Toward Development of Sham Protocols for High-Definition Transcranial Direct Current Stimulation (HD-tDCS)
High-definition transcranial direct current stimulation (HD-tDCS) is a noninvasive cortical stimulation (NICS) technique that, due to the utilization of multi-electrode stimulation, may enable development of sham conditions characterized by indistinguishable scalp sensations compared to active conditions, with little or no cortical influence. We sought to contribute to the development of an optimal sham electrode configuration for HD-tDCS protocols by gathering ratings of overall sensation reported by participants during different electrode configurations and current intensities. Twenty healthy participants completed a magnitude estimation task during which they rated their “overall sensation” in 1-minute intervals during five 5-minute stimulation conditions. A 5 x 5 (Time x Stimulation condition) analysis of variance (ANOVA) was conducted to determine if sensation measurements differed over time, and how this varied by condition. Null hypothesis significance tests and equivalence tests were conducted to determine which sham conditions were statistically indistinguishable from the experimental condition. The ANOVA revealed main effects for Time and Stimulation condition. Planned comparisons, comparing each sham condition to the experimental condition (4x1 ring configuration, 2 mA), revealed differences in sensation ratings for all but one condition (Sham 1x1A); no sham conditions were found to be statistically equivalent to the experimental condition. Our HD-tDCS findings build upon previous NICS reports of differences in sensation ratings between sham versus experimental conditions when traditional “ramping down” approaches were used. Alternative multi-electrode configurations that manipulate electrode placement to shunt current across the scalp warrant further investigation as valid blinding methods.
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