Andermann, M.L. et al. Neuron 80, 900–913 (2013).

An opening in the mouse skull, or cranial window, enables imaging the neurons in the living brain using two-photon microscopy. But using this approach, one can generally only image the most superficial layers of the cortex. To overcome this, Andermann et al. use prisms. They inserted 1-mm-sized glass prisms into the mouse brain, under the cranial window. The hypotenuse of the microprism is coated with aluminum and serves as a mirror that projects the incident light. In this way, the researchers imaged across the vertical plane of the prism and captured neurons across all six layers of the cortex. Having shown previously that the method could be used for acute imaging sessions, Andermann et al. now report that the approach also enables long-term imaging experiments. They use a calcium reporter to simultaneously image the activity of neurons across different cortical layers during locomotion behavior and visual stimulation.