Bettina Pollok

The cerebral oscillatory network associated with auditorily paced finger movements

Motor tasks involve neural activity in a spatially distributed network. It is assumed that coherent activity between these brain structures reflects functional connectivity. The aim of the present study was to investigate brain areas associated with a unimanual auditorily paced finger-tapping task and to characterize their dynamic interplay. We examined cerebromuscular and cerebrocerebral coupling in 10 right-handed subjects using recordings of continuous brain activity with a 122-channel whole-head neuromagnetometer while subjects performed the task with both hands consecutively. Additionally, surface EMG of the first dorsal interosseus was measured. Our data demonstrate that an oscillatory network composed of primary sensorimotor cortex, lateral as well as mesial premotor areas, the posterior parietal cortex and thalamus contralateral, and cerebellum and primary auditory cortex ipsilateral to the tapping hand subserves task execution. Connectivity between these areas and direction of coupling agree well with anatomical findings. During the right-hand condition, additional oscillatory activity in the primary sensorimotor cortex ipsilateral to the tapping hand was evident. This result suggests an asymmetric motor control in right-handers. Cerebrocerebral coupling predominantly occurs at 8-12 Hz. Therefore, our data support the hypothesis that coupling at 8-12 Hz in a cerebello-thalamic-cortical network represents a fundamental characteristic of the motor system and provides evidence for the significance of 8-12 Hz oscillations in a large scale network during the execution of simple motor tasks.

Task-dependent oscillations during unimanual and bimanual movements in the human primary motor cortex and SMA studied with magnetoencephalography

The neural mechanisms subserving uni- and bimanual control of movements are not well understood. Nevertheless, recent studies indicate a functional role of oscillatory activity in movement control and point towards a hemispheric asymmetry in motor control. This study specifically addresses the issues of (i) task-relatedness, (ii) hemispheric symmetry, and (iii) frequency specificity of the measures power, cerebro-muscular coherence, and cerebro-cerebral coherence in bilateral primary motor cortex and supplementary motor area (SMA). We have studied 10 right-handed subjects with simultaneous recordings of magnetoencephalography and surface electromyography during different unimanual and bimanual tasks. Using the analysis technique Dynamic Imaging of Coherent Sources (DICS), left and right primary motor cortex and SMA were functionally localized. Power, cerebro-musclar coherence, and cerebro-cerebral coherence between these areas were computed for four frequency bands in each condition and subjected to ANOVA. Results show a task-specific modulation of power and coherence, and further indicate a hemispheric asymmetry in the control of unimanual and bimanual movements. In addition, different frequency bands showed different task-dependent variations. The gamma band (26-40 Hz) showed strongest modulation for cerebro-muscular coherence and was strongest for the isometric contraction conditions. In contrast, the beta band (13-24 Hz) showed the strongest variations between static and dynamic conditions, and seems to play a particular role in movement control. In summary, our results indicate a differential functional role of oscillatory activity and coupling in the motor system.

The cerebral oscillatory network of voluntary tremor

It has recently been shown that resting tremor in Parkinsons disease is associated with oscillatory neural coupling in an extensive cerebral network comprising a cerebello–diencephalic–cortical loop and cortical motor, somatosensory and posterior parietal areas contralateral to the tremor hand. The aim of the present study was to investigate whether this oscillatory brain network exclusively reflects a pathophysiological state in parkinsonian resting tremor or whether it constitutes a fundamental feature of physiological motor control. We investigated cerebro‐muscular and cerebro‐cerebral coupling in 11 healthy subjects imitating typical antagonistic parkinsonian tremor. We recorded brain activity with a 122‐channel whole‐head neuromagnetometer and surface EMGs of the forearm extensor. Analysis of cerebro‐muscular and cerebro‐cerebral coherence revealed oscillatory coupling in the same brain structures that comprise the oscillatory network of parkinsonian resting tremor. Interestingly, similar to parkinsonian resting tremor, cerebro‐cerebral coherences often showed a significant peak at twice the simulated tremor frequency. The most striking differences between parkinsonian patients, as investigated in a previous study and healthy subjects imitating the antagonistic resting tremor were a reduction of the coupling between primary sensorimotor cortex and a diencephalic structure – most likely the thalamus – and an enhancement of the coupling between premotor and primary sensorimotor cortex. Our results indicate that the coupling of oscillatory activity within a cerebello–diencephalic–cortical loop constitutes a basic feature of physiological motor control. Thus, our data are consistent with the hypothesis that parkinsonian resting tremor involves oscillatory cerebro‐cerebral coupling in a physiologically pre‐existing network.

Effects of 10 Hz and 20 Hz transcranial alternating current stimulation (tACS) on motor functions and motor cortical excitability.

Synchronized oscillatory activity at alpha (8-12 Hz) and beta (13-30 Hz) frequencies plays a key role in motor control. Nevertheless, its exact functional significance has yet to be solved. Transcranial alternating current stimulation (tACS) allows the frequency-specific modulation of ongoing oscillatory activity. The goal of the present study was to investigate the effect of 10 and 20 Hz tACS over left primary motor cortex (M1) on motor functions and cortical excitability in healthy subjects. To this end, tACS was applied for 10 min. Sham stimulation served as control condition. Movement speed and accuracy of the right hand were assessed in 15 right-handed subjects before and after (0, 30 and 60 min) tACS of M1. Cortical silent period (CSP) and motor evoked potentials (MEPs) were determined as measures of M1 excitability. While 10 Hz tACS particularly increased movement variability, especially in tasks requiring internal pacing, 20 Hz tACS resulted in movement slowing. Behavioural effects occurred in distinct time windows. While 10 Hz effects developed over 30 min after stimulation, 20 Hz tACS effects were found immediately after stimulation. Following 10 Hz tACS these effects were significantly correlated with CSP duration, indicating interference with inhibitory pathways. The present findings suggest differential effects of stimulation frequency on motor behaviour and M1 excitability.

Motor-cortical oscillations in early stages of Parkinson's disease

•  Parkinsons disease (PD) is a common movement disorder due to dopaminergic denervation of the basal ganglia. It is characterized by exaggerated oscillatory activity within central motor‐control loops, while cerebro‐muscular coherence is reduced at frequencies below 30 Hz. •  So far, studies investigating the neurophysiological alterations of PD have focused on patients with advanced PD. It remains open to what extent changes of oscillatory activity might occur at early disease stages, representing a characteristic feature of the disease. •  Using magnetoencephalography we show that cerebro‐muscular coherence is unaffected in early PD while beta band oscillations of bilateral primary sensori‐motor cortices are already increased at the earliest stages of PD and, as the disease progresses, evolve a hemispheric imbalance associated with movement execution.

Functional network interactions during sensorimotor synchronization in musicians and non-musicians

Precise timing as determined by sensorimotor synchronization is crucial for a wide variety of activities. Although it is well-established that musicians show superior timing as compared to non-musicians, the neurophysiological foundations - in particular the underlying functional brain network - remain to be characterized. To this end, drummers, professional pianists and non-musicians performed an auditory synchronization task while neuromagnetic activity was measured using a 122-channel whole-head magnetoencephalography (MEG) system. The underlying functional brain network was determined using the beamformer approach Dynamic Imaging of Coherent Sources (DICS). Behaviorally, drummers performed less variably than non-musicians. Neuromagnetic analysis revealed a cerebello-thalamo-cortical network in all subjects comprising bilateral primary sensorimotor cortices (S1/M1), contralateral supplementary motor and premotor regions (SMA and PMC), thalamus, posterior parietal cortex (PPC), ipsilateral cerebellum and bilateral auditory cortices. Stronger PMC-thalamus and PPC-thalamus interactions at alpha and beta frequencies were evident in drummers as compared to non-musicians. In professional pianists stronger PMC-thalamus interaction as compared to non-musicians at beta frequency occurred. The present data suggest that precise timing is associated with increased functional interaction within a PMC-thalamus-PPC network. The PMC-thalamus connectivity at beta frequency might be related to musical expertise, whereas the PPC-thalamus interaction might have specific relevance for precise timing.

Oscillatory activity reflects the excitability of the human somatosensory system

The neuronal activity of the resting human brain is dominated by spontaneous oscillations in primary sensory and motor areas. These oscillations are thought to reflect the excitability of sensory and motor systems that can be modulated according to the actual behavioral demands. However, so far, evidence for an association between oscillatory activity and excitability has been inconsistent. Here, we used magnetoencephalography to reinvestigate the relationship between oscillatory activity and excitability in the somatosensory system on a single trial basis. Brief painful stimuli were applied to relate pain-induced suppressions of oscillatory activity to pain-induced increases in excitability. The analysis reveals a significant negative correlation between sensorimotor oscillatory activity, particularly in the alpha-band, and excitability of somatosensory cortices. Oscillatory activity outside the somatosensory system did not correlate with somatosensory excitability. These findings demonstrate that modulations of sensorimotor oscillatory activity specifically reflect modulations in excitability of the somatosensory system and thus provide direct evidence for the basic tenet of an association between oscillatory activity and cortical excitability.

Perception in action: The impact of sensory information on sensorimotor synchronization in musicians and non-musicians

The present study aimed at investigating to what extent sensorimotor synchronization is related to (i) musical specialization, (ii) perceptual discrimination, and (iii) the movements trajectory. To this end, musicians with different musical expertise (drummers, professional pianists, amateur pianists, singers, and non-musicians) performed an auditory and visual synchronization and a cross-modal temporal discrimination task. During auditory synchronization drummers performed less variably than amateur pianists, singers and non-musicians. In the cross-modal discrimination task drummers showed superior discrimination abilities which were correlated with synchronization variability as well as with the trajectory. These data suggest that (i) the type of specialized musical instrument affects synchronization abilities and (ii) synchronization accuracy is related to perceptual discrimination abilities as well as to (iii) the movements trajectory. Since particularly synchronization variability was affected by musical expertise, the present data imply that the type of instrument improves accuracy of timekeeping mechanisms.

Differential effects of levodopa and subthalamic nucleus deep brain stimulation on bradykinesia in Parkinson's disease

Cardinal symptoms of Parkinsons disease (PD) respond well to treatment with levodopa and deep brain stimulation (DBS) of the subthalamic nucleus (STN). However, it has remained unclear whether levodopa and STN‐DBS have differential effects on bradykinesia. We investigated 8 PD‐patients with STN‐electrodes in four conditions: STN‐DBS and levodopa (ONMED/ONSTIM), STN‐DBS only (OFFMED/ONSTIM), levodopa only (ONMED/OFFSTIM), without STN‐DBS/levodopa (OFFMED/OFFSTIM). Fourteen volunteers served as controls. Subjects performed fastest possible (1) pronation/supination of the forearm (diadochokinesia) and (2) flexion and extension of the index finger (finger movements). Movements were recorded using a 3D‐ultrasound‐system. Maximum frequency, amplitude, and smoothness of movements were determined. During OFFMED/OFFSTIM, all parameters were worser than in all other conditions. In proximal diadochokinesia, OFFMED/ONSTIM significantly improved the amplitude and frequency, whereas ONMED/OFFSTIM had no significant effect. In contrast, we found a stronger effect of levodopa (ONMED/OFFSTIM) on amplitudes of distal finger movement than on amplitudes of diadochokinesia. Combination of treatments during ONMED/ONSTIM further improved both movements. However, maximum frequency remained lower in PD‐patients during ONMED/ONSTIM compared with controls. This study demonstrates a better effect of levodopa on distal finger movements and STN‐DBS on proximal diadochokinesia. Furthermore, a complementary effect of both therapies on brain areas involved in bradykinesia can be assumed.

Right-shift for non-speech motor processing in adults who stutter

INTRODUCTION In adults who do not stutter (AWNS), the control of hand movement timing is assumed to be lateralized to the left dorsolateral premotor cortex (PMd). In adults who stutter (AWS), the network of speech motor control is abnormally shifted to the right hemisphere. Motor impairments in AWS are not restricted to speech, but extend to non-speech orofacial and finger movements. We here investigated the lateralization of finger movement timing control in AWS. METHODS We explored PMd function in 14 right-handed AWS and 15 age matched AWNS. In separate sessions, they received subthreshold repetitive transcranial magnetic stimulation (rTMS) for 20 min at 1Hz over the left or right PMd, respectively. Pre- and post-stimulation participants were instructed to synchronize their index finger taps of either hand with an isochronous sequence of clicks presented binaurally via earphones. Synchronization accuracy was measured to quantify the effect of the PMd stimulation. RESULTS In AWNS inhibition of left PMd affected synchronization accuracy of the left hand. Conversely, in AWS TMS over the right PMd increased the asynchrony of the left hand. CONCLUSIONS The present data indicate an altered functional connectivity in AWS in which the right PMd seems to be important for the control of timed non-speech movements. Moreover, the laterality-shift suggests a compensatory role of the right PMd to successfully perform paced finger tapping.