Neuroimaging is a powerful tool used to delve deep into the far reaches of our brains, uncovering answers to the mysteries posed by our nervous system. This diagnostic has the power to explore normally intangible mechanisms such as emotions, information processing, and memory. Indeed, capturing the brain’s activity in real-time demands resolution, scale, and speed. However, to do this at the desired scale, a system capable of visualizing millions of neurons in mere milliseconds as they interact with each other in a cacophony of chemical and electrical impulses would have to be in existence.
For instance, this system would need to capture millions of neurons in one image. Desirable but unworkable, it would either have to zoom out to capture such a large number of neurons, producing low-resolution pictures. Or amalgamate multiple images of these neuronal networks together, providing a mismatched jigsaw puzzle of neurons imaged at different times. Thus, a neuroimaging system capable of capturing millions of single neurons concurrently at distant parts of the brain is highly desirable.
Now, a study from researchers at Rockefeller University develops a brain imaging technique with the ability to capture the detailed activity of a vast number of cells – across different depths in the brain at high speed and with unprecedented clarity. The team states their system, dubbed light beads microscopy, will allow the scientific community to investigate regions of the brain in a way not possible previously. The study is published in the journal Nature Methods.
The brain is a complex organ that oversees all cognitive processes and overall regulation of our body. As you can imagine, this takes billions of neurons to coordinate – highly compacted in the cortex of our central nervous system. This massive cosmos of neurons sends and receives chemical and electrical signals to control different processes throughout the body. These signals or neurotransmission can make you feel tired, for example, while others make you feel depressed.
Accordingly, this neurotransmission can be local to the central nervous system or use a myriad of nerve tracts to travel throughout the peripheral nervous system. Indeed, scientists have only just begun to untangle this neural network with the help of neuroimaging.
The most sensitive of these is the combination of two-photon scanning microscopy – an imaging technique that uses fluorescence to capture images of living tissue with a resolution of up to 1mm thickness. This technique works by firing a focused laser pulse at a fluorescently tagged target. After a few nanoseconds, the laser pulse hits its mark, and the tag emits fluorescent light, capturing any neuroactivity detected. However, this system has limitations in resolution and speed when used on larger sections of the brain.
The current study develops a deep neuroimaging technique that provides a cortex-wide recording of neuron activity at cellular resolution without compromising resolution or speed.
The groundbreaking technique involves siphoning one main pulse into 30 smaller sub-pulses – each at a different strength enabling them to image at 30 different depths in the mouse brain. Despite operating at different levels in the brain, the same amount of fluorescence is generated at each depth. This is accomplished using a single microscope focusing lens with a cavity of mirrors that staggers the firing of each pulse to ensure they all reach their target depths.
Results show the system is incredibly fast as the footage containing no neuroactivity is deleted. With this approach, the only limit to the speed of the recording is the time it takes the fluorescent tags to flare. Consequently, broad sections of the brain can be recorded within the same time it would take a conventional two-photon microscope to capture a far smaller collection of brain cells.
The lab states the integration of light beads microscopy into a microscopy platform allows for optical access to a large brain volume. As a result, the activity of more than one million neurons across the entire cortex of the mouse brain can be recorded for the first time.
The scientist explain because their method is an innovation that builds on two-photon microscopy, many labs already have or can commercially obtain the technologies necessary to perform light beads microscopy, as described in the paper. They also plan to give labs less familiar with these techniques free access to a simplified version of their system.
Professor Alipasha Vaziri, group leader states: “There’s no good reason why we didn’t do this five years ago,” he says. “It would have been possible—the microscope and laser technology existed. No one thought of it.”
Still, the team doesn’t want their system to replace established neuroimaging techniques. “There are neurobiological questions for which the standard two-photon microscope is sufficient,” Vaziri says. ” But light beads microscopy allows us to address questions that existing methods cannot.”
For the future, the researchers state their new technique provides an opportunity for discovering the neurocomputations underlying cortex-wide encoding and processing of information in the mammalian brain.
Source: Rockefeller University
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