What happens in the brain during mirror therapy?

Although mirror therapy has been shown to be effective for treatment of phantom limb pain, complex regional pain syndrome and other pain conditions, little is known regarding the neural mechanisms responsible. In other words, to the extent that mirror therapy works, why does it work? A recent study imaged the brains of 18 healthy volunteers using fMRI while they moved their right hand. The study was divided into 2 experiments: a no-mirror experiment and a mirror experiment. In both experiments participants performed a series of right-hand finger taps alternating with rests. In the no-mirror experiment, participants could see both hands but were instructed to focus visually on the right hand. In the mirror experiment a mirror was placed between the hands such that it appeared that the right hand was superimposed on the position of the left hand, which was behind the mirror. Participants were instructed to focus visually on the mirror reflection of the right hand (the illusory left hand).

In both experiments, when activation during finger tapping was contrasted with rest, activation was seen in the left precentral gyrus (primary motor cortex) and postcentral gyrus (somatosensory cortex), the left precentral gyrus/superior frontal gyrus (premotor cortex), the right middle temporal gyrus, the left middle occipital gyrus, and the cerebellum.

When cortical activation during the no-mirror experiment was contrasted against activation during the mirror experiment it was found that there was more activation in the mirror experiment than in the no-mirror experiment within the right superior temporal gyrus (STG) and the right superior occipital gyrus.

The STG is an area shown to be involved during the observation of biological motion. The authors indicate that the coordinates of STG activation in this study are very similar to those obtained in a study on imitation of hand movements. It appears then that during the mirror experiment, when participants were observing an illusory left hand (ie. the reflection of their right hand), the mirror neuron system (Rizzolatti & Craighero, 2004) was activated and it was as if they were observing the movement of a left hand distinct from their own.

These results are interesting but run counter to what we might expect. Mirror therapy creates the illusion of the presence of a limb that is in fact missing and appears to result in the correction of the maladaptive cortical migration resulting from deafferentation. Therefore we would expect that mirror therapy would somehow produce activation patterns in the motor cortices contralateral to the illusory limb. Recent TMS (transcranial magnetic stimulation) studies have indeed produced results in support of this hypothesis (Garry, Loftus, & Summers, 2005; Funase, Tabira, Higashi, Liang, & Kasai, 2007). This study, however, found no increased activation of M1 in the right hemisphere (contralateral to the movement of the illusory limb). The authors point out that this divergence in results may be due to the fact that these other studies found increased M1 excitability only when the mirror condition was contrasted against a control condition in which participants moved the right hand but did not directly observe this movement.

Also, I have a feeling that there is something else. In this experiment participants were not asked to mentally image (visualize) the movement of the left hand while observing the illusory left hand. They were therefore not trying to envision movement themselves and this may have accounted for the fact that we only see activity in the mirror neuron system in this study.

So the upshot seems to be that we now have a somewhat better idea of what’s going on when people observe their limbs moving in mirrors but there are still many questions that remain.

• Funase, K., Tabira, T., Higashi, T., Liang, N., & Kasai, T. (2007). Increased corticospinal excitability during direct observation of self-movement and indirect observation with a mirror box. Neurosci Lett, 419(2), 108-12.
• Garry, M. I., Loftus, A., & Summers, J. J. (2005). Mirror, mirror on the wall: Viewing a mirror reflection of unilateral hand movements facilitates ipsilateral M1 excitability. Exp Brain Res, 163(1), 118-22.
• Matthys, K., Smits, M., Van der Geest, J. N., Van der Lugt, A., Seurinck, R., Stam, H. J., et al. (2009). Mirror-Induced visual illusion of hand movements: A functional magnetic resonance imaging study. Arch Phys Med Rehabil, 90(4), 675-81.
• Rizzolatti, G., & Craighero, L. (2004). The mirror-neuron system. Annu Rev Neurosci, 27, 169-92.