‘Mild beads’ technique photographs exercise throughout the mouse mind | Spectrum
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A new imaging technique detects neurons that fire almost simultaneously across large parts of the brain tissue in living mice. The method could help researchers understand how extensive networks of neurons communicate and how these patterns differ between wild-type mice and mouse models of autism. According to one theory of autism, characteristics of the condition are based on unusual signal patterns between distant regions of the brain.
Researchers often monitor brain activity in mice using a technique called two-photon calcium imaging, which involves adding fluorescent protein tags to the calcium ions that rush into cells when neurons fire. You can excite the proteins with a laser and detect the fluorescence with a microscope to watch cells in action.
This method usually limits researchers to a small area of study. To map a larger part of the brain, they often have to scan section by section, which is too slow to capture the interactions of distant neurons. Researchers can speed up the process by illuminating neighboring regions in the brain with a series of consecutive laser beams, each slightly delayed to avoid blurred signals. However, splitting a laser and delaying each beam usually requires complex equipment that makes it difficult to scale. Using this approach, scientists were able to record the activity of only 12,000 neurons.
The new method enables researchers to track the activity of more than 1 million individual neurons in about 16 cubic millimeters of the brain of mice – more than ten times the volume that was imaged using previous techniques. “This is definitely by far the largest volume and number of neurons anyone has captured at the same time,” says Alipasha Vaziri, professor of neurotechnology and biophysics at Rockefeller University in New York City. He directed the new work described in Nature Methods in August.
The technique – called light bead microscopy – consists of splitting a laser beam into 30 beams, each of which excites a point at a slightly different depth in the brain. This division creates a column of “spheres of light” about 500 microns deep that allows researchers to scan an entire block of the brain in the same amount of time that it would normally take to scan a small 2D slice using traditional approaches.
Activity tracker: The fire patterns of a subset of neurons (bright dots) correlate with the timing of a mouse’s spontaneous behavior.
In order to shape the pearls of light, Vaziri and his colleagues created a special cavity lined with a mirror. When a light pulse enters the cavity, it jumps around until it hits a partially reflective mirror: part of the light passes through the mirror towards a microscope, while the rest is reflected back into the cavity for another round trip. Each trip shifts the focus of the beam to a slightly shallower depth and delays the beam by a few nanoseconds. Researchers can then assign the fluorescence detected at different points in time to specific points in the brain.
The team tested the technique by mapping the brains of awake mice whose neurons express a protein that fluoresces in the presence of calcium. They put the mice on a treadmill and fixed the animals’ heads. In one experiment, the researchers recorded the firing of neurons in both hemispheres of the brain. In another test, they recorded from a hemisphere while the mouse was exposed to repeated stimuli, such as brushing the whiskers or presenting a moving image of black and white lines. The team also recorded movements in the animals’ limbs.
While tracking a hemisphere, the team captured more than 200,000 neurons firing in different areas of the brain. By analyzing correlations between the timing of neural activity and the onset of stimuli or movements, the researchers also identified groups of neurons that are attuned to different stimuli or spontaneous behavior.
As expected, neurons in regions of the brain known to be involved in processing sensory or visual information responded to whiskers or visual stimulation, the researchers reported. More surprisingly, neurons in many other cortical regions also lit up in response to these stimuli. And when whiskers and visual stimulation were combined, the additional stimulus modulated the activity of some neurons that were attuned to the other stimulus. The firing patterns of the individual neurons also varied with each presentation of the stimuli.
These results underscore the importance of capturing wide-ranging brain activity at high speed in order to untangle the complexities of neural networks, the researchers say.
Quote this article: https://doi.org/10.53053/MANL1452