image: Researchers developed a tiny, lightweight microscope that captures the electrical spikes of neurons at hundreds of frames per second in awake animals.
Credit: Emily A. Gibson, University of Colorado Denver, Anschutz Medical Campus
WASHINGTON — Researchers have built a tiny, lightweight microscope that captures neuron activity with unprecedented speed that can be used in freely moving animals. The new tool could give scientists a more complete view of how brain cells process information during natural behavior.
The microscope is designed to image genetically encoded voltage indicators — fluorescent dyes that rapidly change brightness when a neuron fires — through a small window in the skull while the animal is awake.
“Unlike most miniature microscopes that track slower calcium signals, ours captures electrical spikes at hundreds of frames per second,” said Emily Gibson from the University of Colorado Anschutz Medical Campus. “This makes it possible to capture the moment a neuron fires as well as the quieter signals that build up inside neurons before firing.”
In the Optica Publishing Group journal Biomedical Optics Express, the researchers describe the new microscope, which is designed to capture very faint changes in brightness. In experiments with mice, they show that it can acquire voltage recordings that closely match those from a standard widefield microscope, reliably measuring activity in individual neurons.
“By capturing these detailed voltage patterns across different parts of the brain, our microscope makes it possible to directly explore how subtle electrical signals influence the timing of brain activity, such as spatial navigation in the hippocampus,” said Gibson. “An increased understanding of how neural circuits guide behavior and cognition could lead to new treatments for a variety of neurological disorders and neurodegenerative diseases.”
Tracking action potential dynamics
Voltage changes in the brain are driven by ion flow across neuron membranes, creating rapid electrical signals called action potentials. The process starts with small voltage changes that eventually reach a threshold, triggering a chain reaction that causes sodium ions to rush in and then potassium ions to rush out. This creates the "spike" of an action potential that travels down the neuron.
Observing these voltage changes can reveal new insights into how the brain’s neural circuits behave during tasks such as learning and forming memories. However, because they occur in just milliseconds, voltage changes can be difficult to capture without using heavy or bulky optical components.
“We took a big step toward tackling these constraints by designing a compact, efficient optical system with high numerical aperture and pairing it with a high-speed sensor to reliably detect action potential spiking,” said co-author Juliet Gopinath from the University of Colorado Boulder. “Our microscope enables recording of both the rapid electrical spikes and the smaller sub-threshold voltage changes that occur inside neurons in freely moving animals.”
Capturing subtle changes
To boost the amount of light collected enough to capture subtle changes in fluorescence brightness, the researchers custom-designed an optical system that achieves a numerical aperture of 0.6 in a small format. They also incorporated a compact, high-efficiency camera that can acquire images at approximately 500 frames per second, fast enough to capture the millisecond timescale of action potentials.
The resulting microscope, called the MiniVolt, has a 250-micron field of view, a 1.3-1.6 mm working distance and a total weight of 16.4 g. Gibson’s team also worked with neuroscientists to pair the microscope with the latest voltage indicator, Voltron2, which is more stable and produces larger fluorescence changes in response to voltage than previous voltage indicators.
To test the microscope, the researchers compared voltage recordings from awake head-fixed mice acquired with MiniVolt to those from a benchtop voltage imaging microscope. The MiniVolt acquired images of in vivo voltage spikes from Voltron2 with a spike peak-to-noise ratio greater than 3 at 530 frames per second. This means that the height of each voltage spike was more than three times larger than the background noise, which was comparable to the signal quality of the benchtop microscope.
The researchers are now working to reduce the microscope’s weight, which is already compatible with imaging in freely moving rats, to enable use in freely moving mice, an essential model for many human diseases. They also want to increase the MiniVolt’s field of view, which is limited by the size of the light source rather than the optical design.
Paper: C. A. Saladrigas, F. Speed, A. Teel, M. Zohrabi, E. J. Miscles, G. L. Futia, L. V. Baker, Y. Zhang, I. Kymissis, V. M. Bright, C. G. Welle, D. Restrepo, J. T. Gopinath, E. A. Gibson, “Miniaturized widefield microscope for high speed in vivo voltage imaging,” Biomed. Opt. Express, 17, 1-11 (2025).
DOI: 10.1364/BOE.576516
About Biomedical Optics Express
Biomedical Optics Express serves the biomedical optics community with rapid, open-access, peer-reviewed papers related to optics, photonics and imaging in biomedicine. The journal scope encompasses fundamental research, technology development, biomedical studies and clinical applications. It is published monthly by Optica Publishing Group and edited by Ruikang (Ricky) Wang, University of Washington, USA. For more information, visit Biomedical Optics Express.
About Optica Publishing Group (formerly OSA)
Optica Publishing Group is a division of Optica, the society progressing light science and technology. It publishes the largest collection of peer-reviewed content in optics and photonics, including 18 prestigious journals, the society’s flagship member magazine, and papers from more than 835 conferences, including 6,500+ associated videos. With over 400,000 journal articles, conference papers and videos to search, discover and access, Optica Publishing Group represents the full range of research in the field from around the globe.
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Biomedical Optics Express
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Miniaturized widefield microscope for high speed in vivo voltage imaging