The Shape of Light Restructures Our Understanding of Vision

Recent work has shown that the process involving the perception of vision occurs very quickly. However, because analysis of this step has been carried out on molecules in a laboratory environment, only limited conclusions can be drawn.

Now, researchers from the University of Geneva (UNIGE), École polytechnique fédérale de Lausanne (EPFL), and the University Hospitals of Geneva have used mice to examine the process in vivo. The noninvasive study demonstrated that light energy alone does not define the response of the retina when it captures light. The shape of light — short or long — also affects the signal sent to the brain to form an image.

The discovery opens avenues of research into vision, diagnostics, and, potentially, new curative possibilities.

“In the eye, the first stage of vision is based on a small molecule — the retinal — which, on contact with light, changes shape,” explained Geoffrey Gaulier, researcher at the Applied Physics Department of the UNIGE Faculty of Science and first author of the study. “When the retinal alters its geometric form, it triggers a complex mechanism that will result in a nerve impulse generated in the optic nerve.”

Artist’s view of femtosecond laser pulses arriving in an eye. Courtesy of Scientify, UNIGE.

This process isn’t instantaneous. There is a short amount of time between the moment the eye perceives the light and the moment the brain decodes it. The researchers took a look at the very first molecule in the chain, which is retinal, to see how long it took to change shape. They isolated the molecule in a cuvette and subjected it to laser pulses to test reaction speed. The test showed the reaction speed to be about 50 femtoseconds.

“By way of comparison, one femtosecond compared to one second is the equivalent of one second compared to the age of the universe,” said Jean-Pierre Wolf, a professor at UNIGE and an author of the paper. “This is so fast that we wondered whether this speed could be achieved by the molecule only when it was isolated, or whether it possessed the same speed in a living organism in all its complexity.”

The physicists called on biologists Ivan Rodriguez and Pedro Herrera, professors at the UNIGE Faculties of Science and Medicine, to continue experimentation. The team placed a contact lens and performed an electroretinogram on mice.

“This method, which is totally noninvasive, makes it possible to measure the intensity of the signal sent to the optic nerve,” Wolf said.

When the light came into contact with the retina, the team observed an electrical voltage at the cornea, thanks to an electronic amplifier. The results showed the same speed as the isolated test. The team continued by varying the shape of the pulses over time.

“We always send the same energy, the same number of photons, but we change the shape of the light pulse. Sometimes the pulse is short, sometimes long, sometimes sliced, etc.,” said UNIGE researcher Geoffrey Gaulier.

Team members did not initially believe that changing the shape of the light would introduce any variation in the retina’s response — only that the number of photons captured by the eye played a role.

“But this is not the case,” Gaulier said.

With the help of computer simulations performed in the group of Ursula Röthlisberger, the team was able to gather clues as to why. The researchers observed that the eye did not react in the same way depending on the shape of the light, even though the light energy was identical.

“We also discovered that the eye’s reaction differed according to the order in which the colors were varied, for example as in a temporal rainbow, even though they follow each other extremely quickly,” Wolf said. The retina believes that there is more or less light, depending on the shape of light, while the energy is similar, and therefore sends a stronger or weaker current to the brain depending on its response.

The researchers believe the discovery will lead to new research fields into vision — potentially leading to new possibilities for diagnosing or potentially treating eye weaknesses.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.abe1911).