From Protocol to Practice: Hidden Sources of Variability in Transcranial Direct Current Stimulation.

Transcranial direct current stimulation (tDCS) is widely used in neuroscience research and clinical contexts because it is non-invasive, relatively inexpensive, technically accessible, and has a highly favorable safety profile. However, its apparent simplicity can obscure implementation challenges and practical sources of variability that influence stimulation quality, participant comfort, reproducibility, and interpretation of results. Standard protocols typically specify current intensity, stimulation duration, electrode size, and montage, but less attention is often given to device selection, electrode preparation, conductive medium, hair and scalp management, impedance troubleshooting, electrode fixation, positioning accuracy, and operator-dependent decisions. These aspects are difficult to fully standardize because they depend on participant-specific factors, devices and consumables, and the operator's skills and experience. Nevertheless, they are central to protocol fidelity and should be treated as methodological variables rather than minor technical details. This review discusses practical challenges in implementing tDCS and provides experience-based recommendations for improving accuracy, quality, and consistency across sessions, participants, and research/clinical sites. The focus is on the "hidden" procedural steps that determine whether a nominally identical stimulation protocol is actually delivered in a comparable way. The discussion on these practical implementation aspects aims to complement existing safety and reporting guidelines and highlight the role of practical training in achieving reliable and reproducible tDCS delivery.