Studies of Cortical Synchrony and Coherence in the Human Sensorimotor System
Keywords: magnetoencephalography
motor
somatosensory
beta

The spatiotemporal dynamics of ongoing beta band (15-30 Hz) cortical oscillations and the modulation of this neural activity by tactile input and movement provide insight into how the brain achieves proper sensorimotor processing. Earlier studies have shown that the synchrony of the cortical beta rhythms within and between central and peripheral neuronal populations is modulated during and following somatosensation or movement, and correlated with effective motor control. In addition, abnormal levels of beta oscillations in the basal ganglia are correlated with motor dysfunction in Parkinson’s disease. Numerous functional roles for the beta rhythm have been proposed – ranging from inhibition to the facilitation of long-range communication. However, the neural network that generates the sensorimotor beta rhythm and the functional significance of this activity have not been fully specified. Thus, I used magnetoencephalography to complete three studies of the beta rhythm in healthy right-handed adults. In the first study, I hypothesized that finger vibration at beta frequencies would generate stimulus-coherent neuronal firing in the neural network that generates the beta rhythm – thus revealing the nodes of this network. Data were analyzed for nineteen subjects (10 females). The coherent activity was revealed using a novel analysis technique that generated whole-brain maps of inter-trial synchrony during passive repetitive finger vibration at 23 Hz. These maps identified contralateral primary somatosensory cortex (SI), posterior parietal cortex, supplementary motor area and primary motor cortex (MI), and ipsilateral brainstem as nodes in the network. In the second study, I correlated changes in focused attention with modulations in beta band cortical responses to specify the functional significance of this activity. Data were analyzed for twelve subjects (7 females). With increased focused attention to the stimulus, I hypothesized that the beta band responses to finger vibration would be enhanced in areas involved in somatosensory processing. A transient increase in the magnitude of beta oscillations in MI (event-related synchronization) following vibration offset was significantly enhanced by attention, as compared to passive stimulation. In addition, attention caused the suppression of beta oscillations (event-related desynchronization, ERD) in ipsilateral SI beginning 1 second prior to vibration offset. Strong attention-modulation of the beta rhythm outside of contralateral SI implies that these changes are indicative of higher-order processing of afferent information. In the third study, I tested the hypothesis that synchrony between beta rhythms in contralateral MI and the relevant muscle supports effective neuronal communication. I correlated changes in task performance with corticomuscular coherence (CMC) during the sustained application of force to match a visually-presented target. Data were analyzed for eighteen subjects (9 females). As predicted, CMC in MI was significantly increased during improved performance in this task. This suggests that central-peripheral synchrony plays an important functional role in sustaining isometric muscle control. Concurrent beta ERD in bilateral SI and primary visual cortices during the contraction indicates the importance of afferent feedback in this task. Gender-related effects were not investigated in these studies. Beta band neuromagnetic responses to movement and somatosensation identify a pervasive neural network that is involved in processing the relevant properties of somatic input and regulating sustained motor output.