Microprisms Offer New Insights into Neural Networks

Thanks to techniques pioneered by Yale research scientist and lecturer Michael Levene and his team, the world may soon know more about how information is processed in the brain.

Levene's research focuses on the cortex, the outermost structure of the brain. As part of the brain's "grey matter," the cortex is responsible for aspects of memory, spatial reasoning, sensory perceptions, and even consciousness.

However, though proximity to the skull might make the cortex seem "accessible," imaging all six layers of the cortex has proven difficult. The most utilized in vivo imaging solution, two-photon microscopy, has trouble imaging the deeper cortical layers with high sensitivity and contrast because the tissue layers disperse the light. Moreover, this approach cannot image multiple layers simultaneously, limiting observation of interactions among the neurons in each layer.

To overcome these challenges, Levene's team implanted a glass microprism into the cortex of a mouse. One face of the prism was exposed along the brain's surface, while the other faces were inserted into the brain matter—one face perpendicular to the surface, another at 45-degree angle and coated with reflective aluminum. When a two-photon microscope was pointed at the exposed face, the microprism acted like a "microperiscope," enabling the researchers to simultaneously view all six cortical layers.

This latest study, published in Neuron, improves on Levene's previous microprism use to enable long-term implantation. Thus, while inserting the microprism did have some effect on the neurons nearest the insertion point, the structural integrity and health of most observed neurons was preserved. Neuron behavior could therefore be repeatedly imaged over a period of months, and, in fact, the clarity of the images progressively improved over time.

Levene's team—which included biomedical engineering postdoctoral fellows Nathan Gilfoy and Markus Wölfel, in addition to professor David McCormick from the Yale School of Medicine and Mark Andermann, Glenn Goldey and Clay Reid of Harvard Medical School—hope their methods will extend the reach of two-photon imaging. "Future integration with other optical approaches, like optogenetics, is going to be really exciting," says Levene. "We think this technique has a great future, and is going to make a big impact in neuroscience."