Svitlana Popovych

Movement-related phase locking in the delta-theta frequency band.

Movements result from a complex interplay of multiple brain regions. These regions are assembled into distinct functional networks depending on the specific properties of the action. However, the nature and details of the dynamics of this complex assembly process are unknown. In this study, we sought to identify key markers of the neural processes underlying the preparation and execution of motor actions that always occur irrespective of differences in movement initiation, hence the specific neural processes and functional networks involved. To this end, EEG activity was continuously recorded from 18 right-handed healthy participants while they performed a simple motor task consisting of button presses with the left or right index finger. The movement was performed either in response to a visual cue or at a self-chosen, i.e., non-cued point in time. Despite these substantial differences in movement initiation, dynamic properties of the EEG signals common to both conditions could be identified using time-frequency and phase locking analysis of the EEG data. In both conditions, a significant phase locking effect was observed that started prior to the movement onset in the δ-θ frequency band (2-7Hz), and that was strongest at the electrodes nearest to the contralateral motor region (M1). This phase locking effect did not have a counterpart in the corresponding power spectra (i.e., amplitudes), or in the event-related potentials. Our finding suggests that phase locking in the δ-θ frequency band is a ubiquitous movement-related signal independent of how the actual movement has been initiated. We therefore suggest that phase-locked neural oscillations in the motor cortex are a prerequisite for the preparation and execution of motor actions.

Complex dynamics in a simple model of interdependent open economies

Based on a simple two-market model, characterized by a demand link between competitive markets for goods, a system of coupled difference equations is used to represent the interdependent structure of a global economy. Relying on numerical and analytical approaches, Various dynamic properties of the proposed model are explored. Among others, a general specification of the regions of stability of the equilibrium and main periodic cycles, the transition to chaos through torus destruction, chaotic synchronization, and the coexistence of different types of attractors in parameter space are described. Typical bifurcation processes are illustrated.

Frequency-specific modulation of connectivity in the ipsilateral sensorimotor cortex by different forms of movement initiation

Abstract A consistent finding in motor EEG research is a bilateral attenuation of oscillatory activity over sensorimotor regions close to the onset of an upcoming unilateral hand movement. In contrast, little is known about how movement initiation affects oscillatory activity, especially in the hemisphere ipsilateral to the moving hand. We here investigated the neural mechanisms modulating oscillatory activity in the ipsilateral motor cortex prior to movement onset under the control of two different initiating networks, namely, Self‐initiated and Visually‐cued actions. During motor preparation, a contralateral preponderance of power over sensorimotor cortex (SM) was observed in &agr; and &bgr; bands during Visually‐cued movements, whereas power changes were more bilateral during Self‐initiated movements. Coherence between ipsilateral SM (iSM) and contralateral SM (cSM) in the &agr;‐band was significantly increased compared to the respective baseline values, independent of the context of movement initiation. However, this context‐independent cSM‐iSM coherence modulated the power changes in iSM in a context‐dependent manner, that is, a stronger cSM‐iSM coherence correlated with a larger decrease in high‐&bgr; power over iSM in the Self‐initiated condition, in contrast to a smaller decrease in &agr; power in the Visually‐cued condition. In addition, the context‐dependent coherence between SMA and iSM in the &agr;‐band and &dgr;‐&THgr;‐band for the Self‐initiated and Visually‐cued condition, respectively, exhibited a similar context‐dependent modulation for power changes. Our findings suggest that the initiation of regional oscillations over iSM reflects changes in the information flow with the contralateral sensorimotor and premotor areas dependent upon the context of movement initiation. Importantly, the interaction between regional oscillations and network‐like oscillatory couplings indicates different frequency‐specific inhibitory mechanisms that modulate the activity in the ipsilateral sensorimotor cortex dependent upon how the movement is initiated.

Age-related changes in oscillatory power affect motor action

With increasing age cognitive performance slows down. This includes cognitive processes essential for motor performance. Additionally, performance of motor tasks becomes less accurate. The objective of the present study was to identify general neural correlates underlying age-related behavioral slowing and the reduction in motor task accuracy. To this end, we continuously recorded EEG activity from 18 younger and 24 older right-handed healthy participants while they were performing a simple finger tapping task. We analyzed the EEG records with respect to local changes in amplitude (power spectrum) as well as phase locking between the two age groups. We found differences between younger and older subjects in the amplitude of post-movement synchronization in the β band of the sensory-motor and medial prefrontal cortex (mPFC). This post-movement β amplitude was significantly reduced in older subjects. Moreover, it positively correlated with the accuracy with which subjects performed the motor task at the electrode FCz, which detects activity of the mPFC and the supplementary motor area. In contrast, we found no correlation between the accurate timing of local neural activity, i.e. phase locking in the δ-θ frequency band, with the reaction and movement time or the accuracy with which the motor task was performed. Our results show that only post-movement β amplitude and not δ-θ phase locking is involved in the control of movement accuracy. The decreased post-movement β amplitude in the mPFC of older subjects hints at an impaired deactivation of this area, which may affect the cognitive control of stimulus-induced motor tasks and thereby motor output.

A mathematical model of dysfunction of the thalamo-cortical loop in schizophrenia

BackgroundRecent experimental results suggest that impairment of auditory information processing in the thalamo-cortical loop is crucially related to schizophrenia. Large differences between schizophrenia patients and healthy controls were found in the cortical EEG signals.MethodsWe derive a phenomenological mathematical model, based on coupled phase oscillators with continuously distributed frequencies to describe the neural activity of the thalamo-cortical loop. We examine the influence of the bidirectional coupling strengths between the thalamic and the cortical area with regard to the phase-locking effects observed in the experiments. We extend this approach to a model consisting of a thalamic area coupled to two cortical areas, each comprising a set of nonidentical phase oscillators. In the investigations of our model, we applied the Ott-Antonsen theory and the Pikovsky-Rosenblum reduction methods to the original system.ResultsThe results derived from our mathematical model satisfactorily reproduce the experimental data obtained by EEG measurements. Furthermore, they show that modifying the coupling strength from the thalamic region to a cortical region affects the duration of phase synchronization, while a change in the feedback to the thalamus affects the strength of synchronization in the cortex. In addition, our model provides an explanation in terms of nonlinear dynamics as to why brain waves desynchronize after a given phase reset.ConclusionOur model can explain functional differences seen between EEG records of healthy subjects and schizophrenia patients on a system theoretic basis. Because of this and its predictive character, the model may be considered to pave the way towards an early and reliable clinical detection of schizophrenia that is dependent on the interconnections between the thalamic and cortical regions. In particular, the model parameter that describes the strength of this connection can be used for a diagnostic classification of schizophrenia patients.

Aging-associated changes of movement-related functional connectivity in the human brain

ABSTRACT Motor performance declines with normal aging. Previous neuroimaging work revealed aging‐related general increases in neural activity, especially in the prefrontal and pre‐motor areas, associated with a loss of hemispheric lateralization. However, the functional mechanisms underlying these changes and their relation to aging‐associated motor decline to date remain elusive. To further elucidate the neural processes underlying aging‐related motor decline, we recorded EEG from younger and older subjects while they performed a finger‐tapping task. As a measure of synchronization between motor areas, we computed the inter‐regional phase‐locking value which reflects functional connectivity between distinct neural populations. Behavioral data revealed increased movement times in older subjects. EEG data showed that phase locking in the &dgr;‐&THgr; frequencies is a general, age‐independent phenomenon underlying the execution of simple finger movements. In stark contrast, the extent of synchronization between motor areas significantly differed dependent upon age of subjects: multiple additional intra‐ and inter‐hemispheric connections were observed in older subjects. Our data shed light upon the results of previous neuroimaging studies showing aging‐related increases in neural activation. In particular, data suggest that the observed aging‐dependent substantial intra‐ and inter‐hemispheric reorganization of connectivity between the corresponding motor areas underlies the previously reported loss of lateralization in older subjects. The changes observed are likely to represent compensatory mechanisms aiming at preserved task performance in older subjects. HighlightsAging affects transient synchronization between motor areas during action.Aging‐associated additional intra‐ and inter‐hemispheric connections were observed.Data provide a mechanism for the loss of lateralization in older subjects (HAROLD).Results support the compensatory network aspects of the HAROLD model.

Mathematical model of the thalamo-cortical loop by dysfunction in schizophrenia

Preceding experimental results suggest that disturbances of auditory information processing within the thalamocortical loop are a core issue relating schizophrenia [1]. Wide differences between schizophrenia patients and healthy controls were found in phase-locking of cortex EEG. We derive a phenomenological mathematical model based on coupled phase oscillators with continuous distributed frequencies to describe the neural activity of the thalamocortical loop. Concerning phase-locking effects observed we examine the influence of the bidirectional coupling strengths between the thalamic and the cortical area. We widen this approach to a model consisting of a thalamic area coupled to three cortical areas, each modeled by a set of nonidentical phase oscillators. At the investigation of our model we use Ott-Antonsen theory [2] and Pikovsky-Rosenblum reduction methods [3]. The results derived from our mathematical model coincide with the experimental data obtained by EEG measurements. The model provides that modifying the coupling strength from the thalamic region to a cortical region effects the duration of phase synchronization and while modifying the coupling back to the thalamic region affects the strength of synchronization in this cortical area. Thus it supports the view that the coupling between the thalamic region and cortical regions is the responsible mechanism for dysfunction of the thalamo-cortical loop in schizophrenia.

Computational model of midbrain dopaminergic neuron activity in ageing and obesity

Obesity is an increasing health problem in the modern world. Feeding behavior is mostly controlled by homeostatic and hedonic systems. It was already demonstrated in the hypothalamus that diet-induced obesity can change the spontaneous activity of cells involved in homeostatic regulation. It is, however, unclear if the hedonic regulation is also affected by diet-induced obersity. The midbrain dopaminergic (DA) neurons are a key component of the hedonic system. Usually, dopaminergic neurons in brain slice preparations show a highly regular pacemaker-like activity pattern. However it was found in [1], that in mice fed on high fat diet (HFD) a significantly increased proportion of DA neurons fired irregularly as compared to the ones in mice that ware fed on normal diet (NCD). A mathematical model of midbrain dopaminergic neuron (DA) has been developed to better understand the mechanisms underlying the different types of firing patterns that these cells exhibit in vitro. The dopaminergic neuron was modeled using a single compartment which includes voltage and Ca2+-dependent currents described by Hodgkin-Huxley kinetics. The model used in this study is based on an existing DA neuron model [2,3] and some parameters were determined using new voltage-clamp data from HFD and NCD mouse brain slices.