Ars Technica

Despite its small size and relatively compact nervous system, the standard lab fruit fly Drosophila manages to engage in some fairly sophisticated behaviours, including a courtship ritual. Researchers have also trained the Drosophila to associate an irrelevant event with an unpleasant experience, causing the fly to form lasting aversions to the event.

Now, researchers have managed to bring all that together and mix it with computing, molecular biology, and lasers. The end result is a system that tracks the flies in real time and targets laser pulses to specific body parts. It can be used to create averse associations so strong that male flies will flee an otherwise available female.

The system itself is pretty impressive. The optical input is provided by a camera that captures a small "fly arena" at 40fps and 512 x 512 pixels. That input is fed into a computer that can track two flies placed in the arena and figure out their bodies' orientation and their wings' positions.

That's pretty impressive on its own, but the system also has a business end in the form of blue and yellow lasers. The computer system controls the shutters that allow the lasers to target the fly arena, as well as mirrors that provide fine control over where the laser beam strikes. The research team has even created a convenient GUI that lets users select which of the conditions should trigger a laser pulse, which of the two lasers to use, which fly to target with, and where on the fly's body to strike.

The system manages to do all of that with only a 25 ms delay between a triggering activity and the laser striking the fly.

When the blue laser is cranked up to sufficiently high power, the Drosophila find that being zapped by it is an unpleasant experience. To test the system, the researchers drew an invisible line on the floor of the arena and chose one of the two flies. If it ever crossed the line, it got zapped. (The second fly, which wasn't targeted, provided a convenient control.) With a bit of time for training, the fly quickly learned to avoid that side of the arena, even though there weren't any obvious landmarks. The avoidance persisted even after the laser was turned off.

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But the system was put to more perverse uses in an experiment the researchers called "restraining order." In this case, the two flies in the arena were male and female, and the system was programmed to zap the male if it ever got within a small distance of the female. Normally, given the chance, male flies would attempt to start the courtship ritual with the female, but getting zapped with a laser whenever they got close gave them other plans. By the end of the training session, the male would flee if the female even approached it.

The researchers showed that the system could do more sophisticated things than simply inflicting discomfort on the flies. The field called optogenetics relies on light-sensitive pores that allow ions into cells. When these pores are expressed in nerve cells, they allow light to short-circuit the normal control of nerve activity. Hit the cell with the right laser and the pores open up, allowing ions to flood in and cause a response just like a normal nerve impulse.

The researchers used flies that expressed one of these channels in specific nerve cells and showed that exposure to the right light could cause the fly to get very jumpy. Literally -- it triggered the nerve response that normally tells the fly to jump. They were also able to use a second channel, triggered by the yellow laser, that suppressed the jumping response. Right now, they can't separate the activation of nerves from the bright blue laser that the flies find unpleasant, but they hope to use a different channel that's sensitive to some other wavelength of light.

Overall, this is really an impressive bit of work, and it should make studying behaviour in a simple system like Drosophila very convenient. Still, I find myself hoping that the researchers don't get interested in trying to get their system to work in mammals.

This article originally appeared on Ars Technica.


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