Cutaneous perception during tDCS: role of electrode shape and sponge salinity.

Transcranial direct current stimulation (tDCS) is a noninvasive method of brain modulation that is increasingly tested for the treatment of neuropsychiatric disorders (Murphy et al., 2009) and cognitive enhancement (Paulus, 2004; Talelli and Rothwell, 2006). Conventional tDCS protocols apply 1–2 mA of current, for several minutes, through conductive-rubber electrodes inserted in sponge wrappers, which are typically soaked in saline, before being placed on the scalp. tDCS has many useful characteristics including low cost, ease of use, portability, and absence of significant side-effects. Indeed, during tDCS, mild tingling or itching sensation are the most common adverse effects (Poreisz et al., 2007), and though isolated cases of skin burns have been reported (Lagopoulos and Degabriele, 2008; Palm et al., 2008), relatively large scale experiences from several active centers, including at Gottingen, suggest that under proper protocols, significant adverse events can be avoided (Dundas et al., 2007; Loo et al., 2010; Poreisz et al., 2007). Acute sensation under electrodes during DC stimulation is well established (Leeming et al., 1970; Mason and Mackay, 1976) and is highly dependent on both stimulation intensity and electrode design (Dundas et al., 2007; Forrester and Petrofsky, 2004; Martinsen et al., 2004; Minhas et al., 2010). Sensation does not simply correlate with either skin damage or brain modulation (Bikson et al., 2009) because of the importance of electrode design and montage (for example, decreasing the distance between electrodes decreases total brain but not skin current). None-the-less, sensation is clinically significant in itself for several reasons including tolerability (especially in vulnerable populations), confounding of experimental and clinical results, and blinding. The report in this issue by Ambrus and colleagues in Gottingen evaluated sensation differences for surface-area matched (35 cm) rectangular and round electrodes. For anodal and cathodal tDCS, as well as tRNS, they found no substantial differences in detection threshold, detection rate, false-positive rate, or quality of sensation. It is well established, including through computational modeling studies, that during electrical stimulation, current distribution at the electrode-tissue (skin) interface is not uniform, with high current density at the electrode edges (Miranda et al., 2006). The current density at an electrode edge is generally undesirable for safety reasons (especially for implanted electrodes; (Merrill et al., 2005)) and may increase sensation during transcutaneous stimulation. Note that during transcranial electrical stimulation, subsequent current dispersion across deeper tis-