Detection of Neural Action Potentials Using Optical Coherence Tomography: Intensity and Phase Measurements with and without Dyes

We review the use of optical coherence tomography (OCT) for detection of neural activity, and present a new approach for depth-localization of neural action potentials (APs) using voltage-sensitive dyes as contrast agents in OCT. A stained squid giant axon is imaged by spectral-domain OCT. Changes in the intensity and phase of back-scattered light coming from regions around the membrane are measured during AP propagation. The depth-resolved change in back-scattered intensity coincides with the arrival of AP at the measurement area, and is synchronous with the changes in transmitted light intensity and reflection-mode cross-polarized light intensity measured independently. The system also provides depth-resolved phase changes as an additional indication of activity. With further investigation our results could open a new era in functional imaging technology to localize neural activity at different depths in situ.