Neural correlates of upper limb unimanual motor practice

Field: Neuroscience (Life Sciences)

The primary control centres for unimanual motor control reside in the contralateral hemisphere. Ipsilateral sensorimotor brain activity has been observed during unimanual motor tasks, and the functional properties of this ipsilateral activity are debated. Cross-education is the interlimb transfer of a practice motor behaviour (skill or strength) to the homologous contralateral limb. A leading theory for cross-education proposes the interlimb transfer manifests from ipsilateral cortical activity during unimanual motor tasks, resulting in motor-related neuroplasticity giving rise to contralateral limb improvements. Cross-education has been effectively utilized in clinical settings for motor recovery in individuals with a stroke that present with a unilateral impairment. Yet, based on stroke-related neuroplastic changes with interhemispheric inhibition, ipsilateral/ipsilesional hemispheric activity would likely be inhibited when the less-affected limb is active. Therefore, investigating ipsilateral brain activity with unimanual tasks is pertinent in neurologically intact and stroke-impaired participants. The purpose of this thesis was to i) investigate the neural correlates of the sensorimotor network with parametrically increasing unimanual handgrip contractions in healthy and stroke-impaired individuals, and ii) determine the effect of handgrip motor fatigue on resting-state cortical activity in the sensorimotor network and motor performance and learning in the contralateral hand. Study One: Two experiments were carried out; experiment one used magnetic resonance imaging (MRI) to investigate the cortical activation and functional connectivity patterns during three different submaximal handgrip contractions (25%, 50%, 75% maximum voluntary contraction [MVC]) with the right hand. In experiment two, the tasks were replicated outside of the MRI using electromyography to measure the muscle activation patterns in the wrist flexors in both limbs during the right-hand motor task. In experiment one, brain activation and functional connectivity within the ipsilateral sensorimotor areas were found to increase parametrically with the increases in handgrip force, and data from experiment two suggest that the increased cortical patterns are not likely driving involuntary muscle contractions in the opposite limb. Study Two: One experiment was carried out to investigate how unimanual parametrically increasing handgrip contractions with the less-affected limb modulates cortical activity in participants with stroke. Higher force contractions increased brain activity in the ipsilesional hemisphere like what was observed in the first experiment in study one with neurologically intact participants. Yet, patterns of functional connectivity differed between groups, with the participants with stroke showing lower connectivity across the three contraction forces with no differences between conditions. Study Three: Study three also involved two separate experiments. In experiment one, resting-state functional connectivity and button press response times with both hands were assessed before and after participants performed nine-minutes of repeated handgrip contractions at either 5% or 50% MVC with their right hand. The handgrip contractions at 50% MVC resulted in motor fatigue and improved response times in the left hand, a phenomenon not observed with the 5% MVC condition. For the 50% MVC condition, increases in functional connectivity were found between the right ipsilateral primary motor cortex (iM1) and the right orbitofrontal cortex and between the supplementary motor areas (SMA) in each hemisphere. M1-M1 connectivity also changed with a subtle decrease after the motor task. In experiment two, the impact of repeated right handgrip contracts on contralateral motor learning and cross-education of a serial reaction time task was investigated. Motor performance and learning with the left hand improved similarly between conditions, however, greater cross-education was observed at post-learning, with the 50% MVC condition exhibiting faster response times without a decrement to performance accuracy. Conclusion: This thesis identifies the involvement and modulation of ipsilateral cortical activation and functional connectivity during and after right handgrip contractions at high-force and/or with fatiguing efforts in healthy and neurologically impaired individuals. These cortical modulations may be utilized to enhance acute motor performance which could have clinical applications for unilateral motor rehabilitation in stroke survivors.