We thank members of the laboratory for and comments. Supported by the Kavli Institute for Brain Science, the NINDS and the NEI.  Key Concept
Calcium indicator dye  Fluorescent molecule which binds to calcium and whose fluorescence spectrum is altered by whether or not it is bound to calcium. By monitoring fluorescence of a population of these molecules it is possible to assess changes in calcium concentration in a compartment, such as a cell. “Calcium imaging” is imaging utilizing a calcium indicator dye and can be used to detect action potential firing since each spike increases the intracellular free calcium concentration.
Photobleaching  Process wherein fluorescent molecules lose their ability to fluoresce after being exposed to excitation light, due to chemical changes induced in those molecules by the excitation light itself.
Two-photon imaging High resolution imaging technique wherein rather than a single photon exciting molecules to fluoresce, the energies of two long-wavelength low-energy photons are combined. Spatial resolution improved by this non-linear excitation since fluorescence only occurs at the regions of very highest photon concentration thereby, for most practical applications, restricting that excitation to within 1 micron of the focal point. In addition, near-infrared light is typically used for two-photon excitation, thus allowing for deeper tissue penetration and imaging of thick biological specimens (including in vivo imaging) due to reduced unspecific scattering and absorption at longer wavelengths.
Fluorescence  Emission of photons by molecules after those molecules absorb and are therefore “excited” by photons. This process occurs via excitation of an orbital electron of the molecule to a higher quantum state (upon absorption of excitation photon). With subsequent relaxation of that same electron back to its ground state, a low energy photon is emitted.
Galvanometer mirrors  Small mirrors mounted on electrical devices which rotate based on the current passing through them (galvanometers) – used in these applications to direct a laser beam. Mirrors are mounted in very close proximity and orthogonally and by rotating independently these mirrors fully control the two dimensional angle of the outgoing laser beam.
Pulsed lasers  Many lasers produce pulses or groups of photons in bursts at some frequency rather than a continuous stream of light. Included in this group are the “ultrafast” lasers (usually based on titanium–sapphire) used in two-photon microscopy; these typically produce pulses of approximately 140 fs duration and at 80 MHz repetition rate. Ultrafast pulsed lasers effectively “pack” the energy of continuous radiation into very short pulses, thus producing extremely high instantaneous intensity of light necessary for efficient non-linear excitation.
Diffraction  Change in direction of propagation of waves such as light (electromagnetic wave) upon encountering an inhomogeneity in its propagation medium. By selectively diffracting different cross-sectional regions of an incoming beam wavefront, those waves will “interfere” or superimpose with each other such that they either cancel each other or reinforce each other to create a desired 2D output pattern.
Brendon O. Watson is a resident in the Psychiatry Department at the Weill Cornell Medical College and New York Presbyterian Hospital in New York City. He completed his M.D. and Ph.D. at Columbia University, where he worked with Rafael Yuste, M.D., Ph.D. focusing on the study of network dynamics in the mouse neocortex and on the refinement of two-photon imaging methods. Along with his residency, he is currently conducting postdoctoral research in the laboratory of Gyorgy Buzsaki, M.D., Ph.D. at Rutgers University in Newark, NJ where he continues to focus on the study of network dynamics with an emphasis on linking them to function and behavior.