What can we learn from the optically recorded epicardial action potential?

Optical mapping using voltage-sensitive fluorescent dyes has become a major tool for studying excitation propagation in the heart. Computational and experimental studies have indicated that the optical upstroke morphology reflects the orientation of the subsurface excitation front. In a recent whole heart computational study performed by Bishop et al. (Bishop, M. J., B. Rodriguez, J. Eason, J. P. Whiteley, N. Trayanova, and D. J. Gavaghan. 2006. Synthesis of voltage-sensitive optical signals: application to panoramic optical mapping. Biophys. J. 90:2938-2945), an example was provided of two different directions of propagation having nevertheless very similar epicardial optical upstrokes. The goal of this comment is to clarify the interpretation of optical upstroke morphologies and reconcile the results obtained by Bishop et al. with previous computational and experimental studies.