Multichannel Systems MEA Manual - Manual (Page 54)

MEA Manual
54
8.2 Capacitive Behavior of Stimulating Electrodes
Regarding the generally used stimulus pulses, stimulating electrodes behave as plate capacitors. The charge
cannot flow back to the stimulus generator due to the high output resistance and thus is kept in the
electrode. The electrode needs a quite long time to discharge itself after stimulation. As a result, stimulus
artifacts interfere with the recording, and electrodes deteriorate over time due to electrolysis. You can avoid
that by choosing an appropriate stimulus protocol that actively discharges the electrode after the pulse.
When using voltage driven stimulation, the electrodes are discharged when the voltage level is set to zero
at the end of the (monophasic) pulse. Not so in current mode. When applying a negative current pulse,
the electrode is charged and needs to be actively discharged by applying an inverted pulse with a matching
product of current and time, that is, you need to stimulate with biphasic pulses for current driven
stimulation to reduce both the stimulus artifact and to avoid an electrode damage. The easiest way is to use
the same signal amplitude and the same duration with an inverse polarity. For voltage driven stimulation,
monophasic pulses are fine.
The following illustration shows the effect of a biphasic current pulse on the discharge of the stimulating
electrode. As you can see, the first monophasic pulse is followed immediately by a pulse of the opposite
polarity and the same product of current and time.