Novel Live Imaging Method Couples Functional and Structural Information

MIT researchers have developed a new way to couple functional maps of the brain precisely and consistently to matching distinctions of physical structure. Their method, demonstrated in live mice performing functions of interest, provided distinguishing structural information for each region all the way through the cortex into deeper tissue below.

"Our study shows for the first time that structural and functional coupling of visual areas in the mouse brain can be detected at sub-cellular resolution in vivo," explained Mriganka Sur, senior author of a paper published recently in Biomedical Optics Express.

The technique combines retinotopic mapping with third-harmonic generation (THG) three-photon microscopy. In retinotopic mapping, researchers can identify functional regions by engineering neurons to flash when they become electrically active in response to a particular stimulation. Three-photon microscopy can finely resolve individual cells and their smaller substructures as deep as a millimeter or more. THG, meanwhile, adds the capability to finely resolve both blood vessels and the fibers of myelin.

Crucially, THG yields an important optical measure called effective attenuation length (EAL), which is a measure of how much the light is absorbed or scattered as it moves through the tissue. In the study, the team showed that EAL specifically depends on each region's unique architecture of cells, blood vessels, and myelin. They measured EAL in each of six visual functional regions and showed that the EAL significantly differed among neighboring visual areas, providing a structural signature of sorts for each functional area. Their measurements were so precise, in fact, that they could show how EAL varied within functional regions.

In other words, by combining the retinotopic mapping with THG three-photon microscopy, first author Murat Yildirim explained, scientists can identify distinct regions by both their function and structure while continuing to work with animals in live experiments. This can produce more accurate and faster results than making observations during behavior and then dissecting tissue in hopes of relocating those same exact positions in preserved brain sections later.

"This advance should enable similar studies of structural and functional coupling in other sensory and non-sensory cortical areas in the brains of mice and other animal models," they wrote. "We believe that the structural and functional correlation in visual areas that we describe for the first time points to crucial developmental mechanisms that set up these areas, thus our work would lead to a better fundamental understanding of brain development, and of disorders such as Alzheimer's, stroke and aging."