Gabriele,

I think when a DC offset is added to tACS protocol, the overall stimulation effect will be composed of both tDCS and tACS effects. This means that the researcher aims to see the combined effect of tDCS and tACS together. However you don't have to combine these two together. I have seen many studies based on pure tACS (with zero offset). As a result depending on your choice you will decide to use pure tACS (without any DC offset) or a tACS with DC offset. 

Ehsan,

thank you very much for your answer.

How is it possible that the same current generator provides direct and alternating current simoultaneously? Despite tDCS and tACS  belong to the same family of neuromodulatory techniques, they are based on two different approaches and I never read about their co-use. May you suggest me any papers in which this is reported?

Thank you again.

Best.

Gabriele

Gabriele,

I am not sure how, but it must be possible to add a dc offset to an AC signal using some analog electronics circuits. Best to ask from an electronic engineer. I suggest emailing to technical teams at one of the current tDCS device producers like NeuroConn.

 I am not fully aware of studies where tACS is used with a nonzrero offset. The only published works might belong to Lisa Marshall (Lubeck). Please check this.

Are you planning to use tACS with a nonzero offset? This might be interesting. The DC part might contribute to long-lasting (plasticity-related) effects while the AC part might contribute to online modifications of endogenous brain rhythms.