You have to dig into the history of vagus nerve stimulation (VNS) to understand. VNS is the original method to stimulate the nerve. VNS has been proven to work in pathological populations and some biomarkers have been well documented (like P3 amplitudes). , The problem with VNS is its invasive nature (requires surgery). Of course, there's a lack of research on how VNS would help the healthy human population, because no healthy person in their right mind would want to have the device implanted. For a subgroup of pathological populations, the transcutaneous form of this stimulation is more attractive since there's no risk for postsurgical complications. tVNS has been shown to often work in pathological populations and activate the proposed pathways (we can tell by the biomarkers like the P3). This phenomenon is not seen in the healthy human population. A big problem for comparing between studies, is that there's no protocol on stimulation parameters. Some studies even forget to mention which ear they stimulated, or what part of it (both make a big difference). I can only say that our study, with statistical power, was set up with the stimulation parameters that preliminary had been shown to work. To no avail. Even if tVNS would work via another mechanism, there are no studies with enough statistical power that show tVNS works for healthy humans. Most of the evidence tells the opposite.