Stimulation of the retina with a multielectrode extraocular visual prosthesis

BACKGROUND: An extraocular approach to developing a retinal prosthesis for blind patients using electrodes placed on the outer surface of the eye is suggested. Experiments were carried out to determine the feasibility of this approach, and evaluate electrode configurations and parameters for stimulation. METHODS: In anaesthetized cats, a 21-electrode extraocular retinal prosthesis (ERP) array was sutured to the sclera over the lateral surface of the eye. Electrically evoked potentials (EEP) were recorded at the visual cortex bilaterally in response to retinal stimulation with the electrode array. Bipolar stimulation of the ERP array electrodes in horizontal and vertical configurations and at different interelectrode separations was investigated with biphasic constant-current pulses. RESULTS: Electrical stimulation of the lateral retina with an ERP elicited EEP that were higher in the ipsilateral visual cortex. The threshold for bipolar retinal stimulation was 500 microA. EEP amplitude increased with increases in stimulus pulse duration and current intensity. Retinal stimulation was slightly more effective with electrodes in a vertical as opposed to horizontal orientation. A larger interelectrode separation resulted in a higher EEP amplitude. CONCLUSIONS: Retinal stimulation with a prototype ERP array is demonstrated. The thresholds for retinal excitation are below safe charge-density limits for chronic neural stimulation. Ipsilateral localization of the EEP suggests that localized retinal stimulation is occurring. An ERP is a new approach to retinal prosthesis research, and might lead to the development of a low-resolution visual prosthesis for blind patients.     

Recording of corticospinal evoked potential for optimum placement of motor cortex stimulation electrodes in the treatment of post-stroke pain

The corticospinal motor evoked potential (MEP) evoked by motor cortex stimulation was investigated as an intraoperative index for the placement of stimulation electrodes in the epidural space over the motor cortex for the treatment of post-stroke pain. A grid of plate electrodes was placed in the epidural space to cover the motor cortex, sensory cortex, and premotor cortex employing a magnetic resonance imaging-guided neuronavigation system in two patients with severe post-stroke pain in the right extremities, a 66-year-old man with dysesthesia manifesting as burning and aching sensation, and a 67-year-old woman with dysesthesia manifesting as pricking sensation. The D-wave of the corticospinal MEP was recorded with a flexible wire electrode placed in the epidural space of the spinal cord during anodal monopolar stimulation of each plate electrode under general anesthesia. The grid electrode was fixed in position with dural sutures and the craniotomy closed. The effect of pain reduction induced by anodal monopolar stimulation of the same plate electrodes was examined using the visual analogue scale (VAS) on a separate day in the awake state without anesthesia. Comparison of the percentage VAS reduction and the recorded amplitude of the D-wave employing the same stimulation electrode revealed significant correlations in Case 1 (r = 0.828, p < 0.01) and Case 2 (r = 0.807, p < 0.01). The grid electrode was then replaced with two RESUME electrodes over the hand and foot areas, and the optimum positions were identified by D-wave recording before electrode fixation. Both patients reported satisfactory pain alleviation with lower stimulation voltages than usually required for patients with similar symptoms. These results indicate the potential of D-wave recording as an intraoperative indicator for the placement of stimulating electrodes over the motor cortex for pain relief.     

Multiple-channel stimulation of the cochlear nucleus

To further test the feasibility of a central nervous system auditory prosthesis, the characteristics of the electrically evoked middle latency response were studied in a series of acutely anesthetized pigmented guinea pigs, with multi-channel penetrating cochlear nucleus electrodes placed into the cochlear nucleus under direct visualization. These stimulating electrodes consisted of a silicone substrate, with five stimulating pads each, sputtered with iridium. Monopolar and bipolar stimulation were used. Threshold, latency, and input-output functions of the electrically evoked middle latency response were studied. Systematic differences were observed, depending on the site and parameters of stimulation. Principally, higher currents were required to produce waves of equal amplitude when the electrodes were closely spaced. For near electrode pairs, the maximum wave amplitudies obtainable within the limits of tissue safety were much lower than for distant electrode pairs. The slope of the growth function curve was steeper for widely spaced electrodes than for adjacent sites. Monopolar stimulation demonstrated maximum wave amplitudes with the lowest current intensity, implying current spread to the entire cochlear nucleus with this stimulation montage. In some cases, threshold differences were observed, higher thresholds being associated with closely spaced electrodes. These findings are consistent with simple models of the electric fields expected to be generated by these electrode arrays. The results support the hypothesis that activation of subpopulations of auditory brainstem neurons with multi-channel penetrating microelectrodes is possible.     

Retinotopy with coordinates of lateral occipital cortex in humans

OBJECT: The lateral occipital cortex in humans is known as the "extrastriate visual cortex." It is, however, an unexplored field of research, and the anatomical nomenclature for its surface has still not been standardized. This study was designed to investigate whether the lateral occipital cortex in humans has retinotopic representation. METHODS: Four right-handed patients with a diagnosis of intractable epilepsy from space-occupying lesions in the occipital lobe or epilepsy originating in the occipital lobe received permanently implanted subdural electrodes. Electrical cortical stimulation was applied directly applied to the brain through metal electrodes by using a biphasic stimulator. The location of each electrode was measured on a lateral skull x-ray study. Each patient considered a whiteboard with vertical and horizontal median lines. The patient was asked to look at the midpoint on the whiteboard. If a visual hallucination or illusion occurred, the patient recorded its outline, shape, color, location, and motion on white paper one tenth the size of, and with vertical and horizontal median lines similar to those on, the whiteboard. Polar angles and eccentricities of the midpoints of the phosphenes from the coordinate origin were measured on the paper. On stimulation of the lateral occipital lobe, 44 phosphenes occurred. All phosphenes were circular or dotted, with a diameter of approximately 1 cm, except one that was like a curtain in the peripheral end of the upper and lower visual fields on stimulation of the parietooccipital region. All phosphenes appeared in the visual field contralateral to the cerebral hemisphere stimulated. On stimulation of the lateral occipital lobe, 22 phosphenes moved centrifugally or toward a horizontal line. From three-dimensional scatterplots and contour maps of the polar angles and eccentricities in relation to the x-ray coordinates of the electrodes, one can infer that the lateral occipital cortex in humans has retinotopic representation. CONCLUSIONS: The authors found that phosphenes induced by electrical cortical stimulation of the lateral occipital cortex represent retinotopy. From these results one can assert that visual field representation with retinotopic relation exists in the extrastriate visual cortex.     

Electrophysiological studies of human face perception. I: Potentials generated in occipitotemporal cortex by face and non-face stimuli.

This and the following two papers describe event-related potentials (ERPs) evoked by visual stimuli in 98 patients in whom electrodes were placed directly upon the cortical surface to monitor medically intractable seizures. Patients viewed pictures of faces, scrambled faces, letter-strings, number-strings, and animate and inanimate objects. This paper describes ERPs generated in striate and peristriate cortex, evoked by faces, and evoked by sinusoidal gratings, objects and letter-strings. Short-latency ERPs generated in striate and peristriate cortex were sensitive to elementary stimulus features such as luminance. Three types of face-specific ERPs were found: (i) a surface-negative potential with a peak latency of approximately 200 ms (N200) recorded from ventral occipitotemporal cortex, (ii) a lateral surface N200 recorded primarily from the middle temporal gyrus, and (iii) a late positive potential (P350) recorded from posterior ventral occipitotemporal, posterior lateral temporal and anterior ventral temporal cortex. Face-specific N200s were preceded by P150 and followed by P290 and N700 ERPs. N200 reflects initial face-specific processing, while P290, N700 and P350 reflect later face processing at or near N200 sites and in anterior ventral temporal cortex. Face-specific N200 amplitude was not significantly different in males and females, in the normal and abnormal hemisphere, or in the right and left hemisphere. However, cortical patches generating ventral face-specific N200s were larger in the right hemisphere. Other cortical patches in the same region of extrastriate cortex generated grating-sensitive N180s and object-specific or letter-string-specific N200s, suggesting that the human ventral object recognition system is segregated into functionally discrete regions.     

A New Cortical Electrode for Neuronavigation-Guided Intraoperative Neurophysiological Monitoring: Technical Note

Intraoperative neurophysiological mapping and monitoring of eloquent brain areas can be combined with image-guided localisation to enhance the safety and efficacy of surgical procedures in the motor cortex. We designed a new type of cortical electrode which can be repeatedly placed on the cortical surface and allows accurate and reproducible stimulation by means of a navigation pointer. The newly designed device consists of a monopolar electrode contact for direct cortical stimulation, housed in a holder which allows placement, easy removal, and precise repeated placement of a surgical navigation pointer. It can be used for navigation-guided, high-frequency anodal monopolar cortical stimulation (MCS) for the mapping of eloquent cortex, and for monitoring of motor pathways. While the cortex is stimulated, compound muscle action potentials (CMAP) are recorded from muscles of the contralateral extremities and are assessed both qualitatively and quantitatively. When the device is used in combination with intraoperative navigation, the stimulation sites may optionally be registered or displayed on the system monitor. This allows repeated pinpointing and obviates the need for strip or grid electrodes in the operative field; although such electrodes may be useful for continuous monitoring, they often are in the surgeon's way. In addition, the primary and supplementary motor cortex can be mapped by determining the location of the sites of stimulation on surface-projected images of the cerebral cortex.     

Far-field responses to stimulation of the cochlear nucleus by microsurgically placed penetrating and surface electrodes in the cat.

OBJECT: A new generation of penetrating electrodes for auditory brainstem implants is on the verge of being introduced into clinical practice. This study was designed to compare electrically evoked auditory brainstem responses (EABRs) to stimulation of the cochlear nucleus (CN) by microsurgically implanted surface electrodes and insertion electrodes (INSELs) with stimulation areas of identical size. METHODS: Via a lateral suboccipital approach, arrays of surface and penetrating microelectrodes with geometric stimulation areas measuring 4,417 microm2 (diameter 75 microm) were placed over and inserted into the CN in 10 adult cats. After recording the auditory brainstem response (ABR) at the mastoid process, the CN, and the level of the inferior colliculus, EABRs to stimulation of the CN were recorded using biphasic, charge-balanced stimuli with phase durations of 80 microsec, 160 microsec, and 240 microsec at a repetition rate of 22.3 Hz. Waveform, threshold, maximum amplitude, and the dynamic range of the responses were compared for surface and penetrating electrodes. The EABR waveforms that appeared for both types of stimulation resembled each other closely. The mean impedance was slightly lower (30 +/- 3.4 kohm compared with 31.7 +/- 4.5 kohm, at 10 kHz), but the mean EABR threshold was significantly higher (51.8 microA compared with 40.5 microA, t = 3.5, p = 0.002) for surface electrode arrays as opposed to penetrating electrode arrays. Due to lower saturation levels of the INSEL array, dynamic ranges were almost identical between the two types of stimulation. Sectioning of the eighth cranial nerve did not abolish EABRs. CONCLUSIONS: Microsurgical insertion of electrodes into the CN complex may be guided and monitored using techniques similar to those applied for implantation of surface electrodes. Lower thresholds and almost equivalent dynamic ranges indicate that a more direct access to secondary auditory neurons is achieved using penetrating electrodes.     

Extraoperative functional mapping of motor areas in epileptic patients by high-frequency cortical stimulation

OBJECT: The aim of this study was to investigate the usefulness of a short train of high-frequency (500 Hz) cortical stimulation to delineate the primary motor cortex (MI), supplementary motor area (SMA), primary somatosensory cortex (SI), supplementary sensory area (SSA), negative motor area (NMA), and supplementary negative motor area (SNMA) in patients with epilepsy who were undergoing functional mapping. METHODS: Seventeen patients were studied, all of whom underwent functional mapping using 50-Hz electrical stimulation. After these clinical evaluations, cortical stimulations with a short train of electrical pulses at 500 Hz were performed through subdural electrodes placed at the MI, SMA, SI, SSA, NMA, and SNMA, which had been identified by 50-Hz stimulation, and surrounding cortical areas, while surface electromyography readings were recorded. RESULTS: Stimulation of the MI elicited motor evoked potentials (MEPs) in contralateral muscles. Stimulation of the SMA also induced MEPs in contralateral muscles but with longer latencies compared with the MI stimulation. Stimulation of the SMA did not elicit MEPs in ipsilateral muscles. Stimulation of the SI, SSA, NMA, and SNMA did not induce MEPs in any muscle. In one patient, MEPs were elicited without seizure induction by 500-Hz stimulation of the electrodes, whereas a 50-Hz stimulation of the same electrodes induced his habitual seizures. CONCLUSIONS: Extraoperative high-frequency stimulation with MEP monitoring is a useful complementary method for cortical mapping without inducing seizure. Stimulation of SMA induces MEPs in contralateral muscles, with longer latencies compared with the stimulation of MI. This finding may be useful for the differentiation between MI and SMA, especially in the foot motor areas.     

Glossopharyngeal nerve evoked potentials after stimulation of the posterior part of the tongue in dogs

OBJECTIVE: Lower cranial nerve palsy is one of the most critical complications after posterior fossa surgery. However, no established monitoring procedures exist for glossopharyngeal nerve function. Therefore, glossopharyngeal nerve evoked potentials after stimulation of the posterior part of the tongue in dogs was studied to analyze whether glossopharyngeal nerve compound action potentials and evoked potentials are useful in the intraoperative monitoring of patients undergoing brainstem and cerebellopontine angle surgery. METHODS: Glossopharyngeal nerve action potentials and cortical potentials were evoked by stimulating the posterior part of the tongue in mongrel dogs. The potentials were evoked by supramaximal constant current electrical stimuli delivered with bipolar stainless steel needle electrodes and recorded with silver ball electrodes. RESULTS: Compound nerve action potentials were recorded from the exposed intracranial portion of the glossopharyngeal nerve. The latency of the initial negative peak of the action potentials was 2.8 +/- 0.6 milliseconds (mean +/- standard deviation; n = 17). Evoked cortical potentials were recorded on the coronal gyrus by stimulating the contralateral side. The latencies of the initial positive peak and negative peak were 20.1 +/- 3.7 and 35.7 +/- 8.2 milliseconds, respectively (n = 6). Ipsilateral tongue stimulation elicited biphasic evoked potentials on the coronal gyrus, which had small amplitudes and delayed latencies. Both compound nerve action potentials and cortical evoked potentials disappeared after sectioning of the glossopharyngeal nerve. CONCLUSION: The glossopharyngeal nerve action potentials and cortical potentials elicited by the stimulation of the posterior one-third of the tongue can be recorded. These evoked potentials represent a new means for intraoperative monitoring of patients undergoing surgery in the brainstem via the cerebellopontine angle, which involves the lower cranial nerves.     

Magnetic and electrical stimulation of the auditory cortex for intractable tinnitus. Case report

Tinnitus is a distressing symptom that affects up to 15% of the population for whom no satisfactory treatment exists. The authors present a novel surgical approach for the treatment of intractable tinnitus, based on cortical stimulation of the auditory cortex. Tinnitus can be considered an auditory phantom phenomenon similar to deafferentation pain, which is observed in the somatosensory system. Tinnitus is accompanied by a change in the tonotopic map of the auditory cortex. Furthermore, there is a highly positive association between the subjective intensity of the tinnitus and the amount of shift in tinnitus frequency in the auditory cortex, that is, the amount of cortical reorganization. This cortical reorganization can be demonstrated by functional magnetic resonance (fMR) imaging. Transcranial magnetic stimulation (TMS) is a noninvasive method of activating or deactivating focal areas of the human brain. Linked to a navigation system that is guided by fMR images of the auditory system, TMS can suppress areas of cortical plasticity. If it is successful in suppressing a patient's tinnitus, this focal and temporary effect can be perpetualized by implanting a cortical electrode. A neuronavigation-based auditory fMR imaging-guided TMS session was performed in a patient who suffered from tinnitus due to a cochlear nerve lesion. Complete suppression of the tinnitus was obtained. At a later time an extradural electrode was implanted with the guidance of auditory fMR imaging navigation. Postoperatively, the patient's tinnitus disappeared and remains absent 10 months later. Focal extradural electrical stimulation of the primary auditory cortex at the area of cortical plasticity is capable of suppressing contralateral tinnitus completely. Transcranial magnetic stimulation may be an ideal method for noninvasive studies of surgical candidates in whom stimulating electrodes might be implanted for tinnitus suppression.     

Recognition of pixelized Chinese characters using simulated prosthetic vision

The rehabilitation of the reading ability of the blind with a limited number of stimulating electrodes is regarded as one of the major functions of the envisioned visual prosthesis. This article systematically studied how many pixels of individual Chinese characters should be needed for correct and economic recognition by blind Chinese subjects. In this study, 40 normal-sighted subjects were tested on a self-developed platform HanziConvertor (Institute for Laser Medicine & Bio-photonics, Shanghai Jiaotong University, China) with digital imaging processing capacities to convert images of printed text into various pixelized patterns made up of discrete dots, and present them orderly on a computer screen. It was found that various complicated factors such as pixel number, character typeface, stroke number, etc., can obviously affect the recognition accuracy. It was also found that optimal recognition accuracy occurs at a specific size of binary pixel array, due to a trade-off between a strictly limited number of stimulation electrodes and character sampling resolution. The results showed that (i) recognition accuracy of pixelized characters is optimal with at least 12 x 12 binary pixels, and therefore it is recommended to apply a minimum of 150 discrete and functioning electrodes for restoring the reading ability of blind Chinese individuals in the visual prosthesis; (ii) fonts of Song Ti and Hei Ti are clearer and more effective than other typefaces; and (iii) characters with fewer strokes lead to better accuracy.     

Chronic motor cortex stimulation for phantom limb pain: a functional magnetic resonance imaging study: technical case report

OBJECTIVE AND IMPORTANCE: Chronic motor cortex stimulation has provided satisfactory control of pain in patients with central or neuropathic trigeminal pain. We used this technique in a patient who experienced phantom limb pain. Functional magnetic resonance imaging (fMRI) was used to guide electrode placement and to assist in understanding the control mechanisms involved in phantom limb pain. CLINICAL PRESENTATION: A 45-year-old man whose right arm had been amputated 2 years previously experienced phantom limb pain and phantom limb phenomena, described as the apparent possibility of moving the amputated hand voluntarily. He was treated with chronic motor cortex stimulation. INTERVENTION: Data from fMRI were used pre- and postoperatively to detect shoulder and stump cortical activated areas and the "virtual" amputated hand cortical area. These sites of preoperative fMRI activation were integrated in an infrared-based frameless stereotactic device for surgical planning. Phantom limb virtual finger movement caused contralateral primary motor cortex activation. Satisfactory pain control was obtained; a 70% reduction in the phantom limb pain was achieved on a visual analog scale. Postoperatively and under chronic stimulation, inhibiting effects on the primary sensorimotor cortex as well as on the contralateral primary motor and sensitive cortices were detected by fMRI studies. CONCLUSION: Chronic motor cortex stimulation can be used to relieve phantom limb pain and phantom limb phenomena. Integrated by an infrared-based frameless stereotactic device, fMRI data are useful in assisting the neurosurgeon in electrode placement for this indication. Pain control mechanisms and cortical reorganization phenomena can be studied by the use of fMRI.     

Enhancement of somatosensory evoked potentials by etomidate in cats: an investigation of its site of action

Enhancement of somatosensory evoked potentials by etomidate has been reported in recent clinical studies. This investigation was designed to investigate the central nervous system site of action responsible for this effect. Six adult cats were anesthetized with halothane (0.8-1%) in a mixture of 50% N2O in O2. A recording electrode was placed stereotactically in the ventral posterior lateral nucleus of thalamus (VPL), and a ball electrode was placed over the surface of the hind limb region of primary sensory cortex. Somatosensory evoked potentials in response to stimulation of tibial nerve thus were simultaneously recorded from cerebral cortex and VPL. The effect of two doses (1 and 3 mg.kg-1) of etomidate given 2 h apart on the latency and amplitude of cortical (positive wave at 15 ms) and thalamic (positive deflection at 10 ms, followed by negative deflection at 17 ms) evoked potentials was studied. There was no significant effect of etomidate on either latency or amplitude of early, positive thalamic potentials. Both doses of etomidate caused a significant increase in the latency and amplitude of cortical potentials. The mean latency of cortical potential increased by 1.72 ms (11%) after the 1 mg.kg-1 dose and 2.3 ms (15.9%) after the 3 mg.kg-1 dose. The maximum mean increase in the amplitude of cortical potentials was 14.3 microV (mean increase 78%, range 28-241%) after 1 mg.kg-1 and 19.1 microV (mean increase 112%, range 28-202%) after 3 mg.kg-1. Cortical amplitude remained significantly elevated for 30 min after 1 mg.kg-1 and for the remainder of the study period (60 min) after 3 mg.kg-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Motor evoked potentials: appropriate positioning of recording electrodes for diagnosis of spinal disorders

Summary The interpretation of normal and pathological findings of motor evoked potentials obtained by the use of transcranial magnetic stimulation depends on adequate examination technique, including the appropriate positioning of the recording electrodes over the muscle. On the basis of knowing the location of the motor end plate zones in muscles, magnetic stimulation of the motor cortex of 30 healthy adults was performed in order to explore the influence of the position of the surface recording electrodes on potential parameters and to establish the standard location of the recording electrodes over the biceps brachii, medial vastus, anterior tibial and abductor hallucis muscles for diagnostic use in spine disorders. The cortical latencies and peak-to-peak amplitudes of the evoked potentials were analysed by varying the location of the recording electrodes and the stimulus intensities. The latencies were significantly shorter when the different electrode lay more proximally over the muscle belly. Reproducible potentials with sharp negative onset and maximum amplitude were recorded with a separation of 5–7.5 cm between the different electrode, located over the motor end plate area, and the indifferent electrode, located over the distal myotendinous junction. This implies that the parameters of evoked potentials depend on the position and separation distance of the recording electrodes over the muscles and that it is possible to record the potentials using a lower stimulus intensity and, above all, on relaxed muscles, which may prove to be applicable for intraoperative monitoring of the spinal cord using magnetic stimulation. Therefore it is recommended to use the verified motor end plate zone location as a standard marker for the recording of motor evoked potentials for diagnosis of lesions of descending motor pathways.Presented at the annual meeting of the European Spine Society, Cambridge, 3–5 September 1992     

Impact of Brain Shift on Intraoperative Neurophysiological Monitoring with Cortical Strip Electrodes

Summary.  Background: Intraoperative neurophysiological monitoring has become the standard procedure for locating eloquent regions of the brain. Such continuous electrical stimulation of motor pathways is usually applied by means of flat silicon-embedded electrodes placed directly on the motor cortex. However, shifting of the silicon strip on the cortical surface as well as electrode displacement due to brain shift underneath the electrode can lead to inaccurate recordings not directly caused by intraoperative impairment of the motor cortex or the motor pathways.  Method: This prospective study was conducted to quantify cortical brain shift during open cranial surgery and to assess its impact on electrode positioning in 31 procedures near the precentral gyrus. Three groups of different lesion volumes were distinguished. Movement of the cortex between opening of the dura and completion of tumor removal as well as cortical electrode shifting were digitally measured and analyzed.  Findings: Cortical surface structures evidenced a significantly larger shift (up to 23.4 mm) in comparison to the electrode strips (up to 4.2 mm) in lesions with a volume of over 20 ml. Cortex shifting highly correlated with lesion volume, whereas strip electrode movement was almost unidirectional and did not differ significantly among the three groups. However, the way they were placed (completely on the cortex vs. partly underlying or overlapping the craniotomy borders) affected the magnitude of their intraoperative displacement. As a consequence, 3 of the 31 cases (9.3%) showed a significant change in the recorded motor responses due to intraoperative dislocation of the stimulating electrode.  Interpretation: Changes in the location of cerebral structures due to intraoperative brain shift may exert a marked influence on intraoperative neurophysiological monitoring if cortical strip electrodes are used. Therefore, long-term monitoring of the central region requires continuous checking of the position of stimulating electrodes and, if necessary, correction of their location. Published online December 5, 2002 Acknowledgments  The authors thank Mr. Udo Warschewske and his co-workers of Functional Imaging Technologies (Waltersdorf, Land Brandenburg, Germany) for their help in establishing the software necessary for the navigation-controlled calculation of intraoperative brain and electrode shifting.  Correspondence: Dr. med. Olaf Suess, Department of Neurosurgery, Benjamin Franklin University Hospital, Free University of Berlin, Hindenburgdamm 30, 12200 Berlin, Germany.     

Mechanical noxious stimuli cause bilateral activation of parietal operculum in callosotomized subjects

The patterns of cortical activation evoked by tactile and mechanical painful stimulation in six normal subjects and three patients with complete resection of the corpus callosum are described and compared, with emphasis on the parietal operculum. Stimulus-related cortical activation was investigated by functional magnetic resonance imaging. In both groups, painful stimulation activated the first somatosensory, insular and cingulate cortices in the contralateral hemisphere, and the parietal opercular cortex in both hemispheres. Comparison between the two patterns of cortical activation demonstrated that ipsilateral activation by unilateral painful stimulation is at least partially independent of the corpus callosum and suggests a different organization of the pain and touch systems.     

Intraoperative monitoring of the corticospinal motor evoked potential (D-wave): clinical index for postoperative motor function and functional recovery

The corticospinal motor evoked potential was investigated as a monitoring index of motor function to perform maximal resection of brain tumors located around the motor cortex in 37 patients with glioma. Tumor resections were performed under general anesthesia with muscle relaxant and completely controlled ventilation. No special arrangements for anesthesia were required. Direct cortical stimulation revealed that if one electrode was placed on the posterior half of the precentral gyrus, the D-wave could be recorded even when using an electrode separation of 10 mm, and the amplitude was larger with anodic rather than cathodic stimulation. Monitoring of the D-wave enabled the function of the corticospinal tract to be evaluated selectively. Postoperative persistent motor disturbance remained in six patients who had a decrease of over 30% in amplitude of the D-wave during tumor resection. A decrease of less than 30% may indicate postoperative preservation of motor function, including transient motor disturbance with subsequent complete recovery. Intraoperative monitoring of the D-wave is suitable for open cranial surgery with general anesthesia, can detect the primary motor cortex, and allow maximal resection of brain tumors located around the motor cortex.     

Long-term potentiation induces expanded movement representations and dendritic hypertrophy in layer V of rat sensorimotor neocortex

While long-term potentiation (LTP) is currently the most widely investigated model of the synaptic mechanisms underlying learning, there is a paucity of reports on the direct effects of LTP on cortical organization. Here we show that strengthening polysynaptic potentiation correlates with an expanded neocortical area that responds to intracortical microstimulation-induced movements of rat forelimb and increased dendritic material in layer V pyramidal cells. Rats carried a stimulating electrode in the corpus callosum (midline), and a recording electrode in the right caudal forelimb area (CFA). Each rat received 15 days of either high frequency stimulation (HFS) or handling. Evoked potentials of the transcallosal pathway were recorded in the right hemisphere before and after 15 days of stimulation or handling. Following the last stimulation, movement representations were determined in the left CFA using high-resolution intracortical microstimulation (ICMS) and then the brains were processed for Golgi-Cox staining. Our results show that synaptic modification results in a recruitment of more neocortical area into movement representations and increases in several measures of dendritic morphology in layers III and V. This study sheds light on the interaction between artificial models of learning, receptive field characteristics and dendritic morphology in the sensorimotor cortex.     

Thalamic stimulation and functional magnetic resonance imaging: localization of cortical and subcortical activation with implanted electrodes. Technical note

The utility of functional magnetic resonance (fMR) imaging in patients with implanted thalamic electrodes has not yet been determined. The aim of this study was to establish the safety of performing fMR imaging in patients with thalamic deep brain stimulators and to determine the value of fMR imaging in detecting cortical and subcortical activity during stimulation. Functional MR imaging was performed in three patients suffering from chronic pain and two patients with essential tremor. Two of the three patients with pain had undergone electrode implantation in the thalamic sensory ventralis caudalis (Vc) nucleus and the other had undergone electrode implantation in both the Vc and the periventricular gray (PVG) matter. Patients with tremor underwent electrode implantation in the ventralis intermedius (Vim) nucleus. Functional MR imaging was performed during stimulation by using a pulse generator connected to a transcutaneous extension lead. Clinically, Vc stimulation evoked paresthesias in the contralateral body, PVG stimulation evoked a sensation of diffuse internal body warmth, and Vim stimulation caused tremor arrest. Functional images were acquired using a 1.5-tesla MR imaging system. The Vc stimulation at intensities provoking paresthesias resulted in activation of the primary somatosensory cortex (SI). Stimulation at subthreshold intensities failed to activate the SI. Additional stimulation-coupled activation was observed in the thalamus, the secondary somatosensory cortex (SII), and the insula. In contrast, stimulation of the PVG electrode did not evoke paresthesias or activate the SI, but resulted in medial thalamic and cingulate cortex activation. Stimulation in the Vim resulted in thalamic, basal ganglia, and SI activation. An evaluation of the safety of the procedure indicated that significant current could be induced within the electrode if a faulty connecting cable (defective insulation) came in contact with the patient. Simple precautions, such as inspection of wires for fraying and prevention of their contact with the patient, enabled the procedure to be conducted safely. Clinical safety was further corroborated by performing 86 MR studies in patients in whom electrodes had been implanted with no adverse clinical effects. This is the first report of the use of fMR imaging during stimulation with implanted thalamic electrodes. The authors' findings demonstrate that fMR imaging can safely detect the activation of cortical and subcortical neuronal pathways during stimulation and that stimulation does not interfere with imaging. This approach offers great potential for understanding the mechanisms of action of deep brain stimulation and those underlying pain and tremor generation.     

Somatotopy in Human Primary Somatosensory Cortex in Pain System

Background: Compared with somatotopical organization (somatotopy) in the postcentral gyrus in the tactile system, somatotopy in the pain system is not well understood. The aim of this study is to elucidate whether there is somatotopy in the human pain system.

Methods: To elucidate the somatotopy of nociceptive neurons in the postcentral gyrus, the authors recorded pain-evoked cortical responses to noxious intraepidermal electrical stimulation applied to the left hand and left foot in 11 male subjects, using magnetoencephalography.

Results: Brief painful stimuli evoked sustained cortical activity in the primary somatosensory cortex (SI) in the hemisphere contralateral to the stimulated side and in the secondary somatosensory cortex in both hemispheres. In SI, representations of the hand and foot were distinctly separated, with a more medial and posterior location for the foot, whereas no significant difference was found in the locations for the secondary somatosensory cortex dipole. The SI arrangement along the central sulcus was compatible with the homunculus revealed by Penfield using direct cortical stimulation during surgery.