Optically teasing apart neural swelling and depolarization.

We measured birefringence, 90 degree scattered light, and voltage sensitive dye changes from lobster walking leg nerves. Systematic application of key chemical agents revealed separate cellular mechanisms underlying fast optical signals. Each agent exhibited mixed effects, some having a greater effect on cellular swelling and refractive index, and some altering membrane potential. Birefringence changes were tightly correlated with voltage sensitive dye signals and were perturbed by those agents that altered membrane potential. Signals from light scattered at 90 degrees corroborated the hypothesis that large angle scattering signals arise from changes in the interstitial spaces and were perturbed by those agents that altered cellular swelling and refractive index. We conclude that multiple cellular mechanisms can be exploited to measure rapid optical signals. Since birefringence produces much larger changes than scattering, the use of polarized light might lead to improvements in imaging neural activity with high temporal resolution, especially since birefringence changes corresponded closely to membrane potential.