Mirror illusion reduces motor cortical inhibition in the ipsilateral primary motor cortex during forceful unilateral muscle contractions

Abstract

Forceful, unilateral contractions modulate corticomotor paths targeting the resting, contralateral hand. However, it is unknown whether mirror-viewing of a slowly moving but forcefully contracting hand would additionally affect these paths. Here we examined corticospinal excitability and short-interval intracortical inhibition (SICI) of the right-ipsilateral primary motor cortex (M1) in healthy young adults under no-mirror and mirror conditions at rest and during right wrist flexion at 60% maximal voluntary contraction (MVC). During the no-mirror conditions neither hand was visible, whereas in the mirror conditions participants looked at the right hand's reflection in the mirror. Corticospinal excitability increased during contractions in the left flexor carpi radialis (FCR) (contraction 0.41 mV vs. rest 0.21 mV) and extensor carpi radialis (ECR) (contraction 0.56 mV vs. rest 0.39 mV), but there was no mirror effect (FCR: P ϭ 0.743, p 2 ϭ 0.005; ECR: P ϭ 0.712, p 2 ϭ 0.005). However, mirror-viewing of the contracting and moving wrist attenuated SICI relative to test pulse in the left FCR by ϳ9% compared with the other conditions (P Ͻ 0.05, d Ն 0.62). Electromyographic activity in the resting left hand prior to stimulation was not affected by the mirror (FCR: P ϭ 0.255, p 2 ϭ 0.049; ECR: P ϭ 0.343, p 2 ϭ 0.035) but increased twofold during contractions. Thus viewing the moving hand in the mirror and not just the mirror image of the nonmoving hand seems to affect motor cortical inhibitory networks in the M1 associated with the mirror image. Future studies should determine whether the use of a mirror could increase interlimb transfer produced by cross-education, especially in patient groups with unilateral orthopedic and neurological conditions.

Heavy‐resistance exercise‐induced increases in jump performance are not explained by changes in neuromuscular function

“…Attribution of changes in these measures to specific sites is problematic, as the response recorded at the muscle is subject to modulation from a range of supraspinal, spinal, peripheral afferent, and motoneuronal inputs (Carroll et al., ). This notwithstanding, changes in the MEP evoked by single‐pulse (Beck et al., ; Griffin & Cafarelli, ; Selvanayagam et al., ; Weier et al., ; Nuzzo et al., ) and paired‐pulse (Weier et al., ; Zult et al., ) TMS have been reported in response to resistance training exercise. For example, Weier et al.…”

“…By varying the interval between stimuli, paired‐pulse TMS can be used to measure the excitability of gamma‐aminobutyric acid type A‐mediated inhibitory (short‐interval intracortical inhibition, SICI) and glutamate‐mediated excitatory (intracortical facilitation, ICF) intracortical circuits (Chen, ). Paired‐pulse TMS has been successfully used to reveal changes in intracortical activity as a result of resistance exercise (Weier et al., ; Zult et al., ), after a period of skill practice (Perez et al., , ) and after fatiguing contractions (Maruyama et al., ; Takahashi et al., ). Collectively these studies demonstrate that TMS can be used to reveal modulations in the central nervous system in response to resistance training exercise, some of which are immediate in nature (Selvanayagam et al., ; Nuzzo et al., ).…”

The Cross-Education Phenomenon: Brain and Beyond

“…Mirror neurons provide a neuroanatomical link between visual sensory inputs and motor neurons that produce descending drive during execution of movements, without explicitly resulting in movement imitation (see Zult et al, 2014 for review). To assess whether visual stimuli has the ability to augment cross-education, several studies have now investigated motor performance and neurophysiological responses to the unilateral contractions performed with a mirror box to simulate movement of the inactive limb (Garry et al, 2005 ; Reissig et al, 2014 , 2015 ; Zult et al, 2015 , 2016 ). One study employed a 3 week high intensity concentric training protocol in the wrist flexors, reporting that the magnitude of strength gain in the untrained limb was almost doubled when training was performed with a mirror when compared to training without visual stimuli (61 and 34% strength increase respectively; Zult et al, 2016 ).…”

“…In contrast, 300 ballistic finger movements performed unilaterally during a single training session produced a similar improvement in contralateral motor performance and corticospinal excitability (measured with TMS), regardless of whether visual mirror feedback was provided (Reissig et al, 2015 ). While there is some evidence from TMS studies for increased corticospinal excitability of the motor pathway to the inactive limb during mirror illusions (Garry et al, 2005 ), more recent work suggests that a reduction in intracortical inhibition of the iM1 may be the primary factor underpinning any benefits associated with mirror training (Reissig et al, 2014 ; Zult et al, 2015 ). Overall, the hypothesis for involvement of the mirror neuron system in unilateral training supports the notion that increased neural output from the “untrained” motor cortex plays a key role in cross-education.…”

The TMS Motor Map Does Not Change Following a Single Session of Mirror Training Either with Or without Motor Imagery

“…This contrasts the observations of Nojima et al ( 2012 ) who observed increased MEP amplitude following mirror training in healthy participants. Other studies also report increases in MEP amplitude (Garry et al, 2005 ; Fukumura et al, 2007 ; Funase et al, 2007 ) and reduction in intracortical inhibition (Zult et al, 2015 ) associated with the passive hand behind the mirror but assessed MEP amplitude whilst the mirror training was performed. Similar increases in MEP amplitude (Fadiga et al, 1995 ; Kasai et al, 1997 ; Rossini et al, 1999 ; Strafella and Paus, 2000 ) or TMS map area (Pascual-Leone et al, 1995 ; Cicinelli et al, 2006 ) have been reported when motor imagery and action observation are performed.…”