(A) Main principle of the VSDi system. Commonly used voltage sensitive dye di-4-ANEPPS, or other dyes, are able to provide measurements of membrane potential of single neurons or large neuronal populations. Dye molecules are localized in membranes with their hydrophobic tails and transduce membrane potentials changes into the optic features. (B) A VSD optical images showing mouse whisker stimulation fluorescence changes . The stimulus onset was at the beginning of first frame number one, the time after stimulus onset is indicated at the bottom left corner of each image.