High-frequency stimulation of the subthalamic nucleus suppresses oscillatory beta activity in patients with Parkinson's disease in parallel with improvement in motor performance

The role of imaging in the surgical treatment of movement disorders

Functional neurosurgery involves precise surgical targeting of anatomic structures to modulate neurologic function. From its conception, advances in the surgical treatment of movement disorders have been intertwined with developments in medical imaging, culminating in the use of stereotactic magnetic resonance imaging (MRI). Meticulous attention to detail during image acquisition, direct anatomic localization, and planning of the initial surgical trajectory allows the surgeon to reach the desired anatomic and functional target with the initial trajectory in most cases, thus reducing the need for multiple passes through the brain, and the associated risk of hemorrhage and functional deficit. This philosophy is of paramount importance in a procedure that is primarily aimed at improving quality of life. Documentation of electrode contact location by means of stereotactic imaging is essential to audit surgical targeting accuracy and to further the knowledge of structure-to-function relationships within the human brain.

A population level computational model of the basal ganglia that generates parkinsonian Local Field Potential activity

Recordings from the basal ganglia's subthalamic nucleus are acquired via microelectrodes immediately prior to the application of Deep Brain Stimulation (DBS) treatment for Parkinson's Disease (PD) to assist in the selection of the final point for the implantation of the DBS electrode. The acquired recordings reveal a persistent characteristic beta band peak in the power spectral density function of the Local Field Potential (LFP) signals. This peak is considered to lie at the core of the causality-effect relationships of the parkinsonian pathophysiology. Based on LFPs acquired from human subjects during DBS for PD, we constructed a computational model of the basal ganglia on the population level that generates LFPs to identify the critical pathophysiological alterations that lead to the expression of the beta band peak. To this end, we used experimental data reporting that the strengths of the synaptic connections are modified under dopamine depletion. The hypothesis that the altered dopaminergic modulation may affect both the amplitude and the time course of the postsynaptic potentials is validated by the model. The results suggest a pivotal role of both of these parameters to the pathophysiology of PD.

Network perspectives on the mechanisms of deep brain stimulation

Deep brain stimulation (DBS) is an established medical therapy for the treatment of movement disorders and shows great promise for several other neurological disorders. However, after decades of clinical utility the underlying therapeutic mechanisms remain undefined. Early attempts to explain the mechanisms of DBS focused on hypotheses that mimicked an ablative lesion to the stimulated brain region. More recent scientific efforts have explored the wide-spread changes in neural activity generated throughout the stimulated brain network. In turn, new theories on the mechanisms of DBS have taken a systems-level approach to begin to decipher the network activity. This review provides an introduction to some of the network based theories on the function and pathophysiology of the cortico-basal-ganglia-thalamo-cortical loops commonly targeted by DBS. We then analyze some recent results on the effects of DBS on these networks, with a focus on subthalamic DBS for the treatment of Parkinson's disease. Finally we attempt to summarize how DBS could be achieving its therapeutic effects by overriding pathological network activity.

The pharmacological blockade of medial forebrain bundle induces an acute pathological synchronization of the cortico-subthalamic nucleus-globus pallidus pathway

Pathological oscillations characterize the firing discharge of different basal ganglia (BG) stations in rat models of Parkinson's disease. Most recent literature focused on the prominence of the beta frequency band in awake rats. Yet, in 6-hydroxydopamine-lesioned animals, the firing discharge of the globus pallidus (GP) and the substantia nigra reticulata are in phase with urethane-induced slow wave cortical activity. The neuronal basis of this pathological synergy at low frequency is widely debated. In order to understand the role of substantia nigra pars compacta (SNc) signalling in the development of pathological synchronization, we performed a pharmacological inactivation of the medial forebrain bundle (MFB) through tetrodotoxin (TTX), which led to a dramatic, but reversible, reduction of the dopamine content in the striatum. This procedure caused a significant contralateral akinesia, detectable as soon as anaesthesia vanished, and lasting about 3-4 h. We sought to determine the electrophysiological counterpart of this transient Parkinsonian-like hypokinetic syndrome. Hence, we obtained the electrocorticogram (ECoG) and single unit recordings from GP and subthalamic nucleus (STN) in normal rats before and after the TTX injection in MFB. Intriguingly, the TTX-mediated inactivation of MFB induced a fast developing coherence between cortex and GP and a significant increase of the cortex/STN synchronization. The intra-GP iontophoretic delivery of haloperidol or the GABA(A) receptor antagonist bicuculline induced a short term cortex/GP synchronization. Strikingly, STN inactivation by local muscimol reversed both haloperidol- and TTX-mediated coherence between cortex and GP. Our data show that an abnormal cortical/BG synchronization, at low frequency, can be reproduced also without SNc neuronal loss and striatal cytoarchitectonic alterations. In addition, our results, which represent an acute and reversible Parkinsonism based upon impaired cable properties, seem compatible with the interpretation of acute changes of the functional interplay between cortex and the STN-GP pathway as a key factor mechanism underlying the fast deep brain stimulation-induced acute Off-On transitions.

Fast oscillations in cortical-striatal networks switch frequency following rewarding events and stimulant drugs

Oscillations may organize communication between components of large-scale brain networks. Although gamma-band oscillations have been repeatedly observed in cortical-basal ganglia circuits, their functional roles are not yet clear. Here I show that, in behaving rats, distinct frequencies of ventral striatal local field potential oscillations show coherence with different cortical inputs. The approximately 50 Hz gamma oscillations that normally predominate in awake ventral striatum are coherent with piriform cortex, whereas approximately 80-100 Hz high-gamma oscillations are coherent with frontal cortex. Within striatum, entrainment to gamma rhythms is selective to fast-spiking interneurons, with distinct fast-spiking interneuron populations entrained to different gamma frequencies. Administration of the psychomotor stimulant amphetamine or the dopamine agonist apomorphine causes a prolonged decrease in approximately 50 Hz power and increase in approximately 80-100 Hz power. The same frequency switch is observed for shorter epochs spontaneously in awake, undrugged animals and is consistently provoked for < 1 s following reward receipt. Individual striatal neurons can participate in these brief high-gamma bursts with, or without, substantial changes in firing rate. Switching between discrete oscillatory states may allow different modes of information processing during decision-making and reinforcement-based learning, and may also be an important systems-level process by which stimulant drugs affect cognition and behavior.

The electrocorticogram signal can be modulated with deep brain stimulation of the subthalamic nucleus in the hemiparkinsonian rat

Electrocorticogram (ECoG) recordings of the 6-hydroxydopamine (6-OHDA)-lesioned parkinsonian rat have shown an increase in the power of cortical beta-band (15-30 Hz) oscillations ipsilateral to the lesion. The power of these oscillations is decreased with dopamine agonist administration. Here, we demonstrate that stimulation of an electrode implanted in the subthalamic nucleus alters the power of cortical beta and gamma oscillations in 6-OHDA-lesioned animals. These alterations are dependent on stimulation frequency, charge, and amplitude/pulse width. Oscillations were significantly reduced during 200- and 350-Hz stimulation. A minimum charge of 4 nC was required to elicit a reduction in oscillation power. A number of amplitude and pulse width combinations that reached 4 nC were tested; it was found that only the combinations of 33 microA/120 micros and 65 microA/60 micros significantly reduced cortical oscillations. The reduction in beta/gamma oscillation power due to deep brain stimulation (DBS) was consistent with a significant reduction in the animals' rotational behavior, a typical symptom of parkinsonism in the rat. A significant shift from high beta to low gamma was observed in the peak frequencies of ECoG recordings while animals were at rest versus walking on a treadmill. However, DBS exhibited no differential effect on oscillations between these two states. EEG recordings from rodent models of DBS may provide surrogate information about the neural signatures of Parkinson's disease relative to the efficacy of DBS.

The role of the subthalamic nucleus in response inhibition: evidence from deep brain stimulation for Parkinson's disease

We measured reaction times during a stop-signal task while patients with Parkinson's disease were on and off unilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN). While reaction times to a "go" stimulus improved, there was no change in reaction times to the "stop" stimulus (SSRTs). However, changes in SSRTs induced by DBS were highly dependent on baseline SSRTs (measured off stimulation), with the greatest improvements being achieved by those with particularly slow reaction times. We therefore selected only those patients whose baseline SSRTs were within the limits of a control sample (N=10). In this group, SSRTs became slower when DBS was on. This finding suggests a role for the STN in response inhibition, which can be interrupted by DBS, observable only when more general improvements in Parkinson's function are minimised. We also compared the effects of unilateral left and right sided stimulation. We found a greater increase in SSRTs after DBS of the left STN.

Resonance in subthalamo-cortical circuits in Parkinson's disease

Neuronal activity within and across the cortex and basal ganglia is pathologically synchronized, particularly at ∼ 20 Hz in patients with Parkinson's disease. Defining how activities in spatially distributed brain regions overtly synchronize in narrow frequency bands is critical for understanding disease processes like Parkinson's disease. To address this, we studied cortical responses to electrical stimulation of the subthalamic nucleus (STN) at various frequencies between 5 and 30 Hz in two cohorts of eight patients with Parkinson's disease from two different surgical centres. We found that evoked activity consisted of a series of diminishing waves with a peak latency of 21 ms for the first wave in the series. The cortical evoked potentials (cEPs) averaged in each group were well fitted by a damped oscillator function (r ≥0.9, P < 0.00001). Fits suggested that the natural frequency of the subthalamo-cortical circuit was around 20 Hz. When the system was forced at this frequency by stimulation of the STN at 20 Hz, the undamped amplitude of the modelled cortical response increased relative to that with 5 Hz stimulation in both groups (P ≤ 0.005), consistent with resonance. Restoration of dopaminergic input by treatment with levodopa increased the damping of oscillatory activity (as measured by the modelled damping factor) in both patient groups (P ≤0.001). The increased damping would tend to limit resonance, as confirmed in simulations. Our results show that the basal ganglia–cortical network involving the STN has a tendency to resonate at ∼ 20 Hz in Parkinsonian patients. This resonance phenomenon may underlie the propagation and amplification of activities synchronized around this frequency. Crucially, dopamine acts to increase damping and thereby limit resonance in this basal ganglia–cortical network.

Synchronisation in the beta frequency-band--the bad boy of parkinsonism or an innocent bystander?

Excessive synchronisation of basal ganglia neuronal activity in the beta frequency band has been implicated in Parkinson's disease. In a recent issue of Experimental Neurology, Bronte-Stewart, H., Barberini, C., Koop, M.M., Hill, B.C., Henderson, J.M., Wingeier, B., 2009. The STN beta-band profile in Parkinson's disease is stationary and shows prolonged attenuation after deep brain stimulation. Exp. Neurol. 215, 20–28. demonstrate that such activity is consistent over time and provide further evidence that deep brain stimulation is associated with its suppression. However, the extent to which beta synchrony has a mechanistic (rather than epiphenomenal) role in parkinsonism remains unclear, and the suppression of this activity by deep brain stimulation is contentious. This commentary discusses the evidence for and against a role for excessive beta synchrony in mediating the parkinsonian phenotype and in providing a possible mechanism to explain the therapeutic effects of deep brain stimulation in Parkinson's disease.

Increased Gamma Oscillatory Activity in the Subthalamic Nucleus During Tremor in Parkinson's Disease Patients

Rest tremor is one of the main symptoms in Parkinson's disease (PD), although in contrast to rigidity and akinesia, the severity of the tremor does not correlate well with the degree of dopamine deficiency or the progression of the disease. Studies suggest that akinesia in PD patients is related to abnormal increased beta (15–30 Hz) and decreased gamma (35–80 Hz) synchronous oscillatory activity in the basal ganglia. Here we investigated the dynamics of oscillatory activity in the subthalamic nucleus (STN) during tremor. We used two adjacent microelectrodes to simultaneously record neuronal firing and local field potential (LFP) activity in nine PD patients who exhibited resting tremor during functional neurosurgery. We found that neurons exhibiting oscillatory activity at tremor frequency are located in the dorsal region of STN, where neurons with beta oscillatory activity are observed, and that their activity is coherent with LFP oscillations in the beta frequency range. Interestingly, in 85% of the 58 sites examined, the LFP exhibited increased oscillatory activity in the low gamma frequency range (35–55 Hz) during periods with stronger tremor. Furthermore, in 17 of 26 cases where two LFPs were recorded simultaneously, their coherence in the gamma range increased with increased tremor. When averaged across subjects, the ratio of the beta to gamma coherence was significantly lower in periods with stronger tremor compared with periods of no or weak tremor. These results suggest that resting tremor in PD is associated with an altered balance between beta and gamma oscillations in the motor circuits of STN.

Deep brain stimulation of the globus pallidus internus in the parkinsonian primate: local entrainment and suppression of low-frequency oscillations

Competing theories seek to account for the therapeutic effects of high-frequency deep brain stimulation (DBS) of the internal globus pallidus (GPi) for medically intractable Parkinson's disease. To investigate this question, we studied the spontaneous activity of 102 pallidal neurons during GPiDBS in two macaques rendered parkinsonian by administration of MPTP. Stimulation through macroelectrodes in the GPi (> or =200 microA at 150 Hz for 30 s) reduced rigidity in one animal and increased spontaneous movement in both. Novel artifact subtraction methods allowed uninterrupted single-unit recording during stimulation. GPiDBS induced phasic (78% of cells) or sustained (22%) peristimulus changes in firing in both pallidal segments. A subset of cells responded at short latency (<2 ms) in a manner consistent with antidromic driving. Later phasic increases clustered at 3- to 5-ms latency. Stimulation-induced decreases were either phasic, clustered at 1-3 ms, or sustained, showing no peristimulus modulation. Response latency and magnitude often evolved over 30 s of stimulation, but responses were relatively stable by the end of that time. GPiDBS reduced mean firing rates modestly and only in GPi (-6.9 spikes/s). Surprisingly, GPiDBS had no net effect on the prevalence or structure of burst firing. GPiDBS did reduce the prevalence of synchronized low-frequency oscillations. Some cell pairs became synchronized instead at the frequency of stimulation. Suppression of low-frequency oscillations did not require high-frequency synchronization, however, or even the presence of a significant peristimulus response. In summary, the therapeutic effects of GPiDBS may be mediated by an abolition of low-frequency synchronized oscillations as a result of phasic driving.

Cumulative and after-effects of short and weak coordinated reset stimulation: a modeling study

We show that the dynamical multistability of a network of bursting subthalamic neurons, caused by synaptic plasticity has a strong impact on the stimulus-response properties when exposed to weak and short desynchronizing stimuli. Intriguingly, such stimuli can reliably shift the network from a stable state with pathological synchrony and connectivity to a stable desynchronized state with down-regulated connectivity. However, unlike in the case of stronger coordinated reset stimulation, after termination of weaker stimulation the network may undergo a transient rebound of synchrony. When the coordinated reset stimulation is even weaker and/or shorter, so that a single stimulation epoch is not effective, the network dynamics and connectivity can still be reshaped in a cumulative manner by repetitive stimulation delivery.

The STN beta-band profile in Parkinson's disease is stationary and shows prolonged attenuation after deep brain stimulation

Producing accurate movements may rely on the functional independence of sensorimotor circuits within basal ganglia nuclei. In parkinsonism there is abnormal synchrony of electrical activity within these circuits that results in a loss of independence across motor channels. Local field potential (LFP) recordings reflect the summation of local electrical fields and an increase in LFP power reflects increased synchrony in local neuronal networks. We recorded LFPs from the subthalamic nucleus (STN) deep brain stimulation (DBS) lead in the operating room in 22 cases from 16 subjects with Parkinson's disease (PD) who were off medication. There was elevated LFP power at beta frequencies (13-35 Hz) at rest. The LFP spectral profile was consistent across several periods of rest that were separated by movement and/or DBS, and appeared to be a relatively stationary phenomenon. The spectral profile and frequencies of the beta-band peak(s) varied among subjects but were similar between the right and left STNs within certain individuals. These results suggest that the LFP spectrum at rest may characterize a "signature" rhythm for an individual with PD. Beta-band power was attenuated after intra-operative STN DBS (p<0.05). The attenuation lasted for 10 s after short periods (30 s) and for up to 50 s after longer periods (5 min) of DBS. The finding that longer periods of DBS attenuated beta power for a longer time suggests that there may be long-acting functional changes to networks in the STN in PD after chronic DBS.