Does the central sulcus divide motor and sensory functions? Cortical mapping of human hand areas as revealed by electrical stimulation through subdural grid electrodes

To clarify the exact anatomic relationship of electrically identified hand areas to the central sulcus, we constructed cortical surface renderings of magnetic resonance images (MRI) to locate the central sulcus accurately and measured the distances of stimulated points from the central sulcus and the Sylvian fissure. We obtained hand responses in 33 patients who underwent implantation of subdural grid electrodes for evaluation and surgical treatment of intractable epilepsy and analyzed these responses according to the presence of motor, sensory, mixed motor and sensory, and arrest responses. Hand motor responses occurred not only in the precentral gyrus but also in the postcentral gyrus, with great variability in superior-to-inferior distribution. Sensory responses also occurred in both the precentral and postcentral gyri with a distribution more ventral than that of motor responses. Mixed motor and sensory responses tended to be limited to the middle part of the central sulcus. Sites where electrical stimulation arrested simple hand repetitive voluntary movements occurred widely throughout the premotor and primary sensorimotor cortices. These data indicate a marked variability in the location of the human cortical hand area, and suggest that motor and sensory hand cortices overlap and are not divided in a simple manner by the central sulcus.

Interleukin (IL)-1 beta, IL-6, tumor necrosis factor-alpha, epidermal growth factor, and beta 2-microglobulin levels are elevated in gingival crevicular fluid during human orthodontic tooth movement

Bone remodeling is a complex process regulated by several mediators. Recent work has revealed that cytokines and growth factors have significant effects on bone cell metabolism. However, little information is available concerning the production of cytokines during orthodontic tooth movement in human subjects, and there is no non-invasive model for determining the production of cytokines. Therefore, the purpose of this study was to identify and quantify the various cytokines in human gingival crevicular fluid (GCF), and to investigate the changes in their levels during orthodontic tooth movement. Twelve patients (mean age, 14.4 years) were used as subjects. An upper canine of each patient having one treatment for distal movement served as the experimental tooth, whereas the contralateral and antagonistic canines were used as controls. The GCF around the experimental and the two control teeth was taken from each subject immediately before activation, and at 1, 24, and 168 hr after the initiation of tooth movement. Cytokine levels were determined by ELISAs. The concentrations of interleukin (IL)-1 beta, IL-6, tumor necrosis factor-alpha, epidermal growth factor, and beta 2-microglobulin were significantly higher in the experimental group than in the controls at 24 hr after the experiment was initiated. All the cytokines remained at baseline levels throughout the experiment for the two control groups. In contrast to cytokine alteration, the amount of total protein in the GCF exhibited a gradual increase, but no significant difference was observed between the control and experimental groups. Since all cytokines in GCF play an important role in the bone remodeling processes in vitro, the present results indicate that the changes in cytokines in GCF are associated with orthodontic tooth movement.

Relation between micellar structure of model bile and activity of esterase

In a model bile solution composed of lecithin (L)-bile salt (B), the solubilization of lipid and the accessibility of enzyme to the lipid were examined by observation of EPR spectra and measurement of enzyme activity. The lifetime of the spin probe in the micellar phase was estimated to be approx. 1 microsecond by means of line shape analysis. Both population and lifetime increased with temperature and the molar ratio of lecithin to bile salt (L/B). The EPR data indicated that simple micelle of bile salt, mixed disk micelle of bile salt-lecithin, and multi-lamellar mixed disk micelle can exist in a model bile solution, depending on the L/B molar ratio across a range from 0 to 1.5. The maximal power of the mixed disk micelle to solubilize cholesteryl ester in the model bile at a L/B molar ratio of 1:1 was confirmed by EPR measurement of cholesteryl 12-DOXYL-stearate. Observation of the enzyme activity on a mixture of model bile and substrate at 37 degrees C revealed selective accessibility of cholesterol esterase (bovine pancreas) to mixed disk micelle, of cholesterol oxidase (Streptomyces cinnamomeus) to both simple and mixed disk micelle, and of pancreatic lipase (porcine pancreas) to both simple micelle and an oil droplet of substrate. The temperature-dependent activity of cholesterol oxidase to cholesterol in mixed disk micelle can be explained in terms of mesomorphic phase transition of lecithin side chains followed with fluidity of liquid crystal phase. Regarding phospholipase C from Bacillus cereus, though the selective accessibility to the micelles was not observed at 37 degrees C, a decrease in activity for mixed disk micelle could be found at lower temperatures.

Brainstem auditory evoked potentials recorded around the human upper brainstem area

The intracranial spatiotemporal distributions of brainstem auditory evoked potentials were analyzed in two patients with epilepsy to study the dipole directions of waves III and V, previously found to have a horizontal and a vertical dipole, respectively. Depth electrodes were implanted bilaterally into the frontal and temporal lobes of these patients, the targets including the amygdala and hippocampus which are close to the upper pons and midbrain. Recordings around the upper pons and from the subcortical area downwards along the dorsal aspect of the upper pons indicated that waves III and V are constructed of mixed dipoles, wave III also having a vertical dipole segment and wave V also a horizontal one. These results suggest that several neural activities from the upper pons to midbrain contribute to the generation of waves III and V.

Induction of apoptotic DNA fragmentation and cell death by natural ceramide

We have found that a solvent mixture of ethanol and dodecane was able to disperse natural ceramide (Cer) into aqueous solution. In U937 cells, addition of natural Cer to medium caused a characteristic pattern of DNA fragmentation, which is indistinguishable from that caused by TNF alpha. The effective concentration of Cer is as low as 25 nM. The present study provides direct evidence that natural Cer functions as a second messenger mediating TNF alpha-induced DNA fragmentation. The use of this solvent to deliver hydrophobic natural Cer to cells will contribute to the elucidation of the biological function of Cer.

Diagnosis of progressive multifocal leukoencephalopathy by stereotactic brain biopsy utilizing immunohistochemistry and the polymerase chain reaction

This report describes the diagnosis of progressive multifocal leukoencephalopathy (PML) in nine patients using cytopathologic and histopathologic examination of computed tomographically guided stereotactic brain biopsies in combination with immunostaining for SV-40-related antigen and the polymerase chain reaction (PCR) for the JC virus. In four patients the diagnosis of PML was based on the microscopic appearance of the biopsies and immunostaining for SV-40-related antigen. In one of these patients the diagnosis was also supported by PCR for the JC virus. In two patients whose biopsies were only suggestive of PML, a definitive diagnosis was possible utilizing immunohistochemistry and PCR. In another case the histopathologic features were atypical of PML, and the diagnosis was established with immunostaining and PCR. The diagnosis of PML was established by PCR alone in two patients whose biopsies showed only suggestive or nonspecific findings. We conclude that the accuracy of stereotactic biopsy in the diagnosis of PML is enhanced by using a combination of light microscopy, immunohistochemistry and PCR.

Clinical and electroencephalographic evidence for sites of origin of seizures with diffuse electrodecremental pattern

A diffuse electrodecremental ictal pattern (DEP) has been associated with tonic seizures and, less often, with other forms of epilepsy and has been considered to reflect a generalized seizure disorder of diffuse cortical or subcortical (brainstem) origin. In some seizures associated with DEP, however, focal ictal manifestations have been observed. We reviewed the records of all patients admitted to our seizure monitoring unit for 3 years and detected 39 patients with seizures associated with DEP. In 23 of 39 patients, clinical ictal behaviors resembled seizures of unilateral supero/mesiofrontal lobe origin and interictal EEG showed a prominent unilateral frontal component. Nine of 39 had complex absences (CA)/complex partial seizures (CPS); 4 of them were of unilateral frontal lobe origin. Seven of 39 patients had tonic or atonic seizures. Seven patients were studied further with subdural electrodes. Ictal onsets showed a high-frequency frontal lobe discharge. We conclude that in a subgroup of patients a generalized electrodecremental pattern on scalp EEG results from a regional cortical high-frequency ictal discharge originating in a single frontal lobe.

Vasoactive peptides and prostaglandin D2 in human cerebromicrovascular endothelium

Prostaglandin D2 (PGD2) is the major prostanoid formed among other prostanoids in cultured microvascular endothelium derived from human brain (HBEC). Angiotensin II, arginine vasopressin and endothelium-1 stimulated the production of PGD2 and PGF2 alpha in a concentration-dependent manner, and this effect was inhibited by their specific receptor antagonists or dexamethasone (inhibitor of phospholipase A2/cyclooxygenase II). Both the peptidergic-induced PGD2 and the exogenously added PGD2 were converted in HBEC to 9 alpha, 11 beta-PGF2, a potent vasoconstrictor. Exogenous PGD2 also dose-dependently enhanced the production of vasoconstrictive PGF2 alpha, thromboxane B2, vasodilatory prostaglandin PGE2, and cAMP in these cells. The PGD2 stimulated formation of the prostanoids was inhibited by acetylsalicylic acid or indomethacin (inhibitors of cyclooxygenase I) but not dexamethasone, demonstrating for the first time that PGD2 may contribute to the production of prostanoids in HBEC. These findings strongly suggest that PGD2 may play a pivotal role in the regulation of cerebromicrovascular function.

Total craniospinal decompression in achondroplastic stenosis

We describe our experience with total craniospinal decompression along the entire neuraxis, extending from the brain stem to the cauda equina, in seven patients with achondroplasia. These patients presented with clinically significant compression at multiple levels. In these patients, there were focal areas of complete myelographic block, typically at the cervicothoracic or thoracolumbar junction, as well as diffuse narrowing of the entire spinal subarachnoid space. In some, there were further complications of basilar impression, Arnold-Chiari malformation, or syringomyelia. Total craniospinal decompression was completed in either one or two stages. Only a small minority of our patients with achondroplasia had critical stenosis over this many levels, requiring total craniospinal decompression. However, with proper preparation and technique, we found that patients can tolerate even such an extensive decompressive procedure and benefit from surgery without suffering postoperative spinal instability.

Electrical stimulation and language

Electrical stimulation has been used for over half a century in clinical settings to facilitate the surgical treatment of patients with intractable seizures and cortical structural lesions. It should be employed as the final result of a stepwise process in patient management but, when called for, can be very accurate in localizing critical functional areas. We discuss the use of this technique in patient evaluation, with an emphasis on the evaluation of patients with seizure disorders.

Vasoconstrictive peptides induce endothelin-1 and prostanoids in human cerebromicrovascular endothelium

Vasoconstrictive peptides and prostanoids have been implicated in the pathogenesis of hypertension and vasospasm. Recently, we have shown that human cerebromicrovascular endothelium [human brain endothelial cells (HBEC)] constitutively produces both endothelin-1 (ET-1) and prostanoids. The vasoactive peptides, arginine vasopressin (AVP) or angiotensin II (ANG II), stimulated secretion of both immunoreactive ET-1 and prostanoids from HBEC by a receptor-mediated induction of phospholipase C (PLC) and PLA2. The release of constitutive or AVP- or ANG II-induced ET-1 occurred at different rates during the 24-h incubation of HBEC in serum-free medium. The temporal profile of AVP-stimulated production of prostanoids differed from that of ANG II. AVP-induced release of prostaglandin D2 (PGD2) persisted for 24 h, whereas ANG II-stimulated PGD2 was only seen during the first 4 h of incubation. ANG II maximally stimulated PGI2 secretion during the 4- to 8-h interval, whereas AVP did not stimulate PGI2 secretion. Dexamethasone (Dxm), indomethacin (Indo), and nordihydroguaiaretic acid, the respective inhibitors of PLA2-cyclooxygenase II, cyclooxygenase, and lipoxygenase, increased both constitutive and AVP- or ANG II-stimulated secretion of ET-1. Dxm also decreased AVP- or ANG II-stimulated production of PGD2 and PGF2 alpha. These results indicate an interrelationship between HBEC production of ET-1 and prostanoids, which may play a role in regulating cerebral microcirculation.

Cortical tongue area studied by chronically implanted subdural electrodes--with special reference to parietal motor and frontal sensory responses

Motor and sensory cortical tongue representations were examined in 40 patients with intractable seizures who underwent chronic subdural electrode grid implantation. Tongue responses were observed in a wide area 4.5 cm anterior and 3 cm posterior to the central sulcus. The distribution of the responses was not influenced by whether the responses were unilateral or bilateral. In patients with fronto-parietal lesions, the tongue motor area was located significantly more superior to the Sylvian fissure and more anterior to the central sulcus than was the tongue motor area of patients without organic lesion. Both motor and sensory responses were found outside of the classic precentral or postcentral area on the lateral surface of the cortex. Motor responses ('parietal motor responses') could occur posterior to the central sulcus and, rarely, sensory responses ('frontal sensory responses') were identified anterior to the central sulcus. These paradoxical parietal motor and frontal sensory responses were seen in 17 out of 40 (42.5%) patients. Nine of these 17 patients had no organic brain lesion on MRI. Clinical factors, such as patient's age, duration of seizures and cognitive functions (IQ, word fluency score), did not influence the frequency of the paradoxical responses. However, patients with brain lesions showed a tendency to have associated paradoxical responses (P < 0.05). In conclusion, paradoxical responses are not uncommon in epilepsy patients, particularly in those with organic lesions. The physiological and clinical implications of the paradoxical responses are discussed.

Endothelin-1 receptor binding and cellular signal transduction in cultured human brain endothelial cells

The kinetic properties of endothelin-1 (ET-1) binding sites and the production of inositol phosphates (IPs; IP1, IP2, IP3), cyclic AMP, thromboxane B2, and prostaglandin F2 alpha induced by various endothelins (ET-1, ET-2, ET-3, and sarafotoxin S6b) were examined in endothelial cells derived from human brain microvessels (HBECs). The presence of both high- and low-affinity binding sites for ET-1 with KD1 = 122 pM and KD2 = 31 nM, and Bmax1 = 124 fmol/mg of protein and Bmax2 = 909 fmol/mg of protein, respectively, was demonstrated on intact HBECs. ET-1 dose-dependently stimulated IP accumulation with EC50 (IP3) = 0.79 nM, whereas ET-3 was ineffective. The order of potency for displacing ET-1 from high-affinity binding sites (ET-1 > ET-2 > sarafotoxin S6b > ET-3) correlated exponentially with the ability of respective ligands to induce IP3 formation. ET-1-induced IP3 formation by HBEC was inhibited by the ETA receptor antagonist, BQ123. The protein kinase C activator phorbol myristate ester dose-dependently inhibited the ET-1-stimulated production of IPs, whereas pertussis toxin was ineffective. Cyclic AMP production by HBECs was enhanced by both phorbol myristate ester and ET-1, and potentiated by combined treatment with ET-1 and phorbol myristate ester. Data indicate that protein kinase C plays a role in regulating the ET-1-induced activation of phospholipase C, whereas interaction of different messenger systems may regulate ET-1-induced accumulation of cyclic AMP. ET-1 also stimulated endothelial prostaglandin F2 alpha production, suggesting that activation of phospholipase A2 is most likely secondary to IP3-mediated intracellular calcium mobilization because both ET-1-induced IP3 and prostaglandin F2 alpha were inhibited by BQ123. These findings are the first demonstration of ET-1 (ETA-type) receptors linked to phospholipase C and phospholipase A2 activation in HBECs.

Arachidonic acid release and permeability changes induced by endothelins in human cerebromicrovascular endothelium

The vasoactive peptide endothelin-1 (ET-1) dose-dependently increased release of 51Cr from human cerebromicrovascular endothelial cells (HBEC), without affecting cell viability as assessed by lactate dehydrogenase release. ET-1 also induced transient accumulation of inositol triphosphate (IP3) and release of [3H] arachidonic acid (AA) from HBEC. The ET-1-induced 51Cr release, formation of IP3, and AA release from HBEC were competitively inhibited by selective ETA subtype receptor antagonist BQ-123. ET-1-stimulated 51Cr- and AA release from HBEC were potentiated by proteinkinase C (PKC) activator phorbol-myristate ester, and abolished by H7, an inhibitor of PKC. Dexamethasone, indomethacin, acetylsalicylic acid, imidazole, as well as the inhibitor of protein kinase A, H8, had no effect on 51Cr release. The results suggest that ETA-receptor mediated activation of PKC and increase in the HBEC 'permeability' for low molecular weight molecules in response to excessive release of endothelins from either HBEC or surrounding tissues during pathologic conditions may contribute to the formation of cerebral edema.

Reliability of amygdalohippocampal volume measured directly on MRI films

Volume measurement of the amygdalohippocampus on MRI may provide important information for epilepsy surgery. This study investigated the reliability of direct volume assessment (DVA) versus computer-assisted measurement (CAM) of amygdalohippocampus. DVA was carried out by counting the number of squares occupied by amygdalohippocampus in a transparent mm2 overlay used on both sagittal and coronal films. CAM was achieved using the 'thresholding and tracing' automated technique. Seventeen patients undergoing language-dominant left temporal lobectomy were studied. Mean volume as determined by the CAM was 4.99 +/- 1.31. In DVA, the mean volume was 5.05 +/- 1.23 (r = 0.974). Reproducibility of the volume measurement was tested by repeating measurements 5 times on 5 patient samples of amygdalohippocampal complex in DVA, and 3 times on 4 samples in CAM. The mean coefficient of variation for amygdalohippocampal volume was 6.9 +/- 3.2% in DVA and 3.9 +/- 2.0 in CAM. We also tested a more simplified method of assessing amygdalohippocampal volume by calculating the left to right volume ratio from MRI scans (2-40 films for each sample). Volume ratios determined directly from MRI films made in different months (14 samples) and of different sections (coronal and sagittal series, 18 samples) correlated well with each other (p < 0.01). Direct assessment of amygdalohippocampal volume can be used effectively for preoperative evaluation of the patient with epilepsy.

Prostaglandin D2 and endothelin-1 induce the production of prostaglandin F2 alpha, 9 alpha, 11 beta-prostaglandin F2, prostaglandin E2, and thromboxane in capillary endothelium of human brain

Endothelial cells derived from human brain capillaries (HBCEC) synthesize prostaglandin D2 (PGD2) which can be stimulated, among other prostanoids, by endothelin 1 (ET-1). Both the PGD2 induced by ET-1 and the exogenously added PGD2 to HBCEC are converted to 9 alpha, 11 beta-prostaglandin F2 (9 alpha, 11 beta-PGF2), a known potent vasoconstrictor. Exogenous PGD2 also dose-dependently enhanced the production of vasoconstrictive PGF2 alpha, thromboxane B2 (TXB2), and the vasodilatory PGE2 as well as cAMP by HBCEC. The PGD2-induced formation of PGF2 alpha, PGE2, and TXB2 was reduced by the cyclooxygenase inhibitors acetylsalicylic acid (ASA) or indomethacin (Indo), indicating for the first time that PGD2 may contribute to the formation of prostanoids in HBCEC. These results strongly suggest that PGD2 may play an important role in the regulation of cerebral capillary function under physiologic and pathologic conditions.

Profile of prostaglandins induced by endothelin-1 in human brain capillary endothelium

The vasoactive peptide, endothelin-1 (ET-1) has been implicated in the pathophysiology of various diseases. Recently, we have shown that human brain endothelial cells both secrete and express immunoreactive ET-1 high-affinity ETA receptors coupled to activation of phospholipase C (PLC). The present study demonstrates concentration-dependent stimulation of prostanoids [thromboxane B2 (TxB2), prostaglandin F2 alpha (PGF2 alpha), 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha) prostaglandin E2 (PGE2), and prostaglandin D2 (PGD2)] production by ET-1 in capillary endothelial cells derived from human brain (HBCEC). The increase in the vasoconstrictive prostanoids TxA2 and PGF2 alpha temporally preceded that of the vasodilatory PGI2, PGE2 and PGD2, and was seen after 15 min of incubation with ET-1 (10 nM). Increased production of vasodilatory prostanoids was observed between 4-8 h of incubation, whereas normalization of both vasoconstrictive and vasodilatory prostaglandins occurred 24 h after addition of ET-1. Both ET-1-stimulated prostanoid and IP3 production were inhibited by BQ123, a specific antagonist of ETA receptors. ET-1-induced prostanoid secretion by HBCEC was also inhibited by dexamethasone (50 microM) and diminished by neomycin (50 microM) and verapamil (10 microM) but not by nifedipine. Phorbol myristate ester potentiated ET-1-stimulated prostanoid secretion, whereas it inhibited IP3 production. Data indicate that ET-1 activates phospholipase A2 (PLA2) and PLC in HBCEC by different intracellular mechanisms. The subsequently induced secretion of vasoactive prostanoids by HBCEC may contribute both qualitatively and temporally to the vasoactive actions of ET-1.

Functional significance of the mu rhythm of human cortex: an electrophysiologic study with subdural electrodes

The existence of the mu rhythm and its general anatomical and physiological relationships are well known. There are few data, however, regarding the details of its anatomical and physiological specificity. We implanted fronto-temporal subdural electrode grids in 9 patients with intractable epilepsy to facilitate their surgical management. A 7-11 Hz cortical mu rhythm was observed in 5-16 electrodes located over the sensorimotor cortex as mapped by electrical stimulation. The mu rhythm was blocked by contralateral face and arm movements, passive movements of contralateral arm, and by ipsilateral arm movements. There was correspondence between the body area movement of which blocked the mu at a given site and the body region that was affected by stimulation at the same site. Power spectral analysis showed an overall decrease in power in all frequency bands. This was less prominent in the 14-100 Hz band resulting in a relative increase in high frequency power in association with movement. We conclude that both the presence and blocking of mu rhythm are specific to the somatic representation of the cortex from which it is recorded. Its functional significance may be similar to other sensory rhythms like the occipital alpha rhythm.

Decreased hippocampal muscarinic cholinergic receptor binding measured by 123I-iododexetimide and single-photon emission computed tomography in epilepsy

Regional binding of 123I-iododexetimide, a muscarinic acetylcholine receptor antagonist, was measured in vivo in the temporal lobes of 4 patients with complex partial seizures using single-photon emission computed tomography. In the anterior hippocampus ipsilateral to the electrical focus, 123I-iododexetimide binding was decreased by 40 +/- 9% (mean +/- SD, p < 0.01) compared with the contralateral hippocampus; 123I-iododexetimide binding in other temporal lobe regions was symmetrical. The data indicate a regionally specific change of muscarinic acetylcholine receptor in anterior hippocampus in complex partial seizures of temporal lobe origin.

Mirth, laughter and gelastic seizures

Little is known about what pathways subserve mirth and its expression laughter. We present three patients with gelastic seizures and laughter elicited by electrical stimulation of the cortex who provide some insight into the mechanisms of laughter and its emotional concomitants. The first patient had seizures manifested by laughter without a subjective feeling of mirth. Magnetic resonance imaging showed a cavernous haemangioma in the left superior mesial frontal region. Ictal subdural electrode recording showed the seizure onset to be in the left anterior cingulate gyrus. Removal of the lesion and of the seizure focus rendered the patient virtually seizure free over 16 months of follow-up. The other two patients had complex partial seizures of temporal lobe origin. Electrical stimulation of the fusiform gyrus and parahippocampal gyrus produced bursts of laughter accompanied by a feeling of mirth. These cases reveal a high likelihood of cingulate and basal temporal cortex contribution to laughter and mirth in humans, and suggest the possibility that the anterior cingulate region is involved in the motor act of laughter, while the basal temporal cortex is involved in processing of laughter's emotional content in man.

Progressive unilateral encephalopathy of childhood (Rasmussen's syndrome): a reappraisal

Twelve children with Rasmussen's encephalitis underwent hemispherectomy and have been followed for an average of 9.15 years. Our decision-making process leads to early, rather than late operation. Because hemiplegia is inevitable with or without operation, earlier operation may allow such children to resume a more normal life. We postulate that earlier operation may prevent some of the intellectual decline that accompanies the continued seizures and their treatment. In view of the variable pathologic findings and lack of evidence of a viral etiology, we suggest either use of the term "progressive unilateral encephalopathy of childhood" or "Rasmussen's syndrome" in recognition of its heritage.

Short latency somatosensory evoked potentials recorded around the human upper brain-stem

We analyzed the intracranial spatiotemporal distributions of the N18 component of short latency median nerve somatosensory evoked potentials (SSEPs) in 3 patients with epilepsy. In these patients, depth electrodes were implanted bilaterally into the frontal and temporal lobes, with targets including the amygdala and hippocampus; the latter two targets are close to the upper pons and midbrain. In this study N18 was divided into the initial negative peak (N18a) and the following prolonged negativity (N18b). Mapping around the upper pons and midbrain showed that: (1) the amplitude of the first negativity, which coincided with scalp N18a, was larger contralateral to the side of stimulation, but showed no polarity change around the upper brain-stem; and (2) the second negativity, which was similar to scalp N18b, did show an amplitude difference or a polarity change. This wave appeared to reflect a positive-negative dipole directed in a dorso-ventral as well as dorso-lateral direction from the midbrain, where positivity arises from the dorsum of the midbrain, contralateral to the side of the stimulation. Recordings from depth electrode derivations oriented in a caudo-rostral direction suggest that N18a and N18b may in part reflect neural activity originating from the upper pons to midbrain region which projects to the rostral subcortical white matter of the frontal lobe as stationary peaks.