One Form of tDCS Does Not Fit All: The intersectionality of severity level and expected tDCS benefit

Abstract

AbstractBackgroundWe investigated if a mixed group of people living with dementia (Alzheimer’s Disease (n = 16) or Frontotemporal dementia (n = 8)) would have faster response times on an N‐Back task (1‐back), a measure of working memory, after practicing the task with 20 minutes of tDCS, versus an analogous placebo condition. Crucially, half of these participants were slower than normal on the task at baseline, while the other half has normal response times. Our primary outcome measure was improvement on the MMSE task.MethodEach participant practiced the n‐back in three separate nine‐session rounds, two months apart, with a different form of stimulation in each round: (1) 4mA unilateral stimulation, with one anode electrode over the left dorsolateral prefrontal cortex area and a cathode electrode over the occipital lobe area; (2) 4mA bilateral stimulation, using the same configuration, but with an extra active anode electrode over the right dorsolateral prefrontal cortex, and (3) SHAM stimulation.ResultWe found the condition x time x severity variable was significant (F (12, 264) = 2.17, p < .05), which suggests the results founds were dependent both of the tDCS montage received and the initial severity level of the participants. More specifically, as supported by post‐hoc tests, participants initially impaired on the N‐Back task improved on the MMSE when they received the bilateral tDCS montage, but those participants with normal response times improved on the MMSE when they received the unilateral tDCS montage.ConclusionConsistent with past studies, results suggest tDCS can be used to improve cognition in people living with dementia. Optimization, however, may depend both on the montage applied and the indiosyncratic nature of the participant. A bilateral montage, for example, that theoretically impacts a wider range of brain areas, may be superior to a unilateral one when people with dementia exhibit a greater degree of impairment.