Screening sham mirror-therapy materials using fNIRS: protocol for an acute within-participant randomised crossover mechanistic study

Abstract Introduction Selecting an appropriate sham control is a key challenge in trials of mirror therapy, specifically paradigms using mirror visual feedback (MVF), because visually similar control conditions may still elicit mirror-related cortical responses. This protocol describes an acute mechanistic, within-participant fNIRS screening study designed to identify the sham mirror-therapy material condition with the most "neutral" neural signature relative to true MVF during a single exposure. Methods and analysis This is a single-centre, within-participant, randomised crossover study conducted at Wuhan Wuchang Hospital (Wuhan, China). Healthy adults aged 18-35 years will complete four conditions in one visit: C1 true MVF and three prespecified sham-material conditions (C2-C4), with condition order counterbalanced using a Latin-square schedule. fNIRS will be acquired during a standardised grasping task. Online acquisition-time quality control (SCI and CV thresholds) will be applied with prespecified re-acquisition rules. The primary outcome is ROI-level task-evoked change in oxygenated haemoglobin (Delta HbO) within prespecified ROIs (PMC and SM1/M1), estimated primarily using GLM-derived beta estimates. Condition effects will be analysed using linear mixed-effects models with prespecified contrasts and Holm multiplicity adjustment to rank sham conditions by a prespecified neutrality decision rule. Ethics and dissemination Ethics approval was obtained from the Ethics Committee of Wuchang Hospital Affiliated to Wuhan University of Science and Technology (Approval No.: 2025-112-01). Findings will be disseminated through publication of this protocol manuscript and a subsequent results manuscript, with key supplementary materials provided as online appendices/supplements as required by the target journal. Trial registration number Chinese Clinical Trial Registry: ChiCTR2600116634. Strengths and limitations of this study - Within-participant randomised crossover design reduces between-participant variability and is well suited for acute mechanistic screening of sham conditions. - Prespecified sham conditions and neutral-ranking decision rule, including prespecified ROIs, contrasts, and Holm multiplicity control, help limit analytic flexibility and support transparent interpretation. - Operational reproducibility safeguards are specified, including standardised task timing/instructions, acquisition-time QC thresholds with re-acquisition rules, and frozen channel-to-ROI mapping and material-definition records in the Supplementary materials. - Single-centre, healthy-participant, single-session paradigm may limit generalisability to clinical stroke populations and to longer-term therapeutic effects. - Blinding may be imperfect because perceptual differences between materials can affect expectancy and attention; blinding assessment is included but residual bias is possible. - fNIRS is susceptible to motion, scalp-coupling variability and physiological noise; despite prespecified QC and preprocessing, residual artefacts may remain and can reduce sensitivity.