Mapping Subcortico-Cortical Coupling-A Comparison of Thalamic and Subthalamic Oscillations

BACKGROUND

The ventral intermediate nucleus of the thalamus (VIM) is an effective target for deep brain stimulation in tremor patients. Despite its therapeutic importance, its oscillatory coupling to cortical areas has rarely been investigated in humans.

OBJECTIVES

The objective of this study was to identify the cortical areas coupled to the VIM in patients with essential tremor.

METHODS

We combined resting-state magnetoencephalography with local field potential recordings from the VIM of 19 essential tremor patients. Whole-brain maps of VIM-cortex coherence in several frequency bands were constructed using beamforming and compared with corresponding maps of subthalamic nucleus (STN) coherence based on data from 19 patients with Parkinson's disease. In addition, we computed spectral Granger causality.

RESULTS

The topographies of VIM-cortex and STN-cortex coherence were very similar overall but differed quantitatively. Both nuclei were coupled to the ipsilateral sensorimotor cortex in the high-beta band; to the sensorimotor cortex, brainstem, and cerebellum in the low-beta band; and to the temporal cortex, brainstem, and cerebellum in the alpha band. High-beta coherence to sensorimotor cortex was stronger for the STN (P = 0.014), whereas low-beta coherence to the brainstem was stronger for the VIM (P = 0.017). Although the STN was driven by cortical activity in the high-beta band, the VIM led the sensorimotor cortex in the alpha band.

CONCLUSIONS

Thalamo-cortical coupling is spatially and spectrally organized. The overall similar topographies of VIM-cortex and STN-cortex coherence suggest that functional connections are not necessarily unique to one subcortical structure but might reflect larger frequency-specific networks involving VIM and STN to a different degree. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Slowing of Frontal β Oscillations in Atypical Parkinsonism

BACKGROUND

Diagnosis of atypical parkinsonian syndromes (APS) mostly relies on clinical presentation as well as structural and molecular brain imaging. Whether parkinsonian syndromes are distinguishable based on neuronal oscillations has not been investigated so far.

OBJECTIVE

The aim was to identify spectral properties specific to atypical parkinsonism.

METHODS

We measured resting-state magnetoencephalography in 14 patients with corticobasal syndrome (CBS), 16 patients with progressive supranuclear palsy (PSP), 33 patients with idiopathic Parkinson's disease, and 24 healthy controls. We compared spectral power as well as amplitude and frequency of power peaks between groups.

RESULTS

Atypical parkinsonism was associated with spectral slowing, distinguishing both CBS and PSP from Parkinson's disease (PD) and age-matched healthy controls. Patients with atypical parkinsonism showed a shift in β peaks (13-30 Hz) toward lower frequencies in frontal areas bilaterally. A concomitant increase in θ/α power relative to controls was observed in both APS and PD.

CONCLUSION

Spectral slowing occurs in atypical parkinsonism, affecting frontal β oscillations in particular. Spectral slowing with a different topography has previously been observed in other neurodegenerative disorders, such as Alzheimer's disease, suggesting that spectral slowing might be an electrophysiological marker of neurodegeneration. As such, it might support differential diagnosis of parkinsonian syndromes in the future. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

J-difference GABA-edited MRS reveals altered cerebello-thalamo-cortical metabolism in patients with hepatic encephalopathy

Hepatic encephalopathy (HE) is a common neurological manifestation of liver cirrhosis and is characterized by an increase of ammonia in the brain accompanied by a disrupted neurotransmitter balance, including the GABAergic and glutamatergic systems. The aim of this study is to investigate metabolic abnormalities in the cerebello-thalamo-cortical system of HE patients using GABA-edited MRS and links between metabolite levels, disease severity, critical flicker frequency (CFF), motor performance scores, and blood ammonia levels. GABA-edited MRS was performed in 35 participants (16 controls, 19 HE patients) on a clinical 3 T MRI system. MRS voxels were placed in the right cerebellum, left thalamus, and left motor cortex. Levels of GABA+ and of other metabolites of interest (glutamine, glutamate, myo-inositol, glutathione, total choline, total NAA, and total creatine) were assessed. Group differences in metabolite levels and associations with clinical metrics were tested. GABA+ levels were significantly increased in the cerebellum of patients with HE. GABA+ levels in the motor cortex were significantly decreased in HE patients, and correlated with the CFF (r = 0.73; p < .05) and motor performance scores (r = -0.65; p < .05). Well-established HE-typical metabolite patterns (increased glutamine, decreased myo-inositol and total choline) were confirmed in all three regions and were closely linked to clinical metrics. In summary, our findings provide further evidence for alterations in the GABAergic system in the cerebellum and motor cortex in HE. These changes were accompanied by characteristic patterns of osmolytes and oxidative stress markers in the cerebello-thalamo-cortical system. These metabolic disturbances are a likely contributor to HE motor symptoms in HE. In patients with hepatic encephalopathy, GABA+ levels in the cerebello-thalamo-cortical loop are significantly increased in the cerebellum and significantly decreased in the motor cortex. GABA+ levels in the motor cortex strongly correlate with critical flicker frequency (CFF) and motor performance score (pegboard test tPEG), but not blood ammonia levels (NH3).

Physiotherapy & Psychosocial Rehabilitation in Postcovid & Postconflict Era: New Roles with same Staff? (dispatch)

Waves of COVID-19 have been managed successfully within US, EU, Southeast Asia and Latin America, however, South Asia and Sub-saharan Africa still suffer new variants and EU fights with UK and United States of ,,new waves of old disease“, postcovide or long covide syndrome. The aim of this communication and research is to prepare our auditors for the size and extent of postcovid systems and the importance of non-doctors and non-medicine experts in management of its consequences.

Weakness Period in Food Assistance as Part of Social Work for Ukrainian Migrants of War Conflicts

Food supplies are leading priorities during reception and first contact with all refugees of war. The aim of the study was to longitudinally document the clients flow in a food and clothing center at SEUC in Bratislava, serving clients sent from various checkpoints between Ukraine, Slovakia and Poland.

MaDoPO: Magnetic Detection of Positions and Orientations of Segmented Deep-Brain Stimulation Electrodes: A Radiation-Free Method Based on Magnetoencephalography

Background: Current approaches to detect the positions and orientations of directional deep-brain stimulation (DBS) electrodes rely on radiative imaging data. In this study, we aim to present an improved version of a radiation-free method for magnetic detection of the position and the orientation (MaDoPO) of directional electrodes based on a series of magnetoencephalography (MEG) measurements and a possible future solution for optimized results using emerging on-scalp MEG systems. Methods: A directional DBS system was positioned into a realistic head–torso phantom and placed in the MEG scanner. A total of 24 measurements of 180 s each were performed with different predefined electrode configurations. Finite element modeling and model fitting were used to determine the position and orientation of the electrode in the phantom. Related measurements were fitted simultaneously, constraining solutions to the a priori known geometry of the electrode. Results were compared with the results of the high-quality CT imaging of the phantom. Results: The accuracy in electrode localization and orientation detection depended on the number of combined measurements. The localization error was minimized to 2.02 mm by considering six measurements with different non-directional bipolar electrode configurations. Another six measurements with directional bipolar stimulations minimized the orientation error to 4°. These values are mainly limited due to the spatial resolution of the MEG. Moreover, accuracies were investigated as a function of measurement time, number of sensors, and measurement direction of the sensors in order to define an optimized MEG device for this application. Conclusion: Although MEG introduces inaccuracies in the detection of the position and orientation of the electrode, these can be accepted when evaluating the benefits of a radiation-free method. Inaccuracies can be further reduced by the use of on-scalp MEG sensor arrays, which may find their way into clinics in the foreseeable future.

Determining the rotational orientation of directional deep brain stimulation electrodes using magnetoencephalography

Objective.The aim of the present study was to evaluate the effect of different electrode configurations on the accuracy of determining the rotational orientation of the directional deep brain stimulation (DBS) electrode with our previously published magnetoencephalography (MEG)-based method.Approach.A directional DBS electrode, along with its implantable pulse generator, was integrated into a head phantom and placed within the MEG sensor array. Predefined bipolar electrode configurations, based on activation of different directional and omnidirectional contacts of the electrode, were set to generate a defined magnetic field during stimulation. This magnetic field was then measured with MEG. Finite element modeling and model fitting approach were used to calculate electrode orientation.Main results.The accuracy of electrode orientation detection depended on the electrode configuration: the vertical configuration (activation of two directional contacts arranged one above the other) achieved an average accuracy of only about 41 ^∘. The diagonal configuration (activation of the electrode tip and a single directional contact at the next higher level of the electrode) achieved an accuracy of 13^∘, while the horizontal electrode configuration (activation of two adjacent directional contacts at the same electrode level) achieved the best accuracy of 6^∘. The accuracy of orientation detection of the DBS electrode depends on the change in spatial distribution of the magnetic field with the rotation of the electrode along its own axis. In the vertical configuration, rotation of the electrode has a small effect on the magnetic field distribution, while in the diagonal or horizontal configuration, electrode rotation has a significant effect on the magnetic field distribution.Significance.Our work suggests that in order to determine rotational orientation of a DBS electrode using MEG, horizontal configuration should be used as it provides the most accurate results compared to other possible configurations.

Parkinson's disease patients benefit from bicycling - a systematic review and meta-analysis

Many Parkinson's disease (PD) patients are able to ride a bicycle despite being severely compromised by gait disturbances up to freezing of gait. This review [PROSPERO CRD 42019137386] aimed to find out, which PD-related symptoms improve from bicycling, and which type of bicycling exercise would be most beneficial. Following a systematic database literature search, peer-reviewed studies with randomized control trials (RCT) and with non-randomized trials (NRCT) investigating the interventional effects of bicycling on PD patients were included. A quality analysis addressing reporting, design and possible bias of the studies, as well as a publication bias test was done. Out of 202 references, 22 eligible studies with 505 patients were analysed. An inverse variance-based analysis revealed that primary measures, defined as motor outcomes, benefitted from bicycling significantly more than cognitive measures. Additionally, secondary measures of balance, walking speed and capacity, and the PDQ-39 ratings improved with bicycling. The interventions varied in durations, intensities and target cadences. Conclusively, bicycling is particularly beneficial for the motor performance of PD patients, improving crucial features of gait. Furthermore, our findings suggest that bicycling improves the overall quality-of-life of PD patients.

Pathomechanisms in hepatic encephalopathy

Hepatic encephalopathy (HE) is a frequent neuropsychiatric complication in patients with acute or chronic liver failure. Symptoms of HE in particular include disturbances of sensory and motor functions and cognition. HE is triggered by heterogeneous factors such as ammonia being a main toxin, benzodiazepines, proinflammatory cytokines and hyponatremia. HE in patients with liver cirrhosis is triggered by a low-grade cerebral edema and cerebral oxidative/nitrosative stress which bring about a number of functionally relevant alterations including posttranslational protein modifications, oxidation of RNA, gene expression changes and senescence. These alterations are suggested to impair astrocyte/neuronal functions and communication. On the system level, a global slowing of oscillatory brain activity and networks can be observed paralleling behavioral perceptual and motor impairments. Moreover, these changes are related to increased cerebral ammonia, alterations in neurometabolite and neurotransmitter concentrations and cortical excitability in HE patients.

DBS electrode localization and rotational orientation detection using SQUID-based magnetoencephalography

The aim of the present study was to investigate the accuracy of localization and rotational orientation detection of a directional deep brain stimulation (DBS) electrode using a state-of-the-art magnetoencephalography (MEG) scanner. A directional DBS electrode along with its stimulator was integrated into a head phantom and placed inside the MEG sensor array. The electrode was comprised of six directional and two omnidirectional contacts. Measurements were performed while stimulating with different contacts and parameters in the phantom. Finite element modeling and fitting approach were used to compute electrode position and orientation. The electrode was localized with a mean accuracy of 2.2 mm while orientation was determined with a mean accuracy of 11°. The limitation in detection accuracy was due to the lower measurement precision of the MEG system. Considering an ideal measurement condition, these values represent the lower bound of accuracy that can be achieved in patients. However, a future magnetic measuring system with higher precision will potentially detect location and orientation of a DBS electrode with an even increased accuracy.

Investigating the 1-year decline in midbrain-to-pons ratio in the differential diagnosis of PSP and IPD

Background

A reliable measure of PSP-specific midbrain atrophy, the midbrain-to-pons ratio (MTPR) has been reported to support the differential diagnosis of progressive supranuclear palsy (PSP) from idiopathic Parkinson’s disease (IPD). Since longitudinal analyses are lacking so far, the present study aimed to evaluate the diagnostic value of the relative change of MTPR (relΔt_MTPR) over a 1-year period in patients with PSP, IPD, and healthy controls (HC).

Methods

Midsagittal individual MRIs of patients with PSP (n = 15), IPD (n = 15), and healthy controls (HC; n = 15) were assessed and the MTPR at baseline and after 1 year were defined. The diagnostic accuracy of the MTPR and its relative change were evaluated using ROC curve analyses.

Results

PSP-patients had a significantly lower MTPR at baseline (M = 0.45 ± 0.06), compared to both non-PSP groups (F (2, 41) = 62.82, p < 0.001), with an overall predictive accuracy of 95.6% for an MTPR ≤ 0.54. PSP-patients also presented a significantly stronger 1-year decline in MTPR compared to IPD (p < 0.001). Though predictive accuracy of relΔ_t_MTPR for PSP (M = − 4.74% ± 4.48) from IPD (M =  + 1.29 ± 3.77) was good (76.6%), ROC analysis did not reveal a significant improvement of diagnostic accuracy by combining the MTPR and relΔ_t_MTPR (p = 0.670). Still, specificity for PSP increased, though not significantly (p = 0.500).

Conclusion

The present results indicate that the relΔ_t_MTPR is a potentially useful tool to support the differential diagnosis of PSP from IPD. For its relative 1-year change, still, more evaluation is needed.

Transesophageal Echocardiography - Dysphagia Risk in Acute Stroke (TEDRAS): a prospective, blind, randomized and controlled clinical trial

BACKGROUND AND PURPOSE

Dysphagia is common in acute stroke and leads to worse overall outcome. Transesophageal echocardiography (TEE) is used in the diagnostic evaluation of stroke with regard to its etiology and is a known cause of postoperative dysphagia in cardiac surgery. The prevalence of dysphagia in acute stroke patients undergoing TEE remains unknown. The aim of the Transesophageal Echocardiography - Dysphagia Risk in Acute Stroke (TEDRAS) study was to assess the influence of TEE on swallowing among patients who have experienced acute stroke.

METHODS

The TEDRAS study was a prospective, blind, randomized, controlled trial that included two groups of patients with acute stroke. Simple unrestricted randomization was performed, and examiners were blinded to each other's results. Swallowing was tested using flexible endoscopic evaluation of swallowing (FEES) at three different time points in the intervention group (24 h before, immediately after and 24 h after TEE) and in the control group (FEES on three consecutive days and TEE earliest after the third FEES). Validated scales were used to assess dysphagia severity for all time points as primary outcome measures.

RESULTS

A total of 34 patients were randomized: 19 to the intervention group and 15 to the control group. The key findings of the repeated-measures between-group comparisons were significant increases in the intervention group for the following dysphagia measures: (1) secretion severity score (immediately after TEE: P < 0.001; 24 h after TEE: P < 0.001) and (2) Penetration-Aspiration Scale score for saliva (immediately after TEE: P < 0.001; 24 h after TEE: P = 0.007), for small (immediately after TEE: P = 0.009) and large liquid boli (immediately after TEE: P = 0.009; 24 h after TEE: P = 0.025).

CONCLUSION

The results indicate a negative influence of TEE on swallowing in acute stroke patients for at least 24 hours.

Deep Brain Stimulation Does Not Modulate Auditory-Motor Integration of Speech in Parkinson's Disease

Deep brain stimulation (DBS) has significant effects on motor symptoms in Parkinson's disease (PD), but existing studies on the effect of DBS on speech are rather inconclusive. It is assumed that deficits in auditory-motor integration strongly contribute to Parkinsonian speech pathology. The aim of the present study was to assess whether subthalamic DBS can modulate these deficits. Twenty PD patients (15 male, 5 female; 62.4 ± 6.7 years) with subthalamic DBS were exposed to pitch-shifted acoustic feedback during vowel vocalization and subsequent listening. Voice and brain activity were measured ON and OFF stimulation using magnetoencephalography (MEG). Vocal responses and auditory evoked responses time locked to the onset of pitch-shifted feedback were examined. A positive correlation between vocal response magnitude and pitch variability was observed for both, stimulation ON and OFF (ON: r = 0.722, p < 0.001, OFF: r = 0.746, p < 0.001). However, no differences of vocal responses to pitch-shifted feedback between the stimulation conditions were found [t₍₁₉₎ = −0.245, p = 0.809, d = −0.055]. P200m amplitudes of event related fields (ERF) of left and right auditory cortex (AC) and superior temporal gyrus (STG) were significantly larger during listening [left AC P200m: F_{(1, 19)} = 10.241, p = 0.005, f = 0.734; right STG P200m: F_{(1, 19)} = 8.393, p = 0.009, f = 0.664]. Subthalamic DBS appears to have no substantial effect on vocal compensations, although it has been suggested that auditory-motor integration deficits contribute to higher vocal response magnitudes in pitch perturbation experiments with PD patients. Thus, DBS seems to be limited in modulating auditory-motor integration of speech in PD.

Longitudinal Recordings Reveal Transient Increase of Alpha/Low-Beta Power in the Subthalamic Nucleus Associated With the Onset of Parkinsonian Rest Tremor

Functional magnetic resonance imaging studies suggest that different subcortico-cortical circuits control different aspects of Parkinsonian rest tremor. The basal ganglia were proposed to drive tremor onset, and the cerebellum was suggested to be responsible for tremor maintenance (“dimmer-switch” hypothesis). Although several electrophysiological correlates of tremor have been described, it is currently unclear whether any of these is specific to tremor onset or maintenance. In this study, we present data from a single patient measured repeatedly within 2 years after implantation of a deep brain stimulation (DBS) system capable of recording brain activity from the target. Local field potentials (LFPs) from the subthalamic nucleus and the scalp electroencephalogram were recorded 1 week, 3 months, 6 months, 1 year, and 2 years after surgery. Importantly, the patient suffered from severe rest tremor of the lower limbs, which could be interrupted voluntarily by repositioning the feet. This provided the unique opportunity to record many tremor onsets in succession. We found that tremor onset and tremor maintenance were characterized by distinct modulations of subthalamic oscillations. Alpha/low-beta power increased transiently immediately after tremor onset. In contrast, beta power was continuously suppressed during tremor maintenance. Tremor maintenance was additionally associated with subthalamic and cortical power increases around individual tremor frequency. To our knowledge, this is the first evidence of distinct subthalamic LFP modulations in tremor onset and tremor maintenance. Our observations suggest the existence of an acceleration signal for Parkinsonian rest tremor in the basal ganglia, in line with the “dimmer-switch” hypothesis.

Chemical exchange saturation transfer imaging in hepatic encephalopathy

Hepatic encephalopathy (HE) is a common complication in liver cirrhosis and associated with an invasion of ammonia into the brain through the blood-brain barrier. Resulting higher ammonia concentrations in the brain are suggested to lead to a dose-dependent gradual increase of HE severity and an associated impairment of brain function. Amide proton transfer-weighted (APT_w) chemical exchange saturation transfer (CEST) imaging has been found to be sensitive to ammonia concentration. The aim of this work was to study APT_w CEST imaging in patients with HE and to investigate the relationship between disease severity, critical flicker frequency (CFF), psychometric test scores, blood ammonia, and APT_w signals in different brain regions.

Whole-brain APT_w CEST images were acquired in 34 participants (14 controls, 20 patients (10 minimal HE, 10 manifest HE)) on a 3 T clinical MRI system accompanied by T₁ mapping and structural images. T₁ normalized magnetization transfer ratio asymmetry analysis was performed around 3 ppm after B₀ and B₁ correction to create APT_w images. All APT_w images were spatially normalized into a cohort space to allow direct comparison. APT_w images in 6 brain regions (cerebellum, occipital cortex, putamen, thalamus, caudate, white matter) were tested for group differences as well as the link to CFF, psychometric test scores, and blood ammonia.

A decrease in APT_w intensities was found in the cerebellum and the occipital cortex of manifest HE patients. In addition, APT_w intensities in the cerebellum correlated positively with several psychometric scores, such as the fine motor performance scores MLS1 for hand steadiness / tremor (r = 0.466; p = .044) and WRT2 for motor reaction time (r = 0.523; p = .022). Moreover, a negative correlation between APT_w intensities and blood ammonia was found for the cerebellum (r = −0.615; p = .007) and the occipital cortex (r = −0.478; p = .045). An increase of APT_w intensities was observed in the putamen of patients with minimal HE and correlated negatively with the CFF (r = −0.423; p = .013).

Our findings demonstrate that HE is associated with regional differential alterations in APT_w signals. These variations are most likely a consequence of hyperammonemia or hepatocerebral degeneration processes, and develop in parallel with disease severity.

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Ammonia is suggested to play a key role in the emergence of HE.

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Increase of ammonia in HE patients might be studied with APT_w CEST.

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HE leads to regionally decreasing APT_w CEST signal.

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APT_w CEST correlates with blood ammonia levels and psychometric test scores.

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APT_w CEST is possibly linked to hyperammonemia or hepatocerebral degeneration.

Impaired Tactile Temporal Discrimination in Patients With Hepatic Encephalopathy

The sensory system constantly receives stimuli from the external world. To discriminate two stimuli correctly as two temporally distinct events, the temporal distance or stimulus onset asynchrony (SOA) between the two stimuli has to exceed a specific threshold. If the SOA between two stimuli is shorter than this specific threshold, the two stimuli will be perceptually fused and perceived as one single stimulus. Patients with hepatic encephalopathy (HE) are known to show manifold perceptual impairments, including slowed visual temporal discrimination abilities as measured by the critical flicker frequency (CFF). Here, we hypothesized that HE patients are also impaired in their tactile temporal discrimination abilities and, thus, require a longer SOA between two tactile stimuli to perceive the stimuli as two temporally distinct events. To test this hypothesis, patients with varying grades of HE and age-matched healthy individuals performed a tactile temporal discrimination task. All participants received two tactile stimuli with varying SOA applied to their left index finger and reported how many distinct stimuli they perceived ("1" vs. "2"). HE patients needed a significantly longer SOA (138.0 ± 11.3 ms) between two tactile stimuli to perceive the stimuli as two temporally distinct events than healthy controls (78.6 ± 13.1 ms; p < 0.01). In addition, we found that the temporal discrimination ability in the tactile modality correlated positively with the temporal discrimination ability in the visual domain across all participants (i.e., negative correlation between tactile SOA and visual CFF: r = -0.37, p = 0.033). Our findings provide evidence that temporal tactile perception is substantially impaired in HE patients. In addition, the results suggest that tactile and visual discrimination abilities are affected in HE in parallel. This finding might argue for a common underlying pathophysiological mechanism. We argue that the known global slowing of neuronal oscillations in HE might represent such a common mechanism.

Ammonia-weighted imaging by chemical exchange saturation transfer MRI at 3 T

Hepatic encephalopathy (HE) is triggered by liver cirrhosis and is associated with an increased ammonia level within the brain tissue. The goal of this study was to investigate effects of ammonia on in vitro amide proton transfer (APT)-weighted chemical exchange saturation transfer (CEST) imaging in order to develop an ammonia-sensitive brain imaging method. APT-weighted CEST imaging was performed on phantom solutions including pure ammonia, bovine serum albumin (BSA), and tissue homogenate samples doped with various ammonia concentrations. All CEST data were assessed by magnetization transfer ratio asymmetry. In addition, optical methods were used to determine possible structural changes of the proteins in the BSA phantom. In vivo feasibility measurements were acquired in one healthy participant and two patients suffering from HE, a disease associated with increased brain ammonia levels. The CEST effect of pure ammonia showed a base-catalyzed behavior. At pH values greater than 5.6 no CEST effect was observed. The APT-weighted signal was significantly reduced for ammonia concentrations of 5mM or more at fixed pH values within the different protein phantom solutions. The optical methods revealed no protein aggregation or denaturation for ammonia concentrations less than 5mM. The in vivo measurements showed tissue specific and global reduction of the observed CEST signal in patients with HE, possibly linked to pathologically increased ammonia levels. APT-weighted CEST imaging is sensitive to changes in ammonia concentrations. Thus, it seems useful for the investigation of pathologies with altered tissue ammonia concentrations such as HE. However, the underlying mechanism needs to be explored in more detail in future in vitro and in vivo investigations.

Connecting occipital alpha band peak frequency, visual temporal resolution, and occipital GABA levels in healthy participants and hepatic encephalopathy patients

Recent studies have proposed a connection between the individual alpha band peak frequency and the temporal resolution of visual perception in healthy human participants. This connection rests on animal studies describing oscillations in the alpha band as a mode of phasic thalamocortical information transfer for low-level visual stimuli, which critically relies on GABAergic interneurons. Here, we investigated the interplay of these parameters by measuring occipital alpha band peak frequency by means of magnetoencephalography, visual temporal resolution by means of behavioral testing, and occipital GABA levels by means of magnetic resonance spectroscopy. Importantly, we investigated a sample of healthy participants and patients with varying grades of hepatic encephalopathy, which are known to exhibit decreases in the investigated parameters, thus providing an increased parameter space. We found that occipital alpha band peak frequency and visual temporal resolution were positively correlated, i.e., higher occipital alpha band peak frequencies were on average related to a higher temporal resolution. Likewise, occipital alpha band peak frequency correlated positively with occipital GABA levels. However, correlations were significant only when both healthy participants and patients were included in the analysis, thereby indicating a connection of the measures on group level (instead of the individual level). These findings provide new insights into neurophysiological and neurochemical underpinnings of visual perception.

Unilateral deep brain stimulation suppresses alpha and beta oscillations in sensorimotor cortices

Deep brain stimulation (DBS) is an established therapy to treat motor symptoms in movement disorders such as Parkinson's disease (PD). The mechanisms leading to the high therapeutic effectiveness of DBS are poorly understood so far, but modulation of oscillatory activity is likely to play an important role. Thus, investigating the effect of DBS on cortical oscillatory activity can help clarifying the neurophysiological mechanisms of DBS. Here, we aimed at scrutinizing changes of cortical oscillatory activity by DBS at different frequencies using magnetoencephalography (MEG). MEG data from 17 PD patients were acquired during DBS of the subthalamic nucleus (STN) the day after electrode implantation and before implanting the pulse generator. We stimulated the STN unilaterally at two different stimulation frequencies, 130 Hz and 340 Hz using an external stimulator. Data from six patients had to be discarded due to strong artefacts and two other datasets were excluded since these patients were not able to finalize the paradigm. After DBS artefact removal, power spectral density (PSD) values of MEG were calculated for each individual patient and averaged over the group. DBS at both 130 Hz and 340 Hz led to a widespread suppression of cortical alpha/beta band activity (8-22 Hz) specifically over bilateral sensorimotor cortices. No significant differences were observed between the two stimulation frequencies. Our finding of a widespread suppression of cortical alpha/beta band activity is particularly interesting as PD is associated with pathologically increased levels of beta band activity in the basal ganglia-thalamo-cortical circuit. Therefore, suppression of such oscillatory activity might be an essential effect of DBS for relieving motor symptoms in PD and can be achieved at different stimulation frequencies above 100 Hz.

Parkinsonian Rest Tremor Is Associated With Modulations of Subthalamic High-Frequency Oscillations

BACKGROUND

High frequency oscillations (>200 Hz) have been observed in the basal ganglia of PD patients and were shown to be modulated by the administration of levodopa and voluntary movement.

OBJECTIVE

The objective of this study was to test whether the power of high-frequency oscillations in the STN is associated with spontaneous manifestation of parkinsonian rest tremor.

METHODS

The electromyogram of both forearms and local field potentials from the STN were recorded in 11 PD patients (10 men, age 58 [9.4] years, disease duration 9.2 [6.3] years). Patients were recorded at rest and while performing repetitive hand movements before and after levodopa intake. High-frequency oscillation power was compared across epochs containing rest tremor, tremor-free rest, or voluntary movement and related to the tremor cycle.

RESULTS

We observed prominent slow (200-300 Hz) and fast (300-400 Hz) high-frequency oscillations. The ratio between slow and fast high-frequency oscillation power increased when tremor became manifest. This increase was consistent across nuclei (94%) and occurred in medication ON and OFF. The ratio outperformed other potential markers of tremor, such as power at individual tremor frequency, beta power, or low gamma power. For voluntary movement, we did not observe a significant difference when compared with rest or rest tremor. Finally, rhythmic modulations of high-frequency oscillation power occurred within the tremor cycle.

CONCLUSIONS

Subthalamic high-frequency oscillation power is closely linked to the occurrence of parkinsonian rest tremor. The balance between slow and fast high-frequency oscillation power combines information on motor and medication state. © 2016 International Parkinson and Movement Disorder Society.

Bicycling suppresses abnormal beta synchrony in the Parkinsonian basal ganglia

OBJECTIVE

Freezing of gait is a poorly understood symptom of Parkinson disease, and can severely disrupt the locomotion of affected patients. However, bicycling ability remains surprisingly unaffected in most patients suffering from freezing, suggesting functional differences in the motor network. The purpose of this study was to characterize and contrast the oscillatory dynamics underlying bicycling and walking in the basal ganglia.

METHODS

We present the first local field potential recordings directly comparing bicycling and walking in Parkinson disease patients with electrodes implanted in the subthalamic nuclei for deep brain stimulation. Low (13-22Hz) and high (23-35Hz) beta power changes were analyzed in 22 subthalamic nuclei from 13 Parkinson disease patients (57.5 ± 5.9 years old, 4 female). The study group consisted of 5 patients with and 8 patients without freezing of gait.

RESULTS

In patients without freezing of gait, both bicycling and walking led to a suppression of subthalamic beta power (13-35Hz), and this suppression was stronger for bicycling. Freezers showed a similar pattern in general. Superimposed on this pattern, however, we observed a movement-induced, narrowband power increase around 18Hz, which was evident even in the absence of freezing.

INTERPRETATION

These results indicate that bicycling facilitates overall suppression of beta power. Furthermore, movement leads to exaggerated synchronization in the low beta band specifically within the basal ganglia of patients susceptible to freezing. Abnormal ∼18Hz oscillations are implicated in the pathophysiology of freezing of gait, and suppressing them may form a key strategy in developing potential therapies. Ann Neurol 2017;82:592-601.

BrainCycles: Experimental Setup for the Combined Measurement of Cortical and Subcortical Activity in Parkinson's Disease Patients during Cycling

Recently, it has been demonstrated that bicycling ability remains surprisingly preserved in Parkinson's disease (PD) patients who suffer from freezing of gait. Cycling has been also proposed as a therapeutic means of treating PD symptoms, with some preliminary success. The neural mechanisms behind these phenomena are however not yet understood. One of the reasons is that the investigations of neuronal activity during pedaling have been up to now limited to PET and fMRI studies, which restrict the temporal resolution of analysis, and to scalp EEG focused on cortical activation. However, deeper brain structures like the basal ganglia are also associated with control of voluntary motor movements like cycling and are affected by PD. Deep brain stimulation (DBS) electrodes implanted for therapy in PD patients provide rare and unique access to directly record basal ganglia activity with a very high temporal resolution. In this paper we present an experimental setup allowing combined investigation of basal ganglia local field potentials (LFPs) and scalp EEG underlying bicycling in PD patients. The main part of the setup is a bike simulator consisting of a classic Dutch-style bicycle frame mounted on a commercially available ergometer. The pedal resistance is controllable in real-time by custom software and the pedal position is continuously tracked by custom Arduino-based electronics using optical and magnetic sensors. A portable bioamplifier records the pedal position signal, the angle of the knee, and the foot pressure together with EEG, EMG, and basal ganglia LFPs. A handlebar-mounted display provides additional information for patients riding the bike simulator, including the current and target pedaling rate. In order to demonstrate the utility of the setup, example data from pilot recordings are shown. The presented experimental setup provides means to directly record basal ganglia activity not only during cycling but also during other movement tasks in patients who have undergone DBS treatment. Thus, it can facilitate studies comparing bicycling and walking, to elucidate why PD patients often retain the ability to bicycle despite severe freezing of gait. Moreover it can help clarifying the mechanism through which cycling may have therapeutic benefits.

Covert hepatic encephalopathy: elevated total glutathione and absence of brain water content changes

Recent pathophysiological models suggest that oxidative stress and hyperammonemia lead to a mild brain oedema in hepatic encephalopathy (HE). Glutathione (GSx) is a major cellular antioxidant and known to be involved in the interception of both. The aim of this work was to study total glutathione levels in covert HE (minimal HE and HE grade 1) and to investigate their relationship with local brain water content, levels of glutamine (Gln), myo-inositol (mI), neurotransmitter levels, critical flicker frequency (CFF), and blood ammonia. Proton magnetic resonance spectroscopy ((1)H MRS) data were analysed from visual and sensorimotor cortices of thirty patients with covert HE and 16 age-matched healthy controls. Total glutathione levels (GSx/Cr) were quantified with respect to creatine. Furthermore, quantitative MRI brain water content measures were evaluated. Data were tested for links with the CFF and blood ammonia. GSx/Cr was elevated in the visual (mHE) and sensorimotor (mHE, HE 1) MRS volumes and correlated with blood ammonia levels (both P < 0.001). It was further linked to Gln/Cr and mI/Cr (P < 0.01 in visual, P < 0.001 in sensorimotor) and to GABA/Cr (P < 0.01 in visual). Visual GSx/Cr correlated with brain water content in the thalamus, nucleus caudatus, and visual cortex (P < 0.01). Brain water measures did neither show group effects nor correlations with CFF or blood ammonia. Elevated total glutathione levels in covert HE (< HE 2) correlate with blood ammonia and may be a regional-specific reaction to hyperammonemia and oxidative stress. Brain water content is locally linked to visual glutathione levels, but appears not to be associated with changes of clinical parameters. This might suggest that cerebral oedema is only marginally responsible for the symptoms of covert HE.

Bicycling and Walking are Associated with Different Cortical Oscillatory Dynamics

Although bicycling and walking involve similar complex coordinated movements, surprisingly Parkinson’s patients with freezing of gait typically remain able to bicycle despite severe difficulties in walking. This observation suggests functional differences in the motor networks subserving bicycling and walking. However, a direct comparison of brain activity related to bicycling and walking has never been performed, neither in healthy participants nor in patients. Such a comparison could potentially help elucidating the cortical involvement in motor control and the mechanisms through which bicycling ability may be preserved in patients with freezing of gait. The aim of this study was to contrast the cortical oscillatory dynamics involved in bicycling and walking in healthy participants. To this end, EEG and EMG data of 14 healthy participants were analyzed, who cycled on a stationary bicycle at a slow cadence of 40 revolutions per minute (rpm) and walked at 40 strides per minute (spm), respectively. Relative to walking, bicycling was associated with a stronger power decrease in the high beta band (23–35 Hz) during movement initiation and execution, followed by a stronger beta power increase after movement termination. Walking, on the other hand, was characterized by a stronger and persisting alpha power (8–12 Hz) decrease. Both bicycling and walking exhibited movement cycle-dependent power modulation in the 24–40 Hz range that was correlated with EMG activity. This modulation was significantly stronger in walking. The present findings reveal differential cortical oscillatory dynamics in motor control for two types of complex coordinated motor behavior, i.e., bicycling and walking. Bicycling was associated with a stronger sustained cortical activation as indicated by the stronger high beta power decrease during movement execution and less cortical motor control within the movement cycle. We speculate this to be due to the more continuous nature of bicycling demanding less phase-dependent sensory processing and motor planning, as opposed to walking.

Low visual cortex GABA levels in hepatic encephalopathy: links to blood ammonia, critical flicker frequency, and brain osmolytes

The pathogenesis of hepatic encephalopathy (HE) is not fully understood yet. Hyperammonemia due to liver failure and subsequent disturbance of cerebral osmolytic balance is thought to play a pivotal role in the emergence of HE. The aim of this in-vivo MR spectroscopy study was to investigate the levels of γ-aminobutyric acid (GABA) and its correlations with clinical symptoms of HE, blood ammonia, critical flicker frequency, and osmolytic levels. Thirty patients with minimal HE or HE1 and 16 age-matched healthy controls underwent graduation of HE according to the West-Haven criteria and including the critical flicker frequency (CFF), neuropsychometric testing and blood testing. Edited proton magnetic resonance spectroscopy ((1)H MRS) was used to non-invasively measure the concentrations of GABA, glutamate (Glu), glutamine (Gln), and myo-inositol (mI) - all normalized to creatine (Cr) - in visual and sensorimotor cortex. GABA/Cr in the visual area was significantly decreased in mHE and HE1 patients and correlated both to the CFF (r = 0.401, P = 0.013) and blood ammonia levels (r = -0.434, P = 0.006). Visual GABA/Cr was also strongly linked to mI/Cr (r = 0.720, P < 0.001) and Gln/Cr (r = -0.699, P < 0.001). No group differences or correlations were found for GABA/Cr in the sensorimotor area. Hepatic encephalopathy is associated with a regional specific decrease of GABA levels in the visual cortex, while no changes were revealed for the sensorimotor cortex. Correlations of visual GABA/Cr with CFF, blood ammonia, and osmolytic regulators mI and Gln indicate that decreased visual GABA levels might contribute to HE symptoms, most likely as a consequence of hyperammonemia.

Patients with manifest hepatic encephalopathy can reveal impaired thermal perception

OBJECTIVES

Previous evoked potential studies indicated central impairments of somatosensory function in patients suffering from hepatic encephalopathy (HE). The aim of this study was to quantify the somatosensory perception in patients with minimal and overt HE.

MATERIALS AND METHODS

Forty-two patients with liver cirrhosis and HE up to grade 2 and 12 age-matched healthy controls underwent a comprehensive graduation of HE including the West Haven criteria, the critical flicker frequency (CFF), and neuropsychometric testing. Quantitative sensory testing, standardized by the German Research Network on Neuropathic Pain, was performed on both hands.

RESULTS

Pain and mechanical detection thresholds were unchanged in HE. Tests of thermal processing revealed that patients with HE of grade 2 perceive cold at lower temperatures (cold detection threshold) and need a higher temperature difference to distinguish between warm and cold (thermal sensory limen). These impairments correlated with the CFF. A correction for attention deficits by performing partial correlations using neuropsychometric test results canceled these correlations.

CONCLUSIONS

The present findings demonstrate an impairment of temperature perception in HE. The extent of this impairment correlates with HE severity as quantified by the CFF. The attenuation of the correlations after correction for attention deficits suggests a strong role of attention deficits for the impaired thermal perception. Thus, it provides initial evidence for a central impairment of thermal processing in HE due to alterations in high-level processes rather than due to peripheral neuropathic processes, which are a frequent complication in patients with liver cirrhosis.

Time-frequency analysis of resting state and evoked EEG data recorded at higher magnetic fields up to 9.4 T

BACKGROUND

Combining both high temporal and spatial resolution by means of simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) is of relevance to neuroscientists. This combination, however, leads to a distortion of the EEG signal by the so-called cardio-ballistic artefacts. The aim of the present study was developing an approach to restore meaningful physiological EEG data from recordings at different magnetic fields.

NEW METHODS

The distortions introduced by the magnetic field were corrected using a combination of concepts from independent component analysis (ICA) and mutual information (MI). Thus, the components were classified as either related to the cardio-ballistic artefacts or to the signals of interest. EEG data from two experimental paradigms recorded at different magnetic field strengths up to 9.4 T were analyzed: (i) spontaneous activity using an eyes-open/eyes-closed alternation, and (ii) responses to auditory stimuli, i.e. auditory evoked potentials.

RESULTS

Even at ultra-high magnetic fields up to 9.4 T the proposed artefact rejection approach restored the physiological time-frequency information contained in the signal of interest and the data were suitable for subsequent analyses.

COMPARISON WITH EXISTING METHODS

Blind source separation (BSS) has been used to retrieve information from EEG data recorded inside the MR scanner in previous studies. After applying the presented method on EEG data recorded at 4 T, 7 T, and 9.4 T, we could retrieve more information than from data cleaned with the BSS method.

CONCLUSIONS

The present work demonstrates that EEG data recorded at ultra-high magnetic fields can be used for studying neuroscientific research question related to oscillatory activity.

Homeostatic structural plasticity can account for topology changes following deafferentation and focal stroke

After brain lesions caused by tumors or stroke, or after lasting loss of input (deafferentation), inter- and intra-regional brain networks respond with complex changes in topology. Not only areas directly affected by the lesion but also regions remote from the lesion may alter their connectivity—a phenomenon known as diaschisis. Changes in network topology after brain lesions can lead to cognitive decline and increasing functional disability. However, the principles governing changes in network topology are poorly understood. Here, we investigated whether homeostatic structural plasticity can account for changes in network topology after deafferentation and brain lesions. Homeostatic structural plasticity postulates that neurons aim to maintain a desired level of electrical activity by deleting synapses when neuronal activity is too high and by providing new synaptic contacts when activity is too low. Using our Model of Structural Plasticity, we explored how local changes in connectivity induced by a focal loss of input affected global network topology. In accordance with experimental and clinical data, we found that after partial deafferentation, the network as a whole became more random, although it maintained its small-world topology, while deafferentated neurons increased their betweenness centrality as they rewired and returned to the homeostatic range of activity. Furthermore, deafferentated neurons increased their global but decreased their local efficiency and got longer tailed degree distributions, indicating the emergence of hub neurons. Together, our results suggest that homeostatic structural plasticity may be an important driving force for lesion-induced network reorganization and that the increase in betweenness centrality of deafferentated areas may hold as a biomarker for brain repair.

Cortical activation associated with asterixis in manifest hepatic encephalopathy

OBJECTIVES

Severe hepatic encephalopathy gives rise to asterixis, a striking motor symptom also called flapping tremor, which is characterized by a sudden ceasing of muscle tone in all muscles of a limb. In this study, we aimed at scrutinizing the cortical activation associated with asterixis and unraveling the underlying pathophysiological mechanisms.

MATERIAL AND METHODS

We recorded simultaneously neural activity with magnetoencephalography (MEG) and muscle activity with surface EMG in nine patients with manifest hepatic encephalopathy showing asterixis. Asterixis events were detected semiautomatically and served as triggers for averaging MEG signals. Evoked responses averaged time-locked to asterixis events were subjected to equivalent current dipole (ECD) modeling. Additionally, we localized the strongest cortico-muscular coherence in the frequency of the co-occurring tremulousness.

RESULTS

Evoked fields averaged time-locked to asterixis events were best explained by a single dipolar source in the contralateral primary motor cortex (M1, Talairach coordinates of mean localization: -40, -20, and 64; Brodmann area 4). This dipole showed a twofold field reversal, that is biphasic wave, with frontal dipole orientation at 49 ms before flap onset and 99 ms after flap onset. Conversely, two maxima with occipital dipole orientation were observed 2 ms and 160 ms after flap onset. Cortico-muscular coherence for the tremulousness was likewise localized in the contralateral M1 confirming earlier findings in the present patient cohort.

CONCLUSIONS

Our results reveal an involvement of M1 in the generation of asterixis. As also tremulousness, also called mini-asterixis, was shown to originate in M1, asterixis and mini-asterixis may share common pathophysiological mechanisms.

Homeostatic structural plasticity increases the efficiency of small-world networks

In networks with small-world topology, which are characterized by a high clustering coefficient and a short characteristic path length, information can be transmitted efficiently and at relatively low costs. The brain is composed of small-world networks, and evolution may have optimized brain connectivity for efficient information processing. Despite many studies on the impact of topology on information processing in neuronal networks, little is known about the development of network topology and the emergence of efficient small-world networks. We investigated how a simple growth process that favors short-range connections over long-range connections in combination with a synapse formation rule that generates homeostasis in post-synaptic firing rates shapes neuronal network topology. Interestingly, we found that small-world networks benefited from homeostasis by an increase in efficiency, defined as the averaged inverse of the shortest paths through the network. Efficiency particularly increased as small-world networks approached the desired level of electrical activity. Ultimately, homeostatic small-world networks became almost as efficient as random networks. The increase in efficiency was caused by the emergent property of the homeostatic growth process that neurons started forming more long-range connections, albeit at a low rate, when their electrical activity was close to the homeostatic set-point. Although global network topology continued to change when neuronal activities were around the homeostatic equilibrium, the small-world property of the network was maintained over the entire course of development. Our results may help understand how complex systems such as the brain could set up an efficient network topology in a self-organizing manner. Insights from our work may also lead to novel techniques for constructing large-scale neuronal networks by self-organization.

A direct relationship between oscillatory subthalamic nucleus-cortex coupling and rest tremor in Parkinson's disease

Electrophysiological studies suggest that rest tremor in Parkinson's disease is associated with an alteration of oscillatory activity. Although it is well known that tremor depends on cortico-muscular coupling, it is unclear whether synchronization within and between brain areas is specifically related to the presence and severity of tremor. To tackle this longstanding issue, we took advantage of naturally occurring spontaneous tremor fluctuations and investigated cerebral synchronization in the presence and absence of rest tremor. We simultaneously recorded local field potentials from the subthalamic nucleus, the magnetoencephalogram and the electromyogram of forearm muscles in 11 patients with Parkinson's disease (all male, age: 52-74 years). Recordings took place the day after surgery for deep brain stimulation, after withdrawal of anti-parkinsonian medication. We selected epochs containing spontaneous rest tremor and tremor-free epochs, respectively, and compared power and coherence between subthalamic nucleus, cortex and muscle across conditions. Tremor-associated changes in cerebro-muscular coherence were localized by Dynamic Imaging of Coherent Sources. Subsequently, cortico-cortical coupling was analysed by computation of the imaginary part of coherency, a coupling measure insensitive to volume conduction. After tremor onset, local field potential power increased at individual tremor frequency and cortical power decreased in the beta band (13-30 Hz). Sensor level subthalamic nucleus-cortex, cortico-muscular and subthalamic nucleus-muscle coherence increased during tremor specifically at tremor frequency. The increase in subthalamic nucleus-cortex coherence correlated with the increase in electromyogram power. On the source level, we observed tremor-associated increases in cortico-muscular coherence in primary motor cortex, premotor cortex and posterior parietal cortex contralateral to the tremulous limb. Analysis of the imaginary part of coherency revealed tremor-dependent coupling between these cortical areas at tremor frequency and double tremor frequency. Our findings demonstrate a direct relationship between the synchronization of cerebral oscillations and tremor manifestation. Furthermore, they suggest the feasibility of tremor detection based on local field potentials and might thus become relevant for the design of closed-loop stimulation systems.

A simple rule for dendritic spine and axonal bouton formation can account for cortical reorganization after focal retinal lesions

Lasting alterations in sensory input trigger massive structural and functional adaptations in cortical networks. The principles governing these experience-dependent changes are, however, poorly understood. Here, we examine whether a simple rule based on the neurons' need for homeostasis in electrical activity may serve as driving force for cortical reorganization. According to this rule, a neuron creates new spines and boutons when its level of electrical activity is below a homeostatic set-point and decreases the number of spines and boutons when its activity exceeds this set-point. In addition, neurons need a minimum level of activity to form spines and boutons. Spine and bouton formation depends solely on the neuron's own activity level, and synapses are formed by merging spines and boutons independently of activity. Using a novel computational model, we show that this simple growth rule produces neuron and network changes as observed in the visual cortex after focal retinal lesions. In the model, as in the cortex, the turnover of dendritic spines was increased strongest in the center of the lesion projection zone, while axonal boutons displayed a marked overshoot followed by pruning. Moreover, the decrease in external input was compensated for by the formation of new horizontal connections, which caused a retinotopic remapping. Homeostatic regulation may provide a unifying framework for understanding cortical reorganization, including network repair in degenerative diseases or following focal stroke.

The adult brain is less hard-wired than traditionally thought. About ten percent of synapses in the mature visual cortex is continually replaced by new ones (structural plasticity). This percentage greatly increases after lasting changes in visual input. Due to the topographically organized nerve connections from the retina in the eye to the primary visual cortex in the brain, a small circumscribed lesion in the retina leads to a defined area in the cortex that is deprived of input. Recent experimental studies have revealed that axonal sprouting and dendritic spine turnover are massively increased in and around the cortical area that is deprived of input. However, the driving forces for this structural plasticity remain unclear. Using a novel computational model, we examine whether the need for activity homeostasis of individual neurons may drive cortical reorganization after lasting changes in input activity. We show that homeostatic growth rules indeed give rise to structural and functional reorganization of neuronal networks similar to the cortical reorganization observed experimentally. Understanding the principles of structural plasticity may eventually lead to novel treatment strategies for stimulating functional reorganization after brain damage and neurodegeneration.

Increased SMA-M1 coherence in Parkinson's disease - Pathophysiology or compensation?

Parkinson's disease (PD) is a common neurodegenerative disorder owing to loss of dopaminergic cells. Akinesia - one of the core symptoms of PD - is associated with exaggerated oscillations at beta frequency (13-30 Hz) within the subthalamic nucleus (STN). Thus, enhanced oscillations below 30 Hz are assumed to represent a pathophysiological marker of PD. However, recent data suggest that OFF medication exaggerated beta oscillations within basal ganglia (BG) cortical networks may serve for the compensation of BG dysfunctions. The STN is functionally connected to mesial prefrontal areas like the supplementary motor area (SMA). But, it is still not fully understood how enhanced beta oscillations within the BG exert dominance over the primary motor cortex (M1) thereby yielding motor impairment. The present study, therefore, investigates the effect of dopaminergic state on SMA-M1 functional connectivity using Magnetoencephalography (MEG). MEG data were recorded in 7 patients suffering from PD with preponderance of akinesia during isometric contraction of the right forearm and during rest. Coherence as a measure of functional connectivity between M1 and SMA was calculated OFF and ON medication and correlated with the motor part of the Unified Parkinson's Disease Rating Scale (UPDRS III) and with disease duration. During rest a significant positive correlation between disease duration and SMA-M1 coherence was found ON but not OFF medication. Conversely, during isometric contraction SMA-M1 coherence and UPDRS III were inversely correlated OFF but not ON medication explaining more than 80% of variance. The results favor the hypothesis that OFF medication exaggerated cortical coherence at beta frequency represents a compensatory mechanism rather than a pathophysiological marker per se.

Changes of cortico-muscular coherence: an early marker of healthy aging?

Cortico-muscular coherence (CMC) at beta frequency (13-30 Hz) occurs particularly during weak to moderate isometric contraction. It is a well-established measure of communication between the primary motor cortex (M1) and corresponding muscles revealing information about the integrity of the pyramidal system. Although the slowing of brain and muscle dynamics during healthy aging has been evidenced, functional communication as determined by CMC has not been investigated so far. Since decline of motor functions at higher age is likely to be associated with CMC changes, the present study aims at shedding light on the functionality of the motor system from a functional interaction perspective. To this end, CMC was investigated in 27 healthy subjects aging between 22 and 77 years during isometric contraction of their right forearm. Neuromagnetic activity was measured using whole-head magnetoencephalography (MEG). Muscle activity was measured by means of surface electromyography (EMG) of the right extensor digitorum communis (EDC) muscle. Additionally, MEG-EMG phase lags were calculated in order to estimate conducting time. The analysis revealed CMC and M1 power amplitudes to be increased with age accompanied by slowing of M1, EMG, and CMC. Frequency changes were particularly found in subjects aged above 40 years suggesting that at this middle age, neurophysiological changes occur, possibly reflecting an early neurophysiological marker of seniority. Since MEG-EMG phase lags did not vary with age, changes cannot be explained by alterations of nerve conduction. We argue that the M1 power amplitude increase and the shift towards lower frequencies might represent a neurophysiological marker of healthy aging which is possibly compensated by increased CMC amplitude.

Differential modulation of STN-cortical and cortico-muscular coherence by movement and levodopa in Parkinson's disease

Previous research suggests that oscillatory coupling between cortex, basal ganglia and muscles plays an important role in motor behavior. Furthermore, there is evidence that oscillatory coupling is altered in patients with movement disorders such as Parkinson's disease (PD). In this study, we performed simultaneous magnetoencephalography (MEG), local field potential (LFP) and electromyogram (EMG) recordings in PD patients selected for therapeutic subthalamic nucleus (STN) stimulation. Patients were recorded (i) after withdrawal of anti-parkinsonian medication (OFF) and (ii) after levodopa administration (ON). We analyzed STN-cortical and cortico-muscular coherence during static forearm contraction and repetitive hand movement in order to evaluate modulations of coherence by movement and medication. Based on previous results from studies investigating resting state coherence in PD patients, we selected primary motor cortex (M1) and superior temporal gyrus (STG) as regions of interest. We found beta coherence between M1 and STN to be suppressed by administration of levodopa. M1-muscular coherence was strongly reduced in the alpha and beta band during repetitive movement compared to static contraction, but was unaffected by administration of levodopa. Strong STG-STN but not STG-muscular coherence could be observed in the alpha band. Coherence with STG was modulated neither by movement nor by medication. Finally, we found both M1-STN and M1-muscular beta coherence to be negatively correlated with UPDRS akinesia and rigidity sub-scores in the OFF state. The present study provides new insights into the functional roles of STN-cortical and cortico-muscular coherence and their relationship to PD symptoms. The results indicate that STN-cortical and cortico-muscular coupling are correlated, but can be modulated independently. Moreover, they show differences in their frequency-specific topography. We conclude that they represent partly independent sub-loops within the motor system. Given their negative correlation with akinesia, neither can be considered "antikinetic".