Wednesday, February 21, 2007
Prosthetic retinas better than expected
New Scientist reports: [edited]
Profoundly blind people could get their best shot yet of restored vision with a more advanced "bionic eye", researchers have announced.
Trials of the new retinal prosthesis will begin shortly, following the success of a prototype that has enabled six blind people to see again. The prototypes were fitted in 2002 to patients who had lost their sight entirely.
Within a few weeks all could detect light, identify objects and even perceive motion again. For one patient, this was the first time he had seen anything in half a century, after his sight was destroyed by retinitis pigmentosa, a virus that attacks retinal cells.
"We hoped they might get some sense of light and dark, but it's really amazing how much they can see - how the brain is able to fill in the gaps," says Mark Humayun, who carried out the implant surgery and developed the device with colleagues at Doheny Eye Institute at the University of Southern California in the US.
For the technique to work, the patient must still have some functioning ganglion cells - nerve cells that transmit visual information from the retinal cells to the optic nerve - as well as a fully-functioning optic nerve. A tiny electronic pad is placed onto the retina of one eye, so that the electrodes are in direct contact with the ganglion cells. Each of the devices' 16 electrodes can stimulate 20 to 30 cells.
The user wears a pair of glasses that contain a miniature camera and that wirelessly transmits video to a cellphone-sized computer in the wearer's pocket. This computer processes the image information and wirelessly transmits it to a tiny electronic receiver implanted in the wearer's head.
When received by the implanted chip, the digital information is transformed into electrical impulses sent into the ganglion cells. From there, the brain takes over as the information travels down the optic nerve to the visual cortex at the back of the brain. The whole process occurs extremely rapidly, so that patients see in real-time. This is important any noticeable lag could stimulate the "vestibular-ocular reflex", making people feel dizzy and sick.
Humayun's team is about to embark on a new trial of an improved device, which they will fit into 50 to 75 people aged over 50, who are also blind as a result of retinitis pigmentosa. The trial will involve monitoring them for two years and will take place in five centres across the US.
The first implant had just 16 electrodes on the retinal pad and, as a result, visual information was limited. The new device has 60 electrodes and the receiver is shrunk to one-quarter of the original's size. It is now small enough to be inserted into the eye socket itself.
Regaining sight has felt like a miracle to those involved in the preliminary trial. At the beginning, it was like seeing assembled dots - "now it's much more than that," says Terry Bryant, aged 58, who received the implant in 2002 after 13 years of blindness. "I can go into any room and see the light coming in through the window. When I am walking along the street I can avoid low hanging branches and I can cross a busy street."