Technical and clinical analysis of microEEG: a miniature wireless EEG device designed to record high-quality EEG in the emergency department

BACKGROUND

We describe and characterize the performance of microEEG compared to that of a commercially available and widely used clinical EEG machine. microEEG is a portable, battery-operated, wireless EEG device, developed by Bio-Signal Group to overcome the obstacles to routine use of EEG in emergency departments (EDs).

METHODS

The microEEG was used to obtain EEGs from healthy volunteers in the EEG laboratory and ED. The standard system was used to obtain EEGs from healthy volunteers in the EEG laboratory, and studies recorded from patients in the ED or ICU were also used for comparison. In one experiment, a signal splitter was used to record simultaneous microEEG and standard EEG from the same electrodes.

RESULTS

EEG signal analysis techniques indicated good agreement between microEEG and the standard system in 66 EEGs recorded in the EEG laboratory and the ED. In the simultaneous recording the microEEG and standard system signals differed only in a smaller amount of 60 Hz noise in the microEEG signal. In a blinded review by a board-certified clinical neurophysiologist, differences in technical quality or interpretability were insignificant between standard recordings in the EEG laboratory and microEEG recordings from standard or electrode cap electrodes in the ED or EEG laboratory. The microEEG data recording characteristics such as analog-to-digital conversion resolution (16 bits), input impedance (>100MΩ), and common-mode rejection ratio (85 dB) are similar to those of commercially available systems, although the microEEG is many times smaller (88 g and 9.4 × 4.4 × 3.8 cm).

CONCLUSIONS

Our results suggest that the technical qualities of microEEG are non-inferior to a standard commercially available EEG recording device. EEG in the ED is an unmet medical need due to space and time constraints, high levels of ambient electrical noise, and the cost of 24/7 EEG technologist availability. This study suggests that using microEEG with an electrode cap that can be applied easily and quickly can surmount these obstacles without compromising technical quality.

Perception of Phosphenes and Flashed Alphabetical Characters is Enhanced by Single-Pulse Transcranial Magnetic Stimulation of Anterior Frontal Lobe: The Thalamic Gate Hypothesis

Single pulses of transcranial magnetic stimulation (sTMS) restricted locally to the primary cortical areas for somatosensory and visual input, unlike the effects of repetitive stimulation, usually fail to elicit projected sensations. We tested the effect of sTMS over anterior frontal cortex in facilitating phosphenes from preceding sTMS over calcarine cortex, which alone was rarely effective in eliciting phosphenes. The combined sTMS elicited complex phosphenes, which changed with the site of frontal sTMS and the interstimulus interval. Our results show that sTMS over anterior frontal cortex also improved reporting of weakly illuminated, flashed four-letter stimuli, which permitted its statistical validation. We propose that the present demonstration of frontal cortical facilitation of visual awareness, when combined with the previous finding of projected paresthesias and sense of movement (Amassian et al, 1991 Brain114 2505 – 2520), provide evidence of a general frontal opening effect on a thalamic gate. Opening this gate facilitates entry of information from primary cortical receiving areas to thalamus. Thereby, the reciprocal thalamocortical interrelations that subserve conscious awareness of sensory stimuli could be fostered.

Role of the calcarine cortex (V1) in perception of visual cues for saccades

OBJECTIVE

To determine the initial level at which the pathways for cue perception, saccades and antisaccades diverge.

METHODS

Two procedures: single pulse transcranial magnetic stimulation (sTMS) over posterior occiput and backward masking were used. A visual cue directed saccades to the left or right, either a pro-saccade (to the side of the cue but beyond it) or an antisaccade, i.e., contraversive saccade. No visual target was presented.

RESULTS

Latencies of the two types of saccades did not differ. Focal sTMS applied unilaterally over V1 suppressed both perception of a cue flashed 80-90ms earlier contralaterally (but not ipsilaterally) and the appropriate saccade. Masking at a delay of 100ms abolished the appropriate saccade and cue perception.

CONCLUSIONS

V1 is essential for the perception of a flashed cue and for executing appropriate pro- and contraversive saccades. Masking may occur beyond V1, where the pathways for perception and for saccades at least to the next visual processing level start separating.

SIGNIFICANCE

VI is needed for rapid, accurate perceptual and motor responses to the crudest (left versus right) cues. It is unlikely that the "where" system can have a major direct input bypassing V1.

Interconnections between cortical areas revealed by transcranial magnetic stimulation

The fact that TMS of cerebral cortex is associated with inhibitory as well as excitatory properties is important because it makes it possible to investigate interconnections between cortical areas and tracing these functional interconnections by a noninvasive excitation or inhibition and temporary interference with the flow of impulses in the cerebral cortex. An important tool is thereby added to the analysis of higher cortical functions.

Cerebral function revealed by transcranial magnetic stimulation

Although transcranial magnetic stimulation (TMS) has been introduced only recently, it is safe and provides a painless, inexpensive noninvasive method for the evaluation of brain function. Determining central motor conduction time (CMCT) permits assessment of the corticospinal pathways. Mapping the central representation of muscles provides a method for investigating the cortical reorganization that follows training, amputation and injury to the central nervous system. Such studies of human plasticity may have important implications for neurorehabilitation. TMS also provides a method whereby cortical excitability can be noninvasively evaluated, which is likely to have important implications in the study of epilepsy, movement disorders and related conditions. TMS is useful in tracking the flow of information from one brain region to another and in investigations of cognition and functional localization, thereby complementing information obtained using functional imaging techniques, which have superior spatial but inferior temporal resolution. Finally, TMS is currently being investigated as a method for establishing cerebral dominance and as a therapeutic tool in the treatment of depression. Investigations for treatment of other neurologic and psychiatric conditions are likely to be undertaken.

Influence of pulse sequence, polarity and amplitude on magnetic stimulation of human and porcine peripheral nerve

1. Mammalian phrenic nerve, in a trough filled with saline, was excited by magnetic coil (MC)-induced stimuli at defined stimulation sites, including the negative-going first spatial derivative of the induced electric field along a straight nerve, at a bend in the nerve, and at a cut nerve ending. At all such sites, the largest amplitude response for a given stimulator output setting was elicited by an induced damped polyphasic pulse consisting of an initial quarter-cycle hyperpolarization followed by a half-cycle depolarization compared with a predominantly 'monophasic' quarter-cycle depolarization. 2. Simulation studies demonstrated that the increased efficacy of the induced quarter-cycle hyperpolarizing-half-cycle depolarizing polyphasic pulse was mainly attributed to the greater duration of the outward membrane current phase, resulting in a greater outward charge transfer afforded by the half-cycle (i.e. quarter-cycles 2 and 3). The advantage of a fast rising initial quarter-cycle depolarization was more than offset by the slower rising, but longer duration depolarizing half-cycle. 3. Simulation further revealed that the quarter-cycle hyperpolarization-half-cycle depolarization showed only a 2.6 % lowering of peak outward current and a 3.5 % lowering of outward charge transfer at threshold, compared with a half-cycle depolarization alone. Presumably, this slight increase in efficacy reflects modest reversal of Na+ inactivation by the very brief initial hyperpolarization. 4. In vitro, at low bath temperature, the nerve response to an initial quarter-cycle depolarization declined in amplitude as the second hyperpolarizing phase progressively increased in amplitude and duration. This 'pull-down' phenomenon nearly disappeared as the bath temperature approached 37 C. Possibly, at the reduced temperature, delay in generation of the action potential permitted the hyperpolarization phase to reduce excitation. 5. Pull-down was not observed in the thenar muscle responses to median nerve stimulation in a normal human at normal temperature. However, pull-down emerged when the median nerve was cooled by placing ice over the forearm. 6. In a nerve at subnormal temperature straddled with non-conducting inhomogeneities, polyphasic pulses of either polarity elicited the largest responses. This was also seen when stimulating distal median nerve at normal temperature. These results imply excitation by hyperpolarizing-depolarizing pulse sequences at two separate sites. Similarly, polyphasic pulses elicited the largest responses from nerve roots and motor cortex. 7. The pull-down phenomenon has a possible clinical application in detecting pathologically slowed activation of Na+ channels. The current direction of the polyphasic waveform may become a significant factor with the increasing use of repetitive magnetic stimulators which, for technical reasons, induce a cosine-shaped half-cycle, preceded and followed by quarter-cycles of opposite polarity.

Transcranial magnetic stimulation in study of the visual pathway

The authors critically reviewed experiments in which transcranial magnetic stimulation (TMS) and repetitive TMS (rTMS) of the higher visual pathway were used. Topics include basic mechanisms of neural excitation by TMS and their relevance to the visual pathway (excitatory and inhibitory effects), TMS and rTMS of calcarine cortex (suppression, unmasking, and phosphenes), TMS of V5 (suppression), TMS and rTMS of higher level temporoparietooccipital areas (perceptual errors, unmasking, and inattention), the role of frontal lobe output in visual perception, and vocalization of perceived visual stimuli (role of consciousness of linguistic symbols).

Serum ionized magnesium in premature and term infants

Magnesium is the element with the second highest concentration in the body and is found almost entirely in the intracellular compartment. The small serum component gives a poor representation of the active, physiologic state of the metal. This state is assessed much better by measuring ionized magnesium in the serum, which can now be performed with a sensitive ion-selective electrode. This study was undertaken to establish the normal serum ionized magnesium levels in newborn infants and to define normal serum ionized calcium/ionized magnesium ratios. Ninety-seven infants were investigated. Six were born before 32 weeks gestation, 28 between 33 and 37 weeks gestation, and 63 were term. Ionized magnesium levels were 0.69 +/- 0.14 mmol/L, 0.63 +/- 0.10 mmol/L, and 0.57 +/- 0.07 mmol/L in each group, respectively. These findings demonstrate a significant decline in serum ionized magnesium with increasing maturity. This decrease may relate to a greater need for magnesium uptake during earlier gestation, more magnesium-induced vasodilation to maintain adequate blood flow to developing tissues and organs, or immature parathormone function earlier in pregnancy. The progressive rise in serum ionized calcium/ionized magnesium ratios found herein supports the latter hypothesis.

Low levels of serum ionized magnesium are found in patients early after stroke which result in rapid elevation in cytosolic free calcium and spasm in cerebral vascular muscle cells

Ninety-eight patients admitted to the emergency rooms of three urban hospitals with a diagnosis of either ischemic stroke or hemorrhagic stroke exhibited early and significant deficits in serum ionized Mg2+ (IMg2+), but not total Mg, as measured with a unique Mg2+-sensitive ion-selective electrode. Twenty-five percent of these stroke patients exhibited >65% reductions in the mean serum IMg2+ found in normal healthy human volunteers or patients admitted for minor bruises, cuts or deep lacerations. The stroke patients also demonstrated significant elevation in the serum ionized Ca2+ (ICa2+)/IMg2+ ratio, a sign of increased vascular tone and cerebrovasospasm. Exposure of primary cultured canine cerebral vascular smooth muscle cells to the low concentrations of IMg2+ found in the stroke patients, e.g. 0.30-0.48 mM, resulted in rapid and marked elevations in cytosolic free calcium ions ([Ca2+]i) as measured with the fluorescent probe, fura-2, and digital image analysis. Coincident with the rise in [Ca2+]i, many of the cerebral vascular cells went into spasm. Reintroduction of normal extracellular Mg2+ ion concentrations failed to either lower the [Ca2+]i overload or reverse the rounding-up of the cerebral vascular cells. These results suggest that changes in Mg2+ metabolism play important roles in stroke syndromes and in the etiology of cerebrovasospasm associated with cerebral hemorrhage.

A new method using neuromagnetic stimulation to measure conduction time within the cauda equina

Using principles derived from electric field measurements and studies of phrenic nerve in vitro, neuromagnetic stimuli in humans were predicted to excite selective low threshold sites in proximal and distal cauda equina. Physical models, in which induced electric fields were recorded in a segment of human lumbosacral spine immersed in a saline filled tank, supported this prediction. Conclusions from the model were tested and confirmed in normal human subjects. Ipsilateral motor evoked potentials were elicited in lower limb muscles and striated sphincters by magnetic coil (MC) stimulation of both proximal and distal cauda equina. Over proximal cauda equina a vertically oriented MC junction and cranially directed induced current elicited a newly identified compound muscle action potential (CMAP). The F response latency and lack of attenuation when the target muscle was vibrated suggest that the proximal response is a directly elicited M response arising near or at the rootlet exit zone of the conus medullaris. Over distal cauda equina, lumbar roots were optimally excited by a horizontally oriented MC junction, and sacral roots by an approximately vertically oriented MC junction, eliciting CMAPs with similar appearance but shorter latency consistent with the known intrathecal lengths of the lower lumbar and sacral nerve roots. The induced current was usually most effective when directed towards the spinal fluid filled thecal sac. Normal subjects showed stable CMAP onset latencies elicited at proximal and distal cauda equina despite wide variation in amplitude. Thus, cauda equina conduction time can be directly calculated. This new method may improve the detection and classification of peripheral neuropathies affecting lower limbs and striated sphincters.

Intravenous magnesium sulfate rapidly alleviates headaches of various types

BACKGROUND

Circumstantial evidence points to the possible role of magnesium deficiency in the pathogenesis of headaches and has raised questions about the clinical utility of magnesium as a therapeutic regimen in some headaches.

METHODS

We evaluated the efficacy of intravenous infusion of 1 gram of magnesium sulfate (MgSO4) for the treatment of patients with headaches and attempted to correlate clinical responses to the basal serum ionized magnesium (IMg2+) level. We also determined if patients with certain headache types exhibit low serum IMg2+ as opposed to total serum magnesium. Using a case-control comparison at an outpatient headache clinic, a consecutive sample of patients presenting with a moderate or severe headache of any type were included in the study. Of the 40 patients in the study (mean age 38.2 +/- 9.4 years; range 14 to 55; 11 men [39.2 +/- 7.3 years] and 29 women [37.8 +/- 10.2 years]), 16 patients had migraines without aura, 9 patients had cluster headaches, 4 patients had chronic tension-type headaches, and 11 had chronic migrainous headaches. Total serum magnesium was measured with atomic absorption spectroscopy and a Kodak Ektachem DT-60. Sensitive ion selective electrodes were utilized to measure serum IMg2+ and ionized calcium (ICa2+); ICa2+/IMg2+ ratios were calculated.

RESULTS

Complete elimination of pain was observed in 80% of the patients within 15 minutes of infusion of MgSO4. No recurrence or worsening of pain was observed within 24 hours in 56% of the patients. Patients treated with MgSO4 observed complete elimination of migraine-associated symptoms such as photophobia and phonophobia as well as nausea. Correlation was noted between immediate and 24-hour responses with the serum IMg2+ levels. Immediate pain relief was observed in 32 (80%) of 40 patients (P < 0.001). In 18. of the 32 patients, pain relief persisted for at least 24 hours (P < 0.005). Of these 18 patients, 16 (89%) had a low serum IMg2+ level. Total magnesium levels in contrast in all subjects were within normal range (0.70-0.99 mmol/L). No side effects were observed, except for a brief flushed feeling. Of the 8 patients with no relief, only 37.5% had a low IMg2+ level. Patients demonstrating no return of headache or associated symptoms within 24 hours of intravenous MgSO4 exhibited the lowest initial basal levels of IMg2+. Non-responders exhibited significantly elevated total magnesium levels compared to responders. Although most subcategories of headache types investigated (ie, migraine, cluster, chronic migrainous) exhibited low serum IMg2+ during headache and prior to intravenous MgSO4, the patients with cluster headaches exhibited the lowest basal levels of IMg2+ (P < 0.01). All headache subjects except for the chronic tension group exhibited rather high serum ICa2+/IMg2+ ratios (P < 0.01, compared to controls).

CONCLUSIONS

Intravenous infusion of 1 gram of MgSO4 results in rapid relief of headache pain in patients with low serum IMg2+ levels. Measurement of serum IMg2+ levels may have a practical application in many types of headache patients. Low serum and brain tissue ionized magnesium levels may precipitate headache symptoms in susceptible patients.

Intravenous magnesium sulphate relieves migraine attacks in patients with low serum ionized magnesium levels: a pilot study

1. We tested the hypothesis that patients with an acute attack of migraine headache and low serum levels (< 0.54 mmol/l) of ionized magnesium are more likely to respond to an intravenous infusion of magnesium sulphate (MgSO4) than patients with higher serum ionized magnesium levels. 2. Serum ionized magnesium levels were drawn immediately before infusion of 1 g of MgSO4 in 40 consecutive patients with an acute migraine headache. 3. Pain reduction of 50% or more as measured on a headache intensity verbal scale of 1 to 10, occurred within 15 min of infusion in 35 patients. In 21 patients, at least this degree of improvement or complete relief persisted for 24h or more. Pain relief lasted at least 24h in 18 of 21 patients (86%) with serum ionized magnesium levels below 0.54 mmol/l, and in 3 of 19 patients (16%) with ionized magnesium levels at or above 0.54 mmol/l (P < 0.001). Mean ionized magnesium levels in patients with relief lasting for at least 24h were significantly lower than in patients with no relief or brief relief (P < 0.01). 4. Measurement of serum ionized magnesium levels may be useful in identifying patients with migraine headaches who may respond to an intravenous infusion of MgSO4.

Predictive value of serum ionized but not total magnesium levels in head injuries

Despite a wealth of recent literature and research on traumatic brain injury, very little has been applicable to diagnosing and treating this syndrome at a tissue level. Part of this problem is the inability to assess rapidly and early in the syndrome the degree or progression of brain injury at a tissue level using simple biochemical analytes. With this in mind, we designed a study in 66 human subjects, who presented with acute blunt head trauma, to determine whether free, ionized serum magnesium (IMg2+) and/or free, ionized serum calcium (ICa2+) levels correlated with the severity of head trauma (HT) and whether any predictive reliable patterns emerge. By using a new ion-selective electrode (ISE) for IMg2+, we have been able to determine IMg2+ and ICa2+ within minutes after sampling in the serum of patients early (1-8 h) after HT. These studies reveal that acute HT is associated with graded deficits (up to 62%, mean = 25%) in serum IMg2+, but not in total serum Mg, which are related to severity of injury based on CT scans and other diagnostic parameters. The greater the degree of injury, the greater the ICa2+/IMg2+ ratio. These ionic findings are compatible with the idea that early ischaemia after head trauma may be important in determining neurological outcome. Our findings provide the first evidence for divalent cation changes in blood after traumatic brain injury, which could be of both diagnostic and prognostic value in patients with traumatic brain injury.

Intravenous magnesium sulfate relieves cluster headaches in patients with low serum ionized magnesium levels

Patients with cluster headaches have been reported to have low serum ionized magnesium levels. We examined the possibility that patients with cluster headaches and low ionized magnesium levels may respond to an intravenous infusion of magnesium sulfate. Thirty-eight infusions of magnesium sulfate were given to 22 patients with cluster headaches. The mean ionized magnesium level prior to 23 infusions which provided relief for at least 2 days and enabled the patient to skip two or more attacks, was 0.521 +/- 0.016 mmol/L; this value was 0.561 +/- 0.016 prior to 15 infusions which were ineffective. These latter 15 infusions were preceded by higher total magnesium levels. The ionized magnesium level prior to the 23 effective infusions was below 0.54 mmol/L in 19 patients. Five of the 15 ineffective infusions were accompanied by basal ionized magnesium levels below 0.54 mmol/L. In 76% of the infusions, there was a correlation between a response and an ionized magnesium level below 0.54 mmol/L. Nine patients (41%) obtained clinically meaningful improvement. Spontaneous remissions and a placebo effect might have accounted for some of the improvement. However, this should have applied equally to all patients, regardless of the ionized magnesium level. Measurements of ionized magnesium may prove useful in elucidating the pathogenesis of cluster headache and in identifying patients who may benefit from treatment with magnesium.

Alcohol-associated acute head trauma in human subjects is associated with early deficits in serum ionized Mg and Ca

Acute head trauma (AHT) (caused by motor vehicle accidents that did not produce loss of consciousness or observed brain lesions on CT scan, or falls) was found to result in early (1-8 h after injury) serum deficits in ionized magnesium (IMg2+) and ionized calcium (ICa2+) assessed with ion-selective electrodes (ISEs). Total Mg (TMg) and other electrolytes as well as serum biochemical analytes were all within the normal reference ranges. AHT patients with acute alcohol intoxication (BAC > or = 150 mg/dl) or alcohol abuse (BAC > 200 mg/dl) demonstrated deficits (15-35% less than normal) in IMg2+, but serum TMg levels were normal as were electrolytes and serum biochemical analytes. AHT patients with alcohol intoxication or alcohol abuse required hospitalization for 1-3 days prior to release, whereas AHT patients without alcohol intoxication were released in less than 24 h. The ICa2+/IMg2+ ratio, a sign of increased vascular tone and vascular reactivity, was significantly elevated in AHT patients with alcohol intoxication but not in AHT patients without alcohol intoxication or abuse. These serum divalent cation changes early after traumatic brain injury could be of considerable practicable diagnostic value in the assessment of alcohol-associated head injury. Use of ion-selective electrodes to accurately measure IMg2+ could serve as a logical basis for monitoring the response of the body to AHT.

The polarity of the induced electric field influences magnetic coil inhibition of human visual cortex: implications for the site of excitation

Human perception of 3 briefly flashed letters in a horizontal array that subtends a visual angle of 3 degrees or less is reduced by a magnetic coil (MC) pulse given, e.g., 90 msec later. Either a round or a double square MC is effective when the lower windings or central junction region, respectively, are tangential to the skull overlying calcarine cortex and symmetrical across the midline. The modeled, induced electric field has peak amplitude at the midline, but the peak spatial derivatives lie many centimeters laterally. Thus, the foveal representation near the midline is closer to the peak electric field than to its peak spatial derivatives, i.e., excitation of calcarine cortex differs from excitation of a straight nerve. With an MC pulse that induces an electric field which is substantially monophasic in amplitude, the lateral-most letter (usually the right-hand letter) in the trigram is preferentially suppressed when the electric field in the contralateral occipital lobe is directed towards the midline. Inferences from using peripheral nerve models imply that medially located bends in geniculo-calcarine or corticofugal fibers are the relevant sites of excitation in visual suppression; end excitation of fiber arborizations or apical dendrites is considered less likely. This conclusion is supported by the fact that the induced electric field polarity in paracentral lobule for optimally eliciting foot movements is opposite to that for visual suppression, the major bends occurring at different portions of the fiber trajectories in the two systems.

Chronic daily headache--one disease or two? Diagnostic role of serum ionized magnesium

The entity of chronic daily headache (CDH) is well documented, but is not included in the current classification. We divided patients with CDH into groups with and without migrainous features. This division resulted in clearly distinguishable syndromes of daily migrainous headaches (DMH) and daily tension-type headaches (DTH). Family history of headaches was more common in patients with DMH. Patients in both groups had a high incidence of caffeine or drug overuse. The clinical division into DMH and DTH was supported by our finding of a higher incidence of disturbed magnesium (Mg) metabolism in patients with DMH. Of 26 patients with DMH, 8 (30.8%) had low serum ionized, but not total, Mg levels, and 16 (61.5%) had high ionized calcium/magnesium ratios. The corresponding numbers for the 22 patients with DTH were 1 (4.5%) and 8 (36.4%). These new laboratory measurements offer possible biological markers for the diagnosis of different headache syndromes.

Clinical studies with the NOVA ISE for IMg2+

The Nova ISE for IMg2+ was utilized to examine IMg2+ in plasma and serum of patients with a variety of pathophysiologic and disease syndromes (e.g., long-term renal transplants [LTRT], during and before cardiac surgery, migraine headaches, head trauma, pregnancy, chronic fatigue syndrome [CFS], non-insulin dependent diabetes mellitus [NIDDM], asthma and after excessive dietary intake of Mg). The results indicate that LTRT treated with cyclosporin A, migraine, head trauma, pregnancy, NIDDM, diseased pregnant, and asthmatic patients all on the average, exhibit significant depression in IMg2+ but not total Mg (TMg). Patients with CFS failed to exhibit changes in serum IMg2+ or TMg levels. Increased dietary load of Mg, for only 6 days, resulted in significant elevations of serum IMg2+ but not TMg. Correlations between the clinical course of several of these syndromes and the fall in IMg2+ were found. The Ca2+/Mg2+ ratio appears to be an important guide for signs of peripheral vasoconstriction and or spasm and possibly enhanced atherogenesis. Overall, the data point to important uses for ISE's for IMg2+ in the diagnosis and treatment of disease states.

Recognition potential: sensitivity to visual field stimulated

The recognition potential (RP) was distinguished from P3 and eye blink responses by its sensitivity to visual area stimulated. Images were flashed in upper and lower hemifields. Current source density profiles were computed, using 16 midline scalp electrodes. For P3 and eye blink profiles, the hemifield stimulated was not a significant factor. For the recognition potential, upper and lower field stimulation produced radically different profiles. An improved recognition potential signal was obtained by a new mathematical procedure. It used the difference in sensitivity to visual area stimulated to reject P3 and eye blink responses.

Measurement of information processing delays in human visual cortex with repetitive magnetic coil stimulation

Previous work disclosed that single magnetic coil (MC) pulses applied over human calcarine cortex could suppress perception of letters briefly presented, e.g. 80-100 ms earlier. Although individual MC stimuli presented 0-60 ms, or more than 140 ms after the visual stimulus were apparently ineffective, combinations of 2 or 3 MC pulses at such intervals temporarily depressed visual perception. Thus, progressing of such language information could be slowed, without being abolished. By contrast, when the first MC pulse was delivered 120 ms or later, a second MC pulse 40 ms later had no detectable effect, implying that calcarine cortex had already transmitted the information. Perceptual recovery of 5-character words initially occurred no earlier than that of random letters, nor or random letters vs. arbitrary linear patterns, implying that the processing delays in calcarine cortex were similar.

Unmasking human visual perception with the magnetic coil and its relationship to hemispheric asymmetry

Visual suppression by a magnetic coil (MC) pulse delivered over human calcarine cortex after a transient visual stimulus 80-100 ms earlier has been used to suppress the representation of a 'masking' visual stimulus and thus to unmask a 'target' visual stimulus given, e.g., 100 ms before the mask. The resulting target unmasking as a function of the interval between mask and MC pulse is approximately the inverse of the visual suppression curve. Arbitrary visual linear patterns can similarly be unmasked. At the long target-mask interval used, the site of masking is deduced to lie beyond calcarine cortex. In several right-handed subjects tested, powerful MC stimulation of the left (but not right) temporo-parieto-occipital cortex also led to (weaker) unmasking.

Deficiency in serum ionized magnesium but not total magnesium in patients with migraines. Possible role of ICa2+/IMg2+ ratio

It has been suggested that magnesium (Mg) may play a role in the pathogenesis of headaches. Serum and intracellular measurements of Mg in headache patients have produced inconsistent results. The recent development of an ion-selective electrode for Mg2+ allowed precise measurement of serum ionized magnesium (IMg2+) in patients with various headache syndromes. Low serum IMg2+ and a high ICa2+/IMg2+ ratio were found in 42% of patients having an attack of migraine, but only in 23% of patients with a severe continuous headache. Total serum Mg was normal in both groups of patients. However, in patients with low serum IMg2+ total serum Mg was lower than in patients with normal serum IMg2+. These results are compatible with the serotonin and vascular concepts of migraine pathogenesis. Low IMg2+ and a high ICa2+/IMg2+ would result in cerebral vasospasm and reduced blood flow in the brain. The activity of serotonin receptors can also be affected by changes in IMg2+ levels. The finding of a difference in IMg2+ levels in two different headache types suggests a possible novel classification of headaches and that migraine patients with a low serum IMg2+ or a high ICa2+/IMg2+ ratio may benefit from Mg supplementation.

Magnetic coil stimulation of straight and bent amphibian and mammalian peripheral nerve in vitro: locus of excitation

1. According to classical cable theory, a magnetic coil (MC) should excite a linear nerve fibre in a homogeneous medium at the negative-going first spatial derivative of the induced electric field. This prediction was tested by MC stimulation of mammalian phrenic and amphibian sciatic nerve and branches in vitro, immersed in Ringer solution within a trough, and identifying the sites of excitation by recording responses of similar latency to local electrical stimulation. Subsequently, the identified sites of excitation were compared with measurements of the induced electric field and its calculated first spatial derivative. A special hardware device was used to selectively reverse MC current direction and to generate predominantly monophasic- or polyphasic-induced pulse profiles whose initial phases were identical in polarity, shape and amplitude. When using the amphibian nerve preparation, a complication was excitation at low threshold points related to cut branches. 2. Reversal of monophasic current resulted in latency shifts corresponding approximately to the distance between induced cathode and anode. The location of each site of excitation was at, or very near, the negative-going first spatial derivative peaks of the induced electric field measured parallel to the straight nerve. Significantly, excitation of the nerve did not occur at the peak of the induced electric field above the centre of the 'figure of eight' MC junction. 3. A polyphasic pulse excited the nerve at both sites, by the negative-going first phase at one location, and approximately 150 microseconds later, by the reversed negative-going second phase at the other location. Polyphasic and monophasic pulses elicited responses with similar latency when the induced current flowed towards the recording electrode. 4. Straddling a nerve with non-coding solid lucite cylinders created a localized spatial narrowing and increase in the induced electric field, resulting in a lowered threshold of excitation. The corresponding closer spacing between first spatial derivative peaks was exhibited by a significant reduction in latency shift when MC current direction was reversed. 5. When a nerve is bent and the induced current is directed along the nerve towards the bend, the threshold of excitation is reduced there. Increasing the angle of the bend from 0 deg to more than 90 deg graded the decrease in threshold. 6. In a straight nerve the threshold was lowest when current was directed towards the cut end.(ABSTRACT TRUNCATED AT 400 WORDS)

Modelling magnetic coil excitation of human cerebral cortex with a peripheral nerve immersed in a brain-shaped volume conductor: the significance of fiber bending in excitation

To help elucidate some basic principles of magnetic coil (MC) excitation of cerebral cortex, a model system was devised in which mammalian phrenic nerve, or amphibian sciatic nerve with its branches was suspended in appropriate Ringer's solution in a human brain-shaped volume conductor, an inverted plastic skull. The nerve was recorded monophasically out of the volume conductor. The site of nerve excitation by the MC was identified by finding where along the nerve a bipolar electrical stimulus yielded a similar action potential latency. MC excitation of hand-related corticospinal (CT) neurons was modelled by giving the distal end of nerve attached to the lateral skull an initial radial (perpendicular) trajectory, with subsequent bends towards the base and posterior part of the skull; this nerve was optimally excited by a laterally placed figure 8 or round MC when the induced electric field led to outward membrane current at the initial bend. By contrast, nerve given a trajectory modelling CT neurons related to the foot was optimally excited when the coil windings were across the midline, but again when membrane current flowed outward at the first bend. Corticocortical fibers were modelled by placing the nerve in the anteroposterior axis lateral to the midline; with the round MC vertex-tangentially orientated, optimal excitation occurred at the bend nearest the interaural line, i.e., near the peak electric field. The findings emphasize the importance of orientation and direction of current in the MC and fiber bends in determining nerve excitation. The findings in the peripheral nerve-skull model help explain (1) why lateral and vertex-tangentially orientated MCs preferentially excite arm-related CT neurons directly and indirectly (through corticocortical fibers), respectively, and (2) why the MC orientations for optimally exciting directly arm and leg-related CT neurons differ.

Cerebello-frontal cortical projections in humans studied with the magnetic coil

Focal stimulation over human cerebellum with a figure 8 magnetic coil (MC) results in an evoked wave recorded from bipolar scalp electrodes on the interaural line and more anteriorly. In 3 subjects, the wave responses along the interaural line had latencies of 8.8-13.8 msec, lasted 17.4-29.0 msec and had a maximum amplitude of 14.4-26.8 microV. The responses were recorded more anteriorly from leads midway between the interaural line and frontal leads; responses recorded from frontal leads were up to 3.5 msec later. The evoked wave was preceded by a diphasic EMG response with a latency of 1.2-2.0 msec. Analysis of the averaged responses recorded by adjoining bipolar leads indicated that the response was predominantly surface positive and crossed. Control experiments eliminated eye movement and somatosensory input as explanations of the evoked response, thereby identifying it as a cortical response. The surface positive wave in humans was compared with the responses recorded in cat and monkey to cerebellar stimulation. The responses in humans could reflect dysfacilitation through MC activation of Purkinje cells, or feed-forward facilitation through transsynaptic or antidromic activation of dentate neurons. The latency of the surface positive wave exceeds that of cerebellar inhibition of MC elicited hand muscle responses, but the discrepancy is at least partly accounted for by the extra delay required to set up the indirect cortico-spinal component required for motoneuron discharge. Estimates made of the cerebello-frontal cortical and peripheral feedback loop times suggest that the central has less than one quarter the delay of the peripheral loop, which would be especially advantageous during fast skilled movements of the fingers.

Paraesthesias are elicited by single pulse, magnetic coil stimulation of motor cortex in susceptible humans

A minority of normal humans experience paraesthesias (usually tingling) projected to the contralateral hand in response to individual transcranial magnetic coil (MC) pulses. The cortical source of the paraesthesias was sought by comparing their incidence with that of muscle responses to focal MC stimulation with either a figure 8 MC or with edge stimulation of a tilted round MC in 4 susceptible subjects. In all 4, paraesthesias were best felt with MC stimulation either at, or anterior to sites yielding movement, implying an initial source in precentral gyrus (and possible premotor cortex), rather than parietal cortex. In the two subjects exhibiting the strongest paraesthesias, the threshold for the paraesthesias was less than that for movement in the relaxed arm. The optimal site of the paraesthesias within the hand was usually in the digits, but differed among subjects. Motor responses and paraesthesias following a given stimulus occurred at different sites in the hand, implying that excitation of differing sets of motor cortical neurons subserved sensory and motor responses. In only one subject were the paraesthesias sufficiently reproducible to warrant interacting electrical digital and transcranial MC pulses. The data suggested that central processing of the response to the MC pulse is slowed by an antecedent digital stimulus, but the delay for perception of each type of stimulus does not greatly differ. The central sense of movement (Amassian et al., 1989a) elicited by MC stimulation of motor cortex is compared with the paraesthesias. Both are attributed to brief, high frequency discharge by motor cortical neurons accessing the perceptual system more readily than after excitation of post-central gyrus, which requires prolonged repetitive stimulation (Libet et al., 1964). Given also the normal pattern of muscle responses in the 4 subjects, their paraesthesias are best explained by a heightened sensitivity of the perceptual system to the motor cortical response to MC stimulation.

Measurement of the electric field induced into inhomogeneous volume conductors by magnetic coils: application to human spinal neurogeometry

We measured the electric fields induced by round and figure "8" magnetic coils (MCs) in homogeneous and inhomogeneous volume conductors. In homogeneous media, the round MC held tangential (i.e., flat) to the volume conductor induced an annular electric field. When the round MC was held on-edge (i.e., orthogonal) to the volume conductor, the induced electric field consisted of two loops mainly parallel to the surface of the volume conductor and which approximated each other directly under the contacting edge of the MC. The tangentially oriented figure "8" MC similarly induced two electric field loops which approximated one another maximally under the region of the junction in its long axis. In a complex inhomogeneous volume conductor, such as a segment of human cervical-thoracic vertebral spine located eccentrically within a large cylindrical tank and submerged in isotonic saline, the direction of electric fields within the spinal canal and across the intervertebral neuroforamina was similar to that observed in the homogeneous volume conductor. However, in and near a single neuroforamen, the electric field and especially its first spatial derivative were markedly elevated compared to that recorded within the long central axis of the vertebral canal. Motor unit and compound muscle action potentials elicited in limb muscles by MC stimulation of human cervical spine confirmed predictions derived from the physical model. The predictions included: (1) absence of spinal cord stimulation compared to relative ease of nerve root stimulation by current that is most likely concentrated at the neuroforamina. When stimulating current is directed towards the periphery, the most likely low threshold site of stimulation is inferred to be just distal to the neuroforamina. It is emphasized that with supramaximal stimulation, more distal sites of excitation may occur; (2) invariant latency shifts at threshold intensities when moving the MC along the rostrocaudal axis of the cervical vertebral column; (3) significant effect (on motor unit activation thresholds) of the direction of induced current flow across the neuroforamina; (4) reduced stimulation when the targeted nerve roots are close to the null point of the electric field, i.e., between locations of high electric field intensity, of opposite polarity; and (5) relatively focal nerve root stimulation by the junction of a transversely orientated figure "8" MC, i.e., parallel to the nerve roots.

Intraoperative recordings of spinal somatosensory evoked potentials to tibial nerve and sural nerve stimulation

Somatosensory evoked potentials (SSEPs) to stimulation of the tibial nerve at the knee (TN-K) and ankle (TN-A), and the sural nerve at the ankle (SN-A), were recorded from 3 or 4 spinal levels during surgery for scoliosis in 11 neurologically normal subjects. With stimulation of all 3 nerves, the propagation velocity along the spine was nonlinear: it was faster over cauda equina and midthoracic cord than over caudal spinal cord. Over the mid-thoracic cord, TN-K SSEP propagation was faster than that of TN-A and SN-A SSEPs, whereas over the caudal spinal cord these values were similar on stimulation of all 3 nerves. These data suggest that fast conducting second order afferent fiber systems contribute to spinal cord SSEPs evoked by stimulating both mixed and cutaneous peripheral nerves.

Stimulation of the human nervous system using the magnetic coil

The magnetic coil (MC) is a unique probe that can be used to elucidate basic neurophysiological mechanisms in humans. Either by excitation or inhibition of responding neural elements, we have been able to investigate: (1) the distribution of the electric field induced within isotropic and anisotropic volume conductors by round and figure-eight MCs; (2) the theoretical relationship between electric field distribution and excitation of distal peripheral nerve, nerve root, cranial nerve, and motor cortex; (3) the effect of focal MC stimulation of motor and visual systems; (4) perturbation of sequential digit movements by MC stimulation of human premotor cortex; (5) activation of frontal motor areas related to speech; (6) elicitation of a sense of movement in an ischemic paralyzed limb by focal MC cortical stimulation; and (7) the effect of stimulation of the human visual system to (a) suppress and unmask visual perception using single MC stimuli and (b) prolong visual suppression using short trains of MC stimuli. In the future, prolongation of MC action by using repetitive stimuli should be useful in further investigating functions concerned with language, speech, and cognition.

Spatial distribution of the electric field induced in volume by round and figure '8' magnetic coils: relevance to activation of sensory nerve fibers

The electric fields induced in finite homogeneous volume conductors by a round and a figure '8' magnetic coil (MC) were measured and related to MC stimulation of the median nerve. The volume conductors, filled with isotonic saline, consisted of a large rectangular trough ('unrestricted') and a smaller trough, whose dimensions approximated human forearm ('restricted'). Various MC orientations were applied to the volume conductor. Bipolar recordings were obtained with a coaxial electrode, which measured the voltage gradient between the exposed edge of the cable shield and the central wire at its tip, 1 cm distant (a linear probe). The probe was moved in 3 dimensions, allowing computer reconstruction of the electric field as a function of the 3 spatial axes. When the probe was parallel to the plane of the round MC and tangential to the direction of current in its windings, the induced electric field was maximal; it tended towards zero when the probe was over the center of the MC, or when the probe, remaining parallel to the plane of the MC, was radial (i.e., perpendicular) to the direction of the current in the windings. For a variety of MC orientations, the electric field was consistently increased when the probe was adjacent and parallel to the edge of the trough, indicating the important effect of boundaries. The electric field was greatly increased focally when the round MC was applied orthogonally to the volume conductor, or when the figure '8' MC was applied tangentially (i.e., flat) to the volume conductor. With the figure '8' MC, a sharp central peak parallel to the long axis was bounded on each side by smaller (less than half amplitude) peaks. The findings from physical modeling led to correct predictions as to the most effective orientations of round and figure '8' MCs for eliciting sensory nerve action potentials (SNAPs) from the median nerve.

Brain stimulation revisited

The results of the following selected studies using magnetic coil (MC) stimulation are presented: (1) evidence for focality of MC stimulation, (2) MC stimulation of frontal areas related to speech, (3) transcallosal responses evoked by MC stimulation, and (4) suppression of visual perception with MC stimulation over occipital cortex. The authors served as subjects, and in most studies a standard Cadwell stimulator and round MC were used. Using a more vertical, rather than tangential, MC orientation and threshold stimulation, nearly isolated movements of individual digits were elicited implying focal cortical excitation. MC stimulation of frontal areas of either hemisphere elicited electromyography in contralateral laryngeal muscles. The shortest latency responses that were often accompanied by arm movement were thought to be elicited from intermediate areas of precentral gyrus and longer latency responses, from near Broca's area, extreme lateral precentral gyrus, and the supplementary motor area. MC stimulation over the occipital cortex resulted in suppression of visual perception of letters briefly flashed on a screen. The topography of suppression implicated the geniculocalcarine system as the site of MC effect. Focal MC stimulation of posterior frontal cerebral cortex elicited a transcallosal response from contralateral homologous cortex with a latency similar to that obtained with focal anodic stimulation but with considerably less excitation of cranial muscles.

Focal stimulation of human cerebral cortex with the magnetic coil: a comparison with electrical stimulation

Percutaneous stimulation of human motor cortex electrically (focal anode) and with magnetic coils (MCs) of various designs is compared. The theoretical prediction was confirmed that positioning the standard round MC laterally and orientating it more towards the vertical induces an electric field appropriate for directly exciting corticospinal neurons (cf., the conventional tangential orientation at the vertex). Thus, during voluntary contraction, minimal latency compound motor action potentials (CMAPs) in contralateral arm were elicited both by focal anodic and appropriately orientated MC stimulation. Conduction time from motor cortex to motoneuron was estimated by subtracting peripheral conduction time and monosynaptic delay at the motoneuron from the overall CMAP latency, yielding an estimated corticospinal conduction velocity as high as 66 m/sec. Discontinuous latency variations observed in population CMAPs or individual motor units approximated mono- or polysynaptic cortical synaptic delays and, therefore, are attributed to the intervals between direct and early, or late indirect corticospinal discharges. A TV computer system was used to track movements of individual digits and the hand following MC stimulation. An appropriately orientated MC readily elicited movements predominantly of a single digit, implying focal activation of motor cortex. A double square and a small pointed MC proved especially convenient for eliciting reproducibly single digit movements. Stronger stimulation revealed a topographical gradient in the responses of the different digits. Responses to a given MC stimulus a little above threshold were variable in amplitude, which could not be explained by the relationship of stimulus to phase of the cardiac or respiratory cycle. Overall, our findings indicate the importance of appropriately orientating a standard round MC and using a specially designed MC to obtain the various types of motor response to stimulation of cerebral cortex.

Comparison of human transcallosal responses evoked by magnetic coil and electrical stimulation

Human transcallosal responses (TCRs) were elicited by focal magnetic coil (MC) stimulation of homologous sites in contralateral frontal cortex and compared with those to focal anodic stimulation. With MC stimulation, the TCR consisted of an initially positive wave with an onset latency of 8.8-12.2 msec, a duration of 7-15 msec, and an amplitude which reached up to 20 microV, sometimes followed by a broad low amplitude negative wave. With anodic stimulation, a similar response was obtained in which the positive wave was similar in latency and maximum amplitude, but had a greater duration. With anodic stimulation, not only was the TCR threshold below that for contralateral movement, but it reached substantial size at intensities below motor threshold. With MC stimulation, contralateral arm movement and scalp corticomotor potentials were observed when the MC was displaced posteriorly towards the central sulcus. Unlike with anodic stimulation, the MC evoked TCR was usually not preceded by a prominent EMG potential from temporalis muscle and was not associated with subject discomfort. The TCR provides unique information concerning the functional integrity of callosal projection neurons, their axons and transsynaptic processes in recipient cortex. This information may prove useful in the evaluation of intrinsic cerebral mechanisms and in establishing cortical viability.

Suppression of visual perception by magnetic coil stimulation of human occipital cortex

Magnetic coil (MC) stimulation percutaneously of human occipital cortex was tested on perception of 3 briefly presented, randomly generated alphabetical characters. When the visual stimulus-MC pulse interval was less than 40-60 msec, or more than 120-140 msec, letters were correctly reported; at test intervals of 80-100 msec, a blur or nothing was seen. Shifting the MC location in the transverse and rostro-caudal axes had effects consistent with the topographical representation in visual cortex, but incompatible with an effect on attention or suppression from an eyeblink. The MC pulse probably acts by eliciting IPSPs in visual cortex. The neural activity subserving letter recognition is probably transmitted from visual cortex within 140 msec of the visual stimulus.

Focal stimulation of human peripheral nerve with the magnetic coil: a comparison with electrical stimulation

The hypothenar compound motor action potential (CMAP) response to ulnar nerve stimulation at the elbow was used as a test system in the human to compare excitations by a round magnetic coil (MC), 92 mm in outer diameter, and by electrical bipolar or tripolar stimulation. Optimal focality of excitation was obtained with the MC at 90 degrees to the extended arm and its plane parallel to the arm, i.e., an orthogonal-longitudinal orientation. Tangential orientation of the MC on the arm, i.e., laying it flat on the arm, powerfully excited additional structures. As expected from classical axonology, orthogonal-transverse orientations were the least effective. With orthogonal-longitudinal orientation and submaximal stimulation, the spread of excitation lateral to the median nerve at the wrist was of the order of 10-15 mm, the thickness of the MC being 12 mm. With the same orientation, the site of origin of the distally propagating impulse was estimated by comparing CMAP latencies to bipolar electrical (with cathode distal) and MC stimulation. Tripolar stimulation (with cathode intermediate) had no advantage over bipolar stimulation. The impulse originated 13-22 mm from the midpoint of the contacting edge. Rotating the MC through 180 degrees and thus reversing the field polarity did not significantly change the CMAP latency, indicating that the effective cathode and anode lay within a few mm of each other. Stimulating with a tilted MC resulted in a maximum CMAP when the orthogonal-longitudinal orientation failed to do so. A simple volume conductor model yielded a potential gradient of the right order of magnitude (35 mV/mm) when the MC stimulator output was 25% of maximum, i.e., a little above threshold for exciting ulnar motor axons.

A sense of movement elicited in paralyzed distal arm by focal magnetic coil stimulation of human motor cortex

Two magnetic coils (MCs) of special design (Cadwell Laboratories) were used to elicit movements predominantly of one or a few digits by percutaneous stimulation of human motor cortex. When cortically elicited movements were ischemically blocked, the MC still elicited a discrete sense of digit movement; a sense of movement map was constructed by stimulating at different scalp sites. Our findings support the existence of corollary discharge.

An analysis of peripheral motor nerve stimulation in humans using the magnetic coil

We compared conventional electrical and magnetic coil (MC) stimulation of distal median nerve in 10 normal subjects and 1 patient. Orthogonal (90 degrees to volar forearm)-longitudinal (the plane of the MC aligned with the long axis of nerve or wire), tilted (to 45 degrees) longitudinal, and tangential edge orientations elicited maximal or near maximal compound motor axon potentials (CMAPs) without simultaneous co-activation of ulnar nerve. Transverse and symmetrical tangential orientations were inefficient. A simulation study of an ideal volume conductor confirmed these findings by predicting that the maximum current density was near the outer edge of the MC and not at the center where the magnetic flux intensity is maximal. An orthogonal-longitudinal MC induces a current in the adjacent volume conductor (for example elbow or wrist), which flows in the same circular direction as in the MC. This differs from a tangentially orientated MC which classically elicits current flow in the volume conductor opposite in circular direction to that in the MC. Amplitude and latency of the CMAP were both altered, but not identically, by changing the intensity of MC and cathodal stimuli. Rotating an orthogonal-longitudinal MC through 180 degrees, thus reversing the direction of current flow, elicited single fiber muscle action potentials whose peak latencies differed at most by 100 microseconds. Thus, the (virtual) cathode and anode are significantly closer (i.e., 5-6 mm) with MC than with electrical stimulation where they are at least 20 mm apart. A disadvantage of MC stimulation is the imprecision in defining exactly where the distally propagating nerve impulse originates. In different subjects, using maximum output and orthogonal or tilted (to 45 degrees) longitudinal orientations, the calculated site of excitation in the median nerve varied 2-15 mm distal to the midpoint of the contacting edge of the MC. This limits the usefulness of the MC in its current configuration for determining distal motor latencies. Future advances in MC design may overcome these difficulties.

Comparison of scalp distribution of short latency somatosensory evoked potentials (SSEPs) to stimulation of different nerves in the lower extremity

SSEPs to stimulation of the CPN at the knee and PTN, PN and SN at the ankle were recorded from 15 cephalic sites and compared in 8 normal subjects. The configuration, amplitude, peak latency and distribution of P27, N35 (CPN) and P37, N45 (PTN, PN and SN) were analyzed. The configuration and distribution of SSEPs to stimulation of the 3 nerves at the ankle were similar across subjects. Both P37 and N45 were greatest in amplitude at the vertex and at recording sites ipsilateral to the side of stimulation. At contralateral sites either negative (N37) or negative, positive, negative potentials were recorded. The peak latency of N37 was the same or slightly less than that of P37. CPN-SSEPs were lower in amplitude and their configuration and scalp distribution showed much greater intersubject variability. This suggests that complex mechanisms which variably interact with one another are reflected in scalp SSEPs to CPN stimulation at the knee. The larger amplitude plus the minimal intersubject variability in morphology and topography of PTN-SSEPs indicate that this nerve is the most suitable for routine clinical use.

Intracranial stimulation of facial nerve in humans with the magnetic coil

Using ourselves as subjects, maximal compound motor action potentials (CMAPs) were evoked in ipsilateral nasal and orbicularis oculi muscles (onset latency 4.9-5.4 msec) by a magnetic coil (MC) tangentially oriented over parieto-occipital scalp. The facial nerve was also electrically stimulated sequentially at the posterior tragus near the stylomastoid foramen, anterior tragus and 3 cm more distally. Onset latency of the CMAP elicited at posterior tragus ranged from 1.0 to 1.3 msec less than that elicited by the MC over scalp. Because the measured distal facial nerve motor conduction velocity was 50-60 m/sec, the locus of impulse generation induced by magnetic coil stimulation was estimated to be approximately 6.5 cm proximal to the site of electrical stimulation at the posterior tragus, i.e., closer to the exit of the facial nerve from the brain-stem than to its entrance into the internal auditory meatus. This non-invasive technique should be useful in evaluating patients with peripheral facial nerve disorders including Bell's palsy.

Conduction characteristics of somatosensory evoked potentials to peroneal, tibial and sural nerve stimulation in man

Lumbar spine and scalp short latency somatosensory evoked potentials (SSEPs) to stimulation of the posterior tibial, peroneal and sural nerves at the ankle (PTN-A, PN-A, SN-A) and common peroneal nerve at the knee (CPN-K) were obtained in 8 normal subjects. Peripheral nerve conduction velocities and lumbar spine to cerebral cortex propagation velocities were determined and compared. These values were similar with stimulation of the 3 nerves at the ankle but were significantly greater with CPN-K stimulation. CPN-K and PTN-A SSEPs were recorded from the L3, T12, T6 and C7 spines and the scalp in 6 normal subjects. Conduction velocities were determined over peripheral nerve-cauda equina (stimulus-L3), caudal spinal cord (T12-T6) and rostral spinal cord (T6-C7). Propagation velocities were determined from each spinal level to the cerebral cortex. With both CPN-K and PTN-A stimulation the speed of conduction over peripheral nerve and spinal cord was non-linear. It was greater over peripheral nerve-cauda equina and rostral spinal cord than over caudal cord segments. The CPN-K response was conducted significantly faster than the PTN-A response over peripheral nerve-cauda equina and rostral spinal cord but these values were similar over caudal cord. Spine to cerebral cortex propagation velocities were significantly greater from all spine levels with CPN-K stimulation. These data show that the conduction characteristics of SSEPs over peripheral nerve, spinal cord and from spine to cerebral cortex are dependent on the peripheral nerve stimulated.

Human cerebral cortical responses to contralateral transcranial stimulation

We wished to develop a noninvasive electrophysiological measure of functioning of human cerebral cortex. The response chosen was that elicited by a corticocortical afferent input, specifically the transcallosal response (TCR). We used ourselves as subjects. The large shock artifacts associated with passing large currents through the head were reduced by: stimulating percutaneously with a focal anode (5 cm2) situated midway between F4 and C4 (i.e., frontal region, standard 10-20 nomenclature) and a medially located, semicircular grounded cathode (26 cm2) no closer than 3 to 4 cm away; and recording with the focal scalp electrode over homologous left frontal cortex vs. two or more reference electrodes interconnected through variable resistors and at least 6 cm away. Transcranial stimulation with pulses of 80 to 140 mA and lasting 100 microseconds elicited a brief diphasic deflection (latency, 2 to 4 ms), followed by an initially positive, often bifid wave with a latency of 9 to 14 ms, lasting 18 to 44 ms and reaching 5 to 10 microV. The initial diphasic deflection greatly increased when the stimulating anode was moved laterally over temporalis muscle, indicating an electromyographic (EMG) origin. The EMG, but not the positive wave, was elicited when the interpolar distance was reduced to 1 cm, which greatly reduced the fraction of current traversing the cortex (epicranial stimulation); furthermore, occluding the circulation to the scalp for 30 minutes by rubber tubing above the zygomas and brows dulled sensation, reduced the EMG, but did not alter the latency or rising phase of the positive wave. Early somatosensory components, if present, were small compared with the positive wave.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of conduction in central motor pathways: techniques, pathophysiology, and clinical interpretation

This communication summarizes the highlights of this symposium and reviews our current understanding of the clinical neurophysiology of conduction in central motor pathways produced by transcranial or direct brain stimulation. Direct (D wave) and indirect (I wave) volleys of descending impulses in the pyramidal system result from motor cortex stimulation and determine the measured responses. The advantages and disadvantages of the electrical and the magnetic stimulating modalities are assessed. The increased latency produced by magnetic as opposed to electrical stimulation is considered. Present clinical applications are discussed. The safety record is encouraging, but has aspects that must be addressed. Potential problems of the interpretation of motor evoked potentials in patients with central nervous system disease are emphasized in light of the pathophysiological mechanism underlying these potentials.