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The cognitive abilities of an individual - their memories and thoughts - depend on the connections that are formed between the neurons of the brain. Not only during development, but throughout adult life, new neuronal connections are formed and others are lost. As part of an international collaboration, researchers at the Bernstein Center for Computational Neuroscience Berlin, with funding from the Berlin School of Mind and Brain, have elucidated an important principle underlying these changes in neuronal connectivity. The work, which was conducted by PhD student Joshua Young and Professor Klaus Obermayer from the Berlin University of Technology, has implications for understanding fundamental processes underlying development, as well as how the brain reorganizes in response to injuries such as cerebral stroke or retinal degeneration. The study will appear online on May 27th in Nature Neuroscience and was achieved through a collaboration with scientists from the University of Sydney, Australia (Prof. Bogdan Dreher, Dr Chun Wang), the University of Newcastle, Australia (Prof. Mike Calford) and the Nencki Institute, Poland (Dr Wioletta Waleszczyk).
The brain is a complex network of neurons that communicate with each other via electro-chemical signals. Every neuron receives signals from a multitude of upstream neurons and integrates this input in order to decide whether to send out its own signal. Previous work has indicated that neurons can 'choose' to become more responsive to the signals of specific neurons. When neuron A sends an impulse which elicits a response in neuron B, the contact from A to B is strengthened. This increase in the strength of the connection leads to an increased responsiveness of neuron B to input from neuron A. Through this process cell B begins to adopt cell A's pattern of activity. Because this transfer of activity pattern is asymmetric the researchers have labelled the phenomenon as 'didactic reorganization'.
The creation of a neuronal brain lesion induces a fundamental reorganization of the connections in the surrounding area. This new study has investigated how neurons of the visual cortex reorganize their connections in response to the inactivation of a small area of the retina. By analyzing the responses of the neurons to visual stimuli the researchers found that the neurons appeared to be reorganizing their connectivity in a highly homogenous way. By comparing these experimental results with those of computational models clear evidence was produced indicating that this homogeneity was a consequence of didactic reorganization.
The visual cortex is the primary brain region where visual signals from the retina are analysed for complex features like the orientation and direction of contours. Due to the retinal inactivation, neurons in a small circumscribed zone of the visual cortex lose their 'direct' input from the retina. As a consequence of this loss of input, the ability of these neurons to propagate signals originating from those that still receive direct input from the retina is greatly increased. It is this enhancement that allows that the effects of didactic reorganization to be clearly demonstrated, as in the case of this study. However, the scientists suspect that didactic reorganization occurs during the modification of neuronal connections in the intact brain as well.
As a fundamental process, this discovery is likely to be very important for understanding how basic neuronal circuits emerge during development. The processes that are active during development seem to re-emerge during adult repair and recovery, thus it is possible that didactic reorganization occurs any time cortical neurons begin to reorganise their connectivity due to any input loss arising from direct or peripheral damage. If so, understanding this phenomenon will be essential for developing better treatments for people who suffer from problems like a cerebral stroke or retinal degeneration.
Original publication:
J. M. Young, W. J. Waleszczyk, C. Wang, M. B. Calford, B. Dreher & K. Obermayer: Cortical reorganization consistent with spike timing- but not correlation-dependent plasticity. Nature Neuroscience (online), 27. Mai 2007
Contact:
Joshua Young, Prof. Dr. Klaus Obermayer
Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Franklinstr. 28/29
10587 Berlin
Germany
Phone: 030-314-73442
EMail: sekr@ni.cs.tu-berlin.de
http://ni.cs.tu-berlin.de/
http://www.bccn-berlin.de/
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Biology, Information technology, Mathematics, Medicine, Nutrition / healthcare / nursing, Physics / astronomy
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