Subcallosal cingulate deep brain stimulation evokes two distinct cortical responses via differential white matter activation

Subcallosal cingulate (SCC) deep brain stimulation (DBS) is an emerging therapy for refractory depression. Good clinical outcomes are associated with the activation of white matter adjacent to the SCC. This activation produces a signature cortical evoked potential (EP), but it is unclear which of the many pathways in the vicinity of SCC is responsible for driving this response. Individualized biophysical models were built to achieve selective engagement of two target bundles: either the forceps minor (FM) or cingulum bundle (CB). Unilateral 2 Hz stimulation was performed in seven patients with treatment-resistant depression who responded to SCC DBS, and EPs were recorded using 256-sensor scalp electroencephalography. Two distinct EPs were observed: a 120 ms symmetric response spanning both hemispheres and a 60 ms asymmetrical EP. Activation of FM correlated with the symmetrical EPs, while activation of CB was correlated with the asymmetrical EPs. These results support prior model predictions that these two pathways are predominantly activated by clinical SCC DBS and provide first evidence of a link between cortical EPs and selective fiber bundle activation.

Dissecting deep brain stimulation evoked neural activity in the basal ganglia

Deep brain stimulation (DBS) is an established therapeutic tool for the treatment of Parkinson's disease (PD). The mechanisms of DBS for PD are likely rooted in modulation of the subthalamo-pallidal network. However, it can be difficult to electrophysiologically interrogate that network in human patients. The recent identification of large amplitude evoked potential (EP) oscillations from DBS in the subthalamic nucleus (STN) or globus pallidus internus (GPi) are providing new scientific opportunities to expand understanding of human basal ganglia network activity. In turn, the goal of this review is to provide a summary of DBS-induced EPs in the basal ganglia and attempt to explain various components of the EP waveforms from their likely network origins. Our analyses suggest that DBS-induced antidromic activation of globus pallidus externus (GPe) is a key driver of these oscillatory EPs, independent of stimulation location (i.e. STN or GPi). This suggests a potentially more important role for GPe in the mechanisms of DBS for PD than typically assumed. And from a practical perspective, DBS EPs are poised to become clinically useful electrophysiological biomarker signals for verification of DBS target engagement.

Role of the volume conductor on simulations of local field potential recordings from deep brain stimulation electrodes

OBJECTIVE

Local field potential (LFP) recordings from deep brain stimulation (DBS) electrodes are commonly used in research analyses, and are beginning to be used in clinical practice. Computational models of DBS LFPs provide tools for investigating the biophysics and neural synchronization that underlie LFP signals. However, technical standards for DBS LFP model parameterization remain to be established. Therefore, the goal of this study was to evaluate the role of the volume conductor (VC) model complexity on simulated LFP signals in the subthalamic nucleus (STN).

APPROACH

We created a detailed human head VC model that explicitly represented the inhomogeneity and anisotropy associated with 12 different tissue structures. This VC model represented our "gold standard" for technical detail and electrical realism. We then incrementally decreased the complexity of the VC model and quantified the impact on the simulated LFP recordings. Identical STN neural source activity was used when comparing the different VC model variants. Results Ignoring tissue anisotropy reduced the simulated LFP amplitude by ~12%, while eliminating soft tissue heterogeneity had a negligible effect on the recordings. Simplification of the VC model to consist of a single homogenous isotropic tissue medium with a conductivity of 0.215 S/m contributed an additional ~3% to the error.

SIGNIFICANCE

Highly detailed VC models do generate different results than simplified VC models. However, with errors in the range of ~15%, the use of a well-parameterized simple VC model is likely to be acceptable in most contexts for DBS LFP modeling.

Aperiodic subthalamic activity predicts motor severity and stimulation response in Parkinson disease

INTRODUCTION

Rhythmic beta activity in the subthalamic nucleus (STN) local field potential (LFP) is associated with Parkinson disease (PD) severity, though not all studies have found this relationship. We investigated whether aperiodic 'noise' elements of LFP, specifically slope of the 1/f broadband, predict PD motor symptoms and outcomes of STN-DBS.

METHODS

We studied micro-LFP from 19 PD patients undergoing STN-DBS, relating the aperiodic 1/f slope and the periodic beta oscillation components to motor severity using the UPDRS-III and improvement with DBS at 1 year.

RESULTS

Beta power, not 1/f slope, independently predicted baseline UPDRS-III (r = 0.425, p = 0.020; r = -0.434, p = 0.032, respectively), but multiple regression using both predicted better (F (2, 16) = 6.621, p = 0.008, R² = 0.453). Only multiple regression using both slope and beta power predicted improvement in UPDRS-III at 1 year post-operatively (F (2, 15) = 6.049, R² = 0.446, p = 0.012).

CONCLUSIONS

Both beta synchronization and slope of the 1/f broadband are informative of motor symptoms in PD and predict response to STN-DBS.

Monitoring stimulus-evoked hemodynamic response during deep brain stimulation with single fiber spectroscopy

Deep brain stimulation (DBS) is a revolutionary treatment for movement disorders. Measuring DBS-induced hemodynamic responses may be useful for surgical guidance of DBS electrode implantation as well as to study the mechanism and assess therapeutic effects of DBS. In this study, we evaluated the performance of a single fiber spectroscopic (SFS) system for measuring hemodynamic response in different cortical layers in a DBS animal model. We showed that SFS is capable of measuring minute relative changes in oxygen saturation and blood volume fraction in-vivo at a sampling rate of 22-33 Hz. During stimulation, blood volume fraction increased, while oxygen saturation showed both increases and decreases at different cortical depths across animals. In addition, we showed the potential of using SFS for measuring other physiological parameters, for example, heart rate, and respiratory rate.

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.

Biophysical characterization of local field potential recordings from directional deep brain stimulation electrodes

OBJECTIVE

Two major advances in clinical deep brain stimulation (DBS) technology have been the introduction of local field potential (LFP) recording capabilities, and the deployment of directional DBS electrodes. However, these two technologies are not operationally integrated within current clinical DBS devices. Therefore, we evaluated the theoretical advantages of using directional DBS electrodes for LFP recordings, with a focus on measuring beta-band activity in the subthalamic nucleus (STN).

METHODS

We used a computational model of human STN neural activity to simulate LFP recordings. The model consisted of 235,280 anatomically and electrically detailed STN neurons surrounding the DBS electrode, which was previously optimized to mimic beta-band synchrony in the dorsolateral STN. We then used that model system to compare LFP recordings from cylindrical and directional DBS contacts, and evaluate how the selection of different contacts for bipolar recording affected the LFP measurements.

RESULTS

The model predicted two advantages of directional DBS electrodes over cylindrical DBS electrodes for STN LFP recording. First, recording from directional contacts could provide additional insight on the location of a synchronous volume of neurons within the STN. Second, directional contacts could detect a smaller volume of synchronous neurons than cylindrical contacts, which our simulations predicted to be a ~0.5 mm minimum radius.

CONCLUSIONS

STN LFP recordings from 8-contact directional DBS electrodes (28 possible bipolar pairs) can provide more information than 4-contact cylindrical DBS electrodes (6 possible bipolar pairs), but they also introduce additional complexity in analyzing the signals.

SIGNIFICANCE

Integration of directional electrodes with DBS systems that are capable of LFP recordings could improve localization of targeted volumes of synchronous neurons in PD patients.

Spatiotemporal dynamics of cortical perfusion in response to thalamic deep brain stimulation

Deep brain stimulation (DBS) has revolutionized the treatment of movement disorders. The parameters of electrical stimulation are important to its therapeutic effect and remain a source of clinical controversy. DBS exerts its actions not only locally at the site of stimulation but also remotely through afferent and efferent connections, which are vital to its clinical effects. Yet, only a few studies have examined how cortical activity changes in response to various electrical parameters. Here, we investigated how the parameters of thalamic DBS alter cortical perfusion in rats using intrinsic optical imaging. We hypothesized that thalamic DBS will increase perfusion in primary motor cortex (M1), proportional to amplitude, pulse width, or frequency of the stimulation applied. We applied 45 different combinations of amplitude, pulse width and frequency in the ventro-lateral (VL) nucleus of the thalamus in anesthetized rats while measuring perfusion in M1. VL thalamic DBS reduced cortical reflectance, which corresponds to an increase in cortical perfusion. We computed the maximum change in reflectance (MCR) as well as the spatial spread of MCR in each trial. Both MCR and spatial spread increased linearly with increases in current amplitude or pulse width of stimulation; however, the effect of frequency was non-linear. Stimulation at 20 Hz was significantly different from that at higher frequencies while stimulation at higher frequencies did not differ significantly from each other. Moreover, the effect of pulse width on MCR was larger than the effect of amplitude. The proportional increase in M1 perfusion due to increase in amplitude or pulse width suggests that both activate more neural elements and increase the volume of tissue activated. These results should help clinicians set parameters of DBS. The use of optical imaging to monitor effects of DBS on M1 may not only help understand DBS mechanisms, but may also provide feedback for closed loop DBS devices.