Bimanual coordination and spinal cord neuromodulation in neural motor control of upper limbs
Creators: Parhizi, Behdad
Publisher: name: University of Alberta Library
Year: 2023

<p>The overall goal of this dissertation was to advance knowledge in the underlying neural mechanisms of motor control of upper limb movement. Arm movements are essential for everyday life. It is not surprising that the neural control of arm movement has been the subject of research investigations for many years. This thesis aimed to address two critical questions currently underexplored: 1) How does the cortical neural correlates of bimanual coordination change when the nature of the movement varies? 2) How does spinal cord stimulation modulate the neural substrates of arm movement across different levels of the central nervous system? Of the wide array of movements that humans are capable of producing, this dissertation examines bimanual coordination. Coordinated movement of the upper limbs is sometimes easy to perform but sometimes requires rigorous training to become adept. Thus, bimanual coordination is regarded as a skilled and complex motor ability that only humans have evolved to possess. Yet, this ability is impaired in persons with neurological conditions. Restoration of upper limb function through neuromodulation has recently attracted attention using transcutaneous spinal cord stimulation (tSCS); however, the neural mechanisms driving improvements are not completely understood.</p>

<p>The first question was addressed by focusing on bimanual coordination to either achieve a common-goal or dual-goals. Goal-conceptualization and the effect of cognitive load during bimanual movements were addressed through kinematic and electrophysiological measures. Results showed that dual-goal reaching significantly increased movement time and error compared to common-goal and unimanual reaching. They also demonstrated increased movement time (except in common-goal) and error with the addition of cognitive load. The findings in electrophysiological measures exhibited significantly stronger alpha band sensorimotor cortical activation during common-goal movements relative to dual-goal movements, and elevated interhemispheric connectivity in the common-goal task relative to the dual-goal and unimanual tasks. Our findings suggest that goal-conceptualization and complexity are critical factors in defining the behavioral outcome of bimanual movements, and their effect is also reflected in the underlying neural mechanisms.</p>

<p>The second question was addressed in three steps by investigating the effect of cervical tSCS on i) cortical, ii) cortico-spinal, and iii) propriospinal pathways related to the motor control of upper limb movements. First, the effect of cervical tSCS on bimanual and unimanual movements was highlighted. Electroencephalography (EEG) recordings were used as an unconventional approach to explore the modulation of sensorimotor cortical oscillations. Results showed significant synchronization of neural activity in sensorimotor regions, and increased interhemispheric connectivity in the presence of tSCS. This finding points to the suppressive effect of tSCS at the cortical level while allowing tighter communication between hemispheres.</p>

<p>Second, the modulatory effect of cervical tSCS on the circuitry of the cervical segment of the spinal cord projecting to the upper limb muscles and corticospinal connections to this region was assessed. Given that the cervical and lumbar cord are coupled through propriospinal connections, we also sought to determine the effect of lumbar tSCS on the cervical neural networks. No effect of cervical tSCS on either the flexor carpi radialis (FCR) Hoffman-reflex (H-reflex) or motor evoked potentials (MEP) was found; however, lumbar tSCS significantly facilitated the FCR H-reflex. Interestingly, combining lumbar and cervical tSCS significantly facilitated spinal and corticospinal pathways to the upper limb muscle, as shown by increased FCR H-reflex and MEP. This study highlights the importance of remote spinal segments to the cervical cord facilitated through cervico-lumbar coupling, and how this may be utilized for enhancing the voluntary control of arm movement.</p>

<p>Finally, because cervico-lumbar coupling is bidirectional, the modulatory effect of cervical tSCS on lumbar circuitry was investigated. Affirmative results could mean a potential for engaging cervical spinal cord through tSCS to support voluntary control of the lower limbs. We found that cervical tSCS significantly suppresses the soleus H-reflex. This novel finding demonstrates the potential role activating cervical spinal networks via tSCS to potentiate cervico-lumbar coupling.<br><br>
Collectively, this thesis constitutes a comprehensive study of upper limb movement and its neural correlates at different levels of the central nervous system. Spinal and supraspinal neuromodulatory effects tSCS on neural substrates of upper limb movement were also explored. Findings of this dissertation could pave the way for enhanced and targeted rehabilitation interventions for individuals with neurological conditions.</p>