Closing the loop: Novel quantitative fMRI approach for manipulation of the sensorimotor loop in tremor.

Tremor is thought to be an effect of oscillatory activity within the sensorimotor network. To date, the underlying pathological brain networks are not fully understood. Disentangling tremor activity from voluntary motor output and sensorimotor feedback systems is challenging. To better understand the intrinsic sensorimotor fingerprint underlying tremor, we aimed to disentangle the sensorimotor system into driving (motor) and feedback/compensatory (sensory) neuronal involvement, and aimed to pinpoint tremor activity in essential tremor (ET) and tremor-dominant Parkinson's disease (PD) with a novel closed-loop approach. Eighteen ET patients, 14 tremor-dominant PD patients, and 18 healthy controls were included. An MR-compatible wrist manipulator was employed during functional MRI (fMRI) while muscle activity during (in)voluntary movements was concurrently recorded using electromyography (EMG). Tremor was quantified based on EMG and correlated to brain activity. Participants performed three tasks: an active wrist motor task, a passive wrist movement task, and rest (no wrist movement). The results in healthy controls proved that our experimental paradigm activated the expected motor and sensory networks separately using the active (motor) and passive (sensory) task. ET patients showed similar patterns of activation within the motor and sensory networks. PD patients had less activity during the active motor task in the cerebellum and basal ganglia compared to ET and healthy controls. EMG showed that in ET, tremor fluctuations correlated positively with activity in the inferior olive region, and that in PD tremor fluctuations correlated positively with cerebellar activity. Our novel approach with an MR-compatible wrist manipulator, allowed to investigate the involvement of the motor and sensory networks separately, and as such to better understand tremor pathophysiology. In ET sensorimotor network function did not differ from healthy controls. PD showed less motor-related activity. Focusing on tremor, our results indicate involvement of the inferior olive in ET tremor modulation, and cerebellar involvement in PD tremor modulation.

Directionality of corticomuscular coupling in essential tremor and cortical myoclonic tremor.

OBJECTIVE: A role of the motor cortex in tremor generation in essential tremor (ET) is assumed, yet the directionality of corticomuscular coupling is unknown. Our aim is to clarify the role of the motor cortex. To this end we also study 'familial cortical myoclonic tremor with epilepsy' (FCMTE) and slow repetitive voluntary movements with a known cortical drive. METHODS: Directionality of corticomuscular coupling (EEG-EMG) was studied with renormalized partial directed coherence (rPDC) during tremor in 25 ET patients, 25 healthy controls (mimicked) and in seven FCMTE patients; and during a self-paced 2 Hz task in eight ET patients and seven healthy controls. RESULTS: Efferent coupling around tremor frequency was seen in 33% of ET patients, 45.5% of healthy controls, all FCMTE patients, and, around 2 Hz, in all ET patients and all healthy controls. Ascending coupling, seen in the majority of all participants, was weaker in ET than in healthy controls around 5-6 Hz. CONCLUSIONS: Possible explanations are that tremor in ET results from faulty subcortical output bypassing the motor cortex; rate-dependent transmission similar to generation of rhythmic movements; and/or faulty feedforward mechanism resulting from decreased afferent (sensory) coupling. SIGNIFICANCE: A linear cortical drive is lacking in the majority of ET patients.

Visual cues added to a virtual environment paradigm do not improve motor arrests in Parkinson's disease.

Objective. Elucidating how cueing alleviates freezing of gait (FOG) in Parkinson's disease (PD) would enable the development of more effective, personalized cueing strategies. Here, we aimed to validate a visual cueing virtual environment (VE) paradigm for future use in e.g. neuroimaging studies and behavioral studies on motor timing and scaling in PD patients with FOG.Approach. We included 20 PD patients with FOG and 16 age-matched healthy control subjects. Supine participants were confronted with a VE displaying either no cues, bars or staircases. They navigated forward using alternate suppression of foot pedals. Motor arrests (as proxy for FOG), and measures of motor timing and scaling were compared across the three VE conditions for both groups.Main results. VE cues (bars and staircases) did not reduce motor arrests in PD patients and healthy control subjects. The VE cues did reduce pedal amplitude in healthy control subjects, without effects on other motor parameters.Conclusion. We could not validate a visual cueing VE paradigm to study FOG. The VE cues possibly failed to convey the necessary spatial and temporal information to support motor timing and scaling. We discuss avenues for future research.

Validation of the Auditory Stroop Task to increase cognitive load in walking tasks in healthy elderly and persons with Parkinson's disease.

BACKGROUND: The development of treatments for freezing of gait (FOG) in Parkinson's disease (PD) requires experimental study set-ups in which FOG is likely to occur, and is amenable to therapeutic interventions. We explore whether the 'Auditory Stroop Task' (AST) can be used to increase cognitive load (and thereby elicit FOG), simultaneously with visual cues (as a therapeutic intervention for FOG). We additionally examined how these two contrasting effects might interact in affecting gait and FOG parameters. OBJECTIVES: We investigated whether: (1) the 'Auditory Stroop Task' (AST) influences gait in healthy elderly and persons with PD who experience FOG, and increases the frequency of FOG events among PD patients; (2) the AST and visual cues interact; and (3) different versions of the AST exert different cognitive loads. METHODS: In 'Experiment 1', 19 healthy elderly subjects performed a walking task while performing a high and low load version of the AST. Walking with a random numbers task, and walking without cognitive load served as control conditions. In 'Experiment 2', 20 PD patients with FOG and 18 healthy controls performed a walking task with the AST, and no additional cognitive load as control condition. Both experiments were performed with and without visual cues. Velocity, cadence, stride length, and stride time were measured in all subjects. FOG severity was measured in patients. RESULTS: Compared to the control conditions, the AST negatively affected all gait parameters in both patients and controls. The AST did not increase the occurrence of FOG in patients. Visual cues reduced the decline in stride length induced by cognitive load in both groups. Both versions of the AST exerted similar effects on gait parameters in controls. CONCLUSIONS: The AST is well-suited to simulate the effects of cognitive load on gait parameters, but not FOG severity, in gait experiments in persons with PD and FOG.

Standardized microgel beads as elastic cell mechanical probes.

Cell mechanical measurements are gaining increasing interest in biological and biomedical studies. However, there are no standardized calibration particles available that permit the cross-comparison of different measurement techniques operating at different stresses and time-scales. Here we present the rational design, production, and comprehensive characterization of poly-acrylamide (PAAm) microgel beads mimicking size and overall mechanics of biological cells. We produced mono-disperse beads at rates of 20-60 kHz by means of a microfluidic droplet generator, where the pre-gel composition was adjusted to tune the beads' elasticity in the range of cell and tissue relevant mechanical properties. We verified bead homogeneity by optical diffraction tomography and Brillouin microscopy. Consistent elastic behavior of microgel beads at different shear rates was confirmed by AFM-enabled nanoindentation and real-time deformability cytometry (RT-DC). The remaining inherent variability in elastic modulus was rationalized using polymer theory and effectively reduced by sorting based on forward-scattering using conventional flow cytometry. Our results show that PAAm microgel beads can be standardized as mechanical probes, to serve not only for validation and calibration of cell mechanical measurements, but also as cell-scale stress sensors.

Event-related mu-rhythm desynchronization during movement observation is impaired in Parkinson's disease.

OBJECTIVE: Patients with Parkinson's disease often experience difficulties in adapting movements and learning alternative movements to compensate for symptoms. Since observation of movement has been demonstrated to lead to the formation of a lasting specific motor memory that resembled that elicited by physical training we hypothesize that mu-rhythm desynchronization in response to movement observation is impaired in Parkinson's disease. METHOD: In a pilot study with nine patients with Parkinson's disease at a Hoehn and Yahr stage of I or II and eleven age-matched controls, we tested this hypothesis by comparing the event related desynchronization (ERD) patterns from the EEG recorded during the observation of hand action and baseline videos. RESULTS: Healthy subjects showed normal bilateral ERD of the mu-rhythm. In patients with Parkinson's disease this distinct ERD pattern was lacking. CONCLUSION: The results of this study suggest that event-related mu-rhythm desynchronization is impaired in Parkinson's disease, even at early stages of the disease. SIGNIFICANCE: Studying event-related mu-rhythm desynchronization dysfunction in Parkinson's disease patients may enhance our understanding of symptoms as impaired motor learning.

From Parkinsonian thalamic activity to restoring thalamic relay using deep brain stimulation: new insights from computational modeling.

We present a computational model of a thalamocortical relay neuron for exploring basal ganglia thalamocortical loop behavior in relation to Parkinson's disease and deep brain stimulation (DBS). Previous microelectrode, single-unit recording studies demonstrated that oscillatory interaction within and between basal ganglia nuclei is very often accompanied by synchronization at Parkinsonian rest tremor frequencies (3-10 Hz). These oscillations have a profound influence on thalamic projections and impair the thalamic relaying of cortical input by generating rebound action potentials. Our model describes convergent inhibitory input received from basal ganglia by the thalamocortical cells based on characteristics of normal activity, and/or low-frequency oscillations (activity associated with Parkinson's disease). In addition to simulated input, we also used microelectrode recordings as inputs for the model. In the resting state, and without additional sensorimotor input, pathological rebound activity is generated for even mild Parkinsonian input. We have found a specific stimulation window of amplitudes and frequencies for periodic input, which corresponds to high-frequency DBS, and which also suppresses rebound activity for mild and even more prominent Parkinsonian input. When low-frequency pathological rebound activity disables the thalamocortical cell's ability to relay excitatory cortical input, a stimulation signal with parameter settings corresponding to our stimulation window can restore the thalamocortical cell's relay functionality.

The pedunculopontine nucleus as an additional target for deep brain stimulation.

The pedunculopontine nucleus has been suggested as a target for DBS. In this paper we propose a single compartment computational model for a PPN Type I cell and compare its dynamic behavior with experimental data. The model shows bursts after a period of hyperpolarization and spontaneous firing at 8 Hz. Bifurcation analysis of the single PPN cell shows bistability of fast and slow spiking solutions for a range of applied currents. A network model for STN, GPe and GPi produces basal ganglia output that is used as input for the PPN cell. The conductances for projections from the STN and the GPi to the PPN are determined from experimental data. The resulting behavior of the PPN cell is studied under normal and Parkinsonian conditions of the basal ganglia network. The effect of high frequency stimulation of the STN is considered as well as the effect of combined high frequency stimulation of the STN and the PPN at various frequencies. The relay properties of the PPN cell demonstrate that the combined high frequency stimulation of STN and low frequency (10 Hz, 25 Hz, 40 Hz) stimulation of PPN hardly improves the effect of exclusive STN stimulation. Moreover, PPN-DBS at low stimulation amplitude has a better effect than at higher stimulation amplitude. The effect of PPN output on the basal ganglia is investigated, in particular the effect of STN-DBS and/or PPN-DBS on the pathological firing pattern of STN and GPe cells. PPN-DBS eliminates the pathological firing pattern of STN and GPe cells, whereas STN-DBS and combined STN-DBS and PPN-DBS eliminate the pathological firing pattern only from STN cells.

Electric field-induced effects on neuronal cell biology accompanying dielectrophoretic trapping.

Trapping neuronal cells may aid in the creation of the cultured neuron probe. The aim of the development of this probe is the creation of the interface between neuronal cells or tissue in a (human) body and electrodes that can be used to stimulate nerves in the body by an external electrical signal in a very selective way. In this way, functions that were (partially) lost due to nervous system injury or disease may be restored. First, a direct contact between cultured neurons and electrodes is created. This is realized using a microelectrode array (MEA) which can be fabricated using standard photolithographic and etching methods. Section 1 gives an overview of the human nervous system, methods for functional recovery focused on the cultured neuron probe, and the prerequisites for culturing neurons on a microelectrode array. An important aspect in the selective stimulation of neuronal cells is the positioning of cells or a small group of cells on top of each of the electrode sites of the MEA. One of the most efficient methods for trapping neuronal cells is to make use of di-electrophoresis (DEP). Dielectrophoretic forces are created when (polarizable) cells are located in nonuniform electric fields. Depending on the electrical properties of the cells and the suspending medium, the DEP force directs the cells towards the regions of high field strength (positive dielectrophoresis; PDEP) or towards regions of minimal field intensities (negative dielectrophoresis; NDEP). Since neurons require a physiological medium with a sufficient concentration of Na+, the medium conductivity is rather high (~ 1.6 S/m). The result is that negative dielectrophoretic forces are created over the entire frequency range. With the use of a planar quadrupole electrode sturcture negative forces are directed so that in the center of this structure cell can be collected. The process of trapping cortical rat neurons is described in Sect. 2 theoretically and experimentally. Medium and cell properties are frequency-dependent due to relaxation processes, which have a direct influence on the strength of the dielectrophorectic force. On the other hand, the nonideal material properties of the gold electrodes and glass substrate largely determine the electric field strength created inside the medium. Especially, the electrode-medium interface results in a significant loss of the imput signal at lower frequencies (< 1 MHz), and thus a reduction of the electric field strength inside the medium. Furthermore, due to the high medium conductivity, the electric field causes Joule heating. Local temperature rises result in local gradients in fluid density, which induces fluid flow. The electrode-medium interface and induced fluid flow are theoretically investigated with the use of modeling techniques such as finite elements modeling. Experimental and theoretical results agreed with each other on the occurrence of the effects described in this section. For the creation of the cultured neuron probe, preservation of cell viability during the trapping process is a prerequisite. Cell viability of dielectrophoretically trapped neurons has to be investigated. The membrane potential induced by the external field plays a crucial role in preservation of cell viability. The membrane can effectively be represented by a capaticance in parallel woth a low conductance; with increasing frequency and /or decreasing field strength the induced membrane potential decreases. At high induced membrane potentials ths representation for the membrane is no longer valid. At this point membrane breakdown occurs and the normally insulating membrane becomes conductive and permeable. The creation of electropores has been proposed in literature to be the cause of this high permeability state. Pores may grow or many small pores may be created which eventually may lead to membrane rupture, and thus cell death. Membrane breakdown may be reversible, but a chemical imbalance created during the high permeability state may still exist after the resealing of the membrane. This may cause cell death after several hours or even days after field application. Section 3 gives a detailed description of membrane breakdown. Since many investigations on electroporation of lipid bilayers and cell membranes are based on uniform electric fields, a finite element model is used to investigate induced membrane potentials in the nonuniform field created by the quadropole electrode structure. Modeling results are presented in cmbination with the results of breakdown experiments using four frequencies in the range from 100 kHz to 1MHz. Radomly positioned neuronals cells were exposed to stepwise increasing electric field strengths. The field strength at which membrane rupture occurred gives an indication of the maximum induced membrane potential. Due to the nonuniformity of the electric field, cell collapse was expected to be position-dependent. However, at 100 kHz cells collapsed at a break down level of about 0.4 V, in contradistinction to findings at higher frequencies where more variation in breakdown levels were found. Model simulations were able to explain the experimental results. For examining whether the neuronal cells trapped by dielectrophoresis were still viable after the trapping process, the frequency range was divided into two ranges. First, a high frequency (14 MHz) and a rather low signal amplitude (3 Vpp) were used to trap cells. At this high frequency the field-induced membrane potential is small according to the theoretical model, and therefore no real damage is expected. The experimental analysis included the investigation of the growth of the neurons, number and length of the processes (dendrites and axons), and the number of outgrowing (~ viable) versus nonoutgrowing (~ nonviable) neural cells. The experimental results agreed with the expectation. The effect of the use of driving signals with lower frequencies and/or higher amplitudes on cell viability was investigated using a staining method as described in the second part of Sect. 4. Survival chances are not directly linked to the estimated maximum induced membrane potential. The frequency of the dield plays an important role, decreasing frequency lowering the chance of survival. A lower frequency limit of 100 kHz is preferable at field strengths less than 80 k V/m, while with increasing field strength this limit shifts towards higher frequencies. The theoretical and experimental results presented in this review form the inception of the development of new electrode structures for trapping neuronal cells on top of each of the electrodes of the MEA. New ways to investigate cell properties and the phenomenon of electroporation using electrokinetic methods were developed that can be exploited in future research linking cell biology to technology.

Experimental investigation on neural cell survival after dielectrophoretic trapping.

Negative dielectrophoretic forces can effectively be used to trap cortical rat neurons. The creation of dielectrophoretic forces requires electric fields of high non-uniformity. High electric field strengths, however, can cause excessive membrane potentials by which cells may unrecoverably be changed or it may lead to cell death. In a previous study it was found that cells trapped at 3 Vtt/14 MHz did not change morphologically as compared to cells that were not exposed to the electric field. This study investigates the viability of fetal cortical rat neurons after being trapped by negative dielectrophoretic forces at frequencies up to 1 MHz. A planar quadrupole micro-electrode structure was used for the creation of a non-uniform electric field. The sinusoidal input signal was varied in amplitude (3 and 5 Vtt) and frequency (10 kHz-1 MHz). The results presented in this paper show that the viability of dielectrophoretically trapped postnatal cortical rat cells was greatly frequency dependent. To preserve viability frequencies above 100 kHz (at 3 Vtt) or 1 MHz (5 Vtt) must be used.

Viability of dielectrophoretically trapped neural cortical cells in culture.

Negative dielectrophoretic trapping of neural cells is an efficient way to position neural cells on the electrode sites of planar micro-electrode arrays. The preservation of viability of the neural cells is essential for this approach. This study investigates the viability of postnatal cortical rat cells that were dielectrophoretically trapped. Morphological characteristics as well as the ratio of the number of outgrowing to the number of non-outgrowing cortical cells were used to compare the viability of trapped cells to that of non-exposed cells. The morphological characteristics include the area of the cell, representing adhesive properties, and the number and length of the processes, as a measure for functional recovery. The results presented in this paper show that the viable state of dielectrophoretically trapped postnatal cortical rat cells under the conditions used was similar to that of non-exposed cells.

Dielectrophoretic trapping of dissociated fetal cortical rat neurons.

Recording and stimulating neuronal activity at multiple sites can be realized with planar microelectrode arrays. Efficient use of such arrays requires each site to be covered by at least one neuron. By application of dielectrophoresis (DEP), neurons can be trapped onto these sites. This study investigates negative dielectrophoretic trapping of fetal cortical rat neurons. A planar quadrupole microelectrode structure was used for the creation of a nonuniform electric field. The field was varied in amplitude (1, 3, and 5 V) and frequency (10 kHz-50 MHz). Experimental results were compared with a theoretical model to investigate the yield (the number of neurons trapped in the center of the electrode structure) with respect to time, amplitude and frequency of the field. The yield was a function of time(1/3) according to theory. However, unlike the model predicted, an amplitude-dependent frequency behavior was present and unexpected peaks occurred in the DEP-spectra above 1 MHz. Gain/phase measurements showed a rather unpredictable behavior of the electrode plate above 1 MHz, and temperature measurement showed that heating of the medium influenced the trapping effect, especially for larger amplitudes and higher frequencies.