Neuromag May 2017 - Page 24

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. Photoreceptors have a very impor- tant job: to encode light and trans- duce it into electricity. Photoreceptors are broken into two main cell groups, rods and cones. The rods afford us our light sensitivity, whereas the cones af- ford us our color vision and our visual acuity. These cells have intrinsic light- sensitive mechanisms that upon ac- tivation 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 degen- erative 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 ex- ploring several methods: pharmaco- logical methods (3) and gene replace- ment 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 laborato- Figure 1: Left: An optical coherence tomography image of a healthy retina, Right: An optical co- ries of Eberhart Zrenner in Tübingen herence tomography image of a retina with retinitis pigmentosa. Image source: Eric McDermott and Günther Zeck in Reutlingen re- Light first passes through the lens and is bent toward the back of the eye where it encounters the multiple lay- ers of retinal cells. These cells trans- duce light into electrical signals that propagate along the optic nerve until they meet the lateral geniculate nu- cleus. 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 path- way is not only complex, but also ex- tremely layered with each layer play- ɽ٥ՅɍѥM)͍ѥ͔Ց́Սє)ͽѡչѥ́ѡ͔)̰ȁхѡɔ́ɕЁ(Ё9UI=55)ݽݡѼɕ)ݸ̃qXԽ5Stѡݽɱ)ȁѥĤ%聕ٕ)ѡԁ͕́ݥѡЁѥ) ٕѡ́ͱѱ䁅ѕɕ)ͥѥ )ȁЁ͕ɥ́͹́͡ݥѠ)́ѥե丁Mɕѕ͔)ٕɔձѥ́ݥѠٕ呅䁅ѥ٤)ѥ́ɽͥѡɽȁɥ)ѕQ́ѽ䁥́)ѡՕɥ́Ѽȁ٥Յɍ)ѕɔɔͱݱ䰁Ёɕ䰁)չٕɕ!ݕٕȰٕݥѠѡ)եٕ́ѡݔ)͕́Սȁ)ѡɕѥѡѽɕѽ