From Nature:
The seemingly complex phenomenon by which fruit flies (Drosophila) learn from bad experiences has been reduced to the actions of a mere 12 neurons, according to research by a team of UK- and US-based scientists. Manipulating this cluster of cells with a laser, the scientists were able to trick the flies into having associative memories of events they had not actually experienced. Flies learn from smells and other signals in their environment. Conditioning by, for example, electric shocks, can teach them to avoid certain odours.
Previous experiments had shown that a structure in the fly brain called the mushroom body was essential for storing those memories, but the mechanism by which those memories get stored has not been well understood. To examine the mechanism, a team led by the University of Oxford's Gero Miesenböck took advantage of “optogenetics”, a technique in which they use light to activate particular cell types that have been genetically engineered to express a light-responsive protein. When laser pulses hit the brain, cells expressing the light-sensitive protein activate. “It's like sending a radio signal to a city but only those houses with a radios set to the right frequency will get the signal,” says Miesenböck.
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