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Posted: May 3, 2010
Visual processing: every neuron counts
(Nanowerk News) German scientists have refined and used a method to observe how individual nerve cells process visual information in a living brain. The new microscopic method made it possible for them to study tiny synapses (one micrometre in size) on a single neuron, and to determine that each individual neuron performs an important role in sensory processing. Their results are detailed in the journal Nature.
When we open our eyes, immense volumes of data must be processed to allow us to see. For instance, there are 126 million sensory cells in the human eye that must first convert light that falls on the eye's retina into electrical signals. But while the processing of visual information starts at this point, the complete image of what we are looking at is put together at the back of the brain's cerebrum in the visual cortex.
The aim of the research team from Technische Universität München (TUM) in Germany was to understand the role of the neuron in the visual cortex in detecting motion, and to observe the process in real time using live mice.
Past studies have shown that specific neurons in the visual cortex of mice do in fact respond when a moving bar is placed in front of them. The response pattern of these 'orientation' neurons has also been documented. In the current study, the team's purpose was to look at this input signal in greater detail; not an easy feat given that each neuron is comprised of a complex tree of small branched antennae (known as dendrites) where many other neurons anchor with their synapses (the structures that allow neurons to transmit signals to other cells).
'Up to now, similar experiments have only been carried out on cultured neurons in Petri dishes,' explained TUM's Dr Arthur Konnerth. 'The intact brain is far more complex. Because it moves slightly all the time, resolving individual synaptic input sites on dendrites was extremely difficult.'
The team used a microscopic probe (specifically two-photon fluorescence microscopy) to observe both a single cell and its tiny dendrites at work in brain tissue. They discovered that when a mouse looks at a variety of motions of a bar pattern, each neuron receives input signals from a range of differently oriented nerve cells but sends only one type of output signal. This suggests that the neuron assesses the importance of the various input signals and eliminates unnecessary information, leaving behind the most essential data required for a clear understanding of movement.
Dr Konnerth hopes to build on this research in the future by observing an individual neuron during the learning process. 'Because our method enables us to observe, down to the level of a single synapse, how an individual neuron in the living brain is networked with others and how it behaves, we should be able to make a fundamental contribution to understanding the learning process,' he said.
'Furthermore, because here at TUM we work closely with physicists and engineers, we have the best possible prospects for improving the spatial and temporal resolution of the images,' added Dr Konnerth.