Vision is a complex process that has been successfully deciphered by many disciplines – physics, biochemistry, physiology, neurology, etc. -: the retina captures light, the optic nerve transmits electrical impulses to the brain, which ultimately generates the perception of an image. Although this process takes some time, recent studies have shown that the first stage of vision, the perception of light itself, is extremely rapid. But the analysis of this decisive step was carried out on molecules in solution in the laboratory. Scientists from the University of Geneva (UNIGE), in collaboration with EPFL and the University Hospitals of Geneva (HUG), Switzerland, reproduced the experiment on mice, in order to observe the treatment of light by a living organism in all its complexity. This non-invasive study shows that light energy alone does not define the response of the retina. Its shape – short or long – also has an impact on the signal sent to the brain to form an image. This discovery, published in the journal Science Advances, opens a new field of research on vision, diagnosis and possibly new curative possibilities.
The cellular mechanism of vision has been studied with success thanks to the collaboration of several disciplines. “In the eye, the first stage of vision is based on a small molecule – the retina – which, on contact with light, changes shape”, explains Geoffrey Gaulier, researcher in the applied physics department of the Faculty of Sciences of UNIGE and first author of the study. “When the retina changes its geometric shape, it triggers a complex mechanism that will result in a nerve impulse being generated in the optic nerve.”
This process takes some time between when the eye perceives light and when the brain decodes it. Physicists looked at the very first molecule in the chain, the retina, to see how long it took to change shape. They isolated this molecule in a cuvette and subjected it to laser pulses to test its reaction speed. To their surprise, the molecule reacted in about 50 femtoseconds! “By way of comparison, a femtosecond against a second is equivalent to a second compared to the age of the Universe”, underlines Jean-Pierre Wolf, professor in the physics section of UNIGE and the last author of the research. “It’s so fast that we wondered if this speed could be achieved by the molecule only when it was isolated, or if it had the same speed in a living organism in all its complexity.”
Light intensity and shape define the sensitivity of the eye
To study this first stage of vision in detail, the scientists called on biologists, in particular Ivan Rodriguez and Pedro Herrera, respectively professors of the faculties of science and medicine at UNIGE, who placed a contact lens and produced an electroretinogram in mice. “This completely non-invasive method makes it possible to measure the intensity of the signal sent to the optic nerve,” continues Jean-Pierre Wolf. When the light reached the retina, they were able to observe an electrical voltage in the cornea, thanks to an electronic amplifier. And their results showed that this step took place at the same extreme speed as when the molecule is isolated!
The team continued the study 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 sharp, etc.”, explains Geoffrey Gaulier. Indeed, changing shape should not induce any variation in the response of the retina, because until now it was thought that only the number of photons picked up by the eye played a role. “But this is not the case!” says the Geneva-based researcher. This result could be explained with the help of computer simulations carried out in the group of Ursula Röthlisberger at EPFL.
Scientists have observed that the eye does not react in the same way depending on the shape of the light, even though the light energy is the same. “We also discovered that the reaction of the eye differed depending on the order in which the colors were varied, for example as in a temporal rainbow, even if they followed each other extremely quickly”, continues Jean-Pierre Wolf. . In short, the retina believes that there is more or less light depending on the shape of the light, while the energy is similar, and therefore sends a more or less strong current to the brain depending on its response.
This discovery, made as part of a Sinergia project of the Swiss National Science Foundation (SNSF), opens up a new field of research on vision. “Now that we know that the shape of light plays a role in perception, we can use this new knowledge to make the eye function differently,” suggests Jean-Pierre Wolf. Areas of investigation into new possibilities for diagnosing or possibly treating eye weaknesses can now be developed.
Source of the story:
Material provided by University of Geneva. Note: Content can be changed for style and length.