Doubts cast on imagery as a rehab tool for stroke patients

Wouldn't it be marvellous if brain-damaged stroke patients could use mental practice to rehabilitate their weakened limbs? This isn't as far fetched as it sounds. Merely imagining performing a movement, or watching someone else execute a movement, provokes activity in the same brain areas that are involved when carrying out that movement with your own body. This suggests imagery exercises could help forge new connections in damaged neural networks involved in actual bodily movement. Indeed, several small-scale studies have reported that mental imagery helps stroke patients recover their limb use, above and beyond the benefits from standard physical therapy.

What's been lacking is a larger study with recently afflicted patients, an adequate control condition, and with the imagery intervention kept separate from standard physical therapy. Now psychologist Magdalena Ietswaart and her colleagues have published the results from just such a study. Sadly the outcome is disappointing.

Ietswaart's team recruited 121 patients within one to six months of their having suffered a stroke, all of whom had significant weakness in one of their arms. Forty-one of these patients were then enrolled on an intensive four-week mental imagery intervention, which involved a total of nine hours supervised exercises and four hours of independent work.

The programme was extraordinarily thorough. As well as basic imagination exercises designed to target the damaged brain areas involved in motor control (e.g. imagining opening and closing the hand), there were also mirror and video techniques to aid the imagination process. For example, placing the weakened hand under a video display of a moving healthy hand can create the illusion that the weakened limb is moving, thus triggering activity in relevant brain areas. There was also a mental rotation exercise, involving rotating pictures of hands - again this has been shown to stimulate the desired motor areas of the brain.

Of the remaining patients, 39 were enrolled on a four-week placebo programme designed to match all the mental effort and therapist attention involved in the imagery programme. But instead of using motor imagery, this group spent their time visualising flowers and other static scenes. A final group of 41 patients had care as usual. Patients in all groups underwent standard physical therapy, but this was kept separate from the imagery work.

When tested soon after the intervention phase, patients in all groups had shown improvement in use of their weakened limb compared with baseline. But here's the rub: there was no difference between groups, either in the amount of limb improvement, or in secondary measures such as independent living. This result suggests the positive outcome for imagery found in previous small studies may have been based on non-specific effects, such as increased motivation. Alternatively, it may be that mental imagery only works as an adjunct to physical exercises, helping to consolidate the progress made with specific, related movements. This new study is the first to study mental imagery as a separate intervention in its own right.

The new findings undermine the idea that mental imagery on its own can help the brain forge new functional connections. If imagery only works by consolidating the benefits of related physical exercise, the researchers said this would significantly diminish its value as an rehabilitation intervention. Apart from anything else, they noted, it would suggest mental imagery could only be used to help patients who are already capable of performing physical exercises.
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Ietswaart, M., Johnston, M., Dijkerman, H., Joice, S., Scott, C., MacWalter, R., and Hamilton, S. (2011). Mental practice with motor imagery in stroke recovery: randomized controlled trial of efficacy. Brain, 134 (5), 1373-1386 DOI: 10.1093/brain/awr077

This post was written by Christian Jarrett for the BPS Research Digest.

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