Catching light
Written by Eric James McDermott
Tamaar( flickr. com)
Do you see it? Look around. If you can see— if you can see anything at all— from the words you are reading to the trees outside to your steaming cup of coffee, it is because you have, among other things, a fully functioning retina. The retina is a light-sensitive tissue that is involved in receiving and processing sensory information. As you may very well know, light is composed of a myriad of different wavelengths; the ones we can see generally range from about 400 nanometers, which is perceived as violet, to around 700 nanometers, which is perceived as red. Isaac Newton famously brought attention to these components of sunlight during his prism experiment in the 17th century. He saw these colors the same way you or I would.
Light first passes through the lens and is bent toward the back of the eye where it encounters the multiple layers of retinal cells. These cells transduce light into electrical signals that propagate along the optic nerve until they meet the lateral geniculate nucleus. The signals then travel onward to the visual cortex, then continue to higher visual areas and then work their way to the frontal cortex. This pathway is not only complex, but also extremely layered with each layer playing a role in visual perception. Some fascinating case studies help elucidate some of the functions of these layers, for instance, there is a report of a
woman who had damage to an area known as“ V5 / MT” and the world for her became still( 1). Imagine: everything you see appears without motion. Blink. Everything is in a slightly altered position. Blink. Again, life became for her but a series of snapshots with gaps in continuity. She reported severe difficulties with everyday activities like crossing the road or pouring a cup of tea. This story is one of many, and the blueprints to our visual architecture are slowly, but surely, becoming uncovered. However, even with the building plans in hand, everything we see hinges on one crucial layer of cells in the retina: the photoreceptors.
Figure 1: Left: An optical coherence tomography image of a healthy retina, Right: An optical coherence tomography image of a retina with retinitis pigmentosa. Image source: Eric McDermott
Photoreceptors have a very important job: to encode light and transduce it into electricity. Photoreceptors are broken into two main cell groups, rods and cones. The rods afford us our light sensitivity, whereas the cones afford us our color vision and our visual acuity. These cells have intrinsic lightsensitive mechanisms that upon activation begin a molecular cascade of activity along the visual pathway. Without them, we are effectively blind, and unfortunately, about 1 in 300 people are affected by retinal degenerative diseases leading to progressive photoreceptor loss( 2). This results in many people with intact cortical visual architecture, but without the ability to utilize it. Think of something akin to having a perfectly functioning car without the key to turn it on. This very situation is what researchers around the world are working to solve: how do we start the car? Researchers are exploring several methods: pharmacological methods( 3) and gene replacement therapies( 4, 5) focus on slowing down photoreceptor degeneration, while retinal implants( 6, 7), stem cells( 8, 9), photochemical ligands( 10), and optogenetics( 2, 11, 12) are ways to give the car a new key. The laboratories of Eberhart Zrenner in Tübingen and Günther Zeck in Reutlingen re-
24 | NEUROMAG | May 2017