Averaging over Depth During Optical Mapping

Optical mapping does not measure Vm at the tissue surface, but instead averages over some depth below the surface.168 Efimov et al.169 observed double-humped action potentials during optical mapping, which they interpreted as being caused by three-dimensional scroll waves rotating below the tissue surface. Bray and Wikswo170 simulated such scroll waves and found that shallow reentrant waves can indeed give rise to double-humped signals.

Averaging over depth also affects the measured Vm signal during electrical stimulation. Janks and Roth171 found that if the electrical length constant is less than the optical decay constant, then the averaged signal severely underestimates the true surface transmembrane potential. Several researchers have observed evidence of electroporation after an electric shock,172-174 but the optically measured transmembrane potential deviated from its resting value by less than 100 mV, which should not be sufficient to cause electroporation. Al-Khadra et al.174 suggested that the optical signal might be collected over depth, with only the surface layer actually electroporated. Janks and Roth175 examined this question using a numerical simulation and concluded that averaging over depth may indeed explain why a shock can cause electroporation while appearing to have a small transmembrane potential. However, they could not explain why the deviation of the resting potential, cited by the experimentalists as evidence for electroporation, was not also underestimated because of averaging over depth. Averaging over depth represents one of the pitfalls researchers face when comparing experimental data to numerical simulations,9 but the development of good models of photon diffusion should remove major uncertainties in the comparison of theory and experiment.176-179

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