High-speed microscope captures brain neuroactivities

US and Hong Kong researchers have used a novel high-speed microscope to record the electrical signals in the neurons of an alert mouse, in a significant breakthrough for the field of neuroscience. Their work has been published in the journal Nature Methods.

Our brains contain tens of billions of nerve cells (neurons) which constantly communicate with each other by sending chemical and electrical flashes — each lasting just 1 ms. In every millisecond, these billions of swift-flying flashes form a giant star-map in the brain that lights up in a glittering pattern. They are the origins of all body functions and behaviours — such as emotions, perceptions, thoughts, actions and memories — as well as brain diseases such as Alzheimer’s and Parkinson’s in the case of abnormalities.

One grand challenge for neuroscience in the 21st century is to capture these complex flickering patterns of neural activities — the key to an integrated understanding of brain-wide interactions — which would have previously required a high-speed microscope into the brain. This has now changed, thanks to the development of a two-photon fluorescence microscope by a team led by Dr Kevin Tsia from The University of Hong Kong and Professor Ji Na from the University of California, Berkeley.

At the heart of the microscope is an innovative technique called FACED (free-space angular-chirp-enhanced delay imaging), developed by Dr Tsai’s team back in 2017. FACED makes use of a pair of parallel mirrors, which generate a shower of laser pulses to create a superfast sweeping laser beam at least 1000 times faster than the existing laser-scanning methods.

In the team’s experiment, the microscope projected a beam of sweeping laser over the mouse’s brain and captured 1000–3000 full 2D scans of a single mouse brain layer (the neocortex) every second. To probe the genuine electrical signals that pulse between the neurons, the team inserted a biosensor (protein molecules), developed by Dr Michael Lin of Stanford University, into the neurons of the mouse brain.

“These engineered proteins will light up (or fluoresce) whenever a voltage signal passes through the neurons,” Dr Tsia said. “The emitted light is then detected by the microscope and formed into a 2D image that visualises the locations of these voltage changes.

“This is really an exciting result, as we now can peek into the neuronal activities that were once obscured and could provide the fundamental clues to understanding brain functions and, more importantly, brain diseases.”

Apart from electrical signals, the team also used the microscope to capture the slow motion of chemical signals in the mouse brain, such as calcium and glutamate, a neurotransmitter, as deep as one-third of a millimetre from the brain’s surface.

A notable advantage of this technique is the ability to track the signals that do not trigger the neuron to fire — weak neuronal signals (sub-threshold signals) that are often difficult to capture and detect. These have yet been studied in detail because of the lack of high-speed techniques like the one developed by the team.

Another important feature of the novel technique is that it is minimally invasive. The classical method for recording electrical firing in the brain is to physically embed or implant electrodes in the brain tissue. However, such physical intrusion could cause damage to the neurons and can only detect fuzzy signals from a couple of neurons.

“This is so far a one-of-its-kind technology that could detect millisecond-changing activities of individual neurons in the living brain,” Dr Tsia said. “So this is, I would say, the cornerstone of neuroscience research to more accurately decoding brain signals.”

Dr Tsia said the team would work to advance the capability of the microscope, stating, “We are working to further combine other advanced microscopy techniques to achieve imaging at higher resolution, wider view and deeper into the brain in the neocortex, which is about 1 mm. This will allow us to probe deeper into the brain for a better and more comprehensive understanding of the functions of the brain.”

Pictured: Dr Kevin Tsia, Associate Professor of Department of Electrical and Electronic Engineering, HKU.

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