldn’t not red light. However, they avoided an area beyond the violet – ergo the ants could see ultraviolet light.

In fact, many other insects can see ultraviolet light. Honeybees see intricate patterns on flower petals made by a chemical compound called flavanols, which reflect ultraviolet light. These patterns are called nectar guides because they guide pollinating insects to the flowers.

A Mimulus flower photographed in visible light (left) and UV light (right) showing a dark nectar guide visible to bees but not to humans. Caption and credit: Plantsurfer/Wikimedia Commons, CC BY-SA 3.0

These pleasures are beyond the abilities of the human visual system. As a result, humans remain blissfully unaware of how much beauty lies beyond the limited portion of the universe’s electromagnetic spectrum that they can see.

When we say ‘light reaches our eyeballs’, we’re actually talking about photons reaching a thin layer of cells called the retina at the back of our eyeballs. The retina contains an intricate circuit of different types of cells that work together to convert the photon’s energy into electrical signals.

The cells that perform this conversion are called photoreceptors. We have two kinds of photoreceptors: the rods that work in low light and three kinds of cone receptors, which work in bright light. The cone cells are most sensitive to light of wavelength 400-700 nm, which we see as different colours. This is why we have colour vision during the day and grayscale vision in the dark.

We can see so little of the electromagnetic spectrum because the cone cells are most sensitive to such a narrow range of wavelengths. So to expand this, the Chinese and American scientists designed upconversion nanoparticles – a tiny cluster of molecules that can attach to photoreceptors in the retina. Once in place, they absorb light in the near-infrared part of the spectrum and release light in the visible spectrum (while preserving the encoded information).

In effect, the retina is retrofit with a tiny device that helps it sense near-infrared light. These nanoparticles are designed to be safe, to not need a power source and not interfere with the retina’s normal functions.

However, the nanoparticles are introduced through an invasive procedure: by injection into the retina. This is a common procedure, one that your friendly neighbourhood ophthalmologist can perform, but the hope is to simply this in the future.

‘Seeing’ heat

Once the nanoparticles were in place, the scientists had to test if the mice actually saw near-infrared light.

When near-infrared light was shined on their eyes, pupils of the mice with retinal injections responded, constricting their irises. This meant their visual system was sensitive to this wavelength.

Mice generally avoid lit areas and move into darker ones, like a cupboard or a crack in the flooring. If a mouse is placed in a box that is part lit and part shadow, it will always go to the dark side. In a box lit with only near-infrared light, mice with modified retinae moved to the dark part while ‘normal’ mice didn’t.

In another test, the scientists trained the mice to detect near-infrared patterns (horizontal and vertical lines) and shapes (squares and triangles) to flee from a water-submerged path. Mice with modified photoreceptors made the great escape and the ‘normal’ mice could not.

Third, the scientists applied a tiny electric shock to the mice’s feet accompanied by a flash of infrared light. The mice with enhanced retinae learned to associated the shock and the light, and reacted to the near-infrared light. The ‘normal’ mice didn’t react to the near-infrared light at all.

Credit: Leslee Lazar

Click to enlarge. Credit: Leslee Lazar

These tests show beyond doubt that the nanoparticles have enhanced the mice’s visual perception and behaviour.

Near-infrared perception is useful. Light of such wavelengths connotes heat, and can give away the presence of live, warm-blooded animals in the dark. However, the injectable nanoparticles can also be used to repair retinal damage, design visual prostheses and even deliver genes to fix deficiencies. The possibilities seem endless.

As cool as this invention sounds, it’s humbling to realise that nature remains ahead of us, as usual. Some fish and amphibians can naturally ‘shift’ the sensitivity of their photoreceptors. They change the chemical configuration of their light-detecting molecules to a different shape so they can perceive near-infrared light.

Researchers have seen this happen in aquatic animals. The spectral distribution of light in fresh water is more on the infrared side than in marine and terrestrial ecosystems. So when a salmon moves from marine to aquatic environment, it switches its photoreceptors to make it more sensitive to near-infrared light – without any fancy nanotechnology.

Leslee Lazar is a cognitive neuroscientist and a visual artist. He currently teaches at IIT Gandhinagar and tweets @leslee_lazar.