I've just started recording evoked potentials in mouse neocortical brain slices, using a constant voltage stimulus (0.1Hz at approx 1V). I see that most people use constant current generators for this purpose. Is a constant current stimulus essential to ensure stable responses over 1-2 hours?
It is desirable to use constant current and not constant voltage for the stimulation. The impedance (resistance) of the tissue is changing due to some electrochemical processes (polarization, oxydation/reduction). Thus, in constant voltage stimulation, the current delivered to the tissue will be dependent on the resistance of the electrodes and the tissue, simply by the Ohm's law. However, in the case of constant current stimulation, the changes in resistance of the electrode/tissue will have a little effect on the delivered current. Again, according to the Ohm's law, the resistance could increase significantly and diminish the current injected to the tissue. However, one can keep the current constant, and overcome the changes in resistance by adjusting the voltage. Obviously, there is a limit of how much we can increase the voltage to overcome the resistance changes >> stimulation compliance. Most constant current stimulators have a compliance of tens to hundreds of volts.
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