Department of Exercise Sciences


Lateralisation of bimanual and unimanual motor imagery

Introduction

The areas of the brain that are active when we imagine moving are similar to those that are active when we actually perform the movement. Most previous studies have examined motor cortex function during imagined movement of the dominant hand. Little is known about the activity of the non-dominant motor cortex during motor imagery, or the activity in either hemisphere during the imagined movement of both hands together.

Aim

To examine the modulation of excitability in the dominant and non-dominant corticomotor pathways during motor imagery of the dominant hand, non-dominant hand, and both hands together.

Methods

  • Participants: 15 healthy right-handed adults.
  • Single-pulse Transcranial Magnetic Stimulation (TMS) was used to assess corticomotor excitability during imagined opening and closing of each hand separately, and both hands together. Surface EMG was recorded from the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles bilaterally. Motor evoked potentials (MEPs) were recorded in both muscles. In one session, TMS was applied over the dominant M1, and in a separate session, TMS was applied over the non-dominant M1.
  • Participants imagined abducting and opposing their thumb onto the tip of their first finger in time with a 1 Hz auditory metronome. They were instructed to use a kinesthetic strategy, where they imagined the feeling of the movement. TMS was delivered with the metronome beat, when APB would be activated during actual performance of the task. This is the ‘on’ phase. TMS was also delivered between metronome beats, when APB would be relaxed during actual task performance. This is the ‘off’ phase. F-waves were also recorded in APB under each imagery condition, to check for changes in excitability at the spinal level.
  • The mean MEP amplitudes in each APB and ADM, during the ‘on’ and ‘off’ phases of unimanual and bimanual performance were calculated and expressed relative to the maximum MEP amplitude of all included trials recorded from that muscle under any condition.
imag1fig2

Fig.1 - Group averaged MEP amplitude (%MEP max) recorded from APB (panel A) and ADM (panel B) as a function of hemisphere stimulated and imagery task (R=Right hand; L=Left hand; B= Both hands). The horizontal lines represent the mean rest MEP amplitude for each muscle. MI of phasic thumb abduction of either hand, and both hands, resulted in significant temporal modulation of right APB MEP amplitude, but hand no effect on left APB MEP amplitude. MI of phasic thumb abduction had no effect on AMD MEP amplitude in either hand. Error bars = SE, ^significant facilitation relative to rest MEP amplitude, *P < 0.05, **P<0.01.

Results

  • Motor imagery of either hand and both hands together, facilitated MEP amplitude in the dominant APB (Fig 1).
  • Motor imagery did not facilitate MEP amplitude in the non-dominant APB.
  • The facilitation of MEP amplitude was specific to the APB, which was engaged by actual task performance. ADM MEP amplitude was not facilitated by motor imagery, as it is not engaged in actual task performance.
  • F-wave amplitude and persistence were not affected by motor imagery.

Interpretation

  • The present study supports the hypothesis that the left motor cortex is dominant for kinesthetic motor imagery in right-handed people. The facilitation of excitability was confined to the dominant corticomotor pathway, and was both muscle-specific and temporally modulated, in the absence of changes in excitability at the spinal level.
  • These findings may have implications for the use of motor imagery in rehabilitation following stroke, as the ability to engage corticomotor pathways in imagery may depend on which hemisphere is affected by a stroke.

Acknowledgements

We would like to thank Cheryl Murphy for assistance with data collection. CMS was supported by the Auckland Medical Research Foundation, and MKF was funded by a Summer Scholarship from the Health Research Council of New Zealand.