If your are wondering what exactly happens in the brain when we depend strongly on our eyesight for navigation and when the perception of motion is particularly well developed, here is the answer.
Scientists at the Max Planck Institute of Neurobiology in Martinsried have now established that two different motion detectors are required for this process.
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.
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.
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.
"This amounts to a scientific breakthrough", said 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," added Eichner.