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 Motional layers in the brain

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Date posted: 08/08/2013

Recognising movement and its direction is one of the first and most important processing steps in any visual system. By this way, nearby predators or prey can be detected and even one’s own movements are controlled. More than fifty years ago, a mathematical model predicted how elementary motion detectors must be structured in the brain. However, which nerve cells perform this job and how they are actually connected remained a mystery. Scientists at the Max Planck Institute of Neurobiology in Martinsried have now come one crucial step closer to this “holy grail of motion vision”: They identified the cells that represent these so-called "elementary motion detectors" in the fruit fly brain. The results show that motion of an observed object is processed in two separate pathways. In each pathway, motion information is processed independently of one another and sorted according to its direction.

Ramón y Cajal, the famous neuroanatomist, was the first to examine the brains of flies. Almost a century ago, he thus discovered a group of cells he described as “curious elements with two tufts”. About 50 years later, German physicist Werner Reichardt postulated from his behavioural experiments with flies that they possess “elementary motion detectors”, as he referred to them. These detectors compare changes in luminance between two neighbouring photoreceptor units, or facets, in the fruit fly’s eye for every point in the visual space. The direction of a local movement is then calculated from this. At least, that is what the theory predicts. Since that time, the fruit fly research community has been speculating about whether these “two-tufted cells” described by Cajal are the mysterious elementary motion detectors.

The answer to this question has been slow in coming, as the tufted cells are extremely small – much too small for sticking an electrode into them and capturing their electrical signals. Now, Alexander Borst and his group at the Max Planck- Institute of Neurobiology have succeeded in making a breakthrough with the aid of a calcium indicator. These fluorescent proteins are formed by the neurons themselves and change their fluorescence when the cells are active. It thus finally became possible for the scientists to observe and measure the activity of the tufted cells under the microscope. The results prove that these cells actually are the elementary motion detectors predicted by Werner Reichardt.

See the full Story via external site: www.mpg.de



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