Brain-stem auditory evoked potentials (BAEPs) from basal surface of temporal lobe recorded from chronic subdural electrodes

BAEPs were recorded from the basal surface of the temporal lobe by subdural electrodes chronically implanted in 6 patients who were evaluated for surgical management of intractable partial seizures. Near-field recordings were obtained by recording between the subdural electrode closet and most distant to the brain-stem. Far-field recordings were obtained by recording between the subdural electrodes and an indifferent electrode over the spinal process of the seventh cervical vertebrae. The recordings were compared with standard ear-vertex recordings.After ipsilateral ear stimulation, the subdural electrode closet to the brain-stem recorded large amplitude waves I and II, followed by less well-defined waves of longer latencies. Recordings to contralateral stimulation showed no clearly defined waves I and II and a large amplitude wave Vn. Waves III, IV, V, Vn and VI were of opposite polarity after ipsi- and contralateral stimulation. These findings indicate that waves I and II are generated ipsilaterally to the stimulation side, whereas wave Vn has a contralateral origin. Wave Vn may be generated in the brachium of the inferior colliculus, as suggested from latency and from dipole configuration studies. This conclusion is consistent with the classical anatomical observations that the supracollicular auditory pathways are predominantly crossed.     

颅内电极在感觉运动区皮质发育不良癫痫手术中的应用

目的：探讨长程颅内电极监测及电刺激方法,在感觉运动区皮质发育不良的难治性癫痫外科手术评估中的意义。方法：筛选MRI提示的皮质发育不良区域与重要功能区-感觉运动区位置关系密切的11例难治性癫痫患者,且头皮长程视频脑电监测及PET检查也初步提示癫痫发作与皮质发育不良所在脑区有关,在可疑脑区放置颅内电极,然后进行颅内电极长程视频脑电监测及电刺激检测,对癫痫起源位置及功能区定位,明确癫痫发作起源区域与感觉运动功能区的解剖学关系,在定位结果指导下进行切除术。结果：11例中3例位于左侧半球,8例位于右侧半球,11例感觉运动功能区皮质分布均存在不同程度变异,7例癫痫发作起源区域与感觉运动功能区一定范围重叠,其中5例与感觉区重叠,该5例切除了起源区域与发作有关的部分感觉区,2例部分致痫灶与运动区重叠,该2例仅切除了除与发作有关的运动区以外的癫痫起源区域,4例癫痫发作起源区域与感觉运动功能区相对独立,该4例完全切除癫痫发作起源区域;手术后6例患者发作消失,2例患者发作频率减少90%以上,1例癫痫发作控制无效,2例患者发生部分感觉缺失,但对生活无明显影响。结论：在皮质发育不良的癫痫患者中,有较高比例的病人伴有功能区皮层分布的变异,长程颅内电极监测及电刺激能够实现癫痫起源区域及功能区精确定位,明确功能区变异情况,对于指导病灶切除,避免损伤功能区皮质,减少术后并发症具有重要意义。     

Concurrent Electroencephalography Recording During Transcranial Alternating Current Stimulation (tACS)

Oscillatory brain activities are considered to reflect the basis of rhythmic changes in transmission efficacy across brain networks and are assumed to integrate cognitive neural processes. Transcranial alternating current stimulation (tACS) holds the promise to elucidate the causal link between specific frequencies of oscillatory brain activity and cognitive processes. Simultaneous electroencephalography (EEG) recording during tACS would offer an opportunity to directly explore immediate neurophysiological effects of tACS. However, it is not trivial to measure EEG signals during tACS, as tACS creates a huge artifact in EEG data. Here we explain how to set up concurrent tACS-EEG experiments. Two necessary considerations for successful EEG recording while applying tACS are highlighted. First, bridging of the tACS and EEG electrodes via leaking EEG gel immediately saturates the EEG amplifier. To avoid bridging via gel, the viscosity of the EEG gel is the most important parameter. The EEG gel must be viscous to avoid bridging, but at the same time sufficiently fluid to create contact between the tACS electrode and the scalp. Second, due to the large amplitude of the tACS artifact, it is important to consider using an EEG system with a high resolution analog-to-digital (A/D) converter. In particular, the magnitude of the tACS artifact can exceed 100 mV at the vicinity of a stimulation electrode when 1 mA tACS is applied. The resolution of the A/D converter is of importance to measure good quality EEG data from the vicinity of the stimulation site. By following these guidelines for the procedures and technical considerations, successful concurrent EEG recording during tACS will be realized.     

Performing Behavioral Tasks in Subjects with Intracranial Electrodes

Patients having stereo-electroencephalography (SEEG) electrode, subdural grid or depth electrode implants have a multitude of electrodes implanted in different areas of their brain for the localization of their seizure focus and eloquent areas. After implantation, the patient must remain in the hospital until the pathological area of brain is found and possibly resected. During this time, these patients offer a unique opportunity to the research community because any number of behavioral paradigms can be performed to uncover the neural correlates that guide behavior. Here we present a method for recording brain activity from intracranial implants as subjects perform a behavioral task designed to assess decision-making and reward encoding. All electrophysiological data from the intracranial electrodes are recorded during the behavioral task, allowing for the examination of the many brain areas involved in a single function at time scales relevant to behavior. Moreover, and unlike animal studies, human patients can learn a wide variety of behavioral tasks quickly, allowing for the ability to perform more than one task in the same subject or for performing controls. Despite the many advantages of this technique for understanding human brain function, there are also methodological limitations that we discuss, including environmental factors, analgesic effects, time constraints and recordings from diseased tissue. This method may be easily implemented by any institution that performs intracranial assessments; providing the opportunity to directly examine human brain function during behavior.     

颅内电极在感觉运动区皮质发育不良癫痫手术中的应用

目的:探讨长程颅内电极监测及电刺激方法,在感觉运动区皮质发育不良的难治性癫痫外科手术评估中的意义。方法:筛选MRI提示的皮质发育不良区域与重要功能区-感觉运动区位置关系密切的11例难治性癫痫患者,且头皮长程视频脑电监测及PET检查也初步提示癫痫发作与皮质发育不良所在脑区有关,在可疑脑区放置颅内电极,然后进行颅内电极长程视频脑电监测及电刺激检测,对癫痫起源位置及功能区定位,明确癫痫发作起源区域与感觉运动功能区的解剖学关系,在定位结果指导下进行切除术。结果:11例中3例位于左侧半球,8例位于右侧半球,11例感觉运动功能区皮质分布均存在不同程度变异,7例癫痫发作起源区域与感觉运动功能区一定范围重叠,其中5例与感觉区重叠,该5例切除了起源区域与发作有关的部分感觉区,2例部分致痫灶与运动区重叠,该2例仅切除了除与发作有关的运动区以外的癫痫起源区域,4例癫痫发作起源区域与感觉运动功能区相对独立,该4例完全切除癫痫发作起源区域;手术后6例患者发作消失,2例患者发作频率减少90%以上,1例癫痫发作控制无效,2例患者发生部分感觉缺失,但对生活无明显影响。结论:在皮质发育不良的癫痫患者中,有较高比例的病人伴有功能区皮层分布的变异,长程颅内电极监测及电刺激能够实现癫痫起源区域及功能区精确定位,明确功能区变异情况,对于指导病灶切除,避免损伤功能区皮质,减少术后并发症具有重要意义。     

Distinct temporal patterns of electrical stimulation influence neural recruitment during PTZ infusion: an fMRI study

Our working hypothesis is that constant inter-pulse interval (IPI) electrical stimulation (ES) would resonate with endogenous epileptogenic reverberating circuits, favoring seizure, while random inter-interval ES protocol would promote desynchronization of such neural networks, interfering with the abnormal recruitment of neural structures. Male Wistar rats were stereotaxically implanted with a monopolar ES carbon-fiber electrode (minimizing fMRI artifact) in the amygdala. A 7T fMRI scanner was used to evaluate brain activity during ES, fixed four pulses per second ratio, using either a periodic IPI (ES-P) or random IPI (non-periodic ES-NP) stimulation paradigm. Appropriate imaging protocols were used to compare baseline BOLD (blood oxygen level dependent) MRI with scans during ES. A second series of experiments, both without stimuli and under the same ES paradigms, were evaluated during continuous infusion of pentylenetetrazole (PTZ, 4 mg/ml/min) through an i.v. catheter. Our results show that temporal lobe activation during ES-P or ES-NP did not present any statistical differences during ES. However, during PTZ infusion, PTZ-P facilitated recruitment of the temporal lobe ipsilateral to ES while PTZ-NP showed significantly less activation ipsilateral to ES and, in turn, less inter-hemispheric differences. Altogether, our results support the hypothesis of reverberating circuits being synchronized by ES-P and desynchronized by ES-NP. Time-coded low frequency stimulation may be an interesting alternative treatment for patients with refractory epilepsy.     

Simultaneous fMRI and Electrophysiology in the Rodent Brain

To examine the neural basis of the blood oxygenation level dependent (BOLD) magnetic resonance imaging (MRI) signal, we have developed a rodent model in which functional MRI data and in vivo intracortical recording can be performed simultaneously. The combination of MRI and electrical recording is technically challenging because the electrodes used for recording distort the MRI images and the MRI acquisition induces noise in the electrical recording. To minimize the mutual interference of the two modalities, glass microelectrodes were used rather than metal and a noise removal algorithm was implemented for the electrophysiology data. In our studies, two microelectrodes were separately implanted in bilateral primary somatosensory cortices (SI) of the rat and fixed in place. One coronal slice covering the electrode tips was selected for functional MRI. Electrode shafts and fixation positions were not included in the image slice to avoid imaging artifacts. The removed scalp was replaced with toothpaste to reduce susceptibility mismatch and prevent Gibbs ringing artifacts in the images. The artifact structure induced in the electrical recordings by the rapidly-switching magnetic fields during image acquisition was characterized by averaging all cycles of scans for each run. The noise structure during imaging was then subtracted from original recordings. The denoised time courses were then used for further analysis in combination with the fMRI data. As an example, the simultaneous acquisition was used to determine the relationship between spontaneous fMRI BOLD signals and band-limited intracortical electrical activity. Simultaneous fMRI and electrophysiological recording in the rodent will provide a platform for many exciting applications in neuroscience in addition to elucidating the relationship between the fMRI BOLD signal and neuronal activity.Download video file.^{(95M, mp4)}     

Neurosurgical management of epilepsy: a personal perspective in 1983

The therapeutic goal in the neurosurgical treatment of medically intractable epilepsy is complete seizure control, for both biologic and psychosocial reasons. Cortical resections are more likely to accomplish this than other surgical alternatives for epilepsy. Although abnormalities on new imaging techniques (CT, positron emission scanning) aid in identifying the epileptic focus, interictal epileptiform EEG changes remain the main indicator of focal origin of the seizures. Where this is equivocal, direct brain recording of spontaneous seizures with subdural electrodes is of value in identifying the side and lobe of seizure onset. The cortical resection is then tailored by the extent of the interictal electrocorticographic abnormalities and functional identification of essential areas such as those for language, using an electrical stimulation mapping technique, under local anesthesia. With this approach, half of the patients with temporal lobe foci are seizure-free since the time of operation, over two-thirds become so with time, and over three-quarters have at least very major reductions in seizure frequency.     

Changes in paroxysmal brainwave patterns of epileptics by weak-field magnetic stimulation

In order to assess the effects of weak-field magnetic stimulation on brain electrical activity in epileptics, three patients suffering from mesial temporal lobe epilepsy (MTLE) were exposed to DC magnetic fields of 0.9 and 1.8 millitesla (mT). The EEG activity was recorded simultaneously from intracranial electrodes inserted through the foramen ovale (FO) and scalp electrodes. Significant enhancement of interictal epileptiform activity was observed in two patients, while in one patient, magnetic stimulation resulted in the cessation of interictal spike/wave trains.     

Illusions of Visual Motion Elicited by Electrical Stimulation of Human MT Complex

Human cortical area MT⁺ (hMT⁺) is known to respond to visual motion stimuli, but its causal role in the conscious experience of motion remains largely unexplored. Studies in non-human primates demonstrate that altering activity in area MT can influence motion perception judgments, but animal studies are inherently limited in assessing subjective conscious experience. In the current study, we use functional magnetic resonance imaging (fMRI), intracranial electrocorticography (ECoG), and electrical brain stimulation (EBS) in three patients implanted with intracranial electrodes to address the role of area hMT⁺ in conscious visual motion perception. We show that in conscious human subjects, reproducible illusory motion can be elicited by electrical stimulation of hMT⁺. These visual motion percepts only occurred when the site of stimulation overlapped directly with the region of the brain that had increased fMRI and electrophysiological activity during moving compared to static visual stimuli in the same individual subjects. Electrical stimulation in neighboring regions failed to produce illusory motion. Our study provides evidence for the sufficient causal link between the hMT⁺ network and the human conscious experience of visual motion. It also suggests a clear spatial relationship between fMRI signal and ECoG activity in the human brain.     

Deactivation of Brain Areas During Self-Regulation of Slow Cortical Potentials in Seizure Patients

This study investigates the neurophysiological basis of EEG feedback for patients with epilepsy. Brain areas are identified that become hemodynamically deactivated when epilepsy patients, trained in EEG self-regulation, generate positive slow cortical potentials (SCPs). Five patients were trained in producing positive SCPs, using a training protocol previously established to reduce seizure frequency in patients with drug refractory epilepsy. Patients attempted to produce positive SCP shifts in a functional magnetic resonance imaging (fMRI) scanner. Two patients were able to reliably produce positive SCP shifts. When these successful regulators were prompted to produce positive SCPs, blood oxygen level-dependent (BOLD) response indicated deactivation, in comparison to a control state, around the recording electrode, frontal lobe, and thalamus. Unsuccessful regulators’ BOLD response indicated no deactivation in cortical areas proximal to the active electrode. No thalamic deactivation was found in poor regulators. Decreased seizure frequency from SCP training may be the result of positively reinforced inhibition in cortical areas proximal to active electrode placement, the frontal cortex, and the thalamus.     

La stimulation électrique pallidale dans le traitement des mouvements anormaux : l’expérience montpelliéraine dans l’utilisation de l’imagerie fonctionnelle comme critères de sélection des patients

Magnetic resonance anatomical imaging gives to the practician an important set of criteria for selecting patients with dystonic and dyskinetic syndromes who can get benefice from a treatment by electrical deep brain stimulation. However, anatomical anomalies seen on MRI, whatever their number and morphological extension, do not reflect the effective (residual) functional activity of the corresponding structures. We describe, in this short report, the way how, in our experience, functional imaging techniques used for measuring cerebral perfusion and neurotransmission in Single Photon Emission Computerized Tomography (SPECT), cerebral metabolism in Positron Emission Tomography (PET) and motor neuroactivations in functional Magnetic Resonance Imaging (fMRI) are capable of progressively fulfil this deficiency.     

Combined use of digital subtraction angiography and MRI for radiosurgery and stereoencephalography

The authors report their experience with the combined use of digital subtraction angiography (DSA) and magnetic resonance imaging (MRI) for the stereotactic placement of intracerebral electrodes in epilepsy and for the radiosurgical treatment of otherwise inoperable arteriovenous malformations of the brain. Both imaging techniques, when used in conjunction, have been found most useful and complementary. For deep electrode placement, they permit optimal visualization of the cerebral structures to be reached by the electrode array while allowing the avoidance of vessels in the vicinity. For radiosurgery of arteriovenous malformations, DSA provides optimal visualization of the feeders and of the malformation itself, while the MRI reveals the cerebral structures to be spared by the photon beam of the linear accelerator. A discussion of their respective roles is presented, with the specific question as to whether MRI alone could be used for both procedures.     

Intracellular Neural Recording with Pure Carbon Nanotube Probes

The computational complexity of the brain depends in part on a neuron’s capacity to integrate electrochemical information from vast numbers of synaptic inputs. The measurements of synaptic activity that are crucial for mechanistic understanding of brain function are also challenging, because they require intracellular recording methods to detect and resolve millivolt- scale synaptic potentials. Although glass electrodes are widely used for intracellular recordings, novel electrodes with superior mechanical and electrical properties are desirable, because they could extend intracellular recording methods to challenging environments, including long term recordings in freely behaving animals. Carbon nanotubes (CNTs) can theoretically deliver this advance, but the difficulty of assembling CNTs has limited their application to a coating layer or assembly on a planar substrate, resulting in electrodes that are more suitable for in vivo extracellular recording or extracellular recording from isolated cells. Here we show that a novel, yet remarkably simple, millimeter-long electrode with a sub-micron tip, fabricated from self-entangled pure CNTs can be used to obtain intracellular and extracellular recordings from vertebrate neurons in vitro and in vivo. This fabrication technology provides a new method for assembling intracellular electrodes from CNTs, affording a promising opportunity to harness nanotechnology for neuroscience applications.     

Use of antidromic evoked potentials in placement of dorsal cord disc electrodes

Intraoperative recordings of somatosensory evoked potentials were made in 16 patients undergoing implantation of a dorsal cord stimulation system. Antidromic recordings, obtained by stimulating through the dorsal cord electrode placed in the epidural space and recording over peripheral nerves in the painful region of the body, and much higher signal-to-noise ratios and could be obtained with greater reliability than standard orthodromic recordings. When the placement of the electrode was adjusted to obtain evoked responses in the painful region, paresthesias referred to that region were obtained in virtually every case. Use of this procedure allows implantation and internalization of the electrodes in a single procedure under general anesthesia, and reduces the necessity of subsequent revisions.     

Integration of silicon-via electrodes with different recording characteristics on a glass microprobe using a glass reflowing process

Electrodes on planar type microelectromechanical system (MEMS) microprobes mainly record neurons on the top-side of probe shaft (called a top-side electrode). However, it is often necessary to record neurons other than those on the top-side of the probe shaft. This study uses the glass reflowing technique to embed silicon-vias in a glass probe to implement a microprobe capable of recording neurons around the shaft. The proposed technology makes it possible to fabricate, distribute, and integrate four types of electrodes on the shaft: top-side, back-side, double-side, and sidewall electrodes. These electrodes have different recording characteristics. The in vitro and in vivo (using crayfish and rat brain) experiments in this study shows that the top-side and back-side electrodes are respectively more sensitive to neurons on the top-side and back-side of the probe shaft. In contrast, signals recorded by double-side electrode and sidewall electrode are equally sensitive to neurons around the probe shaft. This study enables the implementation and integration of these four types of electrodes, meeting the requirements of various neural applications.     

The Safety of Using Body-Transmit MRI in Patients with Implanted Deep Brain Stimulation Devices

Background

Deep brain stimulation (DBS) is an established treatment for patients with movement disorders. Patients receiving chronic DBS provide a unique opportunity to explore the underlying mechanisms of DBS using functional MRI. It has been shown that the main safety concern with MRI in these patients is heating at the electrode tips – which can be minimised with strict adherence to a supervised acquisition protocol using a head-transmit/receive coil at 1.5T. MRI using the body-transmit coil with a multi-channel receive head coil has a number of potential advantages including an improved signal-to-noise ratio.

Study outline

We compared the safety of cranial MRI in an in vitro model of bilateral DBS using both head-transmit and body-transmit coils. We performed fibre-optic thermometry at a Medtronic ActivaPC device and Medtronic 3389 electrodes during turbo-spin echo (TSE) MRI using both coil arrangements at 1.5T and 3T, in addition to gradient-echo echo-planar fMRI exposure at 1.5T. Finally, we investigated the effect of transmit-coil choice on DBS stimulus delivery during MRI.

Results

Temperature increases were consistently largest at the electrode tips. Changing from head- to body-transmit coil significantly increased the electrode temperature elevation during TSE scans with scanner-reported head SAR 0.2W/kg from 0.45°C to 0.79°C (p<0.001) at 1.5T, and from 1.25°C to 1.44°C (p<0.001) at 3T. The position of the phantom relative to the body coil significantly impacted on electrode heating at 1.5T; however, the greatest heating observed in any position tested remained <1°C at this field strength.

Conclusions

We conclude that (1) with our specific hardware and SAR-limited protocol, body-transmit cranial MRI at 1.5T does not produce heating exceeding international guidelines, even in cases of poorly positioned patients, (2) cranial MRI at 3T can readily produce heating exceeding international guidelines, (3) patients with ActivaPC Medtronic systems are safe to be recruited to future fMRI experiments performed under the specific conditions defined by our protocol, with no likelihood of confound by inappropriate stimulus delivery.     

The neural basis of the blood-oxygen-level-dependent functional magnetic resonance imaging signal

Magnetic resonance imaging (MRI) has rapidly become an important tool in clinical medicine and biological research. Its functional variant (functional magnetic resonance imaging; fMRI) is currently the most widely used method for brain mapping and studying the neural basis of human cognition. While the method is widespread, there is insufficient knowledge of the physiological basis of the fMRI signal to interpret the data confidently with respect to neural activity. This paper reviews the basic principles of MRI and fMRI, and subsequently discusses in some detail the relationship between the blood-oxygen-level-dependent (BOLD) fMRI signal and the neural activity elicited during sensory stimulation. To examine this relationship, we conducted the first simultaneous intracortical recordings of neural signals and BOLD responses. Depending on the temporal characteristics of the stimulus, a moderate to strong correlation was found between the neural activity measured with microelectrodes and the BOLD signal averaged over a small area around the microelectrode tips. However, the BOLD signal had significantly higher variability than the neural activity, indicating that human fMRI combined with traditional statistical methods underestimates the reliability of the neuronal activity. To understand the relative contribution of several types of neuronal signals to the haemodynamic response, we compared local field potentials (LFPs), single- and multi-unit activity (MUA) with high spatio-temporal fMRI responses recorded simultaneously in monkey visual cortex. At recording sites characterized by transient responses, only the LFP signal was significantly correlated with the haemodynamic response. Furthermore, the LFPs had the largest magnitude signal and linear systems analysis showed that the LFPs were better than the MUAs at predicting the fMRI responses. These findings, together with an analysis of the neural signals, indicate that the BOLD signal primarily measures the input and processing of neuronal information within a region and not the output signal transmitted to other brain regions.     

Magnetic resonance imaging in the assessment and surgical management of epilepsy and functional neurological disorders

Magnetic resonance imaging (MRI) offers significant advantages over computerized tomography (CT) and teleradiographic techniques when used for the evaluation and management of epilepsy and functional neurological disorders. Depth recording and radiofrequency electrodes can be more accurately positioned within structures such as the amygdala and hippocampus. The extent of corpus callosum section, lobectomy, topectomy, and radiofrequency stereotactic lesions can now be readily confirmed and related with seizure, neurological, and behavioral outcome. Occult, usually low grade, intraparenchymal neoplasms not visualized on CT scans can be located by MRI and biopsied or excised by MRI stereotactic techniques.