The work takes advantage of a gene called halorhodopsin found in a bacterium that grows in extremely salty water, such as the Great Salt Lake in Utah. In the bacterium, Natronomas pharaonis, the gene codes for a protein that serves as a light-activated chloride pump, which helps the bacterium make energy.

When neurons are engineered to express the halorhodopsin gene, the researchers can inhibit their activity by shining yellow light on them. Light activates the chloride pumps, which drive chloride ions into the neurons, lowering their voltage and silencing their firing.
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The group also plans to use the new method to study neural circuits. Last year, Boyden devised a technique to stimulate neurons by shining blue light on them, so with blue and yellow light the researchers can now exert exquisite control over the stimulation and inhibition of individual neurons. Learning more about the neural circuits involved in epilepsy could help scientists develop devices that can predict when a seizure is about to occur, allowing treatment (either shock or light) to be administered only when necessary, Boyden said.