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

Language representation in the human brain: evidence from cortical mapping

The manner in which the human brain processes grammatical-syntactic and lexical-semantic functions has been extensively debated in neurolinguistics. The discreteness and selectivity of the representation of syntactic-morphological properties in the dominant frontal cortex and the representation of the lexical-semantics in the temporo-parietal cortex have been questioned. Three right-handed adult male neurosurgical patients undergoing left craniotomy for intractable seizures were evaluated using various grammatical and semantic tasks during cortical mapping. The sampling of language tasks consisted of trials with stimulation (experimental) and without stimulation (control) from sites in the dominant fronto-temporo-parietal cortex The sampling of language implicated a larger cortical area devoted to language (syntactic-morphological and lexical-semantic) tasks. Further, a large part of the fronto-parieto-temporal cortex was involved with syntactic-morphological functions. However, only the parieto-temporal sites were implicated with the ordering of lexicon in sentence construction. These observations suggest that the representation of language in the human brain may be columnar or multilayered.

Subacute electrical stimulation of the hippocampus blocks intractable temporal lobe seizures and paroxysmal EEG activities

PURPOSE

To investigate the clinical, electroencephalographic (EEG), and histopathologic effects of subacute electrical stimulation of the hippocampal formation or gyrus (SAHCS) on 10 patients with intractable temporal lobe seizures.

METHODS

Bilateral, depth, hippocampal or unilateral, subdural, basotemporal electrodes were implanted in all 10 patients for a topographic diagnosis of the site and extent of the epileptic focus before a temporal lobectomy. In all patients, antiepileptic drugs (AEDs) were discontinued from 48 to 72 h before a program of continuous SAHCS, which was performed for 2-3 weeks. Stimulation parameters were biphasic Lilly wave pulses, 130/s in frequency, 450 micros in duration, and 200-400 microA in amplitude. The stimuli were delivered 23 of every 24 h for the 2-3-week SAHCS period. The effects of SAHCS on the number of clinical seizures per day and the percentage of interictal EEG spikes per 10-second samples of maximal paroxysmal activity at the epileptic focus were determined daily during the 16 days of SAHCS. At the completion of this program, patients underwent an en bloc temporal lobectomy, and the histopathologic effects of SAHCS on the stimulated tissue were analyzed by means of light-microscopy studies.

RESULTS

In seven patients whose stimulation electrode contacts were placed within the hippocampal formation or gyrus and who experienced no interruption in the stimulation program, SAHCS abolished clinical seizures and significantly decreased the number of interictal EEG spikes at the focus after 5-6 days. The most evident and fast responses were found by stimulating either the anterior pes hippocampus close to the amygdala or the anterior parahippocampal gyrus close to the entorhinal cortex. Other surface, hippocampal, and basotemporal EEG signs predicted and accompanied this antiepileptic response. These included an electropositive DC shift and monomorphic delta activity at the medial hippocampal and parahippocampal regions, and a normalization of the background EEG activity and signs of slow-wave sleep in surface. depth, and subdural regions. In contrast, no evident antiepileptic responses or no responses at all were found in three patients when stimulation was either interrupted or when it was administered outside the hippocampus. Light microscopy analysis of the stimulated hippocampal tissue showed histopathological abnormalities attributable to the depth-electrode penetration damage or to the pial surface reaction to the subdural, Silastic electrode plate. However, no evident histopathological differences were found between the stimulated and nonstimulated hippocampal tissue.

CONCLUSIONS

SAHCS appears to be a safe procedure that can suppress temporal lobe epileptogenesis with no additional damage to the stimulated tissue.

Pericollicular approaches to the rhomboid fossa. Part II. Neurophysiological basis

OBJECT

The authors describe their technique of electrophysiological mapping to assist pericollicular approaches into the rhomboid fossa.

METHODS

Surgical approaches to the rhomboid fossa can be optimized by direct electrical stimulation of superficially located nuclei and fibers. Electrophysiological mapping allows identification of facial nerve fibers, nuclei of the abducent and hypoglossal nerves, motor nucleus of the trigeminal nerve, and the ambiguous nucleus. Stimulation at the surface of the rhomboid fossa performed using the threshold technique allows localization above the area that is located closest to the surface. Simultaneous bilateral electromyographic (EMG) recordings from cranial motor nerves obtained during stimulation document the selectivity of evoked EMG responses. With respect to stimulation parameters and based on morphometric measurements, the site of stimulation can be assumed to be the postsynaptic fibers at the axonal cone. Strict limitation to 10 Hz with a maximum stimulation intensity not exceeding 2 mA can be considered safe. Direct side effects of electrical stimulation were not observed.

CONCLUSIONS

Electrical stimulation based on morphometric data obtained on superficial brainstem anatomy defines two safe paramedian supra- and infracollicular approaches to the rhomboid fossa and is particularly helpful in treating intrinsic brainstem lesions that displace normal anatomical structures.

Conduction aphasia and the arcuate fasciculus: A reexamination of the Wernicke-Geschwind model

Wernicke, and later Geschwind, posited that the critical lesion in conduction aphasia is in the dominant hemisphere's arcuate fasciculus. This white matter pathway was thought to connect the anterior language production areas with the posterior language areas that contain auditory memories of words (a phonological lexicon). Alternatively, conduction aphasia might be induced by cortical dysfunction, which impairs the phonological output lexicon. We observed an epileptic patient who, during cortical stimulation of her posterior superior temporal gyrus, demonstrated frequent phonemic paraphasias, decreased repetition of words, and yet had intact semantic knowledge, a pattern consistent with conduction aphasia. These findings suggest that cortical dysfunction alone may induce conduction aphasia.

Motor and sensory mapping of the frontal and occipital lobes

In patients with intractable epilepsy, surgical resections are performed with the primary goal of improving seizure control. The risk is that the resections may also remove tissues crucial for normal activities. The goal of surgical planning is therefore to determine as accurately as possible the regions of seizure onset and the regions controlling important functions, so that one can determine what to remove and what to leave in place. Clinical functional localization has been performed using cortical stimulation for over half a century, using both intraoperative and extraoperative methods. Signal averaging also has been widely used. More recently, techniques based on analysis of EEG in the frequency domain have shown promise. The methods appear to accurately indicate the function of the region assessed but do not necessarily predict functional consequences of resection. We review these methods, their indications, and the results obtained by their use.

Risk and safety of repetitive transcranial magnetic stimulation: report and suggested guidelines from the International Workshop on the Safety of Repetitive Transcranial Magnetic Stimulation, June 5-7, 1996

Single-pulse transcranial magnetic stimulation (TMS) is a safe and useful tool for investigating various aspects of human neurophysiology, particularly corticospinal function, in health and disease. Repetitive TMS (rTMS), however, is a more powerful and potentially dangerous modality, capable of regionally blocking or facilitating cortical processes. Although there is evidence that rTMS is useful for treating clinical depression, and possibly other brain disorders, it had caused 7 known seizures by 1996 and could have other undesirable effects. In June 1996 a workshop was organized to review the available data on the safety of rTMS and to develop guidelines for its safe use. This article summarizes the workshop's deliberations. In addition to issues of risk and safety, it also addresses the principles and applications of rTMS, nomenclature, and potential therapeutic effects of rTMS. The guidelines for the use of rTMS, which are summarized in an appendix, cover the ethical issues, recommended limits on stimulation parameters, monitoring of subjects (both physiologically and neuropsychologically), expertise and function of the rTMS team, medical and psychosocial management of induced seizures, and contra-indications to rTMS.

Localizing Arithmetic Processes in the Brain: Evidence from a Transient Deficit During Cortical Stimulation

Although substantial progress has been made in characterizing the cognitive processes involved in simple arithmetic, the localization of these processes in the brain is not yet well understood. In this article we consider the localization of a specific arithmetic process, the retrieval of arithmetic table facts from memory. We report a single-patient study in which cortical stimulation was used to create transient disruption of brain activity in localized regions of the cortex. We show that stimulation at a left parietal site impaired performance on simple multiplication problems and further that the impairment reflected stimulation-induced disruption of arithmetic fact retrieval. Our findings support the hypothesis (e.g., Warrington, 1982) that the left parietal lobe is implicated in the arithmetic fact retrieval process.

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.

Determination of current density distributions generated by electrical stimulation of the human cerebral cortex

With the use of a 3-dimensional finite element model of the human brain based on structural data from MRI scans, we simulated patterns of current flow in the cerebral hemisphere with different types of electrical stimulation. Five different tissue types were incorporated into the model based on conductivities taken from the literature. The boundary value problem derived from Laplace's equation was solved with a quasi-static approximation. Transcranial electrical stimulation with scalp electrodes was poorly focussed and required high levels of current for stimulation of the cortex. Direct cortical stimulation with bipolar (adjacent) electrodes was found to be very effective in producing localized current flows. Unipolar cortical stimulation (with a more distant reference electrode) produced higher current densities at the same stimulating current as did bipolar stimulation, but stimulated a larger region of the cortex. With the simulated electrodes resting on the pia-arachnoid, as usually occurs clinically, there was significant shunting of the current (7/8 of the total current) through the CSF. Possible changes in electrodes and stimulation parameters that might improve stimulation procedures are considered.

Selective Interference with the Representation of Size in the Human by Direct Cortical Electrical Stimulation

A specific category in human cognition, size representation, was disrupted by the application of cortical electrical interference through a recently modified technique involving implantation of indwelling subdural electrode arrays. When subjected to electrical stimulation at a specific site, the subject was unable to access size information when questioned verbally, but showed no deficit if the size discrimination was presented visually. Verbal questions about size were answered correctly when the patient was not subjected to cortical interference. Other measures of verbal and visual comprehension for the categories of color, shape, orientation, movement, texture, and structure, tested under cortical interference, were normal. This clear-cut distinction between verbal and visual access to information about size, shown by a reversible block at a known and anatomically circumscribed site, provides further evidence that higher order neural processing is categorically represented.

Placebo-controlled pilot study of centromedian thalamic stimulation in treatment of intractable seizures

Stimulation of centromedian (CM) thalamic nuclei has been proposed as a treatment for seizures. We implanted programmable subcutaneous (s.c.) stimulators into CM bilaterally in 7 patients with intractable epilepsy to test feasibility and safety. Stimulation was on or off in 3-month blocks, with a 3-month washout period in a double-blind, cross-over protocol. Stimuli were delivered as 90-microseconds pulses at 65 pulses/s, 1 min of each 5 min for 2 h/day, with voltage set to half the sensory threshold. Stimulation was safe and well-tolerated, with a mean reduction of tonic-clonic seizure frequency of 30% with respect to baseline when stimulator was on versus a decrease of 8% when the stimulator was off. There was no improvement in total number of generalized seizures with stimulation, and treatment differences were not statistically significant. Stimulation at low intensity did not alter the EEG acutely, but high-intensity stimulation induced slow waves or 2-3 Hz spike-waves with ipsilateral frontal maximum. In an open-label follow-up segment with stimulator trains continuing for 24 h/day, 3 of 6 patients reported at least a 50% decrease in seizure frequency. There were no side effects. This pilot project demonstrated the feasibility of controlled study of thalamic stimulation in epilepsy, but further study will be needed to demonstrate efficacy.

Computer modelling of brain cortex excitation by magnetic field pulses

In spite of many clinical and experimental applications, the technique of transcranial magnetic stimulation still presents obscure aspects. This especially concerns safety parameters and the exact characterization of the current induced by a single magnetic pulse. The model proposed consists of an equivalent electric network derived by Maxwell's equations and applied to discretized magnetic resonance imaging of a normal subject. This model allows accurate prediction of current distribution, charge per phase and dissipated energy.

Motor and sensory cortex in humans: topography studied with chronic subdural stimulation

Classic neurosurgical teaching holds that once the Rolandic fissure (Rf) has been located, there are distinct differentiated primary motor and sensory functional units confined within a narrow cortical strip: Brodmann's Areas 4 and 6 for primary motor units in front of the Rf and 3, 1, and 2 for sensory units behind the Rf. To test this assumption, we examined in detail the records of cortical mapping done by electrical stimulation of the cerebral cortex via implanted subdural electrode grids in 35 patients with seizure disorders. Of 1381 stimulations of the electrode sites, 346 (25.1%) produced primary motor or motor-arrest and sensory responses in contralateral body parts: 56.8% were primary motor responses; 16.2% were motor-arrest; 22.5% were sensory; and the remaining 4.5% were mixed motor and sensory responses. Two-thirds (65.9%) of the primary motor responses were located within 10 mm of the Rf, and the remaining one-third (34.1%) were more than 10 mm anterior to the Rf or were posterior to the Rf. Furthermore, in the patient group with brain lesions, fewer than one-third (28.1%) of the responses were within the 10-mm narrow anterior strip. Our study reconfirmed that a significant number--at least one-third--of motor responses are distributed outside the classic narrow cortical strip. In patients with brain lesions, the motor representation is further displaced outside the narrow strip. This finding indicates that primary motor cortex may extend beyond the gyrus immediately anterior to the Rf.