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Can mirror imaging fool the brain? And can that trick help people with phantom limb pain?

Professor Liz Franz and her research team have been taking a closer look at the effect of ‘mirror therapy’ — which involves a mirror box reflection of a healthy limb to examine the neural processes at play.

So here's the back story – It is well known that when one hand moves, the motor cortex on the opposite (contralateral) side of the brain becomes active. That is, when the left hand moves, the right motor cortex becomes active. Mirror box research suggests that when you see the reflection of your left hand moving by use of a mirror – making it look like your right hand – the left motor cortex (which is ipsilateral to actual movement) is activated as well. Researchers in Franz's lab, in particular, completing PhD student Ranjan Debnath, have been investigating the nature of this effect, as there has been some debate over how much activation there is, and whether it can be explained in other ways. Further research suggests that this activation - if it in fact happens - could be useful in treatment of phantom limb pain for missing or impaired limbs. Whether it is effective as a therapy following unilateral stroke is also under study. So it is important to establish clearly that this activation which is ipsilateral (on the same brain side) to a moving hand can be measured, quantified, and interpreted in a reliable and meaningful way following use of the mirror reflection technique.


Professor Franz's team has been very conservative in their approach, ensuring that any other reason for the activation is subtracted from the equation. Continuous EEG readings of neural activity were taken and at the same time electrodes measured movements of the arms/hands to make sure that no involuntary movement triggered the ipsilateral brain activity. Debnath's PhD work led to a recent paper in Cortex which summarises:

“After subtraction, clear activation remains on the critical side ipsilateral to movement”

Further work in a recent paper in Restorative Neurology and Neuroscience examined the idea of limb ownership; again, this idea is central to the possibility of using the mirror reflection method for therapeutic treatment of conditions such as persistent phantom pain. The perception that the mirror hand is in fact the subject's actual hand is a key part of the release from pain. But previous research has relied on participants' subjective responses (“yes, I really thought it was my own moving hand”) to questions some time after the actual testing was done. So Franz's team (including postdoctoral fellow Fu Yan, PhD student Matt Moore MSc students Taylor Winter, Terence Mayne, and visiting honours student, Catherine Stringer) aimed to pin that down as well.

They introduced a foot pedal into the research method so that at the precise point where the participant felt like the mirror image was their actual hand, they would tap the foot pedal. This allowed researchers to align the perception with the EEG reading for that moment. Franz explains, “through careful experimental manipulations we worked to subtract other possible factors (such as activity from the foot press) from the equation. This type of subtractive logic was first introduced by Franciscus Cornelius Donders way back in the 1800s and it is a very useful cognitive tool.”

“Other studies have used fMRI to localise brain activation when examining these types of issues, but the EEG paired with the foot pedal methodology allows us to match the perception and the brain activity in time. Capturing precise brain changes in association with perceptions of movement seems critical in any therapeutic developments using the mirror box. It's very exciting!”