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  • Sculpted light captures brain activity

Nov 2013
VIENNA – A high-speed imaging technique that “sculpts” the 3-D distribution of light in a sample can resolve a single neuron in a living worm, opening possibilities for studying the function of the organism’s nervous system and pairing brain function to anatomy.

A major aim of neuroscience today is to understand how an organism’s nervous system processes sensory input and generates behavior by observing the activity of cells across the entire brain. But to do this, scientists need detailed maps of how the nerve cells are wired in the brain as well as information on how these networks interact in real time.

Frontal part of a nematode seen through a microscope: The neurons of the worm’s brain are colored in green. This is an artist’s interpretation of the discs of light generated by the WF-TeFo (wide-field temporal focusing) microscope, scanning the brain area and recording the activity of certain neurons. Photos courtesy of IMP.

Until now, researchers had focused on studying the activity of single neurons and small networks in the C. elegans worm, but hadn’t been able to establish a functional map of the entire nervous system because of the limitations of the imaging techniques used. The activity of single cells can be resolved with high precision, but simultaneously looking at the function of all the neurons in an entire brain has been a major challenge, with the trade-off being between spatial or temporal accuracy.

“Previously, we would have to scan the focused light by the microscope in all three dimensions,” said quantum physicist Robert Prevedel, a member of the University of Vienna team of physicists and neurobiologists that developed the new technique. “That takes far too long to record the activity of all neurons at the same time. The trick we invented tinkers with the light waves in a way that allows us to generate ‘discs’ of light in the sample.”

Artist’s rendering of a differential interference contrast microscope image overlaid with neurons in the head ganglia of the nematode C. elegans. Discs indicate the sculpted-light excitation scheme used for high-speed functional imaging of neural dynamics.

That means they have to scan in only one dimension to get the information they need, he said. “We end up with three-dimensional videos that show the simultaneous activities of a large number of neurons and how they change over time.” But microscopy wasn’t the only challenge to imaging the worm’s brain: Visualizing the neurons requires tagging them with a fluorescent protein that lights up when it binds to calcium, signaling the nerve cells’ activity.

“The neurons in a worm’s head are so densely packed that we could not distinguish them on our first images,” said neurobiologist Tina Schrödel, a doctoral candidate in the lab of Research Institute of Molecular Pathology (IMP) group leader Manuel Zimmer. “Our solution was to insert the calcium sensor into the nuclei rather than the entire cells, thereby sharpening the image so we could identify single neurons.” Schrödel is co-first author of a study on the work published in Nature Methods (doi: 10.1038/nmeth.2637).

The researchers recorded 70 percent of the nerve cells’ activity in the worm’s head with high spatial and temporal resolution, which could enable experiments not possible before. One question that will be addressed is how the brain processes sensory information to plan and execute specific movements.

Answering that will require further refinement of both the microscopy and computational methods to study freely moving animals, the team members say, something they hope to achieve in the next two years.

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