THE next generation of neural implants might use lasers rather than electrodes to connect to the human nervous system. The technique is still in its infancy, but it holds out the promise of activating nerve fibres with exquisite precision.
The discovery that low-power pulses of near-infrared laser light can activate nerves was made by neurosurgeon Peter Konrad at Vanderbilt University in Nashville, Tennessee, together with his colleagues Anita Mahadevan-Jansen and her husband Duco Jansen. They repeated the classic 18th-century experiment of Luigi Galvani, using the laser pulses instead of electricity. 鈥淲hen we got a frog leg to twitch, I couldn鈥檛 believe it,鈥� Konrad says.
Doctors have already developed an extraordinary array of devices for communicating with nerves, from deep brain implants for controlling Parkinson鈥檚 to cochlear implants for restoring hearing. But these devices all rely on electrodes, whose signals activate all nearby neurons indiscriminately. The advantage of using laser light, delivered via fibre-optic cables, is that it can be aimed precisely, allowing single nerve cells to be fired.
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In tests on rats, the Vanderbilt team has shown the laser pulses can activate subsets of nerve fibres, Konrad reported at the Society for Neuroscience meeting in San Diego, California, last week. 鈥淲e can move a toe at a time,鈥� he says. To do this with a microelectrode would mean dissecting out the nerve fibres, whereas Konrad鈥檚 team had only to make a small cut to expose the sciatic nerve.
鈥淭his is certainly a neat idea,鈥� says Gy枚rgy Buzs谩ki, an expert on neural circuits at Rutgers University in Newark, New Jersey. But it is too early to say whether lasers will be safer than electrodes, he cautions. Long-term physical contact with electrodes can cause a reaction in nerve tissue, which means power levels have to be boosted to maintain the same effect. It is possible that prolonged exposure to laser light could also damage nerves. The rats show no signs of damage, but the longest tests have lasted only half an hour.
鈥淭hey got a frog leg to twitch by repeating Galvani鈥檚 classic experiment, using laser pulses insted of electricity鈥�
Many other key questions also remain unanswered. For starters, no one knows why the technique works. Nor is it known whether other wavelengths of laser light would work too, or if even normal light could have this effect.
The first use of the technique is likely to be in surgery and research, where brain tissue is routinely activated using electrodes to test its function 鈥� while removing a brain tumour, for instance. With electrical stimulation, the applied signal partly obscures the electrical response of the neurons. 鈥淚t鈥檚 the nemesis of neural researchers,鈥� Konrad says. Using laser light should eliminate this problem at a stroke.
Konrad hopes the technique could eventually allow far more sophisticated interfaces to be created. In a severed spinal cord, for instance, activating each of the 100,000 fibres individually would not be possible with electrodes, he says 鈥� but it might just become feasible using the laser approach.
