TDCS effects on word production: Limited by design? Comment on Westwood et al. (2017)

Abstract

Transcranial direct current stimulation (tDCS), has sparked enormous scientific, clinical and public interest (Dubljevic, Saigle, & Racine, 2014; Riggall et al., 2015), because of its potential to modulate human brain function and behaviour without significant side effects. It thereby offers exciting prospects for clinical applications (Bikson et al., 2016). However, recent publications have highlighted substantial variability among reported stimulation effects in healthy individuals (e.g., Wiethoff, Hamada, & Rothwell, 2014) or even questioned the potential of tDCS to induce behavioural effects on cognition and motor function (Horvath, Forte, & Carter, 2015a,b). While the latter have attracted criticism for conceptual and methodological reasons (Antal, Keeser, Priori, Padberg, & Nitsche, 2015; Chhatbar & Feng, 2015), they have nonetheless motivated reflections on the use and the efficacy of tDCS and prompted urgent calls for more rigorous methodology, including replication studies (Fertonani & Miniussi, 2017). In this vein, a recent paper by Westwood, Olson, Miall, Nappo and Romani (2017) published in Cortex reported an attempt to replicate previously observed effects of tDCS on semantic interference during spoken word production using continuous and blocked cyclic naming paradigms (Damian, Vigliocco, & Levelt, 2001; Howard, Nickels, Coltheart, & Cole-Virtue, 2006). Across four experiments, active tDCS was administered to frontal and temporal cortical sites with the authors reporting null effects compared to sham stimulation, followed by far-reaching conclusions concerning the utility of tDCS to modulate cognition in healthy participants. In this commentary, we discuss a number of problems with Westwood et al.'s report, including their theoretical assumptions, choice of stimulation sites, use of reading and naming tasks in the same experiment, stimulation protocols, data analyses and interpretation of their null findings as a “test” of tDCS' efficacy. We conclude with a brief reminder concerning the proper use of the term replication, and recommend measures to be taken to ensure greater rigour in tDCS research conduct and reporting.