Optimizing the mirror illusion during mirror therapy: evidence from unimpaired individuals
Jin-Min Kim, Freya O. Challis, Carmen C. Y. Koo, Jade-Cheuk L. Leung, Leo H. Lo, Sang-Hoon Yeo, T. David Punt · Frontiers in Psychology · 2025
Introduction: Mirror therapy has demonstrated functional benefits for patients recovering from hemiparetic stroke, with its effectiveness primarily attributed to the induction of a compelling visual illusion that engages sensorimotor networks. Although previous research has identified various intervention parameters influencing therapeutic outcomes, a comprehensive understanding of their effects on the illusory experience remains limited. This study investigated how four critical parameters-mirror size (large vs. small), object manipulation (present vs. absent), task complexity (simple vs. complex), and movement execution (unilateral vs. bilateral)-modulate the believability of the mirror illusion in neurologically unimpaired individuals. Methods: Forty healthy participants performed movements under 16 different combinations of these parameters while receiving mirror visual feedback and rated the believability of the reflected hand on an 11-point Likert scale. Results: Repeated-measures ANOVA revealed that a large mirror consistently enhanced the illusory experience compared to a small mirror. Although bimanual movements generally resulted in higher believability ratings than unimanual movements, this advantage diminished when complex object manipulation tasks were introduced. Discussion: These findings suggest that the congruency of multisensory information-between visual, proprioceptive, and motor signals-is critical for maximising the strength of the illusory experience. By identifying the optimal conditions for enhancing the mirror illusion in healthy individuals, this study establishes a foundational framework for adapting and refining mirror therapy protocols in clinical populations.