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Probes with tiny LEDs shed light on neural pathways

15 December 2015

With the help of light-emitting diodes as small as neurons, University of Michigan researchers are unlocking the secrets of neural pathways in the brain.

Neural probes developed at the University of Michigan are helping researchers understand the brain's circuitry. The tines of the probe are just under a tenth of a millimetre wide and host minuscule LED lights and electrodes (image: Fan Wu)

The researchers have built and tested in mice neural probes that hold what are believed to be the smallest implantable LEDs ever made. The new probes can control and record the activity of many individual neurons, measuring how changes in the activity of a single neuron can affect its neighbours.

The team anticipates that experiments using probes based on their design could lead to breakthroughs in understanding and treating neurological diseases such as Alzheimer's.

"This is a very big step forward," says researcher, Professor Kensall Wise. "The fact that you can generate these optical signals on the probe, in a living brain, opens up new doors."

A network of around 100 billion neurons power the human brain, and determining how they work together is a monumental and important task.

"Hundreds of millions of people suffer from neurological diseases, but treatment methods and drugs are currently very limited because scientific understanding of the brain is lacking," says co-researcher, Fan Wu. "We have developed a tool that is needed to better understand how the brain works — and why it doesn't work — to try to solve to these problems."

In genetically modified rodents, neurons can be turned on and off with light. Typically, neuroscientists using this 'optogenetics' technique shine light on a region of the brain through implanted optical fibres and record the response with a second device. This helps to reveal which regions of the brain are responsible for which behaviours. But it can't reveal how the neurons communicate with one another. The new probes can.

Each probe array contains 12 LEDs and 32 electrodes. The micro LEDs are as small as a neuron's cell body, so they can turn single neurons on and off. Meanwhile, the micro-electrodes measure activity at the single-neuron level, reporting how a change in one neuron's behaviour affects the surrounding network.

"Now we can know how a group of cells, both adjacent and farther away, are responding to the activation of a single cell," says Wu. "This will help us better understand how these cells are communicating with each other."

Using micro-LED probes, the team hopes to discover how signals propagate inside the neural circuitry and thus improve understanding of how memories are formed, retrieved and replaced. The proof-of-concept experiment found that superficial and deep neurons in the hippocampus produce different kinds of brain waves when stimulated. Future experiments will explore how these waves are related to memory.

The work is described in a YouTube video clip here.

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