Parameters for direct cortical electrical stimulation in the human: histopathologic confirmation

Value of electrical stimulation and high frequency oscillations (80-500 Hz) in identifying epileptogenic areas during intracranial EEG recordings

PURPOSE

Electrical stimulation (ES) is used during intracranial electroencephalography (EEG) investigations to delineate epileptogenic areas and seizure-onset zones (SOZs) by provoking afterdischarges (ADs) or patients' typical seizure. High frequency oscillations (HFOs--ripples, 80-250 Hz; fast ripples, 250-500 Hz) are linked to seizure onset. This study investigates whether interictal HFOs are more frequent in areas with a low threshold to provoke ADs or seizures.

METHODS

Intracranial EEG studies were filtered at 500 Hz and sampled at 2,000 Hz. HFOs were visually identified. Twenty patients underwent ES, with gradually increasing currents. Results were interpreted as agreeing or disagreeing with the intracranial study (clinical-EEG seizure onset defined the SOZ). Current thresholds provoking an AD or seizure were correlated with the rate of HFOs of each channel.

RESULTS

ES provoked a seizure in 12 and ADs in 19 patients. Sixteen patients showed an ES response inside the SOZ, and 10 had additional areas with ADs. The response was more specific for mesiotemporal than for neocortical channels. HFO rates were negatively correlated with thresholds for ES responses; especially in neocortical regions; areas with low threshold and high HFO rate were colocalized even outside the SOZ.

DISCUSSION

Areas showing epileptic HFOs colocalize with those reacting to ES. HFOs may represent a pathologic correlate of regions showing an ES response; both phenomena suggest a more widespread epileptogenicity.

Middle short gyrus of the insula implicated in speech production: intracerebral electric stimulation of patients with epilepsy

PURPOSE

Different lines of evidence have suggested an involvement of the insular cortex in speech production. These have included results from lesion studies, functional imaging techniques, and electrical stimulation of the human insular cortex during invasive evaluation of epileptic patients.

METHODS

We evaluated 25 patients who had drug refractory focal epilepsy with at least one electrode stereotactically implanted in the insular cortex.

RESULTS

Eight responses to insular cortex electrical stimulation were reported by five patients as speech arrest (five responses) and a lowering of voice intensity (three responses).

CONCLUSIONS

Data from this study implicate the middle short gyrus of the insula in the production of speech and show the importance of intrainsular electrode implantation during invasive pre-resection evaluation by stereo-electroencephalography (SEEG) when speech arrest occurs early in seizure semiology.

Somatotopic organization of pain responses to direct electrical stimulation of the human insular cortex

The question whether pain encoding in the human insula shows some somatotopic organization is still pending. We studied 142 patients undergoing depth stereotactic EEG (SEEG) exploration of the insular cortex for pre-surgical evaluation of epilepsy. 472 insular electrical stimulations were delivered, of which only 49 (10.5%) elicited a painful sensation in 38 patients (27%). Most sites where low intensity electric stimulation produced pain, without after-discharge or concomitant visually detectable change in EEG activity outside the insula, were located in the posterior two thirds of the insula. Pain was located in a body area restricted to face, upper limb or lower limb for 27 stimulations (55%) and affected more than one of these regions for all others. The insular cortex being oriented parallel to the medial sagittal plane we found no significant difference between body segment representations in the medio-lateral axis. Conversely a somatotopic organization of sites where stimulation produced pain was observed along the rostro-caudal and vertical axis of the insula, showing a face representation rostral to those of upper and lower limbs, with an upper limb representation located above that of the lower limb. These data suggest that, in spite of large and often bilateral receptive fields, pain representation shows some degree of somatotopic organization in the human insula.

Intracranial stimulation therapy for epilepsy

Epilepsy is a common chronic neurological disorder effecting 1 to 2% of the population. Despite advances in anti-epileptic drug therapy, epilepsy surgery, and vagus nerve stimulation, approximately 30% of patients continue to have seizures. Intracranial stimulation is currently under investigation as an adjunctive treatment to anti-epileptic medications in adults with medically intractable epilepsy. Several different approaches are now being investigated. Specifically, acute and long-term clinical studies have delivered stimulation either to inhibitory regions outside the seizure focus or directly to the seizure focus. These studies have demonstrated the safety of intracranial stimulation and proof of principle in epilepsy patients. In addition to the different locations tested, clinical studies have also used different temporal patterns of stimulation. The majority of studies have used open-loop or scheduled stimulation, in which, stimulation is delivered on a fixed schedule and is independent of electrographic activity. In contrast, a number of recent investigations have demonstrated the feasibility of closed-loop or responsive stimulation, which is stimulation that is contingent upon the detection of epileptiform activity. This chapter will review the acute and long-term clinical studies of intracranial stimulation, including focal, and nonfocal, and open-loop and responsive stimulation. We will also discuss the optimization and safety of therapeutic parameters used in neurostimulation for epilepsy.

Middle short gyrus of the insula implicated in pain processing

Different lines of evidence have suggested an involvement of the insular cortex in pain processing. Direct electrical stimulation (ES) of the human insular cortex during surgical procedure sometimes induces painful sensations and painful stimuli induce activation of the insular cortex as shown by functional neuroimaging. Invasive evaluation of epileptic patients by deep brain stereotactically implanted electrodes provides an opportunity to analyze responses induced by ES of the insular cortex in awake and fully conscious patients. For this study, we included 25 patients suffering from drug refractory focal epilepsy with at least one electrode stereotactically implanted in the insular cortex using an oblique approach (transfrontal or transparietal). Out of the 83 responses induced by insular ES, eight (9.6%) were reported by five patients as painful sensations. Four were restricted to the cephalic region and four were felt on the ipsilateral or bilateral upper limbs, the shoulders and the trunk (pinprick sensations). The eight stimulation sites were anatomically localized via image fusion between pre-implantation 3D MRI and post-implantation 3D CT scans revealing the electrode contacts. All sites inducing painful sensations were restricted to the upper portion of the middle short gyrus of the insula. The findings of this study suggest that middle short gyrus is involved in the processing of pain-producing stimuli.

Responsive cortical stimulation for the treatment of epilepsy

Epilepsy is a common chronic neurological disorder affecting approximately 1-2% of the population. Despite the available treatment options (pharmacotherapy, surgery, and vagus nerve stimulation), a large percentage of patients continue to have seizures. With the success of deep brain stimulation for treatment of movement disorders, brain stimulation has received renewed attention as a potential treatment option for epilepsy. Responsive stimulation aims to suppress epileptiform activity by delivering stimulation directly in response to electrographic activity. Animal and human data support the concept that responsive stimulation can abort epileptiform activity, and this modality may be a safe and effective treatment option for epilepsy. Responsive stimulation has the advantage of specificity. In contrast to the typically systemic administration of pharmacotherapy, with the concomitant possibility of side effects, electrical stimulation can be targeted to the specific brain regions involved in the seizure. In addition, responsive stimulation provides temporal specificity. Treatment is provided as needed, potentially reducing the likelihood of functional disruption or habituation due to continuous treatment. Here we review current animal and human research in responsive brain stimulation for epilepsy and then discuss the NeuroPace RNS System, an investigational implantable responsive neurostimulator system that is being evaluated in a multicenter, randomized, double-blinded trial to assess the safety and efficacy of responsive stimulation for the treatment of medically refractory epilepsy.

Frontal and temporal functional connections of the living human brain

Connections between human temporal and frontal cortices were investigated by intracranial electroencephalographic responses to electrical stimulation with 1-ms single pulses in 51 patients assessed for surgery for treatment of epilepsy. The areas studied were medial temporal, entorhinal, lateral temporal, medial frontal, lateral frontal and orbital frontal cortices. Findings were assumed to be representative of human brain as no differences were found between epileptogenic and non-epileptogenic hemispheres. Connections between intralobar temporal and frontal regions were common (43-95%). Connections from temporal to ipsilateral frontal regions were relatively uncommon (seen in 0-25% of hemispheres). Connections from frontal to ipsilateral temporal cortices were more common, particularly from orbital to ipsilateral medial temporal regions (40%). Contralateral temporal connections were rare (< 9%) whereas contralateral frontal connections were frequent and faster, particularly from medial frontal to contralateral medial frontal (61%) and orbital frontal cortices (57%), and between both orbital cortices (67%). Orbital cortex receives profuse connections from the ipsilateral medial (78%) and lateral (88%) frontal cortices, and from the contralateral medial (57%) and orbital (67%) frontal cortices. The high incidence of intralobar temporal connections supports the presence of temporal reverberating circuits. Frontal cortex projects within the lobe and beyond, to ipsilateral and contralateral structures.

Functional interactions in brain networks underlying epileptic seizures in bilateral diffuse periventricular heterotopia

OBJECTIVE

Our aim was to investigate relationships between heterotopic and remote cortical structures at seizure initiation, in a patient with bilateral periventricular nodular heterotopias (BPNH) explored by intracerebral electrodes.

METHODS

Stereoelectroencephalography (SEEG) was performed in a man with BPNH and refractory epilepsy to investigate the hypothesis of right temporal lobe epilepsy and the possible involvement of heterotopic structures during seizures. SEEG signals were analyzed with quantification of functional coupling between different brain structures during seizures, using nonlinear regression. We have used Z-score transformation of correlation values to reflect the change from the preictal period. Relationships between BPNH and cortical structures were investigated using analysis of stimulation-induced potentials.

RESULTS

Three spontaneous seizures were recorded and analyzed. Signal analysis of interdependencies in two seizures demonstrated a large initial network involving both heterotopia and cortical structures. Stimulations of heterotopia induced responses in remote cortical structures.

CONCLUSIONS

Distinct epileptogenic networks were identified, in which leader structures were either the heterotopic or the mesial temporal structures, with functional connections between heterotopic and cortical areas.

SIGNIFICANCE

These results confirm that a vast epileptogenic network, including heterotopic and cortical neurons, may be responsible for seizure generation in BPNH. This may explain certain surgical failures in this group.

Chronic subdural electrodes in the management of epilepsy

Subdural electrodes play a very important role in the evaluation of a percentage of patients being considered for epilepsy surgery. Electrical activity at very low and very high frequencies, beyond the practical range of scalp EEG, can be recorded subdurally and may contain considerable information not available non-invasively. The recording and stimulating procedures for using chronically implanted subdural electrodes to localize the epileptogenic zone and map eloquent functions of the human cortex are well established, and complication rates are low. Complications include infections, CSF leak, and focal neurologic deficits, all of which tend to be increased with a higher number of electrodes and longer duration of recordings. Careful consideration of the risks and benefits should be coupled with a firm hypothesis about the epileptogenic zone derived from the non-invasive components of the epilepsy workup to guide the decision about whether and where to implant subdural electrodes. When they are employed to answer a specific question in an individual patient, subdural electrodes can optimize the clinical outcome of a candidate for epilepsy surgery.

Cortical language mapping in epilepsy: a critical review

One challenge in dominant hemisphere epilepsy surgery is to remove sufficient epileptogenic tissue to achieve seizure freedom without compromising postoperative language function. Electrical stimulation mapping (ESM) of language was developed specifically to identify essential language cortex in pharmacologically intractable epilepsy patients undergoing left hemisphere resection of epileptogenic cortex. Surprisingly, the procedure remains unstandardized, and limited data support its clinical validity. Nevertheless, ESM for language mapping has likely minimized postoperative language decline in numerous patients, and has generated a wealth of data elucidating brain-language relations. This article reviews the literature on topographical patterns of language organization inferred from ESM, and the influence of patient characteristics on these patterns, including baseline ability level, age, gender, pathology, degree of language lateralization and bilingualism. Questions regarding clinical validity and limitations of ESM are discussed. Finally, recommendations for clinical practice are presented, and theoretical questions regarding ESM and the findings it has generated are considered.

Changes of EEG synchronization during low-frequency electric stimulation of the seizure onset zone

PURPOSE

To assess whether EEG synchronization changes during short-term low-frequency electrical stimulation of the seizure onset zone.

METHODS

In 10 patients (34+/-11 years) with pharmaco-resistant epilepsy the seizure onset zone (9 temporal lobe, 1 frontal lobe) was electrically stimulated at 1Hz for 5min via intracranial electrodes. Bipolar stimuli were applied and four pulse widths (0.05, 0.1, 0.5, and 1.0ms) were tested. Stimulation amplitudes were held fixed at 1mA for strip electrodes and at 2mA for depth electrodes. Changes of EEG synchronization were assessed by the eigenvalue dynamics of the cross-correlation matrix computed from a 2.5s sliding window.

RESULTS

37 stimulations were performed. We observed EEG desynchronization in 49% (18/37), an increase of EEG synchronization in 27% (10/37) and an EEG pattern with no significant change of synchronization in 24% (9/37). EEG synchronization most frequently occurred when stimulating with a pulse width of 0.5ms. In a patient with bilateral independent seizure onsets stimulation effects on EEG synchronization were different for each side. In the patient with the shortest duration of temporal lobe epilepsy, stimulation triggered periodic epileptic spikes phase-locked to stimulation. One patient experienced an aura during stimulation, which did not evolve into a seizure, and in one patient a sub-clincial seizure occurred.

DISCUSSION

Low-frequency stimulation of the seizure onset zone is associated with different changes of EEG synchronization and its effects depend on the widths of the stimulation pulses. It may be an appropriate stimulation technique for long-term studies assessing whether synchronized or desynchronized brain dynamics prevent seizure occurrence.

Variability in cortical representations of speech sound perception

Recent brain mapping studies have provided new insights into the cortical systems that mediate human speech perception. Electrocortical stimulation mapping (ESM) is a brain mapping method that is used clinically to localize cortical functions in neurosurgical patients. Recent ESM studies have yielded new insights into the cortical systems that mediate speech perception and how these systems vary as a function of individual differences. ESM methods are described and findings from recent ESM studies of speech perception are reviewed. The clinical implications of these findings are discussed as they relate to current understanding of how individual differences in listening abilities are reflected in the underlying cortical representations.

The effectiveness of low-frequency stimulation for mapping cortical function

OBJECTIVE

To establish the efficacy and safety of low-frequency electrical stimulation for cortical brain mapping.

METHODS

Cortical function was mapped using electrical stimulation in epilepsy patients with chronically implanted intracranial subdural electrodes. Contacts overlying motor, sensory, visual, and language cortex were stimulated at frequencies of 5, 10, and 50 Hz, using current levels ranging from 1 to 17.5 mA for 3-5 s. The current intensity and incidence at which functional alterations and afterdischarges (ADs) occurred were recorded. The modified McNemar test for nonindependent measures was used to analyze the data.

RESULTS

122 electrode contact pairs were electrically stimulated at least two different frequencies in 14 patients. Functional alterations were obtained at all stimulation frequencies (5, 10, and 50 Hz) at generally similar rates. The likelihood of producing an AD correlated with stimulation frequency, and lower-frequency stimulation was less likely to provoke an AD. Higher current intensity was required to induce both functional responses and ADs at low-frequency stimulation than high-frequency stimulation. While overall rates of producing functional changes were similar, differences in functional response with regard to frequency were noted at individual cortical sites.

CONCLUSION

5- and 10-Hz stimulation are as effective for mapping cortical function as 50-Hz stimulation and produce fewer ADs. We recommend that mapping of cortical function be started with 5-Hz-frequency stimulation. Higher frequencies should be used in suspect cortex if no symptoms or signs are produced with 5-Hz stimulation.

Young patients with focal seizures may have the primary motor area for the hand in the postcentral gyrus

OBJECTIVE

We determined whether the primary motor hand area was most frequently located in the precentral gyrus in young patients with intractable focal seizures.

METHODS

Sixty-five patients with focal seizures aged between 5 months and 20 years who underwent a two-stage epilepsy surgery using chronic subdural-EEG monitoring were studied. Pairs of subdural electrodes were electrically stimulated, and the brain region with contralateral hand movement induced by the lowest-intense stimulus was defined as the primary motor hand area.

RESULTS

Contralateral hand movement was induced without afterdischarges in 50 children but not in the remaining 15 children. The unpaired t-test revealed that failure to induce contralateral hand motor movement was associated with younger age of subjects. Among the 50 patients with a positive motor response, the primary motor hand area was confined to the precentral gyrus in 9 patients, confined to the postcentral gyrus in 24, and located in both the pre- and post-central gyri in the remaining 17. The McNemar's test revealed that the observed frequency of 24 patients showing the primary motor hand area confined to the postcentral gyrus was larger than chance frequency. Logistic regression analysis failed to demonstrate that the observation of the primary motor hand area confined to the postcentral gyrus was associated with the age, the presence of dysplastic lesion or the seizure onset involving the frontal lobe.

CONCLUSION

Our study failed to support the traditionally-accepted notion that the primary motor hand area is most frequently located in the precentral gyrus but rather demonstrated that a substantial proportion of young patients had the primary motor hand area in the postcentral gyrus.

Electrical stimulation of the hippocampal epileptic foci for seizure control: a double-blind, long-term follow-up study

PURPOSE

Our aim was to evaluate the safety and efficacy of electrical stimulation of the hippocampus in a long-term follow-up study, as well as its impact on memory performance in the treatment of patients with refractory mesial temporal lobe epilepsy.

METHODS

Nine patients were included. All had refractory partial complex seizures, some with secondary generalizations. All patients had a 3-month-baseline-seizure count, after which they underwent bilateral hippocampal diagnostic electrode implantation to establish focus laterality and location. Three patients had bilateral, and six, unilateral foci. Diagnostic electrodes were explanted and definitive Medtronic electrodes were implanted directed into the hippocampal foci. Position was confirmed with MRI and afterwards, the deep brain stimulation system internalized. Patients signed the informed consent approved by the Hospital's Ethics Committee and began a double-blind stimulation protocol. Patients attended a medical appointment every 3 months for seizure diary collection, deep brain stimulation system checkup, and neuropsychological testing.

RESULTS

Follow-up ranged from 18 months to 7 years. Patients were divided in two groups: five had normal MRIs and seizure reduction of >95%, while four had hippocampal sclerosis and seizure reduction of 50-70%. No patient had neuropsychological deterioration, nor did any patient show side effects. Three patients were explanted after 2 years due to skin erosion in the trajectory of the system.

CONCLUSIONS

Electrical stimulation of the hippocampus provides a nonlesional method that improves seizure outcome without memory deterioration in patients with hippocampal epileptic foci.

Long-term electrical stimulation-induced inhibition of partial epilepsy. Case report

The aim of this study was to determine the effects of long-term continuous cerebrocortical electrical stimulation in the treatment of partial epilepsy. The authors review the case of a 44-year-old man with medically intractable postencephalitic localization-related epilepsy with ictal onset in the primary motor cortex. For 5 years he was treated using patterned subthreshold electrical stimulation of the ictal site. This therapy has successfully eliminated the jacksonian march of cortical excitability and secondary generalization and reduced seizure frequency and intensity with an immediate postictal return of motor function. Over time, the seizure frequency subsided by more than 90%, with the patient showing no adverse features resulting from focal stimulation. The results in this case support the hypothesis that effective and safe long-term modulation of focal epilepsy is possible with focal cerebrocortical electrical stimulation.

High-frequency monopolar electrical stimulation of the rat cerebral cortex

OBJECTIVE

Intraoperative monitoring of the motor-evoked potential has been widely used in patients undergoing neurosurgery. Direct stimulation of the brain with high-frequency monopolar stimulation (HFMS) is one of the most common methods to produce motor-evoked potential. We studied the influence of HFMS on the rat cerebral cortex.

METHODS

We applied 1.5, 15, 30, 40, or 50 mA of HFMS to the rat sensorimotor cortex by a short sequence of five monopolar, monophasic, anodal rectangular 500-Hz pulses. We delivered one short five-pulse train 100 times every 5 seconds and examined pre- and post-stimulation electroencephalograms and histological changes at the stimulation site.

RESULTS

We observed no spike waves after HFMS in any of the rats. There was no change in the power spectrum or frequency content in any of the rats exposed to HFMS. Histologically, there was significant swelling of the dendrites in rats sacrificed immediately after exposure to 40- and 50-mA stimulation; the 50-mA stimulation group also exhibited slight swelling of the mitochondria. These findings were not obtained in any of the rats sacrificed 30 days after stimulation.

CONCLUSION

In rats exposed to a stimulation intensity of 30-mA or less, no morphological or electrophysiological changes were observed. However, the possibility that HFMS may affect neural tissue cannot be ruled out.

Pediatric Language Mapping: Sensitivity of Neurostimulation and Wada Testing in Epilepsy Surgery

PURPOSE

Functional mapping of eloquent cortex with electrical neurostimulation is used both intra- and extraoperatively to tailor resections. In pediatric patients, however, functional mapping studies frequently fail to localize language. Wada testing has also been reported to be less sensitive in children.

METHODS

Thirty children (4.7 - 14.9 years) and 18 adult controls (18-59 years) who underwent extraoperative language mapping via implanted subdural electrodes at the NYU Comprehensive Epilepsy Center were included in the study. Ten children and 14 adults underwent preoperative Wada testing. Success of the procedures was defined as the identification of at least one language site by neurostimulation mapping and determination of hemispheric language dominance on the Wada test.

RESULTS

In children younger than 10.2 years, cortical stimulation identified language cortex at a lower rate than was seen in children older than 10.2 years and in adults (p<0.05). This threshold, demonstrated by survival and chi2 analysis, was sharply defined in our data set. Additionally, Wada testing was more likely to be successful than was extraoperative mapping in this younger age group (p<0.05).

CONCLUSIONS

Analysis of our series demonstrates that language cortex is less likely to be identified in children younger than 10 years, suggesting that alternatives to the current methods of cortical electrical stimulation, particularly the use of preoperative language lateralization, may be required in this age group.

Intraoperative Risk of Seizures Associated With Transient Direct Cortical Stimulation in Patients With Symptomatic Epilepsy

Direct cortical stimulation--either with the 60-Hz stimulation or the train-of-five technique--is commonly agreed on being the gold standard for intraoperative mapping of the motor cortex and the motor pathways but may result in an intraoperative seizure. The occurrence of intraoperative stimulation associated seizures with respect to symptomatic epilepsy was evaluated in a group of 129 patients undergoing tumor resection within the central region. Data were reviewed with respect to the frequency of seizures with both stimulation techniques and symptomatic epilepsy. Direct stimulation of the motor cortex was performed with a train of five consecutive pulses, an interstimulus interval of 4 ms, an individual pulse width of 0.5 ms, and 40 mA stimulation intensity at maximum. In 1 of 63 patients (1.6%) presenting with symptomatic epilepsy, a stimulation-associated seizure occurred, and 1 of the other 66 patients (1.5%) had a seizure (n.s., not significant). In the literature, stimulation associated seizures are reported in 1.2% with the train-of-five technique and significantly more frequently in 9.5% with the 60-Hz technique (P < 0.001). In summary, there is no increased risk of the occurrence of stimulation-associated seizures during surgery for patients with symptomatic epilepsy compared with those patients without.

Stereo-EEG in children

BACKGROUND

Stereotactic placement of intracerebral multilead electrodes for chronic EEG recording of seizures or stereoelectroencephalography (SEEG) was introduced 50 years ago at Saint Anne Hospital in Paris, France for the presurgical evaluation of patients with drug-resistant focal epilepsy. SEEG explorations are indicated whenever the noninvasive tests fail to adequately localize the epileptogenic zone (EZ).

INDICATIONS

Currently, approximately 35% of our operated-on children require a SEEG evaluation. Arrangement of electrodes is individualized according to the peculiar needs of each child, to verify a predetermined hypothesis of localization of the EZ based on pre-SEEG anatomo-electro-clinical findings. Multilead intracerebral electrodes are designed to sample cortical structures on the lateral, intermediate, and mesial aspect of the hemisphere, as well as deep-seated lesions. Stereotactic stereoscopic teleangiograms and coregistered 3-D MRI are employed to plan avascular trajectories and to accurately target the desired structures. Pre-SEEG stereotactic neuroradiology and electrode implantation are usually performed in separate procedures. Electrodes are removed once video-SEEG monitoring is completed.

INTRACEREBRAL ELECTRICAL STIMULATIONS

Intracerebral electrical stimulations are used to better define the EZ and to obtain a detailed functional mapping of critical cortical and subcortical regions.

MORBIDITY

Surgical morbidity of SEEG is definitely low in children. SEEG-GUIDED RESECTIVE SURGERY: In 90% of evaluated children, SEEG provides a guide for extratemporal or multilobar resections. SEEG-guided resective surgery may yield excellent results on seizures with 60% of patients in Engel's Class I.

Intraoperative Motor Evoked Potential Monitoring: Overview and Update

Amidst controversy about methodology and safety, intraoperative neurophysiology has entered a new era of increasingly routine transcranial and direct electrical brain stimulation for motor evoked potential (MEP) monitoring. Based on literature review and illustrative clinical experience, this tutorial aims to present a balanced overview for experienced practitioners, surgeons and anesthesiologists as well as those new to the field. It details the physiologic basis, indications and methodology of current MEP monitoring techniques, evaluates their safety, explores interpretive controversies and outlines some applications and results, including aortic aneurysm, intramedullary spinal cord tumor, spinal deformity, posterior fossa tumor, intracranial aneurysm and peri-rolandic brain surgeries. The many advances in motor system assessment achieved in the last two decades undoubtedly improve monitoring efficacy without unduly compromising safety. Future studies and experience will likely clarify existing controversies and bring further advances.

Mutually suppressive interrelations of symmetric epileptic foci in bitemporal epilepsy and their inhibitory stimulation

OBJECTIVES

The goal of this study is to analyze the suppressive interaction of symmetric temporal lobe epileptic foci, assess some failures of epilepsy surgery, and evaluate the possibility of terminating focal seizures with stimulation of symmetric epileptic foci.

MATERIALS AND METHODS

One hundred and twenty-nine intractable epilepsy patients (age range 6-53 years) with bitemporal epileptiform abnormalities in multiple scalp EEGs were evaluated with chronically implanted depth and subdural electrodes. Interelectrode coherence and power spectra were studied using internally developed software.

RESULTS

Bitemporal epileptic foci were found in 85/129 (66%) patients with reciprocal relations between these foci in 57/85 (67%) patients. Temporal lobectomy was performed for 67/85 patients. 12/67 patients became free of seizures (Engel's Class I), 32/67 improved (Classes II and III), and 23/67 did not improve. 14/23 patients demonstrated post-surgical activation of the contralateral temporal lobe epileptic focus. For 8/14 of these patients, the stereotactic cryoamygdalatomy was performed in the temporal lobe contralateral to the first surgery. 5/8 patients became free of seizures. It was found that stimulation of temporal lobe deep epileptic focus may terminate focal seizures in the contralateral symmetric structures.

CONCLUSION

A mutually suppressive relationship is one of variants of the interaction of symmetric epileptic foci. Some epilepsy surgery failures may be a result of post-surgical activation of the intact focus. The increase of coherence between both temporal lobes before the seizure onset of the seizure suggests the establishment of functional interrelations between two epileptic foci at an early, "hidden" phase of seizures, and may predict the direction of seizure spread. Mutually suppressive interrelations of symmetric epileptic foci might be employed for chronic therapeutic stimulation.

Brain stimulation for epilepsy: can scheduled or responsive neurostimulation stop seizures?

PURPOSE OF REVIEW

Scheduled and responsive direct brain stimulation may be an effective and safe therapy for medically intractable epilepsy.

RECENT FINDINGS

Scheduled stimulation (open loop) has been provided via electrodes implanted in thalamic nuclei, the cerebellum and the hippocampus using devices commercially available for treatment of tremor and Parkinson's disease. Small pilot trials suggest that seizure frequency is reduced in some patients with intractable epilepsy. Responsive stimulation requires systems that detect abnormal electrographic activity and provide stimulation (closed loop). Studies in inpatients and outpatients suggest that abnormal electrographic discharges can be detected before there is evolution into a clinical seizure, and that focal stimulation of the epileptogenic region terminates electrographic seizures and reduces the frequency of clinically evident seizures.

SUMMARY

Direct brain stimulation appears to be safe and may be efficacious in treating medically intractable epilepsy. The optimal location (deep brain or cortical) and characteristics of the stimulation (frequency, current, duration), and whether stimulation should be focal or responsive are still to be determined. If ongoing studies of a deep brain stimulator and of a cranially implanted responsive neurostimulator demonstrate effectiveness, then neurostimulation may become available as adjunctive therapy for medically intractable epilepsy.

Bifurcation control of a seizing human cortex

We consider as a mathematical model of human cortical electrical activity a system of fourteen ordinary differential equations. With appropriate parameters, the model produces activity characteristic of a seizure. To prevent such seizures, we incorporate feedback controllers into the model dynamics. We show that three controllers--a linear feedback controller, a differential controller, and a filter controller--can be used to eliminate seizing activity in the model system. We show how bifurcations induced by the linear controller alter those present in the original dynamics.

Feasibility of extraocular stimulation for a retinal prosthesis

BACKGROUND

We present a new approach to developing a retinal prosthesis for blind patients based on extraocular stimulation of the eye with disc electrodes.

METHODS

Experiments to assess the feasibility of using extraocular stimulation in a retinal prosthesis were carried out in anaesthetised adult cats (n=6). A craniotomy and lateral orbital dissection were performed. Ball or disc electrodes were placed on the posterior scleral surface of the eye after incision of the periorbita. Cortical potentials evoked by electrical stimulation with these electrodes were recorded at the primary visual cortex. The viability of adapting the Nucleus 24 auditory brainstem implant (ABI) as an extraocular retinal prosthesis was also investigated.

RESULTS

Electrodes placed on the exterior of the eye could reliably evoke visual cortex responses for a variety of configurations. Threshold currents for eliciting an evoked response were lower than 100 microA with single pulses. Strength-duration curves and cortical activation maps were obtained for different stimulus paradigms. It was possible to excite the retina to evoke a cortical response using the electrodes and stimulus capabilities in a standard Nucleus 24 ABI.

INTERPRETATION

It is possible to electrically stimulate the retina with electrodes placed in an extraocular location. Threshold currents required to elicit a response were low, and comparable to epiretinal implants. Prototype electrodes, and a potential implant, were found to be effective at retinal stimulation.

Intracerebral dynamics of saccade generation in the human frontal eye field and supplementary eye field

Recent functional imaging and electrical stimulation studies have localized in humans two frontal regions critical for the production of saccadic and anti-saccadic eye movements: the frontal and supplementary eye fields (FEF and SEF, respectively). We investigated the time course of their activations during the generation of pro- and anti-saccades from direct intracranial EEG recordings of three human epileptic patients. We found the preparation and the production of the saccades to be coincident with focal and transient increases of EEG power above 60 Hz. Those were produced in very specific brain sites distributed in the FEF and the SEF (as identified by previous human studies at a coarser time resolution). Furthermore, the spatio-temporal resolution of those recordings turned out to be sufficient to discriminate anatomically between several types of neural responses, determined either by the visual or by the motor components of the saccade tasks, and within this second category of responses, between some associated with the preparation of the saccades and others associated with their execution. Altogether, this study provides the first evidence of high-frequency neural responses in the generation of saccades in humans, and provides a firm basis for other studies detailing further the functional organization of the human oculomotor system at this level of spatial and temporal resolution.

Somatosensory and pain responses to stimulation of the second somatosensory area (SII) in humans. A comparison with SI and insular responses

Somatosensory and pain responses to direct intracerebral stimulations of the SII area were obtained in 14 patients referred for epilepsy surgery. Stimulations were delivered using transopercular electrodes exploring the parietal opercular cortex (SII area), the suprasylvian parietal cortex (SI area) and the insular cortex. SII responses were compared to those from adjacent SI and insular cortex. In the three areas we elicited mostly somatosensory responses, including paresthesiae, temperature and pain sensations. The rate of painful sensations (10%) was similar in SII and in the insula, while no painful sensation was evoked in SI. A few non-somatosensory responses were evoked by SII stimulation. Conversely various types of non-somatosensory responses (auditory, vegetative, vestibular, olfacto-gustatory, etc.) were evoked only by insular stimulation, confirming that SII, like SI, are mostly devoted to the processing of somatosensory inputs whereas the insular cortex is a polymodal area. We also found differences in size and lateralization of skin projection fields of evoked sensations between the three studied areas, showing a spatial resolution of the somatotopic map in SII intermediate between those found in SI and insula. This study shows the existence of three distinct somatosensory maps in the suprasylvian, opercular and insular regions, and separate pain representations in SII and insular cortex.

Hippocampal-orbitofrontal connectivity in human: an electrical stimulation study

OBJECTIVE

The identification of the pathways involved in seizure propagation remains poorly understood in humans. For instance, the respective role of the orbitofrontal cortex (OFC) and of the commissural pathways in the interhemispheric propagation of mesial temporal lobe seizures (mTLS) is a matter of debate. In order to address this issue, we have directly tested the functional connectivity between the hippocampus and the OFC in 3 epileptic patients undergoing an intra-cranial stereotactic EEG investigation.

METHODS

Bipolar electrical stimulations, consisting of two series of 25 pulses of 1 ms duration, 0.2 Hz frequency, and 3 mA intensity, were delivered in the hippocampus. Evoked potentials (EPs) were analysed for each series, separately. Grand average of reproducible EPs was then used to calculate latency of the first peak of each individual potential.

RESULTS

Hippocampal stimulations evoked reproducible responses in the OFC in all 3 patients, with a mean latency of the first peak of 222 ms (range: 185-258 ms).

CONCLUSIONS

Our data confirm a functional connectivity between the hippocampus and the OFC in human.

SIGNIFICANCE

This connectivity supports the potential role of the OFC in the propagation of mTLS.

Evidence for Corticofugal Modulation of Peripheral Auditory Activity in Humans

Active cochlear micromechanisms, involved in auditory sensitivity, are modulated by the medial olivocochlear efferent system, which projects directly onto the organ of Corti. Both processes can be assessed non-invasively by means of evoked otoacoustic emissions. Animal experiments have revealed top-down control from the auditory cortex to peripheral auditory receptor, supported by anatomical descriptions of descending auditory pathways from auditory areas to the medial olivocochlear efferent system and organ of Corti. Through recording of evoked otoacoustic emissions during presurgical functional brain mapping for refractory epilepsy, we showed that corticofugal modulation of peripheral auditory activity also exists in humans. In 10 epileptic patients, electrical stimulation of the contralateral auditory cortex led to a significant decrease in evoked otoacoustic emission amplitude, whereas no change occurred under stimulation of non-auditory contralateral areas. These findings provide evidence of a cortico-olivocochlear pathway, originating in the auditory cortex and modulating contralateral active cochlear micromechanisms via the medial olivocochlear efferent system, in humans.

Experimental Electrical Stimulation Therapy for Epilepsy

Electrical stimulation of the nervous system is an attractive possible therapy for intractable epilepsy, but only stimulation of the vagus nerve has been subjected to large, controlled, and completed clinical trials. Controlled trials are in progress for intermittent cycling stimulation of the anterior nuclei of the thalamus, and for cortical stimulation at a seizure focus, responsive to detection of seizure onset. Anecdotal experience has been gathered with stimulation of cerebellum, centromedian thalamus, subthalamus, caudate, hippocampus, and brainstem. All stimulation of the central nervous system for epilepsy must be considered experimental.

[Peroperative functional mapping using direct electrical stimulations. Methodological considerations]

The interindividual anatomo-functional variability of the central nervous system implies that brain surgery within eloquent regions may induce neurological sequelae. Consequently, several methods of functional mapping were developed, both preoperative non-invasive neurofunctional imaging and intraoperative cortico-subcortical electrical stimulations. While this technique was reported as safe, accurate, reliable and reproducible in the recent literature, a rigorous methodology is nevertheless mandatory in order to avoid any error in the detection of the so-called eloquent structures. Indeed, an erroneous mapping could lead to prematurely interrupting the resection (false positive), or to generate a postoperative permanent deficit due to the removal of critical areas interpreted as non-essential to function (false negative). The goal of the present review is to recall the electrophysiological principles of direct brain stimulations, and to consider the selection of stimulation parameters according to a theoretical approach, in order to adapt in practice the methods to each patient. The results reported in the literature are then analyzed, concerning the clinico-surgical contribution of intraoperative electrical mapping (in terms of extent of surgical indications, minimization of risk of permanent deficit, and quality of resection), their methodological interest (e.g. validation of neurofunctional imaging techniques), and their contribution to neurosciences (better understanding of the pathophysiology of brain structures, of the connectivity, and of the dynamic mechanisms underlying plasticity).

Evaluation of extraocular electrodes for a retinal prosthesis using evoked potentials in cat visual cortex

OBJECTIVE

To assess the efficacy of a device using extraocular electrodes as a retinal prosthesis by evaluating the responses evoked in the visual cortex to electrical stimulation.

METHODS

In anaesthetised cats, a lateral orbital dissection and ipsilateral parietal craniotomy was performed. Two extraocular retinal prosthesis (ERP) disc electrodes were sutured to the sclera on the lateral and superior aspects of the globe. Retinal stimulation was performed with charge-balanced constant-current pulses. Potentials evoked in the visual cortex were measured with a ball electrode placed on the lateral gyrus after removal of the dura.

RESULTS

Stable attachment of the ERP electrodes to the globe was achieved with scleral sutures. Visual cortex responses were recorded with the electrodes in bipolar and monopolar configurations. The evoked response consisted of an early component with a peak around 8 ms, and a late component with a peak after 50 ms. Thresholds for evoking a response occurred at current intensities as low as 500 microA. Through extrapolation from evoked response amplitude data, thresholds as low as 300 microA were calculated. Cathodal monopolar stimulation demonstrated lower thresholds than anodal stimulation for evoking cortical responses.

CONCLUSIONS

The ERP electrodes can be easily attached to the globe and are effective in electrically stimulating the retina, evoking responses in the primary visual cortex. Threshold charge-density was within safe limits for neural stimulation.

Transcranial magnetic stimulation in persons younger than the age of 18

OBJECTIVES

To review the use of transcranial magnetic stimulation (single-pulse TMS, paired TMS, and repetitive TMS [rTMS]) in persons younger than the age of 18 years. I discuss the technical differences, as well as the diagnostic, therapeutic, and psychiatric uses of TMS/rTMS in this age group.

METHODS

I evaluated English-language studies from 1993 to August 2004 on nonconvulsive single-pulse, paired, and rTMS that supported a possible role for the use of TMS in persons younger than 18. Articles reviewed were retrieved from the MEDLINE database and Clinical Scientific index.

RESULTS

The 48 studies reviewed involved a total of 1034 children ages 2 weeks to 18 years; 35 of the studies used single-pulse TMS (980 children), 3 studies used paired TMS (20 children), and 7 studies used rTMS (34 children). Three studies used both single and rTMS. However, the number of subjects involved was not reported.

CONCLUSIONS

Single-pulse TMS, paired TMS, and rTMS in persons younger than 18 has been used to examine the maturation/activity of the neurons of various central nervous system tracts, plasticity of neurons in epilepsy, other aspects of epilepsy, multiple sclerosis, myoclonus, transcallosal inhibition, and motor cortex functioning with no reported seizure risk. rTMS has been applied to psychiatric disorders such as ADHD, ADHD with Tourette's, and depression. Adult studies support an antidepressant effect from repetitive TMS, but there is only one study that has been reported on 7 patients that used rTMS to the left dorsal prefrontal cortex on children/adolescents with depression (5 of the 7 subjects treated responded). Although there are limited studies using rTMS (in 34 children), these studies did not report significant adverse effects or seizures. Repetitive TMS safety, ethical, and neurotoxicity concerns also are discussed.

Electric cortical stimulation suppresses epileptic and background activities in neocortical epilepsy and mesial temporal lobe epilepsy

OBJECTIVE

To evaluate the suppressive effect of electric cortical stimulation upon the seizure onset zone and the non-epileptic cortex covered by subdural electrodes in patients with neocortical epilepsy and mesial temporal lobe epilepsy (MTLE).

METHODS

Four patients with medically intractable focal epilepsy had implanted subdural electrodes for preoperative evaluation. Cortical functional mapping was performed by intermittently repeating bursts of electric stimulation, which consisted of 50 Hz alternating square pulse of 0.3 ms duration, 1-15 mA, within 5 s. The effect of this stimulation on the seizure onset zones and on the non-epileptic areas was evaluated by comparing spike frequency and electrocorticogram (ECoG) power spectra before and after stimulation. A similar comparison was performed in stimulation of 0.9 Hz of the seizure onset zones for 15 min.

RESULTS

When the seizure onset zone was stimulated with high frequency, spike frequency decreased by 24.7%. Logarithmic ECoG power spectra recorded at stimulated electrode significantly decreased in 10-32 Hz band by high frequency stimulation of the seizure onset zone, and in 14-32 Hz band by high frequency stimulation of the non-epileptic area. Low frequency stimulation of the seizure onset zone produced 18.5% spike reduction and slight power decrease in 12-14 Hz.

CONCLUSIONS

Both high and low frequency electric cortical stimulation of the seizure onset zone have a suppressive effect on epileptogenicity. Reduction of ECoG fast activities after electric cortical stimulation suggests the augmentation of inhibitory mechanisms in human cortex.

Electrical brain-stimulation paradigm for estimating the seizure onset site and the time to ictal transition in temporal lobe epilepsy

OBJECTIVE

To explore and validate a novel stimulation and analysis paradigm proposed to monitor spatial distribution and temporal changes of the excitability state in patients with temporal lobe epilepsy (TLE).

METHODS

We use intermittent pulse stimulation in the frequency range 10-20Hz. A quantitative measure of spectral phase de-modulation, the relative phase clustering index (rPCI) was applied to the evoked EEG signals, measured from electrodes implanted in the hippocampal formation.

RESULTS

We found that in the interictal periods, high values of rPCI recorded from specific sites were correlated with the most probable seizure onset sites (SOS). Furthermore we found that high values of rPCI from certain locations correlated with shorter time intervals to the next seizure.

CONCLUSIONS

Our clinical findings indicate that although the precise moment of ictal transitions is in general unpredictable, it may be possible to estimate the probability of occurrence of some epileptic seizures.

SIGNIFICANCE

The use of the rPCI for probabilistic forecasting of upcoming epileptic seizures is warranted. rPCI measurements may be used to guide interventions with the aim of modifying local tissue excitability that ultimately might prevent ictal transitions.

Intracerebral study of gamma rhythm reactivity in the sensorimotor cortex

The generators and functional correlates of gamma oscillations within the sensorimotor cortex remain unclear. With the goal of locating the oscillations' sources precisely and then studying the relationship between oscillatory reactivity and ongoing movement, we recorded stereoelectroencephalograms with intracerebral electrodes in eight epileptic subjects awaiting surgical treatment. The sensorimotor cortex was free of lesions and was exhaustively explored with the electrodes. Subjects were asked to perform various self-paced movements contralateral to the exploration zone, brief and sustained, distal movements and a pointing movement. We used the event-related desynchronization method to quantify the reactivity of the 40-60-Hz band before, during and after the performance of movement. A very focused, event-related synchronization of gamma rhythms was found in all subjects. It was predominantly observed in the primary sensorimotor area and its distribution was consistent with the functional map established using electrical stimulations. Two different temporal patterns were observed, the event-related synchronization of gamma rhythms was related either to movement onset or to movement offset but was never recorded before movement. This observation suggests that gamma oscillations are more probably related to movement execution than to motor planning. The different patterns argue in favour of multiple functional roles; it has been shown that gamma oscillations may support the efferent drive to the muscles and here we show that they are also likely to be related to somatosensory integration. We therefore suggest that gamma oscillations in the 40-60-Hz band may support afferent sensory feedback to the sensorimotor cortex during the performance of movement.

Electric Stimulation on Human Cortex Suppresses Fast Cortical Activity and Epileptic Spikes

PURPOSE

To investigate underlying mechanisms and adequate parameters for electric cortical stimulation to inhibit epileptic focus in humans.

METHODS

A patient with intractable partial epilepsy had subdural electrodes implanted for preoperative evaluation. Cortical functional mapping was performed by using 50-Hz alternating square pulse of 0.3-ms duration, 1 to 7 mA, within 5 s. Spike frequency and electrocorticogram (ECoG) power spectra were compared before and after the stimulation when epileptic focus and distant area were stimulated. A similar comparison also was performed in low-frequency stimulation of 0.9 Hz applied for 15 min.

RESULTS

Interictal spikes were reduced after electric cortical stimulation of the epileptic area at a frequency of 50 Hz as well as 0.9 Hz, with concomitant decrease in the electrographic fast activities at 50-Hz stimulation.

CONCLUSIONS

These data suggest that electric cortical stimulation at both high and low frequency has a suppressive effect on epileptic activities in human cortex, possibly through distinct mechanisms.

Material-specific recognition memory deficits elicited by unilateral hippocampal electrical stimulation

Although the medial temporal lobe is thought to be critical for recognition memory (RM), the specific role of the hippocampus in RM remains uncertain. We investigated the effects of transient unilateral hippocampal electrical stimulation (ES), subthreshold for afterdischarge, on delayed item RM in epilepsy patients implanted with bilateral hippocampal depth electrodes. RM was assessed using a novel computer-controlled test paradigm in which ES to left or right hippocampus was either absent (baseline) or synchronized with item presentation. Subsequent yes-no RM performance revealed a double dissociation between material-specific RM and the lateralization of ES. Left hippocampal ES produced word RM deficits, whereas right hippocampal ES produced face RM deficits. Our findings provide the first demonstration in humans that selective unilateral stimulation-induced hippocampal disruption is sufficient to produce impairments on delayed RM tasks and provide support for the material-specific laterality of hippocampal function with respect to RM.

Cortical bases of speech perception:evidence from functional lesion studies

Functional lesion studies have yielded new information about the cortical organization of speech perception in the human brain. We will review a number of recent findings, focusing on studies of speech perception that use the techniques of electrocortical mapping by cortical stimulation and hemispheric anesthetization by intracarotid amobarbital. Implications for recent developments in neuroimaging studies of speech perception will be discussed. This discussion will provide the framework for a developing model of the cortical circuitry critical for speech perception.

Reappraisal of the human vestibular cortex by cortical electrical stimulation study

The cortical areas with vestibular input in humans were assessed by electrical stimulation in 260 patients with partial epilepsy who had undergone stereotactic intracerebral electroencephalogram recordings before surgery. Vestibular symptoms were electrically induced on 44 anatomical sites in 28 patients. The patients experienced illusions of rotation (yaw plane: 18, pitch plane: 6, roll plane: 6), translations (n = 6), or indefinable feelings of body motion (n = 8). Almost all vestibular sites were located in the cortex (41/44): in the temporal (n = 19), parietal (n = 14), frontal (n = 5), occipital (n = 2), and insular (n = 1) lobes. Among these sites, we identified a lateral cortical temporoparietal area we called the temporo-peri-Sylvian vestibular cortex (TPSVC), from which vestibular symptoms, and above all rotatory sensations, were particularly easily elicited (24/41 cortical sites, 58.5%). This area extended above and below the Sylvian fissure, mainly inside Brodmann areas 40, 21, and 22. It included the parietal operculum (9/24 TPSVC sites) which was particularly sensitive for eliciting pitch plane illusions, and the mid and posterior part of the first and second temporal gyri (15/24 TPSVC sites) which preferentially caused yaw plane illusions. We suggest that the TPSVC could be homologous with the monkey's parietoinsular vestibular cortex.

Prefrontal cortex transcranial magnetic stimulation does not change local diffusion: a magnetic resonance imaging study in patients with depression

OBJECTIVE

To determine whether transcranial magnetic stimulation over the left dorsolateral prefrontal cortex produces pathologic changes or leakage of the blood-brain barrier in patients with depression by using apparent diffusion coefficient magnetic resonance imaging.

BACKGROUND

Transcranial magnetic stimulation is a new technology for noninvasively stimulating the brain. It appears to be a relatively safe technique, with some important exceptions. Its neurobiologic mechanisms of action are poorly understood. One theory to explain its apparent antidepressant effects involves a potential change in local blood-brain barrier settings, allowing passage of peripheral substances directly into brain parenchyma. Knowing whether transcranial magnetic stimulation changes local brain diffusion is important as well from a safety perspective. To test whether transcranial magnetic stimulation changes local brain diffusion, we used apparent diffusion coefficient magnetic resonance imaging in depressed patients undergoing interleaved transcranial magnetic stimulation/functional magnetic resonance imaging over the left prefrontal cortex.

METHODS

Within a 1.5 Tesla magnetic resonance imaging scanner, 14 depressed patients were stimulated with a figure-eight transcranial magnetic stimulation coil over the left prefrontal cortex. Apparent diffusion coefficient magnetic resonance imaging was acquired before, and immediately after, 1 Hertz transcranial magnetic stimulation (147 stimuli) intermittently delivered at a motor threshold of more than 7.35 minutes. Phase maps of the transcranial magnetic stimulation magnetic fields were used to guide region-of-interest placement.

RESULTS

No significant qualitative apparent diffusion coefficient differences were observed before and after 1 Hertz transcranial magnetic stimulation underneath the coil.

CONCLUSIONS

One Hertz transcranial magnetic stimulation over the left dorsolateral prefrontal cortex as applied in this study did not result in pathologic changes or leakage of the blood-brain barrier in patients with depression. If prefrontal transcranial magnetic stimulation at these usage parameters changes local diffusion, it is not an obvious or large effect.

In vivo modulation of hippocampal epileptiform activity with radial electric fields

PURPOSE

Electric field stimulation can interact with brain activity in a subthreshold manner. Electric fields have been previously adaptively applied to control seizures in vitro. We report the first results from establishing suitable electrode geometries and trajectories, as well as stimulation and recording electronics, to apply this technology in vivo.

METHODS

Electric field stimulation was performed in a rat kainic acid injection seizure model. Radial electric fields were generated unilaterally in hippocampus from an axial depth electrode. Both sinusoidal and multiphasic stimuli were applied. Hippocampal activity was recorded bilaterally from tungsten microelectrode pairs. Histologic examination was performed to establish electrode trajectory and characterize lesioning.

RESULTS

Electric field modulation of epileptiform neural activity in phase with the stimulus was observed in five of six sinusoidal and six of six multiphasic waveform experiments. Both excitatory and suppressive modulation were observed in the two experiments with stimulation electrodes most centrally placed within the hippocampus. Distinctive modulation was observed in the period preceding seizure-onset detection in two of six experiments. Short-term histologic tissue damage was observed in one of six experiments associated with high unbalanced charge delivery.

CONCLUSIONS

We demonstrated in vivo electric field modulation of epileptiform hippocampal activity, suggesting that electric field control of in vivo seizures may be technically feasible. The response to stimulation before seizure could be useful for triggering control systems, and may be a novel approach to define a preseizure state.