Effects of replacement of extracellular sodium. (A) Joseph cell voltage clamped at 0 mV. After the first light stimulus, the solution was switched to 0 Na/NMDG, causing the photocurrent to become outward. (B) Plot of the photocurrent as a function of time for the full experiment, showing the stable outward light-evoked current in sodium-free solution. Light stimulus, 3.7 × 10¹³ photons/cm². (C) Photostimulation (4.45 × 10¹³ photons/cm²) applied at different holding voltages to another Joseph cell, showing outwardly directed photocurrent above −30 mV. (D) l-V relation of the photocurrent in sodium-substituted extracellular solution in the same cell as C. The interpolated reversal potential was ≈−26.4 mV.