The trick to restoring vision in people blinded by injury or disease may be to bypass the eyes entirely. By establishing a connection between a video device and the part of the brain that receives visual stimuli, researchers have shown that the brain can interpret electronic signals in the same way it interprets light waves.
For years, scientists have tried with limited success to provide sight to the blind via prosthetic devices. One approach is to stimulate the remaining healthy neurons in the retina, the light-sensitive lining of the inside of the eyeball, with miniature electrodes that mimic the effects of incoming light. But retinal tissue is so fragile that it is easily damaged. Another tactic involves inserting microelectrodes into the primary visual cortex--the main part of the brain responsible for processing visual signals-- and stimulating visual nerve cells with electrical impulses. So far, however, no one has been able to achieve more than simple behavioral responses in test animals because of the complexity of that cerebral area and the nature of visual signals.
A team from Harvard Medical School has tried a new approach. Reporting online this week in Proceedings of the National Academy of Sciences, the researchers describe how they got lab animals to track preselected artificial visual signals with their eyes--just as though they were watching lights flashing on a real video screen--by precisely inserting two minute electrodes into the lateral geniculate nucleus (LGN) of the thalamus. This part of the brain acts like a relay station for visual information. All signals from the eyes run through the LGN to the visual cortex. The experiment used sighted monkeys so the researchers could compare their responses to real images with artificial signals, says neuroscientist and co-author John Pezaris. Although the team only used two electrodes as proof of concept, he says, the monkeys followed both the real and artificial signals in exactly the same way.
If further animal research is successful, Pezaris says his team hopes to move on to work with human volunteers, using implants containing more and more electrodes to transmit visual signals of increasing complexity. Eventually, the procedure could lead to a full-fledged artificial vision system comprising twin digital video cameras worn as a pair of glasses that transmits signals wirelessly to an implanted neural stimulator, which in turn connects to micro-electrodes planted in the brain.
The research represents "a fundamental advance toward a visual prosthesis," says neurobiologist Nicholas Hatsopoulos of the University of Chicago in Illinois. For one thing, he says, the thalamus is easier to stimulate than the retina and is much less prone to tissue damage. Furthermore, current neurosurgical procedures, such as those used to treat diseases such as Parkinson's, "could be modified relatively easily to stimulate the thalamus."