The traditional view is that learning is based on the strengthening or weakening of the contacts between nerve cells in the brain. However, this has been challenged by new research findings from Lund University in Sweden that indicate the existence of a third mechanism – a kind of clock function that gives individual nerve cells the ability to time their reactions.
“This means a dramatic increase in the brain’s learning capacity. The cells we have studied control the blink reflex, but there are many cells of the same type that control entirely different processes. It is therefore likely that the timing mechanism we have discovered also exists in other parts of the brain”, said Professor of neurophysiology Germund Hesslow. For this research, G. Hesslow and colleagues Fredrik Johansson and Dan-Anders Jirenhed in collaboration with SPECS's PhD student Riccardo Zucca [http://specs.upf.edu], have used the ‘conditioned reflexes’ paradigm.
The principle behind this paradigm comes from the Russian scientist Ivan Pavlov, who, around the turn of the last century, taught dogs to associate a certain sound with food so that they began to drool on hearing the sound.
In the present experiment, the researchers studied animals that learnt to associate a sound with a puff of air in the eye that caused them to blink. If the time between the sound and the puff of air was quarter of a second, the animals blinked after quarter of a second even if the puff of air was removed. If the time was changed to half a second, the animals blinked after half a second, and so on.
The prevalent theories in brain research state that this learnt timing mechanism is a result of strengthening or weakening of the synapses throughout a network of nerve cells. However, using extracellular super-thin electrodes, the Lund group has now shown that a network is not needed: one single cell can learn when it is time to react.
The cells studied for this work are the Purkinje cells located in the cerebellum, an area of the brain responsible for posture, balance and movement. In particular the researchers focused on those cells that control eye blink response.
This work reveals principles underlying learning mechanisms that could have possible future applications including rehabilitation procedures for the recovery of motor capabilities following brain damage. The findings could also have an impact on conditions such as autism, ADHD and language problems, in which the cerebellum is believed to play a part.
“Intelligible speech is dependent on correct timing, so that the pauses between the sounds are right”, explained Germund Hesslow.
The new findings have already attracted attention in the research community: the internationally renowned memory researcher Charles Gallistel came all the way from Rutgers University in the spring to study the group’s work. Work is now continuing to study what transmitter substance and what receptor on the surface of the cell are responsible for the newly discovered timing mechanism.
Fredrik Johansson, Dan-Anders Jirenhed, Anders Rasmussen, Riccardo Zucca, and Germund Hesslow (2014). Memory trace and timing mechanism localized to cerebellar Purkinje cells.[http://www.pnas.org/content/early/2014/09/24/1415371111.short]
PNAS 2014; published ahead of print September 29, 2014, doi:10.1073/pnas.1415371111
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