Two-photon imaging was performed using a laser scanning system Femto2D-uncage (Femtonics, Budapest, Hungary). The imaging laser wavelength was set to 830 nm and to 840 nm in the uncaging experiments (Mai Tai HP Deep See, SpectraPhysics, Santa Clara, California or Chameleon, Coherent, Santa Clara, California). The excitation was delivered to the sample, and the fluorescence signal was collected using an lens (Olympus, Tokyo, Japan, , NA 1.0) and then separated using dichroic mirrors (700dcxru, Chroma Technology, Bellows Falls, Vermont) before the two channel detector unit which was sitting on the objective arm. For more technical details about the two-photon imaging system see Refs. 9, 11, and 12. To minimize photodamage, the laser intensity was always kept at the minimum required to attain a sufficient signal-to-noise ratio.13,20 Free line scans were placed following the curvature of long dendritic segments to monitor the backpropagating action potentials (AP) and uncaging induced signals.13 To measure population activity, neuronal somata were scanned with constant speed using lines which slightly extended the somata to decrease scanning-induced noise artifacts, while intermediate sections were jumped over within using a spline interpolated path (multiple line scanning method).21 As all unnecessary background areas were avoided in this way, the method provided an increased signal-to-noise ratio and a higher measurement speed. The scanning time of about 6 ms allowed us to analyze the spatiotemporal properties of compartments along long dendritic segments. Neuronal cell bodies were usually scanned at a depth of 80 to from the surface of the acute slices.