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 Prevailing View of Brain's Sensory Processing System is Under Fire

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Date posted: 01/07/2013

A series of studies conducted in June 2013 by Randy Bruno, PhD, and Christine Constantinople, PhD, of Columbia University’s Department of Neuroscience, raises deep questions about the previously thought to be well-understood way in which the brain processes incoming sensory data. It turns out that the process is nowhere near as simple and straightforward as we had previously believed.

For decades, scientists have thought that sensory information is relayed from the skin, eyes, and ears to the thalamus and then processed in the six-layered cerebral cortex in serial fashion: first in the middle layer (layer 4), then in the upper layers (2 and 3), and finally in the deeper layers (5 and 6.) This model of signals moving through a layered “column” was largely based on anatomy, following the direction of axons—the wires of the nervous system.

“Our findings challenge dogma,” said Dr. Bruno, assistant professor of neuroscience and a faculty member at Columbia’s new Mortimer B. Zuckerman Mind Brain Behavior Institute and the Kavli Institute for Brain Science. “They open up a different way of thinking about how the cerebral cortex does what it does, which includes not only processing sight, sound, and touch but higher functions such as speech, decision-making, and abstract thought.”

The research used the sensory system of cat whiskers, which is near-identically wired to the sensations of touch in human fingers (pressure, texture, heat, cold, pain). Dr Bruno chose this system because it is a more simplistic system than that of human fingers, and because previous research mapped each whisker to a specific barrel-shaped cluster of neurons in the brain. As such it is extremely easy to pinpoint where we thought the signals were going.

As it turns out, those beliefs were mostly right. However, as the image below demonstrates, the picture is not as clear-cut as we once thought it was.

A nerve cell in the thalamus (blue) sends its axon (red) into cerebral cortex, where it makes synaptic connections with thousands of neurons. While most of these connections are in a middle layer of the cortex (grey rings), some sparse branches connect to deeper layers. It is these deeper connections that turn established models of sensory processes on their heads.

The study relied on a sensitive technique that allows researchers to monitor how signals move across synapses from one neuron to the next in a live animal. Using a glass micropipette with a tip only 1 micron wide (one-thousandth of a millimetre) filled with fluid that conducts nerve signals, the researchers recorded nerve impulses resulting from whisker stimulation in 176 neurons in the cortex and 76 neurons in the thalamus. The recordings showed that signals are relayed from the thalamus to layers 4 and 5 at the same time.  Although 80 percent of the thalamic axons went to layer 4, there was surprisingly robust signalling to the deeper layer.

To confirm that the deeper layer receives sensory information directly, the researchers used the local anaesthetic lidocaine to block all signals from layer 4. Activity in the deeper layer remained unchanged.

“This was very surprising,” said Dr. Constantinople, currently a post-doctoral researcher at Princeton University’s Neuroscience Institute. “We expected activity in the lower layers to be turned off or very much diminished when we blocked layer 4. This raises a whole new set of questions about what the layers actually do.”

“At this point, we still don’t know what, behaviourally, the different layers do,” said Dr. Bruno, whose lab is now focused on finding those answers.

See the full Story via external site: newsroom.cumc.columbia.edu

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