According to this brief, and really cool article from Vice, untreated graphene does not interfere with ongoing neuronal activity in a mouse model brain culture. One problem with brain implants is that they can cause scarring in the brain, and if this occurs, then the tissue buildup could degrade the implant's capacity to deliver electric currents reliably. This would in turn reduce the effectiveness of implants like Deep Brain Stimulation.
The full article is in ACS Nano. So go read it there!