November 23, 2015

No cable spaghetti in the brain

Randomly wired? A fluorescence microscope reveals the largely random network,
that neurons form in a culture dish. An international team of scientists, led by researchers
from the MPI for Dynamics and Self-Organization, investigated whether nerve cells
in the brain wire randomly.
© Manuel Schottdorf, MPI for Dynamics and Self-Organization

(November 23, 2015)  The brain is not relying on random-wiring, but self-organized neural networks for visual information processing

Our brain is a mysterious machine. Billions of nerve cells are connected such that they store information as efficiently as books are stored in a well-organized library. To this date, many details remain unclear, for instance the set of rules that governs the connections between nerve cells and the organization of information therein. An international team led by scientists of the Max Planck Institute for Dynamics and Self-Organization in Göttingen has now shed new light on these long-standing questions and found that networks are not governed by randomness. Some scientists considered this possibility for the organization of the brain, because randomly connected networks work well in computing applications. To test the random wiring hypothesis, the team examined whether the brain is using random connections to process visual stimuli. To this end, they calculated predictions that rely on the random wiring hypothesis and compared the results with precision measurements of the cortical architecture in various mammals. The result: random connections do not suffice to explain the observed layout of the brain. The scientists conclude that initially random connections in the visual cortex are reorganized to a precisely determined layout using self-organization. Random wiring, in the end, plays a small role.

Nerve cells in the human brain are densely interconnected and form a seemingly impenetrable meshwork. A cubic millimeter of brain tissue contains several kilometers of wires. A fraction of this wiring might be governed by random mechanisms, because random networks could at least theoretically process information very well. Let us consider the visual system: In the retina, several million nerve cells provide information for more than 100 Million cells in the visual cortex. The visual cortex is one of the first regions of the brain to process visual information. In this brain area, various features as spatial orientation, color and size of visual stimuli are processed and represented. The way information is sent may be comparable to a library, in which books can easier found if they are sorted not only alphabetically by title, but also by genre and by author. In a library, books are spread to different shelves, but typically not randomly. Similarly, various facets of visual perception are represented separately in the visual cortex. And the organization of this representation might be random. Mathematical modeling suggested that randomly distributed information is very well suited to separate features, in fact, better and better the more features are concerned.

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