That’s the good news. The bad news is that these devices have unavoidable limitations. Perfect 20/20 vision, for example, is out of the question. You need more electrodes to get a better resolution, which means making them smaller or more densely packed. If they get too small, they cannot provide enough charge to the underlying cells. If they get too close, the underlying cells cannot tell that they are separate points, and they might as well be one big electrode. “For electrical stimulation, we have a maximum limit of 20/100 vision,” says Zrenner – this means that from 7m (20ft) away, people could only read letters that were 10cm wide.
Colour vision is unlikely too. The retina contains three types of colour-detecting “cone cells”, which are sensitive to red, blue and green light. Each cone hooks up to a special double-pronged nerve cell called a bipolar cell, which conveys its signals towards the brain. The retinal implants stimulate these bipolar cells, but each electrode is so big that it triggers dozens at a time, including those corresponding to all three colours. The results are monochrome images of white and grey.
And do not expect these devices to remedy all conditions that can rob someone of their sight. Retinal chips can remedy those that partially damage the retina, but all the hardware in the head needs to work. That rules out anyone with glaucoma, one of the most common causes of blindness, as this damages the optic nerve that carries signals to the brain. Nor will the chips benefit people with damage to the visual centres of their brain, acquired through injury or stroke.
But that does still leave millions of people with retinal diseases who could benefit from the devices. Their sight should improve to the point where they could read, and get on with daily tasks. For them, cost is the limiting factor. Second Sight is currently asking around USD $100,000 for its device. For now, only the rich (and those on clinical trials) will see again but as with all electronics, costs will fall as popularity increases.
So, other sight-restoring techniques are sorely needed. Some scientists are trying to stave off the loss of retinal cells, either by bathing them in the right chemicals, or stimulating them with electricity. Others are focusing on replacing lost cells. The US firm Advanced Cell Technology has used stem cells to regenerate a patient’s dying retinal cells, which could be transplanted back into their eyes. In an early study, two patients who were been treated in this way showed “modest improvement in their vision”. Meanwhile, Jean Bennett from the University of Pennsylvania has used gene therapy to rewrite the faulty genes behind a rare inherited eye disease. In her pilot study, three women who were born virtually blind gained the ability to avoid obstacles and recognise faces.
These studies are preliminary ones, and it will take many more years to test the techniques for safety and effectiveness. But Zrenner is thrilled that such possibilities are even on the horizon. He says, “You once had the patient sitting there and you had to say, “Look, I know what the problem is down to the right molecule, but I can’t help you.“ Now, we can say, okay, there are some options.” The future is, it seems, bright.