Seeing the light

People who suffer from the most common causes of blindness could one day see again with glasses that use an imaging chip and a high-density LED array.

The glasses would detect light, process the information and project visual signals on to genetically altered neurons inside the sufferer’s eyes. These neurons, called retinal ganglion cells, receive and transmit visual information from the retina to visual areas of the brain.

The technique could enable sufferers of retinitis pigmentosa, age-related macular degeneration and diabetic retinopathy to see. In the advanced stages, retinas with these disorders can no longer detect light, which means they no longer have usable rods and cones, but all their remaining neurons, such as the ganglion cells, survive. This raises the possibility that a retinal prosthetic could bypass the diseased tissue and stimulate the remaining healthy cells.

A lead investigator in the project, Mark Hankins, Professor of visual and neuroscience at Oxford University, said that without rods and cones, ganglion cells receive no information to output to the visual areas of the brain.

However, scientists have proved that a small percentage of these cells cells are light sensitive. With gene therapy, a patient’s entire population of ganglion cells could be turned into light-sensitive neurons.

A discreet prosthetic in front of the patient’s eyes would capture images with a camera and process the information. It would then stimulate ganglion cells by outputting a series of bright, intense light spots to them.

Even though the patient’s ganglion cells will be altered so that they can detect light on their own, their retina will still need an external prosthetic. The cells will require a certain wavelength and intensity of light that needs to be controlled. The prosthetic will also use retinal algorithms to replace the visual processing lost in the diseased retinal tissue.

‘So essentially we’ve put all the engineering outside the eye, and the device will attempt to talk to the retinal ganglion cells through optical communication,’ said Hankins.

Until now, researchers have tried to develop retinal prosthetics that would be implanted inside patients’ eyes, but this technique has encountered many technical challenges.

The implanted devices would be designed to electrically stimulate the ganglion cells, but it is difficult to provide the amount of power needed to do this effectively.

‘Devices in the eye with eight electrodes would provide some visual sensation but not real vision,’ said Hankins. ‘You’d need hundreds of electrodes to provide real vision, and it would be difficult to incorporate that amount of electrodes in an implanted device.’

The power-consumption issue is easier to handle outside the eye. Hankins suggested that the vision glasses could be connected to a battery pack the size of an iPod that could be charged overnight.

Power consumption is not the only concern, said Hankins — the idea of implanting a silicon chip inside a person’s eye is also problematic.

The only invasive part of Hankins’ technique is the gene therapy required to change a patient’s ganglion cells into light-sensitive neurons. The approach here is likely to use a vector (energy carrier) to inject genetic information into the eye.

‘Gene therapy has been used successfully to make cells express proteins that they don’t normally express,’ he said.

Hankins cautions that human trials for this technique are still many years away. First, the researchers will test their devices with ganglion cells grown in a Petri dish.

But it is likely Hankins’ external retinal prosthetic device will be a reality sooner than other implantable retinal prosthetics being developed. ‘With implantable devices, there are intractable engineering problems,’ he said. ‘The kind of engineering we’re talking about with our device is not science fiction. We can build that device relatively quickly. The problem now is with the biology — can we make the neurons light-sensitive?’

Hankins said he is encouraged by the progress of technology in that area. ‘At the same time our project is moving along, the light simulation technology is advancing,’ he said. ‘We want to use the advances in generating light-sensitive neurons together with the prosthetic approaches. Then we will be able to bring this whole thing together at an appropriate stage.’

The opto-bionic approach to restore vision is being assisted by the Wellcome Trust Centre for Human Genetics and a £350,000 EPSRC grant.