As a mouse explores its environment, millions of neurons in its brain fire in synchrony. If you were to study only a small part at a time you would miss the forest for the trees, but powerful microscopes capable of capturing the entire mouse brain simultaneously are too heavy to mount on a moving mouse.
Now, a new study in Nature Biomedical Technology presents an innovative solution to this problem: a microscope that weighs just a cent, but can capture large parts of brain activity with unprecedented resolution. “The ability to observe the brain as mice engage in natural behaviors, such as social interactions and prey capture, will advance our understanding of how brain-wide distributed neuroactivity relates to naturalistic behavior,” said Rockefeller’s Alipasha Vaziri, who conducted the study. led.
Mouse-sized microscopy
Larger mammals can accommodate standard head-mounted microscopes, and even rats can support technology that weighs about 20 grams, or eight US cents. However, mice, the most commonly used model organisms for understanding brain function, are much smaller. Microscopes designed for this purpose may weigh less than three grams.
“In recent years we have seen an explosion of head-mounted microscopes for mice, but these typically only support fields of view of a few hundred micrometers at cellular resolution, as the design complexity involved for larger fields of view poses an intolerable problem. weight penalty,” says Vaziri. Existing models that are light enough to be carried by mice invariably compromise the field of view, resolution, and depth range of the microscope (or some combination thereof) and are prone to motion-induced artifacts.
Previous attempts to overcome this limitation have focused on reducing the weight of existing technology, for example by replacing metal parts with plastic, while maintaining the basic optical design of microscopes (especially those capable of imaging larger fields of view) in which a heavy lens forms a large part of the weight. Vaziri has addressed this challenge in what he calls “a principled approach.” Rather than trying to make a complex lens-based system weigh less, he clarified what the goals of the technology really were: to solve a high-resolution mapping problem between points in a 3D volume from the sample to points on the 2D surface of a camera. With that in mind, he wanted to create a lightweight system that met these goals without feeling limited by the need to adapt to an image-preserving lens-based system.
“Everyone was taking these heavy, multi-element lenses and trying to make them lighter,” says Vaziri. “Rather than asking how to make lenses lighter, we solved the problem in reverse and sidestepped the problem by developing an essentially lensless strategy and freeing ourselves from the unnecessary limitations of lens-based imaging.”
New thinking = new approach
Enter diffractive optical elements (DOEs). Unlike conventional lenses, which have a continuously curved surface to generate a spherical curvature of the wavefront, DOEs use microstructures to manipulate light through diffraction, allowing precise control of light waves. They are compact, lightweight and effective. In microscopy, the function of a traditional lens is to map the points in space onto an object on an image plane (such as a camera sensor) so that the image formed resembles the actual scene. However, when trying to form an image with an increasingly larger field of view while maintaining resolution, the errors (optical aberrations) caused by a single lens require more lens elements, resulting in a compound lens design.
Using DOEs, the Vaziri lab has shown that it is possible to accurately map the positions between the scene and the sensor without imaging, and then use computational methods to reconstruct the original scene.
Without a heavy compound lens to weigh it down, the mini microscope weighs just 2.5 grams and provides imaging that can capture wide areas of mouse brains across a 3.6 x 3.6 mm² field of view with a lateral resolution of 4 μm and a depth of field of 300 μm and a recording speed of 16 volumes per second. And most parts can be 3D printed or use cheap cell phone camera sensors. “If laboratories are interested, they can build these microscopes easily and at a low cost,” says Vaziri.
Future versions of the minimicroscope could include wireless data transmission (the current model comes with cables that don’t get in the way of a single mouse, but can easily become tangled if multiple mice interact with each other) and a refinement of the technology that will improve the observation enables parts of the brain that are located deeper in the cortex.
“The system comes with some sacrifices, and does not perform nearly as powerfully as larger microscopes,” says Vaziri. “But this is an important innovation, and one that could only have come about by thinking about the problem with fresh eyes and freeing yourself from perceived limitations.”
