Premotor and prefrontal contributions to modulating upper limb somatosensory input into non-primary motor areas
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Upper limb motor control requires the use and integration of afferent somatosensory input from peripheral receptors to help plan and prepare movements. Cortical surface electroencephalography can be used to measure the earliest relay and processing of mixed somatosensory input in primary (SI) and secondary somatosensory (SII) cortices using parietal somatosensory evoked potentials (SEPs) that occur 20 to 100 milliseconds (ms) after median nerve stimulation. Moreover, somatosensory input into non-primary motor areas, such as premotor cortex (PMC) and supplementary motor area (SMA), can be measured by frontal N30 and N60 SEPs. Therefore, frontal N30 and N60 SEPs may provide an important neurophysiological link between somatosensory processing and upper limb motor control. Both PMC and SMA have intracortical connections with primary motor cortex (M1) and prefrontal cortex (PFC) as well as intercortical connections with their contralateral representations. However, it is not fully understood how somatosensory input in non-primary motor areas, represented by frontal SEPs, are modulated in the cortex by contralateral PMC and ipsilateral PFC. A modulatory role of contralateral M1 but not contralateral premotor areas on somatosensory input into non-primary motor areas has been established through contralateral movement paradigms. Furthermore, a modulatory role of the ipsilateral PFC on somatosensory input into non-primary motor areas has been identified through prefrontal lesion patients but it is unclear how PFC functionally modulates this somatosensory input during movement. Thus, the current thesis aimed to evaluate the contributions of the ipsilateral PFC as well as contralateral PMC on somatosensory processing in non-primary motor areas as well as SI/SII. SEP modulations were examined using experimental manipulations of top-down attention and cued contralateral movements to evaluate PFC and PMC contributions, respectively. In addition, continuous theta burst stimulation, a specific type of inhibitory non-invasive transcranial magnetic stimulation technique, was applied over PMC and PFC to evaluate their specific contributions to modulating somatosensory input into non-primary motor areas and SI/SII during a cued movement task. Understanding frontal SEP modulations and their association with upper limb motor control will have important applications for understanding dysfunctional upper limb motor control in various neurological disorders such as Parkinson’s disease (PD) that are known to have irregular frontal SEPs. The main findings from Chapters 2 and 3 revealed that frontal N30 and N60 SEPs were decreased during early response selection and increased during the late stages of preparing finger sequences to attended somatosensory input. In contrast, SI/SII input represented by parietal P50 and P100 SEPs were increased with attention. The main results of Chapter 4 showed that N30 and N60 SEPs were decreased and increased after transiently decreasing excitability in left PMC and right PFC, respectively. Collectively, the results of this thesis revealed temporally-specific modulations of somatosensory input into non-primary motor areas during contralateral upper limb movements that are a result of changes in activity in a network that includes the right PFC and left PMC.