Does mirror box therapy reduce sensitivity to touch?

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 Bultitude BathJanet 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

Comments

  1. Janet, interesting study and I appreciate the follow-up discussion with Tara.

    Understanding of the process and the precision in our language is important. Translating it into something meaningful to the patient and their support system is critical. A glossary of terms with the use of mirror therapy for the patient (and their supports including clinicians) would be invaluable.
    There are good guides on the use of mirror therapy for stroke by Andreas Rothgangel and Susan Brain (July 2013) and for pain, The Graded Motor Imagery Handbook by noigroup (with Moseley, Butler, Beames and Giles).

    In a study by Michielsen et al in 2011, they looked at the neuronal correlates of mirror therapy: an fMRI study in patients with stroke. It is helpful that there is level A evidence for mirror therapy (and visual imagery) for stroke.

    What was helpful to me, was that during bimanual movement, the mirror illusion increased activity in the precuneus and posterior cingulate cortex – areas associated with awareness of the self and spatial attention.
    I struggle with precision in my language especially around brain function. I see a number of people who have disruption of somatotopic, limb-centred and body-centred frames of reference (I believe – stroke, amputees and patients with persistent pain including PCS and CRPS). Integration is important.
    Providing meaningful sensory input for integration starts with the conversation that we have with people and our understanding. I find that even the concept of allodynia is sometimes perceived as a relatively static phenomenon with pain from light touch.
    I often, when I listen, have patients describe how they obtain relief from gentle massage from trusted, loving members of their family vs how they have a painful response from the light touch of a ‘well-meaning’ clinician. I use a tape measure to measure edema with figure 8 measurements (validated measure like volumetric measures) – ‘light touch with a purpose’ seems to produce a different response. Mirror therapy with a purpose that is understandable – the role of vision in preparatory movement (pre-shaping for grasp) is well established (and how it can bias proprioceptive information).

    Perhaps with better understanding of all the factors that play into a person’s body schema (visual, proprioceptive, auditory, vestibular, knowledge) perhaps we will get closer to being on the same page. The COMPACT study looking at core outcome measures for CRPS for comparing data is an example (possibly). Towards better understanding.
    I would appreciate if you know of a reasonable glossary of terms with translations for mirror therapy or GMI, you would update me (ex. IASP, BIM).

  2. Tara Packham says

    I think this is an important question, as it will help us to better select which clients are most likely to benefit from mirror visual feedback. However, there is an important taxonomy issue here…is sensitivity and impaired sensation meant to include hypersensitivity, dysesthesia, allodynia and or numbness? All of these sensory disturbances may be experienced by persons with CRPS, and all can contribute to the pain experience.

    Unfortunately, the published trials of mirror therapy have not measured allodynia or pain qualities, so we only know that the overall pain experience has changed. One of the lingering questions for me, however, is to what extent healthy normal subjects can inform our understanding of allodynia, which is one of the pathognomonic characteristics of central sensitization. There is also a need for clinically meaningful ways to accurately measure allodynia in people with central sensitization (from CRPS or other neuropathic pains, so we can measure the specific changes we are targeting, and compare the effects of different treatments.

    Janet Bultitude Reply:

    You raise some important points, Tara

    Although our research provides evidence that mirror box therapy reduces sensitivity touch, we cannot say for sure whether this would translate directly to reductions in allodynia or hypersensitivity in patients with CRPS.

    Wand and his colleagues (2004) tested the effects of mirror visual feedback with symmetrical and asymmetrical movements on pain thresholds in healthy people and found that neither condition changed pain sensitivity in their participants. However, unlike us, they tested the pain thresholds immediately *after* the mirror visual feedback – it may be that pain sensitivity is affected in healthy people, but only while they are actively undergoing mirror visual feedback.

    It can be useful to test the mechanisms of mirror visual feedback in healthy participants because this allows us to test one sensory process at a time (e.g., sensitivity to touch in the case of our study). Since patients with CRPS experience such a wide variety of symptoms, and even the type of pain varies dramatically between individuals, it can be difficult to know what changes are occurring. However, studies in healthy participants can only take us so far, and these need to be combined with patient research.