Bradley Lega

Human hippocampal theta oscillations and the formation of episodic memories

The importance of the hippocampal theta oscillation (4–8 Hz) to memory formation has been well‐established through studies in animals, prompting researchers to propose comprehensive theories of memory and learning that rely on theta oscillations for integrating information in the hippocampus and neocortex. Yet, empirical evidence for the importance of 4–8 Hz hippocampal theta oscillations to memory formation in humans is equivocal at best. To clarify this apparent interspecies discrepancy, we recorded intracranial EEG (iEEG) data from 237 hippocampal electrodes in 33 neurosurgical patients as they performed an episodic memory task. We identified two distinct patterns of hippocampal oscillations, at ∼3 and ∼8 Hz, which are at the edges of the traditional 4–8 Hz human theta band. The 3 Hz “slow‐theta” oscillation exhibited higher power during successful memory encoding and was functionally linked to gamma oscillations, but similar patterns were not present for the 8 Hz “fast‐theta” oscillation. For episodic memory, slow‐theta oscillations in the human hippocampus appear to be analogous to the memory‐related theta oscillations observed in animals. Both fast‐theta and slow‐theta oscillations exhibit evidence of phase synchrony with oscillations in the temporal cortex. We discuss our findings in the context of recent research on the electrophysiology of human memory and spatial navigation, and explore the implications of this result for theories of cortico–hippocampal communication.

Deep brain stimulation in the treatment of refractory epilepsy: update on current data and future directions.

Deep brain stimulation for epilepsy has garnered attention from epileptologists due to its well-documented success in treating movement disorders and the low morbidity associated with the implantation of electrodes. Given the large proportion of patients who fail medical therapy and are not candidates for surgical amelioration, as well as the suboptimal seizure control offered by vagus nerve stimulation, the search for appropriate brain structures to serve as targets for deep brain stimulation has generated a useful body of evidence to serve as the basis for larger investigations. Early results of the SANTE trial should lay the foundation for widespread implementation of DBS for epilepsy targeting the anterior thalamic nucleus. Other targets also offer promise, including the caudate nucleus, the subthalamic nucleus, the cerebellum, the centromedian nucleus of the thalamus, and the hippocampus. This paper reviews the logic which underlies these potential targets and recapitulates the current data from limited human trials supporting each one. It also provides a succinct overview of the surgical procedure used for electrode implantation.

Choosing the best operation for chronic subdural hematoma: a decision analysis

OBJECT Chronic subdural hematoma (CSDH), a condition much more common in the elderly, presents an increasing challenge as the population ages. Treatment strategies for CSDH include bur-hole craniostomy (BHC), twist-drill craniostomy (TDC), and craniotomy. Decision analysis was used to organize existing data and develop recommendations for effective treatment. METHODS A Medline search was used to identify articles about treatment of CSDH. Direct assessment by health care professionals of the relative health impact of common complications and recurrences was used to generate utility values for treatment outcomes. Monte Carlo simulation and sensitivity analyses allowed comparisons across treatment strategies. A second simulation examined whether intraoperative irrigation or postoperative drainage affect the outcomes following BHC. RESULTS On a scale from 0 to 1, the utility of BHC was found to be 0.9608, compared with 0.9202 for TDC (p = 0.001) and 0.9169 for craniotomy (p = 0.006). Sensitivity analysis confirmed the robustness of these values. Craniotomy yielded fewer recurrences, but more frequent and more serious complications than did BHC. There were no significant differences for BHC with or without irrigation or postoperative drainage. CONCLUSIONS Bur-hole craniostomy is the most efficient choice for surgical drainage of uncomplicated CSDH. Bur-hole craniostomy balances a low recurrence rate with a low incidence of highly morbid complications. Decision analysis provides statistical and empirical guidance in the absence of well-controlled large trials and despite a confusing range of previously reported morbidity and recurrence.

Neuronal Activity in the Human Subthalamic Nucleus Encodes Decision Conflict during Action Selection

The subthalamic nucleus (STN), which receives excitatory inputs from the cortex and has direct connections with the inhibitory pathways of the basal ganglia, is well positioned to efficiently mediate action selection. Here, we use microelectrode recordings captured during deep brain stimulation surgery as participants engage in a decision task to examine the role of the human STN in action selection. We demonstrate that spiking activity in the STN increases when participants engage in a decision and that the level of spiking activity increases with the degree of decision conflict. These data implicate the STN as an important mediator of action selection during decision processes.

Quality of life after hemicraniectomy for traumatic brain injury in adults. A review of the literature.

Decompressive hemicraniectomy is well accepted for the surgical treatment of intractable intracranial hypertension in cases in which medical management fails. Although it is performed as a life-saving procedure when death is imminent from intracranial hypertension, little is known about the functional outcomes for these patients on long-term follow-up. In this study, the authors performed a systematic review of the literature to examine neurological outcome after hemicraniectomy. A literature search revealed 29 studies that reported outcomes using GOS scores. The GOS scores were transformed to utility values for quality of life using a conversion method based on decision analysis modeling. Based on the literature, 1422 cases were analyzed. The average 6-month-postoperative mortality rate was 28.2%. The mean QOL value among survivors was 0.592, which corresponds roughly to a GOS score of 4. Although more studies are needed for validation of long-term neurological outcome after hemicraniectomy, the assumption that most patients remain in a vegetative state after this intervention is clearly incorrect.

Slow-Theta-to-Gamma Phase–Amplitude Coupling in Human Hippocampus Supports the Formation of New Episodic Memories

Phase-amplitude coupling (PAC) has been proposed as a neural mechanism for coordinating information processing across brain regions. Here we sought to characterize PAC in the human hippocampus, and in temporal and frontal cortices, during the formation of new episodic memories. Intracranial recordings taken as 56 neurosurgical patients studied and recalled lists of words revealed significant hippocampal PAC, with slow-theta activity (2.5-5 Hz) modulating gamma band activity (34-130 Hz). Furthermore, a significant number of hippocampal electrodes exhibited greater PAC during successful than unsuccessful encoding, with the gamma activity at these sites coupled to the trough of the slow-theta oscillation. These same conditions facilitate LTP in animal models, providing a possible mechanism of action for this effect in human memory. Uniquely in the hippocampus, phase preference during item encoding exhibited a biphasic pattern. Overall, our findings help translate between the patterns identified during basic memory tasks in animals and those present during complex human memory encoding. We discuss the unique properties of human hippocampal PAC and how our findings relate to influential theories of information processing based on theta-gamma interactions.

Da Vinci Robot-assisted transoral odontoidectomy for basilar invagination.

The transoral approach is an effective way to decompress the craniocervical junction due to basilar invagination. This approach has been described and refined, but significant limitations and technical challenges remain. Specifically, should the transoral route be used for intradural pathology, such as a meningioma, or should an inadvertent durotomy occur during extradural dissection, achieving a watertight closure of the dura in such a deep and narrow working channel is limited with the current microscopic and endoscopic techniques. Even closure of the posterior pharyngeal mucosa can be challenging, and problems with wound dehiscence encountered in some case series may be attributable to this difficulty. These problems, and the corollary aversion to the procedure felt by many neurosurgeons, led our group to investigate an alternative approach.

Microvascular decompression for hemifacial spasm: Long-term results from 114 operations performed without neurophysiological monitoring

OBJECT Microvascular decompression (MVD) of the facial nerve is an effective treatment for hemifacial spasm (HFS), but the procedure is associated with a significant risk of complications such as hearing loss and facial weakness. Many surgeons advocate the use of intraoperative brainstem auditory evoked response (BAER) monitoring in an attempt to improve surgical outcomes. The authors critically assessed a large series of patients with HFS who underwent MVD without neurophysiological monitoring. METHODS The authors retrospectively identified 114 consecutive patients, with a history of HFS and without a history of HFS surgery, in whom MVD was performed by a single surgeon without the use of neurophysiological monitoring. Postoperative outcomes were determined by reviewing records and through telephone interviews. At least 1 year of postoperative follow-up data were available for 91 of the 114 patients, and the median follow-up duration in all cases was 8 years (range 3 months-23 years). A Kaplan-Meier analysis showed that 86% of the patients were spasm free at 10 years postoperatively. RESULTS There were no surgical deaths or major deficits, and complications included 1 case of postoperative deafness, 1 of permanent subtotal hearing loss, and 10 of delayed facial palsy, 2 of which did not completely resolve at last follow-up. The outcomes, rates of hearing loss, and other complications compared well with those reported in studies in which investigators used intraoperative monitoring. CONCLUSIONS The results suggest that MVD without neurophysiological monitoring is a safe and effective treatment option in patients with HFS. Although BAER monitoring may be a valuable adjunct to surgery at centers experienced with the modality, the absence of intraoperative monitoring should not prevent neurosurgeons from performing MVD in patients with HFS.

Transoral Robotic Surgery of the Skull Base: A Cadaver and Feasibility Study

Objective: The goal of this study was to determine the potential role as well as the current limitations of the da Vinci Surgical System robot in transoral surgery of the skull base. Methods: The da Vinci robot was used to perform dissections of the skull base on 7 cadaver heads with their neck and clavicles intact. Neurosurgeons and otolaryngologists familiar with all facets of the open microscopic, minimally invasive, endoscopic and transoral robotic surgical procedure proceeded with the approach to and dissection of the human skull base. Results: The da Vinci robot provided superb illumination and 3-dimensional depth perception. The 30- degree endoscope improved cephalad visualization, and the ‘intuitive’ nature of the da Vinci surgical robot arms provided an advantage by their ability to suture the dura at the level of the clivus. An entirely transoral route provides access to the middle and lower clivus as well as the infratemporal fossa, but access to the sellar region and anterior cranial fossa is limited via a purely transoral route. Tremor-free dural closure was successfully performed. Conclusion: Our findings suggest that transoral robotic surgery utilizing the da Vinci robot system holds great potential for skull base surgical resection of extradural and intradural tumors of the middle and lower clivus and infratemporal fossa. A collaborative approach with neurosurgeon and otolaryngologist alternating at the master console and bedside is a successful strategy. Further instrument development is necessary, and continued investigation is warranted.

Direct Brain Stimulation Modulates Encoding States and Memory Performance in Humans

People often forget information because they fail to effectively encode it. Here, we test the hypothesis that targeted electrical stimulation can modulate neural encoding states and subsequent memory outcomes. Using recordings from neurosurgical epilepsy patients with intracranially implanted electrodes, we trained multivariate classifiers to discriminate spectral activity during learning that predicted remembering from forgetting, then decoded neural activity in later sessions in which we applied stimulation during learning. Stimulation increased encoding-state estimates and recall if delivered when the classifier indicated low encoding efficiency but had the reverse effect if stimulation was delivered when the classifier indicated high encoding efficiency. Higher encoding-state estimates from stimulation were associated with greater evidence of neural activity linked to contextual memory encoding. In identifying the conditions under which stimulation modulates memory, the data suggest strategies for therapeutically treating memory dysfunction.