Modulation of hippocampal activity with fornix Deep Brain Stimulation

Neuronal Network Activation Induced by Forniceal Deep Brain Stimulation in Mice

Background: The fimbria-fornix is a nerve fiber bundle that connects various structures of the limbic system in the brain and plays a key role in cognition. It has become a major target of deep brain stimulation (DBS) to treat memory impairment in both dementia patients and animal models of neurological diseases. Previously, we have reported the beneficial memory effects of chronic forniceal DBS in mouse models of intellectual disability disorders. In Rett syndrome and CDKL5 deficiency disorder models, DBS strengthens hippocampal synaptic plasticity, reduces dentate inhibitory transmission or increases adult hippocampal neurogenesis that aids memory. However, the underlying neuronal circuitry mechanisms remain unknown. This study we explored the neural network circuits involved in forniceal DBS treatment. Methods: We used acute forniceal DBS-induced expression of c-Fos, an activity-dependent neuronal marker, to map the brain structures functionally connected to the fornix. We also evaluated the mouse behavior of locomotion, anxiety, and fear memory after acute forniceal DBS treatment. Results: Acute forniceal DBS induces robust activation of multiple structures in the limbic system. DBS-induced neuronal activation extends beyond hippocampal formation and includes brain structures not directly innervated by the fornix. Conclusions: Acute forniceal DBS activates multiple limbic structures associated with emotion and memory. The neural circuits revealed here help elucidate the neural network effect and pave the way for further research on the mechanism by which forniceal DBS induces benefits on cognitive impairments.

Deep brain stimulation for Alzheimer's disease - current status and next steps

INTRODUCTION

Alzheimer's disease (AD) requires novel therapeutic approaches due to limited efficacy of current treatments.

AREAS COVERED

This article explores AD as a manifestation of neurocircuit dysfunction and evaluates deep brain stimulation (DBS) as a potential intervention. Focusing on fornix-targeted stimulation (DBS-f), the article summarizes safety, feasibility, and outcomes observed in phase 1/2 trials, highlighting findings such as cognitive improvement, increased metabolism, and hippocampal growth. Topics for further study include optimization of electrode placement, and the role of stimulation-induced autobiographical-recall. Nucleus basalis of Meynert (DBS-NBM) DBS is also discussed and compared with DBS-f. Challenges with both DBS-f and DBS-NBM are identified, emphasizing the need for further research on optimal stimulation parameters. The article also reviews alternative DBS targets, including medial temporal lobe structures and the ventral capsule/ventral striatum.

EXPERT OPINION

Looking ahead, a phase-3 DBS-f trial, and the prospect of closed-loop stimulation using EEG-derived biomarkers or hippocampal theta activity are highlighted. Recent FDA-approved therapies and other neuromodulation techniques like temporal interference and low-intensity ultrasound are considered. The article concludes by underscoring the importance of imaging-based diagnosis and staging to allow for circuit-targeted therapies, given the heterogeneity of AD and varied stages of neurocircuit dysfunction.

Sensing-Enabled Deep Brain Stimulation in Epilepsy

Deep brain stimulation has demonstrated efficacy in reducing seizure frequency in patients with drug-resistant epilepsy who may otherwise not be candidates for other surgical procedures. Recently, a clinical device that can monitor neural activity in the form of local field potentials around the deep brain stimulator lead implant site has been introduced. While this technology has been clinically adopted in other disorders treated with deep brain stimulation, such as Parkinson's disease, its application in epilepsy remains unclear. Previous research using investigational devices has suggested that specific frequency bands may correlate with clinical response to deep brain stimulation in epilepsy, but features of the clinical device may prevent its use. The authors present their experience with using this technology in epilepsy patients and describe some of its limitations. Ultimately, novel biomarkers will need to be identified to elucidate how neural activity at deep brain stimulation sites may change with clinical response.

Deep brain stimulation of fornix in Alzheimer's disease: From basic research to clinical practice

Alzheimer's disease (AD) is one of the most common progressive neurodegenerative diseases associated with the degradation of memory and cognitive ability. Current pharmacotherapies show little therapeutic effect in AD treatment and still cannot prevent the pathological progression of AD. Deep brain stimulation (DBS) has shown to enhance memory in morbid obese, epilepsy and traumatic brain injury patients, and cognition in Parkinson's disease (PD) patients deteriorates during DBS off. Some relevant animal studies and clinical trials have been carried out to discuss the DBS treatment for AD. Reviewing the fornix trials, no unified conclusion has been reached about the clinical benefits of DBS in AD, and the dementia ratings scale has not been effectively improved in the long term. However, some patients have presented promising results, such as improved glucose metabolism, increased connectivity in cognition-related brain regions and even elevated cognitive function rating scale scores. The fornix plays an important regulatory role in memory, attention, and emotion through its complex fibre projection to cognition-related structures, making it a promising target for DBS for AD treatment. Moreover, the current stereotaxic technique and various evaluation methods have provided references for the operator to select accurate stimulation points. Related adverse events and relatively higher costs in DBS have been emphasized. In this article, we summarize and update the research progression on fornix DBS in AD and seek to provide a reliable reference for subsequent experimental studies on DBS treatment of AD.

Changes in the Brain in Temporal Lobe Epilepsy with Unilateral Hippocampal Sclerosis: An Initial Case Series

Temporal lobe epilepsy (TLE) is a network disorder of the brain. Network disorders predominately involve dysregulation of hippocampal function caused by neuronal hyperexcitability. However, the relationship between the macro- and microscopic changes in specific brain regions is uncertain. In this study, the pattern of brain atrophy in patients with TLE and hippocampal sclerosis (HS) was investigated using volumetry, and microscopic changes in specific lesions were observed to examine the anatomical correspondence with specific target lesions using diffusion tensor imaging (DTI) with statistical parametric mapping (SPM). This retrospective cross-sectional study enrolled 17 patients with TLE and HS. We manually measured the volumes of the hippocampus (HC), amygdala (AMG), entorhinal cortex, fornix, and thalamus (TH) bilaterally. The mean diffusivity and fractional anisotropy of each patient were then quantified and analyzed by a voxel-based statistical correlation method using SPM8. In right TLE with HS, there was no evidence of any abnormal diffusion properties associated with the volume reduction in specific brain regions. In left TLE with HS, there were significant changes in the volumes of the AMG, HC, and TH. Despite the small sample size, these differences in conditions were considered meaningful. Chronic left TLE with HS might cause structural changes in the AMG, HC, and TH, unlike right TLE with HS.

Long Term Performance of a Bi-Directional Neural Interface for Deep Brain Stimulation and Recording

Background: In prior reports, we described the design and initial performance of a fully implantable, bi-directional neural interface system for use in deep brain and other neurostimulation applications. Here we provide an update on the chronic, long-term neural sensing performance of the system using traditional 4-contact leads and extend those results to include directional 8-contact leads.

Methods: Seven ovine subjects were implanted with deep brain stimulation (DBS) leads at different nodes within the Circuit of Papez: four with unilateral leads in the anterior nucleus of the thalamus and hippocampus; two with bilateral fornix leads, and one with bilateral hippocampal leads. The leads were connected to either an Activa PC+S^® (Medtronic) or Percept PC^°ledR (Medtronic) deep brain stimulation and recording device. Spontaneous local field potentials (LFPs), evoked potentials (EPs), LFP response to stimulation, and electrode impedances were monitored chronically for periods of up to five years in these subjects.

Results: The morphology, amplitude, and latencies of chronic hippocampal EPs evoked by thalamic stimulation remained stable over the duration of the study. Similarly, LFPs showed consistent spectral peaks with expected variation in absolute magnitude dependent upon behavioral state and other factors, but no systematic degradation of signal quality over time. Electrode impedances remained within expected ranges with little variation following an initial stabilization period. Coupled neural activity between the two nodes within the Papez circuit could be observed in synchronized recordings up to 5 years post-implant. The magnitude of passive LFP power recorded from directional electrode segments was indicative of the contacts that produced the greatest stimulation-induced changes in LFP power within the Papez network.

Conclusion: The implanted device performed as designed, providing the ability to chronically stimulate and record neural activity within this network for up to 5 years of follow-up.

Evidence supporting deep brain stimulation of the medial septum in the treatment of temporal lobe epilepsy

Electrical brain stimulation has become an essential treatment option for more than one third of epilepsy patients who are resistant to pharmacological therapy and are not candidates for surgical resection. However, currently approved stimulation paradigms achieve only moderate success, on average providing approximately 75% reduction in seizure frequency and extended periods of seizure freedom in nearly 20% of patients. Outcomes from electrical stimulation may be improved through the identification of novel anatomical targets, particularly those with significant anatomical and functional connectivity to the epileptogenic zone. Multiple studies have investigated the medial septal nucleus (i.e., medial septum) as such a target for the treatment of mesial temporal lobe epilepsy. The medial septum is a small midline nucleus that provides a critical functional role in modulating the hippocampal theta rhythm, a 4-7-Hz electrophysiological oscillation mechanistically associated with memory and higher order cognition in both rodents and humans. Elevated theta oscillations are thought to represent a seizure-resistant network activity state, suggesting that electrical neuromodulation of the medial septum and restoration of theta-rhythmic physiology may not only reduce seizure frequency, but also restore cognitive comorbidities associated with mesial temporal lobe epilepsy. Here, we review the anatomical and physiological function of the septohippocampal network, evidence for seizure-resistant effects of the theta rhythm, and the results of stimulation experiments across both rodent and human studies, to argue that deep brain stimulation of the medial septum holds potential to provide an effective neuromodulation treatment for mesial temporal lobe epilepsy. We conclude by discussing the considerations necessary for further evaluating this treatment paradigm with a clinical trial.

Low-frequency stimulation of a fiber tract in bilateral temporal lobe epilepsy

OBJECTIVE

Pharmacoresistant bilateral mesial temporal lobe epilepsy often implies poor resective surgical candidacy. Low-frequency stimulation of a fiber tract connected to bilateral hippocampi, the fornicodorsocommissural tract, has been shown to be safe and efficacious in reducing seizures in a previous short-term study. Here, we report a single-blinded, within-subject control, long-term deep-brain stimulation trial of low-frequency stimulation of the fornicodorsocommissural tract in bilateral mesial temporal lobe epilepsy. Outcomes of interest included safety with respect to verbal memory scores and reduction of seizure frequency.

METHODS

Our enrollment goal was 16 adult subjects to be randomized to 2-Hz or 5-Hz low-frequency stimulation of the fornicodorsocommissural tract starting at 2 mA. The study design consisted of four two-month blocks of stimulation with a 50%-duty cycle, alternating with two-month blocks of no stimulation.

RESULTS

We terminated the study after enrollment of five subjects due to slow accrual. Fornicodorsocommissural tract stimulation elicited bilateral hippocampal evoked responses in all subjects. Three subjects underwent implantation of pulse generators and long-term low-frequency stimulation with mean monthly seizures of 3.14 ± 2.67 (median 3.0 [IQR 1-4.0]) during stimulation-off blocks, compared with 0.96 ± 1.23 (median 1.0 [IQR 0-1.0]) during stimulation-on blocks (p = 0.0005) during the blinded phase. Generalized Estimating Equations showed that low-frequency stimulation reduced monthly seizure-frequency by 0.71 per mA (p < 0.001). Verbal memory scores were stable with no psychiatric complications or other adverse events.

SIGNIFICANCE

The results demonstrate feasibility of stimulating both hippocampi using a single deep-brain stimulation electrode in the fornicodorsocommissural tract, efficacy of low-frequency stimulation in reducing seizures, and safety as regards verbal memory.

Thalamic deep brain stimulation modulates cycles of seizure risk in epilepsy

Chronic brain recordings suggest that seizure risk is not uniform, but rather varies systematically relative to daily (circadian) and multiday (multidien) cycles. Here, one human and seven dogs with naturally occurring epilepsy had continuous intracranial EEG (median 298 days) using novel implantable sensing and stimulation devices. Two pet dogs and the human subject received concurrent thalamic deep brain stimulation (DBS) over multiple months. All subjects had circadian and multiday cycles in the rate of interictal epileptiform spikes (IES). There was seizure phase locking to circadian and multiday IES cycles in five and seven out of eight subjects, respectively. Thalamic DBS modified circadian (all 3 subjects) and multiday (analysis limited to the human participant) IES cycles. DBS modified seizure clustering and circadian phase locking in the human subject. Multiscale cycles in brain excitability and seizure risk are features of human and canine epilepsy and are modifiable by thalamic DBS.

Inducing neuroplasticity through intracranial θ-burst stimulation in the human sensorimotor cortex

The progress of therapeutic neuromodulation greatly depends on improving stimulation parameters to most efficiently induce neuroplasticity effects. Intermittent θ-burst stimulation (iTBS), a form of electrical stimulation that mimics natural brain activity patterns, has proved to efficiently induce such effects in animal studies and rhythmic transcranial magnetic stimulation studies in humans. However, little is known about the potential neuroplasticity effects of iTBS applied through intracranial electrodes in humans. This study characterizes the physiological effects of intracranial iTBS in humans and compare them with α-frequency stimulation, another frequently used neuromodulatory pattern. We applied these two stimulation patterns to well-defined regions in the sensorimotor cortex, which elicited contralateral hand muscle contractions during clinical mapping, in patients with epilepsy implanted with intracranial electrodes. Treatment effects were evaluated using oscillatory coherence across areas connected to the treatment site, as defined with corticocortical-evoked potentials. Our results show that iTBS increases coherence in the β-frequency band within the sensorimotor network indicating a potential neuroplasticity effect. The effect is specific to the sensorimotor system, the β band, and the stimulation pattern and outlasted the stimulation period by ∼3 min. The effect occurred in four out of seven subjects depending on the buildup of the effect during iTBS treatment and other patterns of oscillatory activity related to ceiling effects within the β band and to preexistent coherence within the α band. By characterizing the neurophysiological effects of iTBS within well-defined cortical networks, we hope to provide an electrophysiological framework that allows clinicians/researchers to optimize brain stimulation protocols which may have translational value.NEW & NOTEWORTHY θ-Burst stimulation (TBS) protocols in transcranial magnetic stimulation studies have shown improved treatment efficacy in a variety of neuropsychiatric disorders. The optimal protocol to induce neuroplasticity in invasive direct electrical stimulation approaches is not known. We report that intracranial TBS applied in human sensorimotor cortex increases local coherence of preexistent β rhythms. The effect is specific to the stimulation frequency and the stimulated network and outlasts the stimulation period by ∼3 min.

Development and in vivo assessment of a novel MRI-compatible headframe system for the ovine animal model

BACKGROUND

The brain of sheep has primarily been used in neuroscience as an animal model because of its similarity to the human brain, in particular if compared to other models such as the lissencephalic rodent brain. Their brain size also makes sheep an ideal model for the development of neurosurgical techniques using conventional clinical CT/MRI scanners and stereotactic systems for neurosurgery.

METHODS

In this study, we present the design and validation of a new CT/MRI compatible head frame for the ovine model and software, with its assessment under two real clinical scenarios.

RESULTS

Ex-vivo and in vivo trial results report an average linear displacement of the ovine head frame during conventional surgical procedures of 0.81 mm for ex-vivo trials and 0.68 mm for in vivo tests, respectively.

CONCLUSIONS

These trial results demonstrate the robustness of the head frame system and its suitability to be employed within a real clinical setting.

NGF and the Amyloid Precursor Protein in Alzheimer's Disease: From Molecular Players to Neuronal Circuits

Alzheimer's disease (AD), one of the most common causes of dementia in elderly people, is characterized by progressive impairment in cognitive function, early degeneration of basal forebrain cholinergic neurons (BFCNs), abnormal metabolism of the amyloid precursor protein (APP), amyloid beta-peptide (Aβ) depositions, and neurofibrillary tangles. According to the cholinergic hypothesis, dysfunction of acetylcholine-containing neurons in the basal forebrain contributes markedly to the cognitive decline observed in AD. In addition, the neurotrophic factor hypothesis posits that the loss nerve growth factor (NGF) signalling in AD may account for the vulnerability to atrophy of BFCNs and consequent impairment of cholinergic functions. Though acetylcholinesterase inhibitors provide only partial and symptomatic relief to AD patients, emerging data from in vivo magnetic resonance imaging (MRI) and positron emission tomography (PET) studies in mild cognitive impairment (MCI) and AD patients highlight the early involvement of BFCNs in MCI and the early phase of AD. These data support the cholinergic and neurotrophic hypotheses of AD and suggest new targets for AD therapy.Different mechanisms account for selective vulnerability of BFCNs to AD pathology, with regard to altered metabolism of APP and tau. In this review, we provide a general overview of the current knowledge of NGF and APP interplay, focusing on the role of APP in regulating NGF receptors trafficking/signalling and on the involvement of NGF in modulating phosphorylation of APP, which in turn controls APP intracellular trafficking and processing. Moreover, we highlight the consequences of APP interaction with p75NTR and TrkA receptor, which share the same binding site within the APP juxta-membrane domain. We underline the importance of insulin dysmetabolism in AD pathology, in the light of our recent data showing that overlapping intracellular signalling pathways stimulated by NGF or insulin can be compensatory. In particular, NGF-based signalling is able to ameliorates deficiencies in insulin signalling in the medial septum of 3×Tg-AD mice. Finally, we present an overview of NGF-regulated microRNAs (miRNAs). These small non-coding RNAs are involved in post-transcriptional regulation of gene expression , and we focus on a subset that are specifically deregulated in AD and thus potentially contribute to its pathology.

Long-term seizure outcome during continuous bipolar hippocampal deep brain stimulation in patients with temporal lobe epilepsy with or without mesial temporal sclerosis: An observational, open-label study

OBJECTIVE

We present the findings related to seizure outcome during hippocampal deep brain stimulation (Hip-DBS) in patients with refractory temporal lobe epilepsy.

METHODS

Twenty-five patients submitted to Hip-DBS were studied. All patients were evaluated with interictal and ictal electroencephalography (EEG) and high-resolution 1.5 T magnetic resonance imaging (MRI). The hippocampus was targeted directly on MRI using a posterior occipital burr hole approach. Bipolar continuous stimulation was ramped up until 3.0 V (300 µs, 130 Hz). Patients were considered responders if at least 50% seizure frequency reduction was obtained.

RESULTS

Median age was 39 years; median follow-up time was 57 months (16 women). All patients had focal with impaired awareness seizure (FIAS) and 23 patients had focal aware seizure (FAS). Baseline median FAS and FIAS frequency was 8. Ictal EEG showed unilateral (n = 10) or bilateral (n = 15) seizure onset. MRI showed unilateral (n = 11) or bilateral (n = 8) mesial temporal sclerosis (MTS) and was normal in six6 patients. Fifteen patients were submitted to bilateral and 10 patients to unilateral Hip-DBS. Median reduction in FAS frequency was 66%. Eighteen patients with FAS were considered responders and five (21%) were free of FAS. Median FIAS frequency (n = 25) reduction was 91%. Twenty-two patients were considered responders and eight (32%) were free of FIAS. FIAS were significantly more reduced then FAS (P = .017). There was no relation between any contact's position within the hippocampus and outcome for either FAS (P = .727) or FIAS (P = .410). There was no difference in outcome in patients submitted to either unilateral or bilateral Hip-DBS regarding FAS (P = .978) or FIAS (P = .693).

SIGNIFICANCE

Hip-DBS significantly reduced the frequency of both FAS and FIAS in this cohort of patients with refractory temporal lobe epilepsy. Hip-DBS might represent a good therapeutic option in such patients not amenable to resective surgery.

Coherence between the hippocampus and anterior thalamic nucleus as a tool to improve the effect of neurostimulation in temporal lobe epilepsy: An experimental study

BACKGROUND

Although the mechanisms by which deep brain stimulation (DBS) modifies the activity of the ictal network are mostly undefined, recent studies have suggested that DBS of the anterior nucleus of the thalamus (ANT) can be an effective treatment for mesial temporal lobe epilepsy (MTLE) when resective surgery cannot be performed. In a nonhuman primate (NHP) model of MTL seizures, we showed that the ANT was actively involved during interictal and ictal periods through different patterns and that the hippocampus (HPC) and ANT synchronously oscillate in the high beta-band during seizures.

OBJECTIVE

Based on those findings, we evaluated whether the frequency of stimulation is an important parameter that interferes with seizures and how to adapt stimulation protocols to it.

METHODS

We investigated the effects of low-frequency (40 Hz - determined as the ictal frequency of correlation between structures) and high-frequency (130 Hz - as commonly used in clinic) ANT stimulation in three monkeys in which MTLE seizures were initiated.

RESULTS

Low-frequency stimulation had a strong effect on the number of seizures and the total time spent in seizure, whereas high-frequency stimulation had no effect. The coherence of oscillations between the HPC and the ANT was significantly correlated with the success of low-frequency stimulation: the greater the coherence was, the greater the antiepileptic effect of ANT-DBS.

CONCLUSION

Our results suggest that low-frequency stimulation is efficient in treating seizures in a nonhuman primate model. More importantly, the study of the coherence between the ANT and HPC during seizures can help to predict the anti-epileptic effects of ANT stimulation. Furthermore, the DBS paradigm could be customized in frequency for each patient on the basis of the coherence spectral pattern.

Modulation of Human Memory by Deep Brain Stimulation of the Entorhinal-Hippocampal Circuitry

Neurological disorders affecting human memory present a major scientific, medical, and societal challenge. Direct or indirect deep brain stimulation (DBS) of the entorhinal-hippocampal system, the brain's major memory hub, has been studied in people with epilepsy or Alzheimer's disease, intending to enhance memory performance or slow memory decline. Variability in the spatiotemporal parameters of stimulation employed to date notwithstanding, it is likely that future DBS for memory will employ closed-loop, nuanced approaches that are synergistic with native physiological processes. The potential for editing human memory-decoding, enhancing, incepting, or deleting specific memories-suggests exciting therapeutic possibilities but also raises considerable ethical concerns.

Acute modulation of the limbic network with low and high-frequency stimulation of the human fornix

Targeted stimulation of white matter has opened newer perspectives in the field of neuromodulation, towards an attempt to improve memory or as a therapy for epilepsy. Stimulation of the fornix, being a part of the Papez circuit, is likely to modulate the limbic network excitability. However, the stimulation-frequency dependent variability in network excitability is unknown. In the case study, which involved stereo electroencephalographic (SEEG) recording of field potentials in a 48-year old left-handed woman with suspected temporal lobe epilepsy, we demonstrated the network effects of acute low (1 and 10 Hz) and high (50 Hz) frequency electrical stimulation of fornix. Mapping the short-latency evoked responses to forniceal stimulation confirmed the SEEG target localization within the Papez circuit. Low and high-frequency stimulation of the fornix produced opposite effects in the post-stimuli excitability, with the latter causing increased excitability in the limbic network that culminated in a clinical seizure. A distinct spectral peak around 8 Hz confirmed that sensing field potentials from the forniceal white matter is feasible. This is the first case study that provided an insight into how the temporal patterning of forniceal stimulation altered the downstream limbic network excitability.

Frequency-dependent electrical stimulation of fimbria-fornix preferentially affects the mesolimbic dopamine system or prefrontal cortex

BACKGROUND

The fimbria/fornix fiber system is an essential part of the hippocampal-VTA loop, and therefore activities that are propagated through this fiber system control the activity of the mesolimbic dopamine system.

OBJECTIVES/HYPOTHESIS

We hypothesized that stimulation of the fimbria/fornix with an increasing number of electrical pulses would cause increasing activity of the mesolimbic dopamine system, which coincides with concurrent changes in neuronal activities in target regions of the mesolimbic dopaminergic system.

METHODS

Right fimbria/fornix fibers were electrically stimulated with different pulse protocols. Stimulus-induced changes in neuronal activities were visualized with BOLD-fMRI, whereas stimulus-induced release of dopamine, as measured for the activity of the mesolimbic dopamine system, was determined in the nucleus accumbens with in vivo fast-scan cyclic voltammetry.

RESULTS

Dependent on the protocol, electrical fimbria/fornix stimulation caused BOLD responses in various targets of the mesolimbic dopamine system. Stimulation in the low theta frequency range (5 Hz) triggered significant BOLD responses mainly in the hippocampal formation, infralimbic cortex, and septum. Stimulation in the beta frequency range (20 Hz) caused additional activation in the medial prefrontal cortex (mPFC), nucleus accumbens, striatum, and VTA. Stimulation in the high-gamma frequency range (100 Hz) caused further activation in the hippocampus proper and mPFC. The strong activation in the mPFC during 100 Hz stimulations depended not only on the number of pulses but also on the frequency. Thus, short bursts of 5 or 20 high-frequency pulses caused stronger activation in the mPFC than continuous 5 or 20 Hz pulses. In contrast, high-frequency burst fimbria/fornix stimulation did not further activate the mesolimbic dopamine system when compared to continuous 5 or 20 Hz pulse stimulation.

CONCLUSIONS

There exists a frequency-dependent dissociation between BOLD responses and activation of the dopaminergic system. Low frequencies were more efficient to activate the mesolimbic dopamine system, whereas high frequencies were more efficient to trigger BOLD responses in target regions of the mesolimbic dopamine system, particularly the mPFC.

In vivo Diffusion Tensor Magnetic Resonance Tractography of the Sheep Brain: An Atlas of the Ovine White Matter Fiber Bundles

Diffusion Tensor Magnetic Resonance Imaging (DTI) allows to decode the mobility of water molecules in cerebral tissue, which is highly directional along myelinated fibers. By integrating the direction of highest water diffusion through the tissue, DTI Tractography enables a non-invasive dissection of brain fiber bundles. As such, this technique is a unique probe for in vivo characterization of white matter architecture. Unraveling the principal brain texture features of preclinical models that are advantageously exploited in experimental neuroscience is crucial to correctly evaluate investigational findings and to correlate them with real clinical scenarios. Although structurally similar to the human brain, the gyrencephalic ovine model has not yet been characterized by a systematic DTI study. Here we present the first in vivo sheep (ovis aries) tractography atlas, where the course of the main white matter fiber bundles of the ovine brain has been reconstructed. In the context of the EU's Horizon EDEN2020 project, in vivo brain MRI protocol for ovine animal models was optimized on a 1.5T scanner. High resolution conventional MRI scans and DTI sequences (b-value = 1,000 s/mm², 15 directions) were acquired on ten anesthetized sheep o. aries, in order to define the diffusion features of normal adult ovine brain tissue. Topography of the ovine cortex was studied and DTI maps were derived, to perform DTI tractography reconstruction of the corticospinal tract, corpus callosum, fornix, visual pathway, and occipitofrontal fascicle, bilaterally for all the animals. Binary masks of the tracts were then coregistered and reported in the space of a standard stereotaxic ovine reference system, to demonstrate the consistency of the fiber bundles and the minimal inter-subject variability in a unique tractography atlas. Our results determine the feasibility of a protocol to perform in vivo DTI tractography of the sheep, providing a reliable reconstruction and 3D rendering of major ovine fiber tracts underlying different neurological functions. Estimation of fiber directions and interactions would lead to a more comprehensive understanding of the sheep's brain anatomy, potentially exploitable in preclinical experiments, thus representing a precious tool for veterinaries and researchers.

Field potential 1/f activity in the subcallosal cingulate region as a candidate signal for monitoring deep brain stimulation for treatment-resistant depression

Subcallosal cingulate cortex deep brain stimulation (SCC-DBS) is an experimental therapy for treatment-resistant depression (TRD). Refinement and optimization of SCC-DBS will benefit from increased study of SCC electrophysiology in context of ongoing high-frequency SCC-DBS therapy. The study objective was a 7-mo observation of frequency-domain 1/f slope in off-stimulation local field potentials (SCC-LFPs) alongside standardized measurements of depression severity in 4 patients undergoing SCC-DBS. SCC was implanted bilaterally with a combined neurostimulation-LFP recording system. Following a 1-mo off-stimulation postoperative phase with multiple daily recordings, patients received bilateral SCC-DBS therapy (130 Hz, 90 μs) and weekly resting-state SCC-LFP recordings over a 6-mo treatment phase. 1/f slopes for each time point were estimated via linear regression of log-transformed Welch periodograms. General linear mixed-effects models were constructed to estimate pretreatment sources of 1/f slope variance, and 95% bootstrap confidence intervals were constructed to estimate treatment phase 1/f slope association with treatment response (50% decrease in preimplantation symptom severity). Results show the time of recording was a prominent source of pretreatment 1/f slope variance bilaterally, with increased 1/f slope magnitude observed during night hours (2300-0659). Increase in right 1/f slope was observed in the setting of treatment response, with bootstrap analysis supporting this observation in 3 of 4 subjects. We conclude that 1/f slope can be measured longitudinally in a combined SCC-DBS/LFP recording system and likely conforms to known 1/f circadian variability. The preliminary evidence of 1/f slope increase during treatment response suggests a potential utility as a candidate biomarker for ongoing development of adaptive TRD-neuromodulation strategies.NEW & NOTEWORTHY In four patients with treatment-resistant depression undergoing therapeutic deep brain stimulation (DBS), we present the first longitudinal observations of local field potentials (LFP) from the subcallosal cingulate region outside the postoperative period. Specifically, our results demonstrate that frequency-domain 1/f activity is measurable in a combined DBS-LFP recording system and that right hemisphere recordings appear sensitive to mood state, thus suggesting a potential readout suitable for consideration in ongoing efforts to develop adaptive DBS delivery systems.

Automated Steerable Path Planning for Deep Brain Stimulation Safeguarding Fiber Tracts and Deep Gray Matter Nuclei

Deep Brain Stimulation (DBS) is a neurosurgical procedure consisting in the stereotactic implantation of stimulation electrodes to specific brain targets, such as deep gray matter nuclei. Current solutions to place the electrodes rely on rectilinear stereotactic trajectories (RTs) manually defined by surgeons, based on pre-operative images. An automatic path planner that accurately targets subthalamic nuclei (STN) and safeguards critical surrounding structures is still lacking. Also, robotically-driven curvilinear trajectories (CTs) computed on the basis of state-of-the-art neuroimaging would decrease DBS invasiveness, circumventing patient-specific obstacles. This work presents a new algorithm able to estimate a pool of DBS curvilinear trajectories for reaching a given deep target in the brain, in the context of the EU's Horizon EDEN2020 project. The prospect of automatically computing trajectory plans relying on sophisticated newly engineered steerable devices represents a breakthrough in the field of microsurgical robotics. By tailoring the paths according to single-patient anatomical constraints, as defined by advanced preoperative neuroimaging including diffusion MR tractography, this planner ensures a higher level of safety than the standard rectilinear approach. Ten healthy controls underwent Magnetic Resonance Imaging (MRI) on 3T scanner, including 3DT1-weighted sequences, 3Dhigh-resolution time-of-flight MR angiography (TOF-MRA) and high angular resolution diffusion MR sequences. A probabilistic q-ball residual-bootstrap MR tractography algorithm was used to reconstruct motor fibers, while the other deep gray matter nuclei surrounding STN and vessels were segmented on T1 and TOF-MRA images, respectively. These structures were labeled as obstacles. The reliability of the automated planner was evaluated; CTs were compared to RTs in terms of efficacy and safety. Targeting the anterior STN, CTs performed significantly better in maximizing the minimal distance from critical structures, by finding a tuned balance between all obstacles. Moreover, CTs resulted superior in reaching the center of mass (COM) of STN, as well as in optimizing the entry angle in STN and in the skull surface.

A Chronically Implantable Neural Coprocessor for Investigating the Treatment of Neurological Disorders

Developing new tools to better understand disorders of the nervous system, with a goal to more effectively treat them, is an active area of bioelectronic medicine research. Future tools must be flexible and configurable, given the evolving understanding of both neuromodulation mechanisms and how to configure a system for optimal clinical outcomes. We describe a system, the Summit RC+S "neural coprocessor," that attempts to bring the capability and flexibility of a microprocessor to a prosthesis embedded within the nervous system. This paper describes the updated system architecture for the Summit RC+S system, the five custom integrated circuits required for bi-directional neural interfacing, the supporting firmware/software ecosystem, and the verification and validation activities to prepare for human implantation. Emphasis is placed on design changes motivated by experience with the CE-marked Activa PC+S research tool; specifically, enhancement of sense-stim performance for improved bi-directional communication to the nervous system, implementation of rechargeable technology to extend device longevity, and application of MICS-band telemetry for algorithm development and data management. The technology was validated in a chronic treatment paradigm for canines with naturally occurring epilepsy, including free ambulation in the home environment, which represents a typical use case for future human protocols.