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