Random Walk
This fruit fly has a dye-filled glass electrode (pink) inserted into its brain. The fly’s head is clamped to the underside of a reservoir filled with a sterile saline solution (colored blue here) that bathes the electrode and the brain. At rest, the fly clings to the reservoir; a gentle puff of air starts it flapping its wings in tethered flight.
Getting Inside a Fly's Head
What goes on in the tiny brain of a fruit fly? We’re beginning to find out, now that Michael Dickinson, the Zarem Professor of Bioengineering, and postdocs Gaby Maimon and Andrew Straw have succeeded in recording the activity of individual brain cells as the fly flies. This is no mean feat, considering that each fly is only about 2.5 millimeters long.
“Researchers have recorded the neural-cell activity of fruit flies before, but only in animals that had been stuck or glued down,” Dickinson explains. “Gaby was able to develop a preparation where the animal is tethered”—its head clamped into place—“but free to flap its wings.” By slicing off a patch of the hard cuticle covering the brain, “we were able to target our electrodes onto genetically marked neurons,” he says. As the electrodes took data, high-speed digital cameras simultaneously recorded the flies’ behavior.
The study focused on a set of visual-system neurons that “basically help the fly detect when its body posture changes” in order to maintain stable flight, Dickinson says. When the wings started flapping, these cells immediately ramped up their activity. “The neurons’ responses to visual motion roughly double when the flies begin to fly, which suggests that the system is more sensitive during flight,” Dickinson says. “The increase is very abrupt. It’s not at all a subtle change, and so we suspect that there is a neurochemical quickly released during flight that sets the animal’s brain in this different state.”
Previous studies in locusts—which are far bigger and thus far easier to study—had suggested the existence of this effect. However, the genetics of locusts are not nearly as well understood as those of Drosophila. Now, says Dickinson, it should be possible to “figure out specifically what causes the change in sensitivity. Is the system turned off when the fly is on the ground? What neurochemicals are involved? We can use all the genetic tricks that are available in fruit flies to get a better idea of what is going on.”Adds Maimon, “Sensory neurons in many species—including birds, rodents, and primates—change their response strength depending on the behavioral state of the animal, but why these changes take place is not entirely clear.”
The researchers also plan to spy on olfactory and motor cells to see if they display similar behavior.“The question is, ‘Is the entire brain completely different in flight?’” Dickinson says. “We suspect that this phenomenon is not unique to the visual cells we have studied. Most cells care whether the animal is flying or not.”
A paper describing the research was published in the March issue of Nature Neuroscience; the work was funded by the National Science Foundation and a Caltech Della Martin Fellowship.—KS
