The Brain of the Fly

Photo: Fly under microscope

A fly can see the movements of a flying soccer ball in slow motion. The minute brains of these aeronautic acrobats process visual movements in only fractions of a second. Just how the brain of the fly manages to perceive motion with such speed and precision is predicted quite accurately by a mathematical model.

Back in 1956, a mathematical model was developed that predicts how movements in the brain of the fly are recognized and processed. Countless experiments have since endorsed all of the assumptions of this model.

What remains unclear, however, is the question as to which nerve cells are wired to each other in the fly brain for the latter to function as predicted in the model. "We simply did not have the technical tools to examine the responses of each and every cell in the fly's tiny, but high-powered brain", as Dierk Reiff from the Max Planck Institute of Neurobiology explains. The brain area that is responsible for the fly´s motion detection is sixth of a cubic millimetre. Although it seems almost impossible to single out the reaction of a certain cell to any particular movement stimulus, this is precisely what the researchers have now succeeded in doing.

The electrical activity of individual nerve cells is usually measured with the aid of extremely fine electrodes. In the fly, however, most of the nerve cells are simply too small to be measured using this method. Nevertheless, since the fly is the animal model in which motion perception has been studied in most detail, the scientists were all the more determined to prize these secrets from the insect's brain. A further incentive is the fact that, albeit the number of nerve cells in the fly is comparatively small, they are highly specialized. Flies can therefore process a vast amount of information in their tiny brain about proper motion and movement in their environment in real time. So it's no wonder that deciphering this system is a worth-while undertaking.

"We had to find some way of observing the activity of these tiny nerve cells without electrodes", Dierk Reiff explains one of the challenges that faced the scientists. In order to overcome this hurdle, the scientists used the fruit fly and some of the most up-to-date genetic methods available. They succeeded in introducing the indicator molecule TN-XXL into individual nerve cells. By altering its fluorescent properties, TN-XXL indicates the activity of nerve cells. To examine how the brains of fruit flies process motion, the neurobiologists presented the insects with moving stripe patterns on a light-diode screen. The nerve cells in the flies' brains react to these LED light impulses by becoming active, thus causing the luminance of the indicator molecules to change. Although TN-XXL's luminance changes are much higher than that of former indicator molecules, it took quite some time to capture this comparatively small amount of light and to separate it from the LED-light impulse. The solution: The 2-photon-laser microscope needed to be synchronized with the LED-screen at a tolerance of merely a few microseconds. The TN-XXL signal could subsequently be separated from the LED-light and selectively measured using the 2-photon-microscope.

"At long last, after more than 50 years of trying, it is now technically possible to examine the cellular construction of the motion detector in the brain of the fly", reports a pleased Alexander Borst, who has been pursuing this goal in his department for a number of years.; Source: Max-Planck-Institut für Neurobiologie