Photons

Stosiek, C., Garaschuk, O., Holthoff, K. & Konnerth, A. In vivo two-photon calcium imaging of neuronal networks. Proc Natl Acad Sci U S A 100, 7319-7324 (2003).
Homma, R. et al. Wide-field and two-photon imaging of brain activity with voltage- and calcium-sensitive dyes. Philos Trans R Soc Lond B Biol Sci 364, 2453-2467 (2009).
Garaschuk, O., Linn, J., Eilers, J. & Konnerth, A. Large-scale oscillatory calcium waves in the immature cortex. Nat Neurosci 3, 452-459 (2000).