London, Nov 18 : Ever wondered why is it easier to re-learn riding a bicycle, even if we haven't practiced for years? Well, now scientists at the Max Planck Institute of Neurobiology have provided an answer by showing what really goes on in the brain when it learns or forgets something.
They have shown that nerve cell contacts established during a learning process stay even when they are no longer required. The reactivation of this temporarily inactivated 'stock of contacts' allows a faster learning of things forgotten.
In order to learn something, nerve cells make new connections with each other. When faced with an unprecedented piece of information, for which no processing pathway yet exists, filigree appendages begin to grow from the activated nerve cell towards its neighbours.
Whenever a special point of contact, called synapse, forms at the end of the appendage, information can be transferred from one cell to the next - and new information is learned. Once the contact breaks down, we forget what we have learned.
However, what happens when the brain learns something, forgets it after a while and then has to learn it again later?
It is known that "relearning" tends to be easier than starting "from scratch" but researchers wanted to find out if this subtle difference also have its origins in the structure of the nerve cells.
The researchers have shown that there are considerable differences in the number of new cell contacts made - depending on whether a piece of information is new or is being learned second time around.
Nerve cells that process visual information, for instance, produced a considerably higher number of new cell contacts if the flow of information from their "own" eye was temporarily blocked.
After approximately five days, the nerve cells had rearranged themselves so as to receive and process information from the other eye - the brain had resigned itself to having only one eye at its disposal.
Once information flowed freely again from the eye that had been temporarily closed, the nerve cells resumed their original function and now more or less ignored signals from the alternative eye.
"What surprised us most, however, was that the majority of the appendages which developed in response to the information blockade, continued to exist, despite the fact that the blockade was abolished," Nature quoted lead author Mark Huener, as saying.
Everything seems to point to the fact that synapses are only disabled, but not physically removed.
When the same eye was later inactivated again, the nerve cells reorganized themselves much more quickly - because they could make use of the appendages that had stayed in place.
Many of the appendages that develop between nerve cells are thus maintained and facilitate later relearning.
According to researchers, this insight is crucial to the understanding of the fundamental processes of learning and memory.