Surely, everybody knows this phenomenon: an animal doesn't stand out against its background and becomes visible to us only when it moves. The reason behind this is that we depend strongly on our eyesight for navigation, and the perception of motion is particularly well developed. But what exactly happens in the brain during this process? How must the nerve cells be interconnected for movements to be recognized as such? Scientists at the Max Planck Institute of Neurobiology in Martinsried have now established that two different motion detectors are required for this process in the fly brain.
The fact that we are able to see movement may sound trivial. After all, we perceive motion the very second we open our eyes. However, we are only beginning to grasp the complexity of the nerve cell circuits necessary to perceive motion. Decoding these circuits down to the cell level, that is the aim of Alexander Borst and his team at the Max Planck Institute of Neurobiology. Yet it is not the human brain, with its approximately one hundred billion nerve cells, on which they concentrate, but the brain of the minute fruit fly Drosophila.
Flies and humans have more in common than meets the eye
Despite being less than half a millimetre in size, the brain of the fruit fly is not only highly efficient but also fairly straightforward -- containing a "mere" hundred thousand nerve cells. Here, the scientists see a chance to succeed in breaking the nerve cell circuits down into their individual components. The findings are also relevant for humans since, when it comes to the brain, the difference between humans and fruit flies is not as great as one might expect. Just recently, the Martinsried neurobiologists have demonstrated that fruit flies process optical information in much the same way as all other vertebrates examined so far: information is broken down into different image channels immediately after the photoreceptors. While the photoreceptors react to every change in light, nerve cells in the next layer convey only "light-on" (ON) or "light-off" (OFF) alterations. But how does this enable the brain to calculate motion?
Neon signs in the fly cinema
To get to the bottom of motion perception, the neurobiologists used apparent motion to outsmart the fly's visual perception. In a sort of "fly cinema," the animals watched how first one and then an adjacent stripe in their visual field became brighter or darker. Anyone who has ever had the opportunity of watching the "moving" luminous advertising on New York's Times Square knows that, when stationary lights are switched on and off in quick succession, you gain the impression that movement is involved. In the fly cinema, the drosophila has the same perception.
The scientists chose the width of the stripes for the fly cinema such that only a small number of photoreceptors were stimulated. The fly sees an apparent motion when first one and then an adjacent photoreceptor perceives an ON- or OFF- contrast change. OFF-OFF impulses, for example, would indicate that a dark edge is passing across their visual field. But what happens when neighbouring photoreceptors report an ON-OFF or OFF-ON change? Is motion then calculated by two motion detectors (one for ON-ON and one for OFF-OFF) or by four detectors (i.e. one for each combination)?
Such a task seems almost too complicated just for two nerve cell detectors. However, the construction and maintenance of four circuits is much more elaborate than that of two. In other words, from the evolutionary point of view, two motion detectors are expected to be preferable to four. To establish whether this is the case, the neurobiologists recorded the electrical responses of those nerve cells that reacted to motion while the flies saw how the stripes changed contrast in the fly cinema. In addition, the scientists also conducted diverse computer simulations to predict and analyze the results. All these investigations came to the clear conclusion that the information about ON- and OFF-contrast changes is relayed to two motion detectors only.
The cells behind the detector
"This amounts to a scientific breakthrough," says Hubert Eichner, commenting the results of his study. "For over 50 years now, the scientific world has been trying to work out how many detectors are necessary in order to perceive motion." Now that the number of motion detectors has been determined, the neurobiologists can set about tracking down the cells that constitute these two detectors. Chances are good that the brain analysis of these tiny flies will help us to understand also our own motion perception in the future.
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