I think the answer is neither here nor on Wikipedia but forgive me if it is, I don't read maths well. This question applies specifically to current density during transcranial stimulation with either DC or AC currents.
I know that current density for DC currents can be calculated as current/area (A/m^2). e.g. if I have 1mA (anodal or cathodal) current running between 2 electrodes with a surface area of 35cm^2 each, I'll get a current density of 1mA/35cm^2=0.0285mA/cm^2 at each electrode.
For alternating currents (with polarity change), however, it is not clear to me whether I should use the peak to peak amplitude (i.e. the difference between max.anodal current and max. cathodal current), or half peak amplitude (the absolute difference between 0 current and max. anodal/cathodal current intensity - which is equivalent if the AC current is symmetrical) in this calculation. Is it the maximum value or the slope (max. value minus min. value) that is important?
e.g. if I apply an AC current, alternating between -1 and 1mA, is my current density the same as for DC stimulation (because of absolute current being maximally 1mA in either cathodal or anodal direction) or do I have to double it (because of a net difference of 2mA between maximal anodal and maximal cathodal current)?
In the academic literature on transcranial AC stimulation either definition has been applied but I suspect only one can be correct.