Perceptual effects of the mirror box training in normal subjects

BACKGROUND AND PURPOSE: The mirror box (MB) was developed in the early 1990 s to relieve phantom limb sensations and chronic pain. Although its efficacy has been shown in several clinical populations, the mechanisms underpinning effects still have to be fully understood. METHODS: 48 healthy subjects participated in 4 behavioral experiments where kinesthetic sensitivity of the hand inside the MB was tested during the observation of mirror-reflected finger movements, executed with the hand outside the MB. RESULTS: We identified two effects on the hand hidden inside the MB: diminished kinesthetic sensitivity for passive movements, and the occurrence of unconscious, involuntary movements with the finger inside the box, suggesting reduced motor awareness. Such sensory-motor effects were somatotopically specific to the finger moved, were influenced by the contextual presence of the whole hand in the mirror, and occurred for both active and passive movements of the hand outside the MB. CONCLUSIONS: The present results suggest that the MB induce a somatotopically and contextually specific overriding of kinesthetic control by vision, compatible with a process of embodiment of the mirror-reflected hand image and provide novel clues to the understanding of the mechanisms underlying MB effects.