Scientists Figure Out Retinal Code

In the Western world, the two most common degenerative diseases of the eye are Retinitis Pigmentosa (RP) and Age-Related Macular Degeneration (AMD). These can be very debilitating conditions and, in their worst forms, these eye diseases can cause the sufferer to gradually go blind. These diseases target the super-fine layer of cells at the back of our eyes which help us to see, the retinal ganglion cells (RGC) of the retina.

A person who lives with with either of these eye conditions gradually loses the use of these RGCs and without them, slowly goes blind. But recently, scientists and ophthalmologists across the globe believe that they have cracked the retinal code and, with the use of some nifty neuroengineering and ophthalmic genetics, are on the way to restoring the sight to tens of millions of people who have lost their vision in this way.

How Does the Eye Work?

Our eyes see by using a complex pathway of different types of cells which fire off nerve signals to each other in rapid succession. To demonstrate how fast these signals fire, imagine that you switch on a light bulb. How quickly do you see the light? It is the time taken for the light to reach your eye, and for that signal to travel along cells in the retina, optic nerve, into the brain and then to the back of your head where the visual system is found. That’s fast!

The retina is made up of around 15 million RGCs and other types of cells which all feed in to the optic nerve. The optic nerve is a long insulated pipe of about a million cells, which constantly sends the signals from all the different types of cells which are responsible for vision.  Think of the optic nerve as a big electricity cable which is insulated. Inside, the copper wires conducting the electricity are like the long axons of the optic nerve fibres, except that there are about a million in each eye.

Now think about the different types of information about our environment that these cells send to our brains. Look around for a second, you take in information about:

• Light and darkness in the environment
• Colors
• Visual textures
• Hues and shades
• Movement
• Distance and spatial relationships
• Patterns

This is just the beginning of the story. A visual scientist could list many more factors the eye has to be able to see for a person to perceive their environment entirely.

AMD and RP

The area of the macula in the retina is where we see fine detail. When we focus our vision on something, we are using the macula and this has the highest number of retinal cells. With AMD, degeneration of these cells can occur with age and it means that we can no longer focus on things, but retain our peripheral vision. Some scientists believe that this may be due to ‘light toxicity’, and many types of medicines are available to slow down this change.

RP is the name given to a collection of inherited diseases of the retina which begin with the loss of peripheral vision and night blindness, but can end with a loss of even central vision. Inherited genetic defects affect at least 40 different genes which are responsible for making sure that the cells in the eye and retina function properly.

Giving Vision to the Blind

Neuroengineering

Once some of these diseases become completely understood, then it is possible to start to work on developing a cure. Another key to begin to explain this process is the miniaturization of technology. We could not have done this twenty years ago, even though scientists (and sci-fi writers) were dreaming of the advent of the sort of technology which could make it possible.

A Californian company called Second Sight has developed an external visual system which enhances the eyesight of retinally damaged patients.

It consists of a pair of glasses wired up to detect visual stimuli which send impulses to an array of receptors which are surgically attached to the retina. These receptors only consist of a 6 by 10 array right now, but no doubt as this technology is enhanced, the number of receptors can grow into the millions of cells they replace.

The receptors then send signals in the usual way along the optic nerve to reach the brain. Such signals at present allow a legally blind person to be able to detect changes in light and dark or large movements in front of them and even be able to count objects placed in front of them, and it is a good start.

Optogenetics

Sheila Nirenberg, a neuroscience professor in Cornell University and one of her students, Chethan Pandarinath, have recently demonstrated breakthroughs in the field of optogenetics.

Optogenetics can be used to target single populations of cells in the retina. For example, Nirenberg has been experimenting with a group of retinal cells which only express a certain type of chemical, in this case channelrhodopsin-2 (ChR2). They are able to do this because the different types of visual stimulus that we discussed earlier are all decoded by a certain type of cell in the retina. So, for example, a group of cells respond when a light is switched on – and only on. Another group responds when a light is switched off. And so on. We have Nobel Prize winning scientists Hubel and Wiesel to thank for these painstaking discoveries.

Nirenberg and Pandarinath infect only a certain type of retinal cell with a mutated version of ChR2 which is a chemical which responds to blue light. A prosthetic device attached to the retina is then able to stimulate this group of cells into producing action potential (nerve transmission) in the same way that undamaged cells do. Amazingly, because of the advances in understanding the retinal code, it does this reliably and efficiently, so a person can not only see rough movements, but also landscapes, animals, and even recognise faces.

These amazing advances can only get better as our technology improves and shrinks. Watch this space!