Role of the frontal lobes in the propagation of mesial temporal lobe seizures

The depth ictal electroencephalographic (EEG) propagation sequence accompanying 78 complex partial seizures of mesial temporal origin was reviewed in 24 patients (15 from the University of Pittsburgh Epilepsy Center and 9 from UCLA). All patients were monitored with bilateral mesial frontal and mesial temporal depth electrodes and later received anterior temporal lobectomy. Ictal EEG records were categorized according to sequence of spread from the temporal focus to the other regions. Although propagation patterns varied both within and between patients, certain features were notable: (a) It was very common for seizure activity to spread initially to the ipsilateral frontal lobe (observed in 22 of 24 patients). (b) The most common mode of spread (15 of 24 patients) was initiating temporal lobe----ipsilateral frontal lobe----contralateral frontal lobe----contralateral temporal lobe. (c) Occasionally, seizure discharges invaded the frontal lobes but failed to invade the contralateral temporal lobe (2 of 24 patients). (d) Seizure activity occasionally invaded the contralateral temporal lobe prior to invading the frontal lobes (2 of 24 patients). Other notable features included (i) a clear tendency for mesial temporal seizure discharges initially to invade orbitofrontal (as opposed to anterior cingulate) cortex and (ii) the emergence of a period of clear asymmetry in the frontal lobes during which high-amplitude, rapid discharges were present on the side ipsilateral to the initiating temporal lobe. These results suggest that the prefrontal region, especially the orbitofrontal cortex, is strongly influenced by mesial temporal ictal activity. This region appears to be frequently involved in the propagation of seizures initiated in the mesial temporal lobe and may play a role in the interhemispheric propagation of mesial temporal seizures.

Prognostic significance of independent auras in temporal lobe seizures

We performed a retrospective study of auras that occurred independently of complex partial seizures in patients undergoing continuous EEG monitoring with stereotactically implanted depth electrodes placed in mesiotemporal structures. Forty of 54 patients had a history of independent auras, and 32 of these individuals had independent auras while being monitored. Two hundred ninety-two independent auras were recorded, and EEG characteristics and subjective symptoms were analyzed with regard to surgical outcome. Most patients had ictal EEG changes with all or some of their auras. Four patients had nonlocalized or multifocal complex partial seizures despite the presence of well-localized auras, indicating that if intracranial ictal recording is used as a gold standard for localization, complex partial seizures must be recorded. Presence or absence of EEG change with auras did not affect prognosis nor did variability of ictal EEG pattern, spatial extent of initial field potential, aura duration, or character of symptoms correlate with prognosis or postoperative persistence of auras.

Quantitative comparison of cell loss and thiopental-induced EEG changes in human epileptic hippocampus

Thiopental-induced EEG beta activity recorded from mesial and lateral temporal lobe sites was analyzed both visually and by computer in 30 patients with complex partial seizures. All patients later received anterior temporal lobectomy. Volumetric cell densities and percentage of principal cell loss relative to normal controls were determined in several different areas of each resected temporal lobe. The percentage of drug-induced beta activity was then compared with the percentage of cell loss by (a) making precise topographic correlations between induced beta and cell loss, (b) relating the overall degree of left/right beta asymmetry to cell loss averaged over wide regions, and (c) comparing the likelihood of left/right beta asymmetries in patients with and without extreme cell loss. The results obtained were consistently negative and were unaffected by whether EEG data were analyzed by computer or visual inspection. These results indicate that the well-known loss of drug-induced beta activity found in epileptogenic regions is not an indicator of the degree of cell loss or sclerosis. The underlying focal dysfunction measured by EEG beta activity remains unknown, and factors such as focal hypoperfusion, hypometabolism, and the microanatomic features of the cells remaining in the focus should be investigated.

Computer enhancement of scalp-sphenoidal ictal EEG in patients with complex partial seizures

Color topographic maps of scalp/sphenoidal ictal EEG records were computer-generated in 5 patients with medically refractory complex partial seizures of suspected temporal lobe origin. Seven ictal EEG records were analyzed by parsing them into a series of 1.28 sec epochs. User-interactive, computer techniques were utilized to replace eye movement and other artifactual segments in the peri-ictal records with nearby uncontaminated EEG segments. Artifact replacement techniques were designed to minimize or eliminate EEG discontinuities in those epochs in which artifact was removed. Significance probability maps consisting of z scores were constructed by comparing theta band power for each 1.28 sec epoch composing the peri-ictal period with the mean and standard deviation of theta power computed from a pre-ictal baseline period consisting of 50-75 epochs. The resulting maps were compared with available independent clinical measures in order to validate their usefulness. The independent measures consisted of non-invasive data (visual impressions of raw scalp/sphenoidal ictal EEGs and positron emission tomography) and invasive data (depth ictal recordings, pathological findings in resected temporal lobes, and surgical outcome). Ictal topographic maps appeared to either confirm or provide new localizing information in all 5 patients. In 3 of 5 patients, several seconds of localized theta suppression preceded localized theta augmentation during the peri-ictal period. The transition between the 2 states was very rapid (approximately 2 sec). The spatial locus of suppression was always in agreement with the spatial locus of augmentation. In general, the maps were considerably easier to interpret than the raw scalp/sphenoidal ictal EEGs: (1) Localized theta suppression in the maps was not evident from visual inspection of the raw ictal scalp/sphenoidal EEG. (2) Clear localized theta augmentation in the maps occurred well before the appearance of unambiguous phase reversals in the raw scalp/sphenoidal ictal EEG. (3) Mapping data were correctly lateralizing in 1 patient in whom visual interpretation of the raw scalp/sphenoidal ictal EEGs was considered non-localizing but often suggestive of a seizure origin in the hemisphere contralateral to that implicated by depth recordings, PET, and pathological data. These results suggest that topographic mapping of scalp/sphenoidal ictal EEGs in patients with complex partial seizures may eventually prove to be a useful adjunct to the interpretation of raw ictal recordings.

Inter-hemispheric propagation of human mesial temporal lobe seizures: a coherence/phase analysis

Intra- and inter-hemispheric propagation of ictal discharges was analyzed with computer techniques in 10 patients with complex partial seizures of mesial temporal lobe origin in whom depth electrodes had been stereotaxically implanted. Coherence and phase analysis of seizure discharges was used to detect the emergence of linear relationships between all possible pairs of surface and depth recording derivations both between and within hemispheres. This analysis included mesial temporal, lateral temporal, and frontal lobe sites during both the onset and inter-hemispheric propagation of 28 ictal episodes. Although strong intra-hemispheric coherences and linear phase spectra reliably emerged in both the epileptogenic and non-epileptogenic hemispheres during seizure onset and contralateral spread, these relationships were usually not observed for inter-hemispheric comparisons. Only 3 of 10 patients demonstrated some degree of consistency in the emergence of significant wideband coherences and linear phase spectra between left and right mesial temporal sites during the inter-hemispherics propagation of ictal discharges. Mesial temporal lobe sites which demonstrated such a relationship included the amygdala, pes hippocampi, and parahippocampal gyrus. In 7 of 10 patients, lateral temporal derivations were sampled during ictal events; the emergence of linear relationships between left and right lateral temporal derivations during inter-hemispheric propagation was observed for only two. Various frontal lobe sites were monitored in 3 of the 10 patients; the emergence of linear relationships was observed only between left and right orbitofrontal derivations in the one patient for whom this region was sampled. These results suggest that the hippocampal commissure, parts of the corpus callosum, and parts of the anterior commissure may be relatively unimportant for the inter-hemispheric propagation of mesial temporal seizures in man. Future studies in non-human primates may reveal that ictal discharges which originate in the mesial temporal region propagate preferentially via brain-stem pathways to contralateral homologous regions.

Visual versus computer evaluation of thiopental-induced EEG changes in temporal lobe epilepsy

Thiopental-induced EEG beta activity was analyzed both visually and by computer in 33 patients with complex partial epilepsy. Studies were done in 16 patients with depth electrodes in limbic structures and 17 patients with scalp and sphenoidal electrodes. The percentage of drug-induced change in beta activity was quantified by computer using spectral analysis. The statistical significance of asymmetries between homologous sites in the amount of change was determined. The spatial distribution of significant asymmetries was used for localization and compared with the results of independent visual analysis of the thiopental EEG. Concordance between computer and visual evaluation occurred in 10 of 17 scalp/sphenoidal and 10 of 16 depth electrode tests. The accuracy of visual and computer localization was determined by comparing them with locus of itcal EEG onset, interictal spikes, and positron emission tomography. In scalp/sphenoidal studies, computer analysis indicating asymmetry appeared more likely to correlate with independent clinical criteria than visual analysis. In depth studies the reverse appeared to be true. Scalp/sphenoidal tests yielded positive results in 25-30% of patients whereas depth electrode tests were positive in 50-70% of patients. The results indicate that computer analysis of surface thiopental tests is an accurate and useful supplement to visual evaluation of these tests.

Interhemispheric propagation time of human hippocampal seizures: II. Relationship to pathology and cell density

The time required for hippocampal seizure activity to propagate to the contralateral hippocampal formation was evaluated in 57 patients with complex partial seizures and related to histological findings following temporal lobectomy. Interhemispheric propagation times were found to be significantly longer (greater than 20 s) in those patients diagnosed as having hippocampal sclerosis than in those patients without hippocampal sclerosis. Quantitative analysis of cell densities was made in 28 patients. Dentate fascia granule densities and hippocampal pyramidal cell densities were found to be lower in those patients with longer interhemispheric propagation times. However, this relationship was not observed when only patients with hippocampal sclerosis were considered. Thus, a tendency for hippocampal seizures to exhibit interhemispheric propagation times greater than 20 s is suggestive of underlying hippocampal sclerosis. However, information about interhemispheric propagation time does not appear useful for estimating the actual degree of cell loss within the sclerotic hippocampus. The association of longer interhemispheric propagation times with hippocampal sclerosis supports the hypothesis that direct commissural connections between the hippocampal formations in humans are either unimportant or missing.

Interhemispheric propagation time of human hippocampal seizures. I. Relationship to surgical outcome

This study evaluated ictal stereotaxic electroencephalogram (SEEG) records in 75 patients with complex partial seizures who later received anterior temporal lobectomy and were evaluated for long-term seizure relief. The time required for seizures to propagate from the putatively epileptogenic hippocampal formation to the contralateral hippocampal formation was measured from 615 ictal SEEG records. These interhemispheric propagation times were then compared with the degree of post-lobectomy seizure relief. Poor postsurgical seizure relief was associated with seizure propagation times of less than or equal to 5 s. Relief or reduction of seizures after surgery was associated with seizure propagation times greater than 50 s. These relationships were also found to occur in a subset of 56 patients who did not exhibit interhemispheric propagation times of less than 0.5 s, thus indicating that interhemispheric propagation times in the range of 0.5-5 s is a negative prognostic sign even in the absence of "bilaterally synchronous" ictal SEEG onsets. The finding of longer interhemispheric propagation times in patients who were improved by surgery may be accounted for by the greatly reduced size, or absence, of the hippocampal commissure in humans and suggests that the corpus callosum is a major, albeit indirect, route by which hippocampal foci may propagate seizure activity contralaterally. The finding of shorter interhemispheric propagation times in patients who did poorly after surgery may be accounted for by the existence of foci outside the region of excision with more direct access to callosal pathways or, alternatively, by the presence of damage in a more seizure-prone contralateral hippocampus.

Distribution of pyramidal cell density and hyperexcitability in the epileptic human hippocampal formation

Pyramidal cell densities in various regions of the anterior and posterior hippocampal formation were measured from en bloc temporal lobe resections and compared with presurgical stereoelectroencephalography (SEEG) data derived from depth electrodes in 12 patients with temporal lobe epilepsy. These data were compared with cell densities observed in four nonepileptic control patients. Patients who consistently exhibited anterior focal changes in the SEEG accompanying onset of ictus had cell densities that were selectively reduced in the anterior hippocampal formation but normal with respect to controls in the posterior hippocampal formation. Patients who exhibited more regional changes in the SEEG at onset of ictus had reduced cell densities in both the anterior and posterior hippocampal formation. Patients who exhibited focal spike activity in the anterior hippocampal formation as their predominant interictal SEEG pattern also had selectively reduced cell densities in the anterior hippocampal formation, while patients with widespread spiking throughout the hippocampal formation had reduced cell densities both anteriorly and posteriorly. These data support the concept that epileptogenesis occurs in or near those areas of epileptic hippocampus that are most damaged. Hippocampal sclerosis must be viewed as related to adjacent hyperexcitable or epileptogenic neurons and not solely as a passive result of repeated anoxia or ischemia.

Temporal lobe volumetric cell densities in temporal lobe epilepsy

Volumetric cell densities in 13 different subfields of the temporal lobe were calculated to test various hypotheses about mesial and lateral temporal lobe sclerosis in patients with complex partial epilepsy. In patients benefitting (primary group) from anterior temporal lobectomy (ATL), sclerosis was greater (fewer cells) in anterior than in posterior hippocampus. By contrast, the patients lacking full benefit (nonprimary group) from ATL had decreased numbers of neurons equally distributed from anterior to posterior hippocampus, indicating that zones of mesial temporal cell loss are linked to zones of epileptogenicity. These data support a model of focal hippocampal epilepsy originating from zones of cell loss and synaptic reorganization that is epileptic. There were no differences in cell densities in gyrus hippocampi or in lateral temporal gyri when patients with temporal lobe epilepsy and controls were compared. Hippocampal cell densities in mesial temporal lobe were not reduced in psychomotor epileptic patients with extrahippocampal foci consisting of foreign tissue. Variables in seizure histories were not correlated with Ammon's horn cell densities, indicating that most of the sclerosis preceded the seizures, which did virtually no significant further damage to hippocampus with repeated partial or generalized seizures.

Temporo-spatial patterns of pre-ictal spike activity in human temporal lobe epilepsy

The statistical properties of pre-ictal EEG spike activity in medial temporal lobe sites were analyzed in 6 patients with medically refractory complex partial seizures. A total of 24 1 h pre-ictal periods (2-6 periods per patient) were evaluated by quantifying the rate of occurrence of individual spatial patterns of spike activity derived from a subset (n = 6) of the recording channels. The channels chosen for analysis always included those medial temporal lobe sites which were documented to be most likely to initiate seizures, as well as their respective contralateral homologues. Each 1 h pre-ictal period was divided into 360 10 sec bins which were then visually classified into 1 of 64 possible spatial patterns of spike activity. These patterns, in turn, were grouped into 1 of 5 general spatial patterns and evaluated for trends across 3 20 min pre-ictal segments. Pooling these data across patients yielded the following results: (1) Focal patterns of spike activity tended to decline significantly in rate of occurrence several minutes prior to seizures, while the rate of independent contralateral patterns did not change. (2) The rate of occurrence of patterns of bilateral loosely coupled spike activity (involving focal and contralateral sites) increased significantly across the 20 min pre-ictal segments and was clearly augmented during the 20 min prior to seizures. These findings indicate that the degree of bilateral independence in medial temporal lobe spike activity tends to decrease several minutes prior to the localized onset of temporal lobe seizures; such changes may reflect the mechanisms responsible for the inter-ictal-ictal transition.

Changes in intelligence following temporal lobectomy: relationship to EEG activity, seizure relief, and pathology

Pre- and posttemporal lobectomy measures of intelligence and memory in 36 patients with medically refractory complex partial seizures were compared with (1) various aspects of presurgical ictal and interictal EEG activity derived from surface and deep electrodes, (2) postlobectomy seizure relief, and (3) pathological findings in the resected lobe. With respect to interictal EEG data, bilaterally synchronous surface spikes (accompanied or unaccompanied by simultaneous deep spikes) and sharp waves were significantly correlated with lower prelobectomy intelligence scores and a drop in these scores following lobectomy. With respect to ictal EEG data, bilaterally synchronous and multifocal onsets were significantly correlated with a postlobectomy drop in intelligence scores. Patients with poor postlobectomy seizure relief tended to have lower presurgical intelligence scores and a drop in intelligence scores following lobectomy. The patients most likely to show a postlobectomy drop in intelligence were those demonstrating some combination of poor seizure relief, an absence of pathology in the resected specimen, or EEG signs indicative of poor seizure relief. Postlobectomy changes in intellectual status are therefore not necessarily exclusively attributable to the amount of postlobectomy seizure relief experienced by these patients, but might be due to a combination of factors.

Surface and deep EEG correlates of surgical outcome in temporal lobe epilepsy

Interictal and ictal EEG characteristics derived from limited surface montages and medial temporal lobe sites were compared with long-term seizure relief following anterior temporal lobectomy in 52 epileptics. Patients were classified into one of four surgical outcome groups, ranging from seizure free to no clinical improvement. For each patient, interictal records were analyzed according to deep and surface spike characteristics and background activity. Ictal records were analyzed according to the proportion of episodes initiated in a unilateral or bilaterally synchronous fashion, the proportion of surface or deep onsets, the variability of onset location, and the morphology of seizures onsets. Interictal EEG variables that correlates with surgical outcome included: (a) various types of bilaterally synchronous surface/deep spikes; (b) diffuse background slowing; (c) sharp waves; and (d) the presence of multiple independent deep spike patterns in the lobe chosen for resection. Relevant ictal EEG variables included: (a) episodes initiated in a bilaterally synchronous fashion; (b) variability in seizure onset location; (c) the proportion of precisely focal onsets from deep sites; (d) the proportion of surface onsets; and (e) the proportion of onsets from the side chosen for resection. Multivariate analysis of these data with linear, stepwise, discriminate analysis and adaptive, nonlinear, distribution-free pattern recognition demonstrated that: (a) both interictal and ictal EEG characteristics can independently predict surgical outcome at levels significantly better than chance; (b) ictal and interictal EEG data contain nonredundant information for making such predictions; and (c) nonlinear pattern recognition techniques are capable of deriving the most accurate rules for predicting the effects of surgery.

Neuropathological findings following temporal lobectomy related to surface and deep EEG patterns

Interictal and ictal characteristics of preoperative EEG recordings, derived from limited surface montages, and medial temporal lobe sites, were compared with the results of pathological studies done on resected lobes obtained from 44 patients with complex partial seizures. Pathological material was divided into four groups: (a) sclerosis (mesial temporal or restricted to pes hippocampi); (b) neoplasia (mainly hamartomas); (c) miscellaneous lesions; and (d) no significant lesions. Interictal EEG correlates of no pathology included bilaterally synchronous surface spikes (with or without simultaneous deep spikes) and independent surface spikes (with or without simultaneous deep spikes) on the sides of lobectomy. Ictal EEG correlates of no pathology included unilateral surface or surface/deep onsets, bilaterally synchronous surface onsets, more than one onset location, and suppression at onset. Focal onsets correlated with sclerosis. Frequent interictal spike activity in the nonlobectomized lobe and fast buildup at onset of ictus suggested neoplasia. Many of the EEG correlates of no pathology are known to correlate with poor postsurgical seizure relief, due probably in part to the fact that absence of pathology in the resected specimen is a negative prognostic sign. Patients with sclerosis could be distinguished from patients with no demonstrable pathology with 81% cross-validation classification accuracy using a distribution-independent, nonlinear classifier. Both interictal and ictal EEG measures were used by the classifier, and one may conclude that ictal and interictal EEG recordings contain nonredundant information for predicting the presence and type of underlying pathology.

Surface and deep EEG correlates of surgical outcome in temporal lobe epilepsy

Interictal and ictal EEG characteristics derived from limited surface montages and medial temporal lobe sites were compared with long-term seizure relief following anterior temporal lobectomy in 52 epileptics. Patients were classified into one of four surgical outcome groups, ranging from seizure free to no clinical improvement. For each patient, interictal records were analyzed according to deep and surface spike characteristics and background activity. Ictal records were analyzed according to the proportion of episodes initiated in a unilateral or bilaterally synchronous fashion, the proportion of surface or deep onsets, the variability of onset location, and the morphology of seizures onsets. Interictal EEG variables that correlates with surgical outcome included: (a) various types of bilaterally synchronous surface/deep spikes; (b) diffuse background slowing; (c) sharp waves; and (d) the presence of multiple independent deep spike patterns in the lobe chosen for resection. Relevant ictal EEG variables included: (a) episodes initiated in a bilaterally synchronous fashion; (b) variability in seizure onset location; (c) the proportion of precisely focal onsets from deep sites; (d) the proportion of surface onsets; and (e) the proportion of onsets from the side chosen for resection. Multivariate analysis of these data with linear, stepwise, discriminate analysis and adaptive, nonlinear, distribution-free pattern recognition demonstrated that: (a) both interictal and ictal EEG characteristics can independently predict surgical outcome at levels significantly better than chance; (b) ictal and interictal EEG data contain nonredundant information for making such predictions; and (c) nonlinear pattern recognition techniques are capable of deriving the most accurate rules for predicting the effects of surgery.

Correlation of criteria used for localizing epileptic foci in patients considered for surgical therapy of epilepsy

Criteria for anterior temporal lobectomy, performed on seven patients with partial complex seizures, were derived from a battery of fourteen presurgical tests. Seven tests were routine studies aimed at identifying a focus of epileptic excitability, while seven were designed to reveal areas of focal functional deficit. Conflicting information was frequently obtained from the tests of epileptic excitability, suggesting that it is probably inaccurate to view patients with partial complex seizures as having a single epileptogenic focus. Presurgical evaluation must therefore be aimed at identifying the focus most responsible for the patient's habitual seizures. Tests of focal functional deficit provided useful nonconflicting confirmatory information in each of the seven patients studied. The most reliable information was obtained from depth electrode implantation, and this procedure should be considered essential except when all evidence of surface-recorded epileptic excitability, including ictal onset, and evidence of focal functional deficit agree.

Sleep state and seizure foci related to depth spike activity in patients with temporal lobe epilepsy

Depth spike activity was evaluated from medial temporal lobe sites using computer spike recognition techniques in all-night sleep records derived from 10 patients with medically refractory complex partial seizures. Sleep stages were classified into 1 of 4 groups: wakefulness, REM sleep, light sleep and deep sleep. Some disturbance in the periodicity of the sleep cycle was noted in most patients, but the relative proportions of REM sleep, light sleep and deep sleep were close to that reported for normals. Depth spike activity was observed to be most frequent in a majority of sites during deep sleep in 6 patients and during light sleep in 3 patients. In 1 patient equal numbers of sites showed maximal activation during light sleep and deep sleep. In 4 patients, certain sites in the more epileptogenic lobe demonstrated a maximal rate during waking or REM sleep. All patients reported in this study were considered to be suitable for temporal lobectomy. In contrast to the results obtained from a previous study, the side with the site demonstrating maximal mean spike rate did not necessarily correspond to the side chosen for lobectomy. Significant correspondence across patients between the more epileptogenic lobe and maximal spike rate was not found during waking and was further reduced during light sleep and deep sleep. The correspondence was, however, significant during REM sleep and for the side containing the site demonstrating the smallest activation in mean spike rate during light sleep or deep sleep relative to waking. These results indicate that an analysis of sleep induced changes in depth spike activity can be useful in improving predictions concerning epileptogenicity. Quantification of other aspects of the interictal EEG, such as background activity, may further improve such predictions.

Procaine-induced seizures in epileptic monkeys with bilateral hippocampal foci

Intravenous procaine HCl given at low doses (0.5-2.5 mg/kg) to two monkeys with bilateral alumina hippocampal foci depressed interictal spiking or had little effect. At 5.0 mg/kg unilateral limbic activation occurred. At 10.0 mg/kg unilateral or bilateral limbic activation and generalized seizures could be evoked within 3-10 min. At higher doses (15 and 20 mg/kg) bilateral limbic activation or brief (one min) generalized seizures occurred. The unilateral-onset psychomotor seizures were not identical to spontaneous psychomotor seizures, and the generalized seizures never occurred spontaneously in these monkeys. However, these results do indicate that procaine challenges may selectively activate limbic epileptogenic areas without activation of debilitating generalized tonic-clonic seizures.

Effects of chronic cerebellar stimulation on chronic limbic seizures in monkeys

No influences of chronic cerebellar stimulation were found in 10 different controlled experiments in 5 different monkeys with chronic alumina-induced psychomotor seizures. The stimulation parameters used were comparable to those used in human epileptics, and continuous daily EEG and behavioral monitoring allowed all seizures to be measured for daily frequency and duration over the several weeks of the experiments. Nocturnal seizures were similarly quantified in 3 monkeys to verify that cerebellar stimulation did not affect them. Motor cortex potentials evoked by cerebellar pulses confirmed that the stimulations were activating the cerebellum throughout the experiments, and measures of electrode access resistance and impedance verified that the electrodes remained in contact with the cerebellum. In one monkey given phenobarbital medication, interictal morbid behavior appeared to be improved by chronic stimulation of either cerebellum or dorsolateral frontal cortex, thus indicating an arousal influence of brain stimulation not due to cerebellum per se.

Neuropsychologic correlates of depth spike activity in epileptic patients

The statistical properties of depth spiking in the interictal EEG were assessed in 12 patients with intractable temporal lobe epilepsy in whom electrodes had been stereotactically implanted for diagnostic purposes. Prior to the implantation surgery each patient was administered a battery of neuropsychologic tests. Correlational analyses showed that the total amount of depth spike activity (TA) negatively correlated with measures of intelligence, while the laterality of the spike activity (LAT) appeared to be positively related to a memory score and the degree of psychologic independence. The results of this study are consistent with previous findings of a significant relationship between surface EEG recordings and performance. In addition, our data suggest a relationship between specific neuropsychologic functions and the patterns of in-depth EEG epileptiform discharges.

A sample and hold amplifier system for stimulus artifact suppression

A sample and hold amplifier system has been described which is capable of eliminating stimulus artifacts from a variety of biological recordings which would otherwise be impossible to interpret during electrical stimulation. Factors contributing to the prolongation of stimulus artifact are discussed in relation to the design requirements of this system. The application of this artifact suppression circuit to monitoring EEG seizures during electrical stimulation is demonstrated.

Tissue reactions to long-term electrical stimulation of the cerebellum in monkeys

Light and electron microscopic analyses were carried out on the stimulated and unstimulated paravermal cortices of six rhesus monkeys that had electrodes implanted on their cerebella for 2 months. The electrodes and the stimulation regime (10 p.p.s.: 8 min on, 8 min off) were similar to those used to stimulate the human cerebellum for treatment of certain neurological disorders. Mere presence of the electrode array in the posterior fossa for 2 months resulted in some meningeal thickening, attenuation of the molecular layer, and loss of Purkinje cells immediately beneath the electrode array. There was no evidence of scarring. After 205 hours of stimulation (7.35 X 10(6) pulses) over 18 days, a charge of 0.5 muC/ph or estimated charge density of 7.4 muC/sq cm/ph resulted in no damage to the cerebellum attributable to electrical stimulation per se. Such a charge/phase is about five times the threshold for evocation of cerebellar efferent activity, and might be considered "safe" for stimulation of human cerebellum. Charge density/phase and charge/phase were directly related to increased cerebellar injury in the six other cerebellar cortices stimulated. Leptomeningeal thickening increased with increased charge density. Injury included severe molecular layer attenuation, ongoing destruction of Purkinje cells, gliosis, ongoing degeneration of myelinated axons, collagen intrusion, and increased levels of local polysaccharides. In all cases, even with damage that destroyed all conducting elements beneath the electrodes, there was no damage further than 1 to 2 mm from the edges of the electrode arrays.

Electrophysiological studies of long-term electrical stimulation of the cerebellum in monkeys

Six rhesus monkeys were stimulated on the paravermal cortex for 205 hours (18 days) with different charge densities in order to determine the electrode performance and neural damage that may result from long-term cerebellar stimulation comparable to that being used in man. The electrode-tissue interface was relatively stable and no neural damage was found when the charge/phase (0.5 muC/ph) or charge density (7.4 muC/sq cm/ph) was very low. At all higher charge levels tested (2.4, 4.8, 10, and 22 muC/ph), changes in the electrode-tissue interface, meningeal encapsulation, and neural damage were directly related to the charge density delivered. Unstimulated electrodes on the opposite paravermal cortex exhibited mild tissue reactivity and cell damage, probably due to mechanical compression of the molecular layer and pial vessels. Motor cortex field potentials could be evoked by charges as low as 0.1 muC/ph delivered to paravermal cortex; for a given charge/phase longer pulses were more effective than short pulses. After neural damage resulting from 205 hours of 4.8 muC pulses at 10 per second (total charge 14.76 C), the threshold for the motor cortex evoked potential increased by a factor of four or more. With the charge held constant to different-sized electrodes placed bilaterally in the same monkey, damage was greater under the smaller electtrode. This finding suggests that the charge density to cerebellar cortex must be controlled to avoid neural damage.

A comparison of EEG seizure patterns recorded with surface and depth electrodes in patients with temporal lobe epilepsy

Surface and depth EEG seizure patterns were compared in 34 patients with intractable temporal lobe epilepsy in whom depth EEG electrodes had been chronically implanted in order to localize epileptogenic sites with a view to surgery. EEG records accompanied by clinical seizures, auras, no behavioral changes, as well as records for which no behavioral observations had been made, were judged with respect to the manner in which seizure activity originating unilaterally in the depth of one of the temporal lobes spread to the surface. For each EEG record, the onset of seizure activity in depth was classified as being focal or regional in form, and seizure activity was judged as: (1) not spreading to the surface, (2) spreading bilaterally and synchronously to the surface, (3) spreading initially to the surface ipsilateral to the depth site(s) in which the electrographic seizure first appeared, or (4) spreading initially to the surface contralateral to the depth site(s) in which the seizure activity initially occurred. EEG seizure activity was found to be less likely to propagate to the surface for those records that were either unaccompanied by behavior changes or accompanied only by auras than for those records accompanied by clinical seizures. In records accompanied by clinical seizures, seizure activity commonly propagated to the surface in a bilateral and synchronous fashion and was also found to spread initially to the ipsilateral but not to the contralateral surface. Anatomical and electrophysiological data accounting for the occurrence of ipsilateral spread were discussed. Diagnostic usefulness of surface recordings during clinical seizures in temporal lobe epilepsy was discussed.

A comparison of EEG seizure patterns recorded with surface and depth electrodes in patients with temporal lobe epilepsy

Surface and depth EEG seizure patterns were compared in 34 patients with intractable temporal lobe epilepsy in whom depth EEG electrodes had been chronically implanted in order to localize epileptogenic sites with a view to surgery. EEG records accompanied by clinical seizures, auras, no behavioral changes, as well as records for which no behavioral observations had been made, were judged with respect to the manner in which seizure activity originating unilaterally in the depth of one of the temporal lobes spread to the surface. For each EEG record, the onset of seizure activity in depth was classified as being focal or regional in form, and seizure activity was judged as: (1) not spreading to the surface, (2) spreading bilaterally and synchronously to the surface, (3) spreading initially to the surface ipsilateral to the depth site(s) in which the electrographic seizure first appeared, or (4) spreading initially to the surface contralateral to the depth site(s) in which the seizure activity initially occurred. EEG seizure activity was found to be less likely to propagate to the surface for those records that were either unaccompanied by behavior changes or accompanied only by auras than for those records accompanied by clinical seizures. In records accompanied by clinical seizures, seizure activity commonly propagated to the surface in a bilateral and synchronous fashion and was also found to spread initially to the ipsilateral but not to the contralateral surface. Anatomical and electrophysiological data accounting for the occurrence of ipsilateral spread were discussed. Diagnostic usefulness of surface recordings during clinical seizures in temporal lobe epilepsy was discussed.