The contribution of interindividual factors to variability of response in transcranial direct current stimulation studies

Abstract

There has been an explosion of research using transcranial direct current stimulation (tDCS) for investigating and modulating human cognitive and motor function in healthy populations. It has also been used in many studies seeking to improve deficits in disease populations. With the slew of studies reporting “promising results” for everything from motor recovery after stroke to boosting memory function, one could be easily seduced by the idea of tDCS being the next panacea for all neurological ills. However, huge variability exists in the reported effects of tDCS, with great variability in the effect sizes and even contradictory results reported. In this review, we consider the interindividual factors that may contribute to this variability. In particular, we discuss the importance of baseline neuronal state and features, anatomy, age and the inherent variability in the injured brain. We additionally consider how interindividual variability affects the results of motor-evoked potential (MEP) testing with transcranial magnetic stimulation (TMS), which, in turn, can lead to apparent variability in response to tDCS in motor studies.

Effects of Transcranial Direct Current Stimulation over Left Dorsolateral pFC on the Attentional Blink Depend on Individual Baseline Performance

“…This effect was only observed during (but not after) anodal stimulation, was not found for ctDCS, and importantly, could not be explained by regression to the mean. No effects of tDCS were observed at the group level, emphasizing the importance of taking individual differences in baseline performance into account, as others have recently argued (Benwell, Learmonth, Miniussi, Harvey, & Thut, 2015;Li, Uehara, & Hanakawa, 2015). tDCS did not affect distractor inhibition, but post hoc analyses revealed low test-retest reliability of our priming measure, rendering it difficult to interpret this result.…”

“…Thus, ctDCS may not always modulate performance. Moreover, as in this study, a growing number of studies report that the effects of stimulation on behavior can vary considerably from individual to individual (Benwell et al, 2015;Krause & Cohen Kadosh, 2014;López-Alonso, Cheeran, Río-Rodríguez, & Fernández-Del-Olmo, 2014; for a review, see Li et al, 2015). Thus, the effects of tDCS on behavior are complex and are difficult to predict without taking into account preexisting individual differences.…”

Intra-Subject Consistency and Reliability of Response Following 2 mA Transcranial Direct Current Stimulation

“…To date, a number of inter-subject factors have been identified as influencing tDCS including anatomical structure Datta et al, 2012) age (Fujiyama et al, 2014;Moliadze et al, 2014), and potentially even genetic profile (for review see Li et al (2015)). …”

Transcranial direct current stimulation in post stroke aphasia and primary progressive aphasia: Current knowledge and future clinical applications

“…Although there is increased interest in tDCS research, very little attention is focused on inter-individual variability in response to tDCS and how this can impact the outcome of a study (see Li, Uehara, & Hanakawa, 2015; Wiethoff, Hamada, & Rothwell, 2014 for detailed reviews). This is especially important in aphasia research because of variability in lesion site, size, recovery mechanisms, and deficit severity levels (e.g., Torres, Drebing, & Hamilton, 2013).…”