Mirror box therapy is best known for its use in treating phantom limb pain, Complex Regional Pain Syndrome (CRPS), and paralysis after stroke. It only appears to help a small proportion of patients [1] and understanding the sensory processes that occur during the therapy may help us to discover why this is the case and how it could be adapted to help more people.
Our recently-published study [2] may help shed light on the effect that mirror box therapy has on sensations in the treated limb. We set out to test the idea that conflict between actual and predicted sensations leads to sensory problems in conditions such as CRPS [3]. Specifically, we were interested in whether creating such conflict in pain-free volunteers could reduce their sensitivity to touch. Like many researchers before us [e.g. 4,5], we used a mirror to induce conflict between actual and predicted sensations in the arm.
Rather than using the therapeutic form of mirror visual feedback in which people move their forearms up and down at the same time, our key manipulation was to ask our volunteers to look at the reflective side of the mirror while they moved their arms in opposite directions. That is, our participants moved one arm up while they moved the other down (and vice versa). This meant that the arm behind the mirror moved in the opposite direction to the hand that could be seen in the mirror’s reflection. This form of mirror visual feedback is thought to simulate sensory conflict since, thanks to the mirror, the arm is “seen” to move in the opposite direction to both the felt movement direction and the movement direction that would be predicted based on the motor commands.
We presented touch stimuli to the wrist of the arm that was behind the mirror using a computer-controlled device that was about the size of a 10p piece. For a few seconds at a time, the participants made the arm movements and the device vibrated. In half of the trials, there was a gap in the vibration – that is, the vibration stopped for a fraction of a second. For each trial the participant had to say if there was a gap or not.
If conflict between actual and predicted sensations impairs sensitivity to touch then making asymmetrical movements with mirror visual feedback should make people worse at telling whether or not there was a gap. People’s tactile sensitivity while they performed symmetrical arm movements with mirror visual feedback was also measured.
Unexpectedly, we found that our participants were worse at detecting the gap during the symmetrical movement condition – that is, the form of mirror visual feedback that is used for therapy. In contrast, their sensitivity to touch during the asymmetrical movement condition was no different to two control conditions in which they made symmetrical and asymmetrical movements with a board placed between their arms rather than a mirror. This suggests that sensitivity to touch is reduced while someone performs mirror box therapy.
What was not surprising is that on average participants reported that they had a stronger sense that they were watching the hand behind the mirror (rather than the reflection of the other hand) during the symmetrical movement condition as compared to the asymmetrical movement condition. In other words, the mirror visual feedback illusion was more convincing when they moved both arms in the same direction at the same time.
Although our results were not what we expected, we speculate that they may nonetheless reflect the consequences of sensory conflict. When a person performs mirror box therapy it is unlikely that the movement of the two arms are perfectly coordinated. Instead, there are bound to be small differences in the rate and size of the movement of each arm and their distances from the mirror. This means that even symmetrical movements with mirror visual feedback could produce small degrees of sensory conflict.
In contrast, when a person makes asymmetrical movements with mirror visual feedback there may be less conflict because the differences between the arm that is “seen” in the mirror and the arm that is felt behind the mirror might be so big that the two sources of information are not integrated. Remember – the participants found the mirror visual feedback illusion to be less convincing during the asymmetrical movement condition than in the symmetrical movement condition.
So what does this mean for mirror box therapy? One possibility is that this reduction in sensitivity helps to suppress abnormal sensations such as tingling and numbness in people with CRPS and related conditions. It is also possible that our results do not at all reflect what happens during the therapeutic application of mirror visual feedback, and sensitivity is only affected in people who have otherwise normal sensory integration. In the end, we are yet to crack mirror box therapy.
About Janet Bultitude
Janet is a Lecturer in Cognitive and Experimental Psychology at the University of Bath. She investigates the relationships between sensations and movements and how these interact with attention. Her early career focussed on understanding and treating problems with these processes in stroke patients. More recently she has been studying changes in cognitive processes that are sometimes observed in CRPS, and whether these can be targeted for treatment.
References
[1] Moseley GL, Gallace A, Spence C. Is mirror therapy all it is cracked up to be? Current evidence and future directions. Pain. 2008;138(1):7–10.
[2] Bultitude JH, Juravle G, Spence C. Tactile Gap Detection Deteriorates during Bimanual Symmetrical Movements under Mirror Visual Feedback. PLoS One. 2016;11(1):e0146077.
[3] McCabe CS, Blake DR. An embarrassment of pain perceptions? Towards an understanding of and explanation for the clinical presentation of CRPS type 1. Rheumatology. 2008;47(11):1612–6.
[4] Wand BM, Szpak L, George PJ, Bulsara MK, O’Connell NE, Moseley GL. Moving in an environment of induced sensorimotor incongruence does not influence pain sensitivity in healthy volunteers: a randomised within-subject experiment. PLoS One. 2014 Jan;9(4):e93701.
[5] McCabe CS, Haigh RC, Halligan PW, Blake DR. Simulating sensory-motor incongruence in healthy volunteers: implications for a cortical model of pain. Rheumatology. 2005;44(4):509–16.
Commissioning Editor: Neil O’Connell