Impulse Response EEG Amplifier
“Impulse” response is hard to measure in practice. It’s more commonly calculated as the derivative of the much more practical step response. 

It’s possible, though somewhat risky, to use a nerve stimulus generator to generate the step. After all those things can produce >400V while your EEG amplifier maxes out at  &lt;&lt;100mV. Adjusting the current and using a divider to protect the amplifier a ~1mV step should be possible. 

But, your stimulator does not generate steps, just pulses. So you would likely have to max out the duration and deal with a bit more complicated math. 

It might be much more practical/easier to buy/put together a simple slow square wave generator with a calibrated voltage output.