Electrical Stimulation Artifacts - open-ephys/wiki GitHub Wiki

Electrical stimulation artifacts are unwanted electrical signals that appear in neural recordings that are conducted during ongoing electrical stimulation. They occur because charge from the stimulation circuit leaks into the recording circuit through parasitic paths.

Where do stimulus artifacts come from?

Your stimulation electrode is, more or less, a small capacitor. When you apply a biphasic current pulse to a capacitor, you linearly ramp its voltage up and down. If your pulses are perfectly charge balanced (the time integral of the current pulse is zero coulombs ), then you should have no artifact. But, this is not going to be the case. All electrical circuits have imperfections. This means that the positive and negative phases of your pulses are not perfectly balanced. Because there is no discharge path for this extra charge, it just hangs around on the electrode slowly leaking out for a long period of time. The accumulated voltage can be in the range of 10s of volts (~6 orders of magnitude larger than the neural signals you are interested in!). This of course is going to force its way into your recordings through various parasitic paths and the result is what you see.

Here is a simulation of your situation that illustrates the point. Its a current source driving a leaky capacitor (your electrode).

The current waveform is on the top traces. The positive pulse is 100 uA and the negative ranges from 100 uA to -150uA. The resulting electrode voltage is shown on the bottom traces. You can see that as the current pulse becomes less balanced, you have to wait longer and longer for the 10s of volts on the electrode to slowly discharge.

Note the log time scale.

What is the purpose of a stimulus isolator?

A stimulus isolator's job is to take the stimulation circuit off the common ground of your recording system. Because the stimulus pulse (ideally, but not perfectly in the real world) is isolated (i.e. on a separate floating circuit with no current path in common with your acquisition system save some capacitive coupling), it should be far less likely to affect it, even if the pulse is unbalanced. If you are grounding one terminal of your stimulus isolator, then you would completely lose this benefit, so don't do that!

Active discharge

To deal with the imperfections of electrical isolation, modern stimulus isolators include a circuit that actively discharges the electrode after a pulse has concluded. To do this, they short the stimulator's electrodes briefly after a pulse to remove the excess charge that has accumulated following a stimulus pulse. Because modern microcontrollers are fast, this can even be done on a pulse by pulse basis allowing recovery on sub-millisecond time scales. For instance, the Open Source Stimjim stimulus isolator and waveform generator provides this capability.

The multiplexer in the above schematic is used to short stimulator electrodes when the stimulus pulses are not being delivered. This will have a large mitigating effect on stimulus artifacts. The stimjim paper provide more information on the circuit's operation and capabilities.