Mirror therapy (MT) has been shown to be a useful intervention for rehabilitation of upper limb functions following stroke, since the first attempt by Altschuler et al. (1). The neural correlate of MT remains under investigation. Three main theories explaining the neural mechanism underlying the clinical efficacy of MT have been proposed (2).
The first theory hypothesizes that the neural correlate of MT is the mirror neuron system (MNS), which is defined as a class of neurons that fire during action observation and action execution (3). It is assumed that the MNS can be triggered when people are observing mirror visual feedback (MVF) generated in MT (4, 5). The affected cortical motor system can be accessed via the MNS owing to their functional connections (6). The second theory, supported by several studies with transcranial magnetic stimulation (TMS), suggests that a potential neural mechanism underlying the effect of MT can be the recruitment of the ipsilesional corticospinal pathway. Indeed, many TMS studies have demonstrated the increment of motor-evoked potentials of the ipsilesional primary motor cortex in participants with stroke when viewing MVF (7), which indicates a facilitatory effect of MVF on the ipsilesional corticospinal pathway. The last theory attributes the effect of MT to the compensation of restricted proprioception input from the affected limb and the enhancement of attention toward the paretic upper limb (8), which may contribute to the reduction of the learned non-use in patients with stroke (1).
A substantial number of randomized controlled trials (RCTs) have demonstrated that MT is useful in improving upper limb functions after stroke (9–12). A recently published meta-analytic review identified a moderate level of evidence supporting the effects of MT on improving upper limb motor functions (Hedges’ g = 0.47) and activities of daily living (ADLs) (Hedges’ g = 0.48) in patients with stroke (13). In the meta-analysis (13), the heterogeneity of conducting MT was obvious across studies. One major category of MT is movement-based MT (MMT), in which participants practice simple movements such as wrist flexion and extension, or finger flexion and extension, with their unaffected hands when viewing the MVF generated by a physical mirror placed at their mid-sagittal plane (14–16). Another category of MT is task-based MT (TMT), in which participants perform specific motor tasks with their unaffected hands, such as squeezing sponges, placing pegs in holes, and flipping a card, while they are viewing the MVF (12, 17). In some studies, researchers applied MMT in the first few sessions and subsequently applied TMT in the following sessions, constituting a hybrid MT protocol (9, 10, 18). MMT and TMT were also described as intransitive and transitive movements in some studies (9, 10). However, a sub-group meta-analysis comparing MMT and TMT was not carried out in the meta-analysis study (13).
Initially, MMT was used for alleviating phantom pain after amputation and for treating upper limb hemiplegia after stroke (1, 19). Subsequently, the effect of MMT in stroke upper limb rehabilitation has been systematically investigated by many clinical trials (14–16, 20). Arya et al. were the first to compare the effects of TMT with those of conventional rehabilitation on upper limb motor recovery after stroke, and they found a superior effect of TMT (12). The main rationale that Arya et al. mentioned was that the response of the MNS was better for object-directed actions than for non-object actions (12, 21). In a recent study comparing the effects of action observation training and MT on gait and balance in patients with stroke, the results showed that action observation training had significantly better effects on the improvement of balance functions than MT (22), indicating that action observation may be different from MT in terms of their neural mechanisms. In other studies in which TMT was introduced or combined with MMT, the authors did not explain why they employed TMT (9–11).
Thus far, no RCT has systematically investigated the difference between the effects of MMT and TMT. Therefore, we aimed to conduct an RCT to directly compare the effect of MMT and TMT, on improving hemiplegic upper limb motor functions, spasticity, and ADLs, in a group of patients with stroke.[…]