Ever since the late 1700s when the Italian physician Luigi Galvani found that static electricity could induce a deceased frog’s leg to move, scientists have – to paraphrase Whitman – ‘sung the body electric’. In the years that have followed, researchers including Hodgkin, Huxley and Katz, to name but a few key players – these men are to electrophysiology as David Beckham’s left foot is to football – have sought and succeeded to elucidate the mechanisms by which our peripheral nerves, and at their origin, our brain and spinal cord, function on electricity. With the field having progressed in leaps and bounds, a number of their contemporaries are now concerned with illuminating the contributions of individual neurons, using perhaps the most quintessential – in the eyes of humans, at least – of electromagnetic radiation: light. The technology in question, named optogenetics, is in simple terms brain control in a flashlight, allowing particular groups of nerve cells to be turned on (or off) at will, without affecting the activity of adjacent neurons.

“Optogenetic implants might… offer patients a way to activate their own neurotransmitters to alleviate pain, epileptic seizures, and even Parkinson’s tremors”

The technique works by harnessing specific ion channels, a permutation of cellular proteins, that can act as light-activated molecular switches. These are inserted into a set of neurons by genetic manipulation, where they quietly sit shut, without any effect on the cells’ electricity activity, until the researcher chooses to open them by illumination with an intense pulse of laser light of an appropriate wavelength. The proteins utilised are known as opsins. When light shines on an opsin, it absorbs a photon and changes. This opens the channel, leading to an influx of positively charged ions that stimulate the cell into activity. Since both the timing and the duration of the light pulse can be controlled with precision, it is possible to mimic the activity of individual nerve cells and thus investigate how different patterns of activity influence behaviour.

The applications of such molecular machinery are likely to be multiple and far-reaching. As yet, some of the most tantalising experiments have been carried out by the neuroscientist Gero Miesenböck and his former colleagues at the Sloan-Kettering Cancer Center in New York City. Using one of the biologist’s most beloved test subjects, the common fruit fly, the researchers have found that by switching on a particular group of neurons with a light pulse, female flies could be coaxed into producing the male courtship song. It is, it seems, as if the fruit fly may have a ‘unisex’ brain that can be directed to produce different patterns of behaviour – male or female – by a few neuronal master switches.

The situation in the mammal is somewhat more complex, since such a modality does not have the ability to penetrate the skull. Instead, light must be delivered by a fibre-optic cable implanted in the brain. Experiments with this technology have so far concentrated on constellations of the neurons that are known to release dopamine, a neurotransmitter conferring pleasurable sensation. By infecting mice with a protein that would make them react to light and implanting a cable, neuroscientists have observed that mice let loose in a maze with a ‘dopamine button’ (sending a signal that turned on the light in their brain and triggered a rush of the hormone) will return almost obsessively it: they gain, and liberally employ, the ability to activate neurotransmitters that gave them satisfaction. Whilst there is an undeniable leap between mouse and human, it is not unreasonable to believe that such implants in people might one day offer patients a way to activate their own neurotransmitters to alleviate pain, epileptic seizures, and even Parkinson’s tremors. For optogenetics, the future is undoubtedly bright