Combined Voltage and Calcium Epifluorescence Imaging In Vitro and In Vivo Reveals Subthreshold and Suprathreshold Dynamics of Mouse Barrel Cortex

1 Laboratory of Sensory Processing and 2 Flavour Perception Group, Brain Mind Institute, Ecole Polytechnique Federale de Lausanne, Lausanne; and 3 Institute of Physiology, University of Bern, Bern, Switzerland Submitted 6 November 2006; accepted in final form 10 March 2007 Cortical dynamics can be imaged at high spatiotemporal resolution with voltage-sensitive dyes (VSDs) and calcium-sensitive dyes (CaSDs). We combined these two imaging techniques using epifluorescence optics together with whole cell recordings to measure the spatiotemporal dynamics of activity in the mouse somatosensory barrel cortex in vitro and in the supragranular layers in vivo. The two optical signals reported distinct aspects of cortical function. VSD fluorescence varied linearly with membrane potential and was dominated by subthreshold postsynaptic potentials, whereas the CaSD signal predominantly reflected local action potential firing. Combining VSDs and CaSDs allowed us to monitor the synaptic drive and the spiking activity of a given area at the same time in the same preparation. The spatial extent of the two dye signals was different, with VSD signals spreading further than CaSD signals, reflecting broad subthreshold and narrow suprathreshold receptive fields. Importantly, the signals from the dyes were differentially affected by pharmacological manipulations, stimulation strength, and depth of isoflurane anesthesia. Combined VSD and CaSD measurements can therefore be used to specify the temporal and spatial relationships between subthreshold and suprathreshold activity of the neocortex. Address for reprint requests and other correspondence: C. Petersen, Laboratory of Sensory Processing, Brain Mind Institute, SV-BMI-LSENS, Station 15, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland (E-mail: carl.petersen{at}epfl.ch )