Fig. 1. Optical mapping of electrical activation in the embryonic chick heart. (A) Setup consisting of epifluorescence microscope, temperature-controlled oxygenated organ bath and computer-controlled intensified high-speed camera. (B) Brightfield image of E6.5 heart immediately after recording. (C) Epifluorescence signals of di-4-ANEPPS from the same heart captured by the high-speed camera (80x80 pixels). (D) An example of changes in fluorescence intensity over time from a 4x4 pixel region of the ventricle (raw 12 bit data). Drops in fluorescent intensity level correspond to depolarization. (E) A typical action potential recording from a single pixel that was inverted, digitally filtered (white line), and the first derivative (yellow) calculated. Peak of the first derivative, corresponding to maximum upstroke velocity, was determined (orange) to mark the time of activation of individual pixels. x axis scale in milliseconds. (F) An example of two-dimensional array of the optical mapping data. la, left atrium; ra, right atrium; lv, left ventricle; rv, right ventricle.