Pre- and post-stimulus alpha activity shows differential modulation with spatial attention during the processing of pain

Extensive work using magneto- and electroencephalography (M/EEG) suggests that cortical alpha activity represents a top-down controlled gating mechanism employed by processes like attention across different modalities. However, it is not yet clear to what extent this presumed gating function of alpha activity also applies to the processing of pain. In the current study, a spatial attention paradigm was employed requiring subjects to attend to painful laser stimuli on one hand while ignoring stimuli on the other hand. Simultaneously, brain activity was recorded with MEG. In order to disentangle pre- and post-stimulus effects of attention, alpha activity was analyzed during time windows in anticipation of and in response to painful laser stimulation. Painful laser stimuli led to a suppression of alpha activity over both ipsi- and contralateral primary somatosensory areas irrespective if they were attended or ignored. Spatial attention was associated with a lateralization of anticipatory pre-stimulus alpha activity. Alpha activity was lower over primary somatosensory areas when the contralateral hand was attended compared to when the ipsilateral hand was attended, in line with the notion that oscillatory alpha activity regulates the flow of incoming information by engaging and/or disengaging early sensory areas. On the contrary, post-stimulus alpha activity, for stimuli on either hand, was consistently decreased with attention over contralateral areas. Most likely, this finding reflects an increased cortical activation and enhanced alerting if a painful stimulus is attended. The present results show that spatial attention results in a modulation of both pre- and post-stimulus alpha activity associated with pain. This flexible regulation of alpha activity matches findings from other modalities. We conclude that the assumed functional role of alpha activity as a top-down controlled gating mechanism includes pain processing and most likely represents a unified mechanism used throughout the brain.

High frequency oscillations in the subthalamic nucleus: a neurophysiological marker of the motor state in Parkinson's disease

Increasing evidence suggests that abnormal oscillatory activity in basal ganglia and cortex plays a pivotal role in the pathophysiology of Parkinson's disease. Recordings of local field potentials from subthalamic nucleus of patients undergoing deep brain stimulation have focused on oscillations occurring at frequencies below 100 Hz in the alpha, beta and gamma range and suggested that, in particular, an increase of beta band oscillations underlies slowing of movement in Parkinson's disease. Recent findings showing that the amplitude of high frequency oscillations (>200 Hz) couples with the phase of beta activity have raised the important question about the role of subthalamic high frequency oscillations in Parkinson's disease. To investigate functional characteristics and clinical relevance of high frequency oscillations, we recorded local field potentials from 18 subthalamic nuclei of 9 akinetic-rigid Parkinsonian patients with implanted deep brain stimulation electrodes and still externalised leads before and after intake of levodopa. We identified two distinct bands of high frequency oscillations, one centred around 250 Hz and another one around 350 Hz that show characteristic levodopa dependent amplitude and coupling behaviours. Administration of levodopa changed the power ratio between the two high frequency bands towards the component centred around 350 Hz in all 18 nuclei under study (p<10(-4)). Moreover, this power ratio correlated significantly with the Unified Parkinson's Disease Rating Scale hemibody akinesia/rigidity subscore (r=0.3618, p=0.015), but interestingly not with beta peak power (p=0.1) suggesting that levodopa induced changes in high frequency and beta oscillations are at least potentially independent of each other. Accordingly, a combined parameter composed of power ratio of high frequency oscillations and beta peak power significantly increased the correlation with the motor state (r=0.45, p=0.004). These results indicate that a shift from slower to faster frequencies of the spectrum greater than 200 Hz represents a prokinetic neurophysiological marker underlying levodopa induced motor improvement in Parkinson's disease.

Self-organized criticality in developing neuronal networks

Recently evidence has accumulated that many neural networks exhibit self-organized criticality. In this state, activity is similar across temporal scales and this is beneficial with respect to information flow. If subcritical, activity can die out, if supercritical epileptiform patterns may occur. Little is known about how developing networks will reach and stabilize criticality. Here we monitor the development between 13 and 95 days in vitro (DIV) of cortical cell cultures (n = 20) and find four different phases, related to their morphological maturation: An initial low-activity state (≈19 DIV) is followed by a supercritical (≈20 DIV) and then a subcritical one (≈36 DIV) until the network finally reaches stable criticality (≈58 DIV). Using network modeling and mathematical analysis we describe the dynamics of the emergent connectivity in such developing systems. Based on physiological observations, the synaptic development in the model is determined by the drive of the neurons to adjust their connectivity for reaching on average firing rate homeostasis. We predict a specific time course for the maturation of inhibition, with strong onset and delayed pruning, and that total synaptic connectivity should be strongly linked to the relative levels of excitation and inhibition. These results demonstrate that the interplay between activity and connectivity guides developing networks into criticality suggesting that this may be a generic and stable state of many networks in vivo and in vitro.

Motor impairment in liver cirrhosis without and with minimal hepatic encephalopathy

AIM

Manifest hepatic encephalopathy (HE) goes along with motor symptoms such as ataxia, mini-asterixis, and asterixis. The relevance of motor impairments in cirrhotics without and with minimal HE (mHE) is still a matter of debate.

PATIENTS AND METHODS

We tested three different groups of patients with liver cirrhosis: no signs of HE (HE 0), mHE, and manifest HE grade 1 according to the West Haven criteria (HE 1). All patients (n = 24) and 11 healthy control subjects were neuropsychometrically tested including critical flicker frequency (CFF), a reliable measure for HE. Motor abilities were assessed using Fahn Tremor Scale and International Ataxia Rating Scale. Fastest alternating index finger movements were analyzed for frequency and amplitude.

RESULTS

Statistical analyses showed an effect of HE grade on tremor and ataxia (P < 0.01). Additionally, both ratings yielded strong negative correlation with CFF (P < 0.01, R = -0.5). Analysis of finger movements revealed an effect of HE grade on movement frequency (P < 0.03). Moreover, decreasing movement frequency and increasing movement amplitude parallel decreasing CFF (P < 0.01, R = 0.6).

CONCLUSION

Our results indicate that ataxia, tremor, and slowing of finger movements are early markers for cerebral dysfunction in HE patients even prior to neuropsychometric alterations becoming detectable.

Modality specific functional interaction in sensorimotor synchronization

Movement execution strongly relies on precise sensorimotor synchronization. In a finger-tapping task that requires subjects to synchronize their finger taps to regular pacing signal synchronization accuracy varies with respect to pacing signal's modality. This study aimed at elucidating functional brain dynamics associated with modality specific behavioral synchronization accuracy. To this end, 10 right-handed subjects performed a finger-tapping task with respect to regular auditory and visual pacing, respectively, whereas neuromagnetic activity was recorded using a 122-channel whole-head neuromagnetometer. Visual pacing was associated with significantly reduced tap-to-pacer asynchrony and increased intertap variability as compared to auditory pacing. The brain dynamics associated with task execution were analyzed using the frequency domain beamformer approach dynamic imaging of coherent sources (DICS). Both tasks were shown to be associated with comparable networks. However, during visual pacing involvement of the ventral premotor cortex (PMv) was shown, whereas during auditory pacing the dorsal premotor cortex (PMd) was concerned with task execution. Synchronization with respect to visual pacing was associated with significantly increased functional interaction between thalamus and PMv at beta frequency as compared to functional interplay between thalamus and PMd during auditory pacing. Auditory synchronization was associated with increased functional interaction between left superior temporal gyrus and PMd at alpha frequency. Furthermore, functional interaction between thalamus and premotor cortex at beta frequency was significantly correlated with synchronization accuracy. All in all the present data suggest that modality specific synchronization differences are associated with frequency and connectivity specific changes of functional interaction in distinct brain networks.

A model for cortical rewiring following deafferentation and focal stroke

It is still unclear to what extent structural plasticity in terms of synaptic rewiring is the cause for cortical remapping after a lesion. Recent two-photon laser imaging studies demonstrate that synaptic rewiring is persistent in the adult brain and is dramatically increased following brain lesions or after a loss of sensory input (cortical deafferentation). We use a recurrent neural network model to study the time course of synaptic rewiring following a peripheral lesion. For this, we represent axonal and dendritic elements of cortical neurons to model synapse formation, pruning and synaptic rewiring. Neurons increase and decrease the number of axonal and dendritic elements in an activity-dependent fashion in order to maintain their activity in a homeostatic equilibrium. In this study we demonstrate that synaptic rewiring contributes to neuronal homeostasis during normal development as well as following lesions. We show that networks in homeostasis, which can therefore be considered as adult networks, are much less able to compensate for a loss of input. Interestingly, we found that paused stimulation of the networks are much more effective promoting reorganization than continuous stimulation. This can be explained as neurons quickly adapt to this stimulation whereas pauses prevents a saturation of the positive stimulation effect. These findings may suggest strategies for improving therapies in neurologic rehabilitation.

Levodopa affects functional brain networks in Parkinsonian resting tremor

Resting tremor in idiopathic Parkinson's disease (PD) is associated with an oscillatory network comprising cortical as well as subcortical brain areas. To shed light on the effect of levodopa on these network interactions, we investigated 10 patients with tremor-dominant PD and reanalyzed data in 11 healthy volunteers mimicking PD resting tremor. To this end, we recorded surface electromyograms of forearm muscles and neuromagnetic activity using a 122-channel whole-head magnetometer (MEG). Measurements were performed after overnight withdrawal of levodopa (OFF) and 30 min after oral application of fast-acting levodopa (ON). During OFF, patients showed the typical antagonistic resting tremor. Using the analysis tool Dynamic Imaging of Coherent Sources, we identified the oscillatory network associated with tremor comprising contralateral primary sensorimotor cortex (S1/M1), supplementary motor area (SMA), contralateral premotor cortex (PMC), thalamus, secondary somatosensory cortex (S2), posterior parietal cortex (PPC), and ipsilateral cerebellum oscillating at 8 to 10 Hz. After intake of levodopa, we found a significant decrease of cerebro-cerebral coupling between thalamus and motor cortical areas. Similarly, in healthy controls mimicking resting tremor, we found a significant decrease of functional interaction within a thalamus-premotor-motor network during rest. However, in patients with PD, decrease of functional interaction between thalamus and PMC was significantly stronger when compared with healthy controls. These data support the hypothesis that (1) in patients with PD the basal ganglia and motor cortical structures become more closely entrained and (2) levodopa is associated with normalization of the functional interaction between thalamus and motor cortical areas.

Inverse relationship between adult hippocampal cell proliferation and synaptic rewiring in the dentate gyrus

Adult neurogenesis is a key feature of the hippocampal dentate gyrus (DG). Neurogenesis is accompanied by synaptogenesis as new cells become integrated into the circuitry of the hippocampus. However, little is known to what extent the embedding of new neurons rewires the pre-existing network. Here we investigate synaptic rewiring in the DG of gerbils (Meriones unguiculatus) under different rates of adult cell proliferation caused by different rearing conditions as well as juvenile methamphetamine treatment. Surprisingly, we found that an increased cell proliferation reduced the amount of synaptic rewiring. To help explain this unexpected finding, we developed a novel model of dentate network formation incorporating neurogenesis and activity-dependent synapse formation and remodelling. In the model, we show that homeostasis of neuronal activity can account for the inverse relationship between cell proliferation and synaptic rewiring.

Trends in suspected and recognized occupational respiratory diseases in Germany between 1970 and 2005

BACKGROUND

Respiratory diseases represent a major proportion of occupational diseases in many countries. Little information is available about their incidences over the past several decades.

METHODS

Based on the reports of the three German federal accident insurance agencies, the numbers of suspected and recognized cases of occupational respiratory diseases between 1970 and 2005 were collected and combined. The trends in the rates per 100,000 insured workers were calculated.

RESULTS

In total, a decline in occupational respiratory diseases since 1998 could be observed. This trend is mainly based on the decrease in non-malignant respiratory diseases due to silica and obstructive airway diseases. In contrast, asbestos-induced diseases showed a leveling off or an increase (mesothelioma) during the last 10years.

CONCLUSIONS

Although trends in occupational disease may be influenced by several factors, the presented data indicate that prevention has been effective in reducing some ofthe most frequent occupational respiratory diseases in Germany.