A Mini Microscope Illuminates Neuron Spikes
By capturing neurons’ electrical activity in fine detail across time and space, researchers aim to deepen our grasp of how the brain works and to pave the way for new treatments for neurological disorders. Yet achieving sufficient sensitivity and temporal precision with a device small and light enough to be mounted on live rodents—the quintessential model for human disease—has remained a significant hurdle.
Researchers in the United States have now built a miniature, high-performance microscope from readily available optics and sensors that can ride on mice while they are awake but restrained in place. The device weighs so little that it could, in the future, be adapted to follow the behavior of free-moving mice (Biomed. Opt. Express, doi: 10.1364/BOE.576516). The team anticipates that with further customization, tracking the neural activity of freely roaming mice will become feasible.
Advances in neuronal imaging
Currently, scientists can monitor mouse brain activity in vivo using genetically encoded calcium indicators. These indicators glow when they bind calcium ions, which participate in many voltage changes across cell membranes, i.e., spikes.
However, calcium-based measurements reflect spike activity over timescales of hundreds of milliseconds. Neuronal voltages, by contrast, can shift in under a millisecond. Relying on calcium signals to infer fast spikes can sacrifice temporal precision, and while computer-led post-processing can uncover the underlying spikes, this approach can introduce errors.
In the new work, Emily Gibson and colleagues—affiliated with the University of Colorado Anschutz Medical Campus, the University of Colorado Boulder, and Columbia University—employ genetically encoded voltage indicators. These proteins fluoresce in response to voltage changes across cell membranes, offering a more direct, millisecond-accurate readout of neuron firing.
As the researchers caution, these voltage signals generate only small fluctuations atop a large baseline fluorescence, making it difficult to detect spikes. The main challenge is to design a microscope with a large numerical aperture to gather light efficiently and to pair it with a fast, sensitive image sensor, all while keeping the device light enough to be carried on a living animal.
A remarkably small, capable microscope
Calcium-based signals can still be analyzed later to infer spikes, but that post-processing can introduce inaccuracies.
Gibson and her team built such a microscope that they named MiniVolt, assembling it from off-the-shelf components. It uses four lenses to achieve a numerical aperture around 0.6 and a sensor capable of operating at more than 500 frames per second. The assembly is housed in a casing created via 3D printing and, with filters integrated, measures just a few centimeters across and weighs a mere 16 grams—roughly the weight of a light battery and a small gadget.
To test MiniVolt, the scientists created a small skull opening in a mouse engineered to express a voltage indicator called Voltron2 in its visual cortex. The microscope was mounted on the mouse’s head, with the head fixed in place, and illuminated via a green laser delivered through a fiber optic. The fluorescence signal was recorded to monitor the firing of individual neurons.
When benchmarked against a traditional bench-top wide-field microscope, MiniVolt tracked comparable voltage spike patterns in a given neuron and delivered similar spike peak-to-noise ratios—at least 3—while imaging at 530 frames per second.
Gibson and colleagues say the device is already light enough to be mounted on freely moving rats, which can bear on their heads loads up to about 35 grams. However, mice remain the most valuable model for understanding the human brain, and they typically tolerate devices weighing only around 4 grams.
Reducing the weight further should be achievable. The researchers expect to trim mass by customizing optics to preserve the same numerical aperture with smaller lenses, and by using a smaller, purpose-built CMOS sensor while maintaining performance, since they are currently using only a fraction of the sensor’s capacity. If successful, these refinements could enable high-performance neural imaging in even smaller animals while preserving the quality of the data needed to study brain function and disease mechanisms.
