High-definition transcranial direct current stimulation (HD-tDCS) is a non-invasive brain stimulation technique shown to modulate neuronal networks. In order for HD-tDCS to be used in randomized, placebo-controlled clinical trials, it is critical to have methods that enable blinding. Some research has shown that sham stimulation is an effective blind in tDCS. However, few studies have investigated the double-blinding of HD-tDCS, especially at intensities greater than 2mA. We address this knowledge gap by examining the blinding and double-blinding of HD-tDCS among a mixed neurologic sample of older adults.

A sample of 240 older adults (Mage = 72.21±8.94) with various clinical diagnoses (Normal Cognition = 34, Amnestic MCI [aMCI] = 172, Dementia-Alzheimer’s Type [DAT] = 27, Other = 7) were recruited through five double-blind, randomized controlled trials. All participants were stimulation naive at their first session and received one to thirty sessions of 20- or 30-minutes of active (n=1472) or sham (n=681) stimulation at total amplitudes of 2mA, 4mA, or 6mA. At the start of each stimulation session, a study team member entered a code into the tDCS unit, and the electrical current was gradually ramped up to the specified (blinded) amplitude over a period of 30 seconds. The current remained at this level for the specified amount of time in the active condition (e.g., 20-minutes) but was ramped down over the next 30 seconds for those in the sham condition. This ramp up/down process was repeated in the final minute (e.g., 20th minute) in the sham session to provide both primacy and recency effects. After each active or sham session, participants were asked whether they received 'real’ or sham stimulation. One study also asked a study team member if they believed the participant received real or sham stimulation at two primary outcome endpoints.

We used Fisher’s Exact tests to evaluate the efficacy of our blinding and double-blinding procedures. In stimulation naive participants receiving their first session, there were no differences in accuracy, suggesting adequate blinding. We also examined participant blinding across all sessions to determine whether repeated HD-tDCS exposure might impact blinding. Across all sessions, participants in the sham condition were more likely to endorse being in the 'real’ (active) condition, again suggesting adequate blinding. There were no significant group differences for active versus sham in the frequency of the study team correctly stating the participant’s condition, suggesting sufficient double-blinding. No significant differences were found in study team blinding when data from the 2mA versus 4mA to 6mA were analyzed separately.

These results suggest that the HD-tDCS sham method is an effective blind and double-blind for HD-tDCS in clinical trials, even at total amplitudes as high as 6mA.