Jülich, 10 October 2013 – The human brain keeps changing throughout a person’s lifetime. New connections are continually created while synapses that are no longer in use degenerate.
To date, little is known about the mechanisms behind these processes. Jülich neuroinformatician Dr. Markus Butz has now been able to ascribe the formation of new neural networks in the visual cortex to a simple homeostatic rule that is also the basis of many other self-regulating processes in nature. With this explanation, he and his colleague Dr. Arjen van Ooyen from Amsterdam also provide a new theory on the plasticity of the brain – and a novel approach to understanding learning processes and treating brain injuries and diseases.
The brains of adult humans are by no means hard wired. Scientists have repeatedly established this fact over the last few years using different imaging techniques. This so-called neuroplasticity not only plays a key role in learning processes, it also enables the brain to recover from injuries and compensate for the loss of functions. Researchers only recently found out that even in the adult brain, not only do existing synapses adapt to new circumstances, but new connections are constantly formed and reorganized. However, it was not yet known how these natural rearrangement processes are controlled in the brain. In the open-access journal PLOS Computational Biology, Butz and van Ooyen now present a simple rule that explains how these new networks of neurons are formed.
"It’s very likely that the structural plasticity of the brain is the basis for long-term memory formation," says Markus Butz, who has been working at the recently established Simulation Laboratory Neuroscience at the Jülich Supercomputing Centre for the past few months. "And it’s not just about learning. Following the amputation of extremities, brain injury, the onset of neurodegenerative diseases, and strokes, huge numbers of new synapses are formed in order to adapt the brain to the lasting changes in the patterns of incoming stimuli."
These results show that the formation of new synapses is driven by the tendency of neurons to maintain a 'pre-set' electrical activity level. If the average electric activity falls below a certain threshold, the neurons begin to actively build new contact points. These are the basis for new synapses that deliver additional input – the neuron firing rate increases. This also works the other way round: as soon as the activity level exceeds an upper limit, the number of synaptic connections is reduced to prevent any overexcitation – the neuron firing rate falls. Similar forms of homeostasis frequently occur in nature, for example in the regulation of body temperature and blood sugar levels.
"It was previously assumed that structural plasticity also follows the principle of Hebbian plasticity. The findings suggest that structural plasticity is governed by the homeostatic principle instead, which was not taken into consideration before," says Prof. Abigail Morrison, head of the Simulation Laboratory Neuroscience at Jülich. Her team is already integrating the new rule into the freely accessible simulation software NEST, which is used by numerous scientists worldwide.
These findings are also of relevance for the Human Brain Project. Neuroscientists, medical scientists, computer scientists, physicists, and mathematicians in Europe are working hand in hand to simulate the entire human brain on high-performance computers of the next generation in order to better understand how it functions. “Due to the complex synaptic circuitry in the human brain, it’s not plausible that its fault tolerance and flexibility are achieved based on static connection rules. Models are therefore required for a self-organization process,” says Prof. Markus Diesmann from Jülich’s Institute of Neuroscience and Medicine, who is involved in the project. He heads Computational and Systems Neuroscience (INM-6), a subinstitute working at the interface between neuroscientific research and simulation technology.
Via Dr. Stefan Gruenwald