The script for brain development
Each of the two hemispheres of fish brains only processes information arriving from the opposing eye. Similar observations can be made in newborn mammals: Almost all neuronal inputs into the visual cortex originate from the opposing eye, it is only later in development that this task is reorganized until the two hemispheres finally process information from both eyes. Up to now there was no explanation for this phenomenon. Is this merely an evolutionary relic - similar to gills found in the embryonic development of humans? What is the purpose of this development? Researchers from the Max Planck Institute for Dynamics and Self-Organization, the Bernstein Center for Computational Neuroscience (BCCN) and the University of Jena are now able to answer these questions. "It is essential for the correct development of the visual center in the human brain" says Fred Wolf, head of the study. (Physical Review Letters, May 18 2009).
Cells in the visual cortex respond to selected optical elements, such as edges and contours. Every cell has a "orientation preference", it responds best to edges with a certain direction. Assigning the same color to cells with the same orientation, one gets a so called "orientation preference map". Usually nearby cells have a similar orientation preference. But there are exceptions to this rule, so-called "pinwheels": regions in which cells with different preferences meet, as if they were in the center of a windmill. This alignment allows correct analysis of visual data while keeping processing distances minimal, as there have to be neural cells closely connected to cells with the same or similar orientation preference, but also cells connected to cells with different orientation preferences.
Furthermore, cells in the visual cortex respond to stimuli that reach only one of the two eyes. The corresponding "ocular dominance map" changes in early stages of development. Scientists have been wondering for some time now how these maps get formed. "They emerge by self-organization of the cells, there is no engineer overseeing this development", says Fred Wolf from the Max Planck Institute of Dynamics and Self-Organization and the Bernstein Center for Computational Neuroscience in Göttingen.
Various models for the self-organization of the brain were able to explain the formation of the orientation preference map, but not its stability: Pinwheels with different orientation appeared to cancel out. Trying to understand the stability of pinwheels, the scientists analyzed whether the different maps somehow influenced each other. In their models, pinwheels try to keep as much distance as possible from the boundaries of the ocular dominance maps. Computer simulations showed that under these assumptions pinwheels survive. The ocular dominance map stabilizes the orientation preference map. However this only happens under one condition: It is vital that the process starts with an asymmetrical ocular dominance map - the feature where the mammal brain is similar to the fish brain. In newborn children the majority of cells in the visual cortex responds to stimuli originating from the opposite eye, information from the other eye becomes more important only later on. The scientists were able to show that this archetypical development is crucial even today: It is essential for the correct formation of the ocular dominance map, and can therefore also influence the development of the orientation preference map.
In the visual cortex occur both occular dominance and orientation preference. Similarly, different representations are likely to occur also in other areas of the cerebral cortex responsible for the processing of complex information. This is how humans gather images and words, acoustic and visual information. Research on how ocular dominance and orientation preference influence each other may be a good model of this more general activity.
For further information, see also
Lars Reichl, Siegrid Lowel, Fred Wolf.
Pinwheel stabilization by ocular dominance segregation.
Phys. Rev. Lett. 102, 208101 (2009)