Short-latency somatosensory evoked potentials

Effects of short-term immobilization of the upper limb on the somatosensory pathway: a study of short-latency somatosensory evoked potentials

[Purpose] Previous studies have reported that the nervous system is influenced during short-term cast immobilization. However, the effects of short-term inactivity on somatosensory information processing systems are not well understood. This study investigated the effect of 10 h of upper limb immobilization on the somatosensory pathway using short-latency somatosensory evoked potentials. [Participants and Methods] Twenty right-handed healthy participants (mean age 21.7 years) were enrolled in this study. The participants’ left hands and forearms were wrapped in a soft bandage at a 90° elbow flexed position. The participants were instructed not to move their left hand for 10 h. To obtain short-latency somatosensory evoked potentials, we used a multimodal evoked potential system. The left median nerve was electrically stimulated at a rate of 5 Hz for a duration of 0.2 ms. The intensity of the stimulus was adjusted to induce mild twitches of the thumb. The amplitudes and latencies of the short-latency somatosensory evoked potential components (N9, N13, and N20) were measured before and after immobilization. [Results] The amplitude of the N9 component significantly increased after immobilization. [Conclusion] Our results indicated that the changes in the excitability of the peripheral somatosensory nerve were due to 10 h of inactivity.

Electrophysiological correlates of changes in reaction time based on stimulus intensity

BACKGROUND

Although reaction time is commonly used as an indicator of central nervous system integrity, little is currently understood about the mechanisms that determine processing time. In the current study, we are interested in determining the differences in electrophysiological events associated with significant changes in reaction time that could be elicited by changes in stimulus intensity. The primary objective is to assess the effect of increasing stimulus intensity on the latency and amplitude of afferent inputs to the somatosensory cortex, and their relation to reaction time.

METHODS

Median nerve stimulation was applied to the non-dominant hand of 12 healthy young adults at two different stimulus intensities (HIGH & LOW). Participants were asked to either press a button as fast as possible with their dominant hand or remain quiet following the stimulus. Electroencephalography was used to measure somatosensory evoked potentials (SEPs) and event related potentials (ERPs). Electromyography from the flexor digitorum superficialis of the button-pressing hand was used to assess reaction time. Response time was the time of button press.

RESULTS

Reaction time and response time were significantly shorter following the HIGH intensity stimulus compared to the LOW intensity stimulus. There were no differences in SEP (N20 & P24) peak latencies and peak-to-peak amplitude for the two stimulus intensities. ERPs, locked to response time, demonstrated a significantly larger pre-movement negativity to positivity following the HIGH intensity stimulus over the Cz electrode.

DISCUSSION

This work demonstrates that rapid reaction times are not attributable to the latency of afferent processing from the stimulated site to the somatosensory cortex, and those latency reductions occur further along the sensorimotor transformation pathway. Evidence from ERPs indicates that frontal planning areas such as the supplementary motor area may play a role in transforming the elevated sensory volley from the somatosensory cortex into a more rapid motor response.

Characteristics of mid- to long-latency spinal somatosensory evoked potentials following spinal trauma in the rat

The purpose of this study was to develop and implement a new technique for repeated monitoring of spinal mid- to long-latency somatosensory evoked potentials (SpSEPs) during sciatic nerve stimulation following recovery from spinal cord injury (SCI) in rats. Results of this study showed significant reproducibility of SpSEP components between specimens (analysis of variance [ANOVA], p > 0.05) and recording days (ANOVA, p > 0.700) using this technique. SpSEP amplitudes were significantly reduced (approximately 50% of uninjured amplitude, ANOVA, p < 0.001) following SCI and remained depressed for 10 weeks post-injury. SpSEP amplitude following high-intensity stimuli (> 1 mA) correlated with BBB locomotor score (Pearson, R > 0.353, P < 0.001). Characteristics of the mid- to long-latency SpSEPs suggest these components may reflect the integrity of the lateral pain pathway within the spinothalamic tract (STT). The technique and data presented in this study may be useful in future studies aimed at quantifying spinal cord integrity following injury and treatment using the rat model of SCI.

Median nerve somatosensory evoked potentials recorded with cephalic and noncephalic references in central and peripheral nervous system lesions

Somatosensory evoked potentials (SSEP) to electrical stimulation of the median nerve by using cephalic and noncephalic references were studied to detect the generator sources of short latency evoked potentials in 29 patients with cerebral, brainstem, spinal and peripheral nerve lesions. Patients were divided into six groups according to the localization of their lesions: group 1: cortical and subcortical lesions, group 2: basal ganglion lesions, group 3: pons and mesencephalon lesions, group 4: diffuse cerebral lesions, group 5: cervical cord lesions, group 6: brachial plexus lesions. Potentials were recorded using cephalic and noncephalic references after median nerve stimulation. Evidence obtained from patients suggested the following origins for these short latency SSEPs: P9 may arise in brachial plexus, P11 in dorsal basal ganglions or dorsal column, P13 and P14 in the nucleus cuneatus and lemniscal pathways, N16 in subthalamic structures and most likely mid and lower pons, N18 from the thalamus and thalamocortical tract, and N20 from primary somatosensory cortex.

Effects of different concentrations of sevoflurane and desflurane on subcortical somatosensory evoked responses in anaesthetized, non-stimulated patients

Twenty-four patients were recruited and given either sevoflurane or desflurane as their sole anaesthetic. Each patient was given sequentially increasing or decreasing doses at 0.5 MAC intervals, and the median nerve somatosensory evoked response recorded after an equilibration at each concentration. The N20-P25 and P25-N35 amplitudes decreased with increasing agent concentration. However, for both agents the P15-N20 amplitude response was quadratic in shape. The peak inflection points were at 3.2% for sevoflurane and 4.9% for desflurane. There were no differences between the ascending and descending groups. This increase in activity in the midbrain at 'surgical' end-tidal anaesthetic concentrations suggests more complex neuroelectrical responses to anaesthesia than simple global suppression.

Anatomic origin of P13 and P14 scalp far-field potentials

After median nerve stimulation, noncephalic or earlobe reference montages enable one to record over the scalp a well-defined, positive far-field response, which has been labeled the P14 or P13-P14 complex. It has been ascertained that this wave is generated in the caudal brainstem. Its use is reliable and sometimes mandatory in assessing a number of diseases that affect primarily the brainstem, such as multiple sclerosis or coma. Because of its complex shape as well as discrepant findings in the literature, it is still debated whether this potential is produced by a single or by multiple serial generators. The authors present these different views and summarize the different recording methods, while bearing in mind that some recording techniques are more suitable for routine purposes and others are preferred in selected cases, when more information regarding caudal brainstem function is required.

Anatomic origin and clinical application of the widespread N18 potential in median nerve somatosensory evoked potentials

N18 is a broad negativity, with a duration of approximately 20 msec after positive far-field potentials and is recorded widely over the scalp using a noncephalic reference. Its origin has been controversial but its preservation after pontine or upper medullary lesion while loss after high cervical lesions suggested its medullary origin. Comparison with animal studies and direct recording studies in humans leads the authors to conclude that N18 is most likely generated at the cuneate nucleus by primary afferent depolarization. Namely, dorsal column afferents send collaterals to interneurons within the cuneate nucleus, which in turn synapse on presynaptic terminals of dorsal column fibers and depolarize them as a mechanism of presynaptic inhibition. In this way, an electrical sink is formed on presynaptic terminals, whereas their dorsocaudally situated axons serve as a source. The ventrorostral negative pole of the resultant dipolar potential must correspond to N18. The authors obtained a measure to evaluate medullary function objectively, and therefore N18 may be useful as a diagnostic tool for brain death. Usage of a C2S reference is essential for the accurate estimation of N18. Origins of other somatosensory evoked potential components related to the cuneate nucleus are also discussed.

Generators of short latency human somatosensory-evoked potentials recorded over the spine and scalp

Somatosensory evoked potentials (SEPs) are most commonly obtained after stimulation of the median nerve and the posterior tibial nerve. SEPs reflect conduction of the afferent volley along the peripheral nerve, dorsal columns, and medial lemniscal pathways to the primary somatosensory cortex. Short-latency SEPs are recorded over the spine and scalp. After posterior tibial nerve stimulation, the following waveforms are recorded: N22, W3, the dorsal column volley, N29, P31, N34, and P37. After median nerve stimulation, the brachial plexus volley, dorsal column volley (N11), N13, P14, N18, N20, and P22 potentials are recorded. We discuss the current state of knowledge about the generators of these SEPs. Such information is crucial for proper interpretation of SEP abnormalities.

Detailed analysis of the latencies of median nerve somatosensory evoked potential components, 2: Analysis of subcomponents of the P13/14 and N20 potentials

Detailed analysis of P13/14 and N20 wavelets was performed for 62 normal subjects and patients with various lesions along the somatosensory pathway. A histogram of the latencies of all the identified P13/14 wavelets (measured from P13/14 onset) demonstrated three latency-groups, which were named P13, P14a and P14b subcomponents. The relationship between the three newly identified subcomponents and the conventional naming of P13 and P14 was inconstant, indicating the ambiguity of the latter. P14b was most prominent in the contralateral central region, and therefore a P15 positivity slightly after P14b was often recorded in the CPc-Fz and CPc-CPi leads (CPc and CPi are centroparietal electrodes contralateral and ipsilateral to the stimulation). P14b/P15 was lost even in patients with cortical lesions, and thalamocortical fibers were assumed for its origin. The CPc-Fz and CPi-Fz leads registered a low negativity named broad N13', suggesting frontal predominance of the overall P13/14 complex. Both P13 and P14a were identified in a patient with a pontine lesion, and a caudal brainstem origin for both was suspected due to the onset of two repetitive bursts of the ascending lemniscal volley. We refuted the presynaptic origin of the scalp P13 potential and pointed out that a prolonged and/or polyphasic P11 frequently observed in patients with high cervical lesions can be mistaken as scalp P13. A histogram of the latencies of all the identified negative wavelets of N20 in the CPc-Fz lead (measured from N20 onset) revealed five definite latency-groups, which were named N20a, N20b, N20c, N20d and N20e subcomponents. The highest peak of N20 actually corresponded to either N20b, N20c or N20d, and this uncertainty, which must be related to intracortical processes, resulted in a large instability of the N20 peak latency as well as the age and sex dependence of the N20 onset-peak interval, both of which were demonstrated by our preceding study (Sonoo, M., Kobayashi, M., Genba-Shimizu, K., Mannen, T. and Shimizu, T. Detailed analysis of the latencies of median nerve SEP components, 1: selection of the best standard parameters and the establishment of the normal values. Electroenceph. clin. Neurophysiol., 1996b, 100: 319-331). Negative subcomponents in the CPc-NC lead and positive subcomponents in the Fz-NC lead constituted mirror images of each other, which suggested that these subcomponents were generated within area 3b.

Reduced variance of latencies in pudendal evoked potentials after normalization for body height

The value of the Somatosensory Evoked Potentials (SEP) in the assessment and detection of neurological disorders could be considerably enhanced if the normative standards of (SEP) characteristic parameters were normalized taking into account all other systematic sources of variance. The present study examines the influence of body height on the peak and interpeak latencies of the pudendal somatosensory evoked potentials. We examined the peak latency (L1) of the evoked potential recorded at the L1 vertebra and the onset latency (ONc) of the cortical evoked potentials, after stimulation of the pudendal nerve, as a function of body height in 40 normal male subjects (age 20-40 years). Significant positive correlation was found between both (ONc) latency and ONc-L1 interpeak latency and body height (H). Assuming that the latter is proportional to the length of the neural pathways, the experimental data were fitted using a theoretical model representing the conduction in the sensory neuraxis as a function of body height. Using the estimated fitting functions, we normalized our data with regard to a typical value of body height. The normalized values of the aforementioned latencies reveal a significantly reduced variance, as compared to the original ones, and consequently their diagnostic importance is significantly increased. Similar procedures applied to the L1 (spinal) latencies and the latencies of the bulbocavernosus reflex (BCR) reveal no correlation with body height and this is discussed on the basis of neuroanatomical considerations.

[Evoked potentials by median nerve stimulation (SSEP): subcortical components]

Este estudo constitui uma revisão de literatura realizada com a finalidade de se relacionar a designação, as características dos campos de potencial e os geradores implicados, para os componentes subcorticais do potencial evocado somatossensorial por estimulação do nervo mediano no punho.

Epilepsy in schizencephaly: abnormal cortical organization studied by somatosensory evoked potentials

Median nerve short-latency somatosensory evoked potentials (MN-SSEP) are recorded from the scalp to assess parietal lobe function and from the cortex to identify primary sensory and motor areas before epilepsy surgery. Nevertheless, the origins of many of the MN-SSEP waveforms and the reliability of this technique for localizing the central sulcus are not definitively known. We studied a child with a unilateral, closed, right parietal schizencephalic cleft and frequent simple partial seizures before the child underwent cortical resection. The sensory examination, neuroimaging, and electrical brain stimulation findings indicated a normal thalamus and an abnormal parietal lobe. Scalp-recorded MN-SSEPs showed intact widespread N18 potentials bilaterally, but absent right, although normal left parietal N20 and P27 waveforms. Cortically recorded MN-SSEPs could not localize the central sulcus owing to an absence of the expected negative potential over the right postcentral gyrus and the presence of waves with abnormal latencies over the precentral cortex. These findings suggest that: (a) the N18 potential probably originates at or below the level of the thalamus, (b) the N20 and P27 peaks are most likely generated by parietal cortex or white matter, and (c) cortically recorded MN-SSEPs can fail to localize the central sulcus before epilepsy surgery when congenital anomalies exist in the parietal lobe.

Possible model for generation of P9 far-field potentials

The distribution of P9 far-field somatosensory evoked potentials, after stimulation of the median nerve with a knee reference, was examined to determine the mechanism of the generation of P9 potentials. In addition to positive potentials (P9s), we found a negative potential (N9) recorded from the chest ipsilateral to the stimulation. The simulation of the distribution of these P9/N9 potentials by an electrical circuit diagram suggested the validity of this model for generation of the P9s.

Median nerve somatosensory evoked potentials: studies on latency variability as a function of subject height, limb length and nerve conduction velocity

Report on the results of regression analysis studies concerning median nerve somatosensory evoked potentials (SEPs) latencies, as dependent variables, and subject height, limb length and nerve conduction velocity (NCV), as independent variables. The tests were performed on 23 normal volunteers. Absolute SEP latencies could be predicted by a linear regression model when the independent variable was arm length; when it was subject height, however, both exponential and polynomial models proved better, the latter showing the best coefficients of determination, R 2. Multiple linear regression with two independent variables (arm length and NCV) was found to be better than simple linear regression for predicting P/N13 latency. The regression line for EP-P/N13 latency on height was found to be a polynomial curve; although the regression was found to be significant by the "F" test (alpha = 1%), the model had a low R 2 value (0.41). The same applies to the P/N13-N19 interpeak latency regression curve, but the regression was significant for alpha = 5% in that case. Although interwave latencies are the most useful parameters for clinical interpretation of median SEPs, absolute latencies may occasionally be important, and should be corrected for body size. In unusually tall subjects, it might be useful to double-check EP-P/N13 interwave latency prolongation by estimating the maximum expected P/N13 latency, using a model that takes into account both limb length and NCV.

Direct recording of somatosensory evoked potentials in the vicinity of the dorsal column nuclei in man: their generator mechanisms and contribution to the scalp far-field potentials

Somatosensory evoked potentials (SEPs) in the vicinity of the dorsal column nuclei in response to electrical stimulation of the median nerve (MN) and posterior tibial nerve (PTN) were studied by analyzing the wave forms, topographical distribution, effects of higher rates of stimulation and correlation with components of the scalp-recorded SEPs. Recordings were done on 4 patients with spasmodic torticollis during neurosurgical operations for microvascular decompression of the eleventh nerve. The dorsal column SEPs to MN stimulation (MN-SEPs) were characterized by a major negative wave (N1; 13 msec in mean latency), preceded by a small positivity (P1) and followed by a large positive wave (P2). Similar wave forms (P1'-N1'-P2') were obtained with stimulation of PTN (PTN-SEPs), with a mean latency of N1' being 28 msec. Maximal potentials of MN-SEPs and PTN-SEPs were located in the vicinity of the ipsilateral cuneate and gracile nuclei, respectively, at a level slightly caudal to the nuclei. The latencies of P1 and N1 increased progressively at more rostral cervical cord segments and medulla, but that of P2 did not. A higher rate of stimulation (16 Hz) caused no effects on P1 and N1, while it markedly attenuated the P2 component. These findings suggest that P1 and N1 of MN-SEPs, as well as P1' and N1' of PTN-SEPs, are generated by the dorsal column fibers, and P2 and P2' are possibly of postsynaptic origin in the respective dorsal column nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of somatosensory evoked potentials: simultaneous stimulation of two nerves

To analyse the mechanism by which sensory inputs are integrated, interactions of somatosensory evoked potentials (SEPs) in response to simultaneous stimulation of two nerves were examined in 12 healthy subjects. Right, left and bilateral median nerves were stimulated in random order so that a precise comparison could be made among the SEPs. The arithmetical sum of the independent right and left median nerve SEPs was almost equal within 40 msec of stimulus onset to that evoked by the simultaneous stimulation of bilateral median nerves. However, a difference emerged after 40 msec. The greatest difference was recorded after 100 msec. Sensory information from right and left median nerves may interact in the late phase of sensory processing. Left median, left ulnar, and both nerves together were stimulated. The sum of the SEPs of left median and ulnar nerves was not equal to that evoked by the simultaneous stimulation of the two nerves even at early latencies. Differences between them were first recorded at 14-18 msec and became greater after 30-40 msec. It is suggested that the neural interactions between impulses in the median and ulnar nerves begin below the thalamic level.

Bifurcation of P9 far-field potentials induced by changes in the shoulder position

Somatosensory evoked potentials to stimulation of the left median nerve were recorded from normal adults with reference to the right knee in the usual shoulder position and in an elevated shoulder position. A single peak of the P9 potentials in the former position bifurcated into two peaks in the latter position without changing the onset latency. This waveform change can be accounted for by changes in the resistance of the volume conductor around the nerve trunk.

The effect of stimulus frequency on post- and pre-central short-latency somatosensory evoked potentials (SEPs)

We assessed the influence of the stimulus frequency on short-latency SEPs recorded over the parietal and frontal scalp of 26 subjects to median nerve stimulation and 16 subjects to digital nerve stimulation. When the stimulus frequency is increased from 1.6 Hz to 5.7 Hz, the amplitude of the N13 potential decreases whereas the P14 remains stable. The amplitude of the N20 is not changed significantly whereas the P22, the P27 and the N30 decrease significantly. In 50% of the subjects 2 components can be seen within the frontal negativity that follows the P22: an early 'N24' component, which is not affected by the stimulus rate, and the later N30, which is highly sensitive to the stimulus frequency. The distinct amplitude changes of the N20 and P22 with increasing stimulus frequency is one among other arguments to show that these potentials arise from separate generators.

Origin of scalp far-field N18 of SSEPs in response to median nerve stimulation

To identify the origin of scalp-recorded far-field negativity of short-latency somatosensory evoked potentials to median nerve stimulation (designated N18), direct records were made from the thalamus and ventricular system during 4 stereotaxic and 3 posterior fossa operations. In the thalamus a negative potential with almost the same latency as the scalp N18 was restricted to the Vim nucleus, but there was a large positive potential in the VC nucleus and medial lemniscus. Vim negativity increased in amplitude when high frequency stimulation was given to the median nerve, indicative of a facilitation effect. In contrast, the amplitude of scalp N18 decreased at high frequency stimulus. Direct recordings made through the medulla oblongata to the mid-brain showed a negative potential with gradually increasing latency. Above the upper pons, there was stationary negativity with no latency shift. The similarity between this negative potential and N18 is shown by their having the same latency and same response to the amplitude reduction and latency prolongation produced by high frequency stimulus. Our data suggest that scalp N18 comes from brain-stem activity between the upper pons and the mid-brain rather than from the thalamus.

Selective effects of repetition rate on frontal and parietal somatosensory evoked potentials (SEPs)

The effects of changing the stimulus presentation rate on early parietal (N20-P25) and frontal (P22-N30) somatosensory potentials (SEPs), evoked by median nerve stimulation, were investigated in 15 normal subjects. Stimuli were presented at 0.1, 0.4, 1.0, 4.0 and 10/sec. Only minor latency changes, mainly for the frontal P22 component, were observed when the stimulus rate was increased up to 10/sec: while the frontal P22-N30 complex was more rapidly and severely reduced in amplitude than the parietal N20-P25 complex. The differential effects of stimulus presentation rate on early frontal and parietal SEPs support the hypothesis of separate neural generators and suggest that the choice of the stimulation frequency may be critical for the interpretation of diagnostic SEP studies.

Bilateral somatosensory evoked potentials in four patients with long-standing surgical hemispherectomy

Four patients were studied electrophysiologically 8 to 24 years after surgical removal of one cerebral hemisphere without damage to the striatum or diencephalon. Somatosensory evoked potentials (SEPs) to electrical stimulation of the median nerve on the left or right side were averaged and mapped out over the scalp. Stimulation on the side opposite to the missing hemisphere evoked brief P9 and P14 farfields and a slow N18 negative potential of 15- to 25-msec duration bilaterally. No additional focal response was detected over the remaining (ipsilateral) hemisphere for 60 msec after the stimulus. Because long-standing hemispherectomy entails massive retrograde degeneration of thalamocortical neurons, the preserved P14 and N18 responses must reflect neural activities generated below the thalamus that are volume conducted to the scalp bilaterally. The data clarify several current issues in the evaluation of SEP components.

Epidurally recorded cervical spinal activity evoked by electrical and mechanical stimulation in pain patients

Spinal SEPs to electrical and mechanical stimulation of the upper limb of the non-painful side in 7 pain patients were recorded from the cervical epidural space. In response to electrical stimulation of the median nerve, the longitudinal distribution of the spinal postsynaptic negativity (N13) along the cord had a distinct level of maximal amplitude at the C5 vertebral body. When recorded at increasing distances cranial or caudal to this level, the latency of N13 was successively prolonged, in agreement with a spread-out near-field generator in the dorsal horn. Similar patterns of distribution and levels of maximal amplitude were demonstrated for the N13 wave evoked by electrical stimulation of the ulnar and thumb nerves as well as by mechanical stimulation of the thumb ball. The amplitude ratios of the N13 waves evoked by electrical stimulation of the median nerve and the thumb nerves, and by mechanical stimulation of the thumb ball were 3.9 to 1.4 to 1. The slow positive wave (P18), which has been assumed to represent recurrent presynaptic activity, had a somewhat different distribution, with a lower maximal amplitude and a less marked falling off in amplitude along the cord, as compared to the N13 component. The initial presynaptic positivity (P10) appeared with an almost constant amplitude along the cord. Tactile stimuli produced responses with considerably longer latency and duration than those obtained with electrical stimulation. There seemed to be a non-linear relationship between the amplitude of the response and the depth of skin indentation. The presented data contribute a more detailed picture of epidurally recorded spinal SEPs than previous studies. They will serve as a reference for further analysis of SEPs evoked by stimulation of the affected side in pain patients, to explore whether the painful state is associated with altered SEPs before or after therapeutic intervention.

Use of somatosensory evoked responses in the prediction of outcome from coma

Present data on 60 comatose patients with head trauma, hypoxia and cerebrovascular disease suggested that SER may yield quantitative, useful information concerning the functional state of the cerebral cortex. To assess the prognosis of individual patients we propose to classify patients from various etiologies of coma into the following categories: I. If there is bilateral absence of cortical responses, irrespective of the etiology of coma, none of these patients recover. II. If the initial cortical responses in the first 24 hours are normal, then it is imperative that these should be repeated in the first week before any definitive prognosis can be given, (since as in one case, we noted on the fifth day there was distortion of amplitude of his response and eventually the cortical responses were unobtainable, therefore indicating a poor prognosis). III. Patients who have normal responses throughout the acute course of illness carry an excellent prognosis from coma of all etiologies, except with ischemic etiology. The prognosis remains favorable for recovery from coma, but these patients may remain with significant neurological deficits. IV. When there is a 75% drop in the amplitude of the responses, it indicates a poor prognosis for ultimate neurological recovery, and the majority of these patients will remain in a persistent vegetative state. V. In patients with intermediate reduction in amplitude, 25-50% carried a moderate prognosis, and the majority of these cases in our series were able to perform activities of daily living.

Origin of frontal N15 component of somatosensory evoked potential in man

Origin of the frontal somatosensory evoked potential (SEP) by median nerve stimulation was investigated in normal volunteers and in patients with localized cerebrovascular diseases, and the following results were obtained. (1) In normal subjects, SEPs recorded at F3 (or F4) contralateral to the stimulating median nerve were composed of P12, N15, P18.5 and N26. Similar components were recognized in SEP recorded at Fz. (2) In patients in whom putaminal or thalamic hemorrhages had destroyed the posterior limbs of the internal capsules, frontal N15 and parietal N18 (N20) disappeared. These components were also absent in patients with cortical (parietal) infarctions. Among these patients, the thalamus was not affected in cases with putaminal hemorrhages and cortical infarctions. These facts indicate that the generator of the frontal N15 does not exist in the thalamus but that it originates from the neural structure central to the internal capsule, which suggests a similarity to the generator of the parietal N18. Because N15 was recorded in the midline of the frontal region with shorter latency than parietal N18, the frontal N15 might represent a response to the sensory input of the frontal lobe via the non-specific sensory system.

Motor and somatosensory evoked potentials recorded from the rat

An accurate neurophysiological technique that is able to monitor both the sensory and motor tracts of the spinal cord is required to assess patients with injury or other lesions of the cord, and for the evaluation of experimental studies of cord injury. We have recorded and characterized the motor and somatosensory evoked potentials (MEPs and SSEPs) from 20 normal rats and from 16 rats with cord lesions. MEPs were elicited by applying constant current anodal stimuli to the sensorimotor cortex (SMC) with the responses recorded from microelectrodes in the spinal cord at T10 (MEP-C) and from a bipolar electrode placed on the contralateral sciatic nerve (MEP-N). SSEPs were elicited by stimulating the sciatic nerve and were recorded from the cord at T10 and the contralateral SMC. The MEP-C consisted of an initial D wave (mean latency 1.21 +/- 0.12 msec and 4 subsequent I waves, 11-14). The D wave was elicited at stimulation frequencies exceeding 100 Hz. The initial positive wave of the MEP-N (mean latency 3.09 +/- 0.19 msec) was followed by several slower components which were attenuated by repetition rates exceeding 8.2 Hz. The grand mean SSEP consisted of 7 peaks. Sectioning of the dorsal columns abolished the SSEP but spared the MEP. Complete cord transection abolished both the MEP and SSEP. These experiments demonstrate that the combined recording of MEPs and SSEPs is an accurate and easily performed method of monitoring the functional integrity of the rat cord, and suggest that this technique would be of value in patients, especially those undergoing operative treatment of spinal lesions.

Specific gating of the early somatosensory evoked potentials during active movement

The gating effect of self-paced rapid flexion movements of the fingers on the early somatosensory evoked potentials following electrical stimulation of the median nerve at the wrist was studied in normal volunteers. Triggering of the median nerve stimulation by the EMG signals with a delay of 100 msec showed that the slow positive wave of the movement-associated potential was not directly responsible for the SEP amplitude variations observed. The nerve action potential at Erb's point as well as far-field components P9 and P11 were unchanged by the active movements. Far-field components P13-P14, which are presumably generated in the medial lemniscus, were not significantly modified. An enhancing effect on the widespread N18 component was found, which is in favour of a subcortical gating process. The parietal component N20 was unchanged by active movement interference whereas the frontal P22 component showed a marked suppression. A fronto-parietal dissociation was thus disclosed which could be in favour of separate cortical generators in the debate on the origin of SEP components. An important gating effect was observed on parietal P27 and frontal N30 components, the latter being considerably reduced in amplitude. The parietal P45 component showed no significant alteration. Each component of the early SEPs was thus distinctly influenced by the gating process during active movement interference.

Somatosensory evoked potentials from the thalamic sensory relay nucleus (VPL) in humans: correlations with short latency somatosensory evoked potentials recorded at the scalp

Somatosensory evoked potentials (SEPs) were recorded in humans from an electrode array which was implanted so that at least two electrodes were placed within the nucleus ventralis posterolateralis (VPL) of the thalamus and/or the medial lemniscus (ML) of the midbrain for therapeutic purposes. Several brief positive deflections (e.g., P11, P13, P14, P15, P16) followed by a slow negative component were recorded from the VPL. The sources of these components were differentiated on the basis of their latency, spatial gradient, and correlation with the sensory experience induced by the stimulation of each recording site. The results indicated that SEPs recorded from the VPL included activity volume-conducted from below the ML (P11), activity in ML fibers running through and terminating within the VPL (P13 and P14), activity in thalamocortical radiations originating in and running through the VPL (P15, P16 and following positive components) and postsynaptic local activity (the negative component). The sources of the scalp-recorded SEPs were also analyzed on the basis of the timing and spatial gradients of these components. The results suggested that the scalp P11 was a potential volume-conducted from below the ML, the scalp P13 and P14 were potentials reflecting the activity of ML fibers, the small notches on the ascending slope on N16 may potentially reflect the activity of thalamocortical radiations, and N16 may reflect the sum of local postsynaptic activity occurring in broad areas of the brain-stem and thalamus.

Atlantoaxial instability in Down syndrome: roentgenographic, neurologic, and somatosensory evoked potential studies

To identify patients with Down syndrome and asymptomatic atlantoaxial instability who are at increased risk for developing neurologic symptoms, we studied 27 patients with this skeletal disorder and compared them with an age- and sex-matched group of 27 patients with Down syndrome without atlantoaxial instability. A third group of six patients had symptomatic atlantoaxial instability. The mean atlanto-dens intervals and the mean spinal canal widths among the three groups were significantly different. There were no significant differences in mean composite neurologic scores and somatosensory evoked responses between patients in the asymptomatic group and those in the control group. However, when a subsample of patients with high and low latencies (greater than 1 SD below and above the mean) was formed and comparisons were made with roentgenographic findings, there was a high correspondence between somatosensory evoked potential latencies and atlanto-dens interval measurements. We conclude that no single assessment technique, but a combined approach using roentgenographic, CT scan, neurologic, and neurophysiologic investigations, will provide information of the risk status of patients with Down syndrome and atlantoaxial instability.

Scalp-recorded short and middle latency peroneal somatosensory evoked potentials in normals: comparison with peroneal and median nerve SEPs in patients with unilateral hemispheric lesions

Peroneal somatosensory evoked potentials (SEPs) were performed on 23 normal subjects and 9 selected patients with unilateral hemispheric lesions involving somatosensory pathways. Recording obtained from right and left peroneal nerve (PN) stimulations were compared in all subjects, using open and restricted frequency bandpass filters. Restricted filter (100-3000 Hz) and linked ear reference (A1-A2) enhanced the detection of short latency potentials (P1, P2, N1 with mean peak latency of 17.72, 21.07, 24.09) recorded from scalp electrodes over primary sensory cortex regions. Patients with lesions in the parietal cortex and adjacent subcortical areas demonstrated low amplitude and poorly formed short latency peroneal potentials, and absence of components beyond P3 peak with mean latency of 28.06 msec. In these patients, recordings to right and left median nerve (MN) stimulation showed absence or distorted components subsequent to N1 (N18) potential. These observations suggest that components subsequent to P3 potential in response to PN stimulation, and subsequent to N18 potential in response to MN stimulation, are generated in the parietal cortical regions.

Comparison in man of short latency averaged evoked potentials recorded in thalamic and scalp hand zones of representation

Recordings were performed in the thalamus of 13 patients suffering from either abnormal movements or intractable pain, with the aim of delimiting the region to be destroyed or stimulated in order to diminish the syndrome. In 11 of these patients averaged evoked potentials were recorded simultaneously from the scalp and specific thalamus (VP) hand area levels following median nerve stimulation. These recordings were done during the operation or afterwards when an electrode was left in place for a program of stimulation. The latencies of onsets and peaks on the scalp 'P15' were compared with those of the VP wave; a clear correspondence was found. Moreover, when increased stimulation was used, both waves began to develop in parallel. Thus in the contralateral 'P15' a component exists due to the field produced by the thalamic response. To explain the presence of an ipsilateral scalp 'P15' wave, we propose that a second wave having the same latency and a slightly shorter peak exists on the scalp due to a field produced by a brain-stem response. This double origin of 'P15' is also shown by the different changes which the ipsilateral and contralateral waves present during changes in alertness. The scalp 'N18-N20' is also composed of at least 2 components. The first peak appears on the scalp with a latency shorter than that of the negativity which develops in the thalamus. The N wave, moreover, increases in latency with rapid stimulus repetition. We propose with others that 'N18' is a cortical event reflecting the arrival of the thalamo-cortical volley. The second component, 'N20,' has a peak latency closely correlated to that of the thalamic negativity. This component was present alone in 'N' when rapid stimulation (greater than 4/sec) was used, which did not change the thalamic response. It must be a field produced by the thalamic negativity.

Simulation of 'stationary' SAP and SEP phenomena by 2-dimensional potential field modelling

In order to model the distribution of potentials in the hand due to antidromic SAP propagation and in the body due to afferent conduction of the median nerve volley, 2-dimensional matrices of the appropriate shape were constructed, each containing a 'generator' consisting of up to 3 'source' and 3 'sink' points. The value of the field potential at other sites was calculated using a finite difference method. It was shown that the potential gradient is virtually zero in matrix zones which are separated from the region containing the generator by a constriction in the boundary of the conductor. Points on the far side of the constriction remain virtually equipotential, at a level determined by the potential at the junction. This is naturally influenced by the proximity of the generator, so that as the generator approaches the constriction a potential difference will develop between points on the far side, irrespective of their distance from the junction, and other remote parts of the matrix. In the context of human SAPs and SEPs, such factors may be of paramount importance in the generation of so-called 'stationary' or 'far-field' potentials. With additional postulates concerning the manner in which the SAP is attenuated by the termination of axons as it propagates through the hand, and the course taken by the median nerve volley between the arm and neck, it was possible to model the majority of stationary SAP phenomena described by Kimura et al. (1984), and also the distribution and latency of the P9 SEP component following median nerve stimulation.

Short latency SEPs in infants and children: developmental changes and maturational index of SEPs

Short latency SEPs were studied in 56 normal subjects, aged 1 month to 34 years. To identify wave components definitely, simultaneous recordings not only with bipolar leads but also with superimposing methods (comparing the recordings with postcentral- and frontal-non-cephalic references (NC), contralateral to the stimulation side) were obtained in addition to routine recordings with postcentral-NC leads. Four positive (P1, P2, P3 and P4) and 2 negative (N' and N1) peak components were usually identified in all age groups, which represented the developmental changes in their features. By the method of serial representation of SEPs at each age, P1, P2, P3, P4, N' and N1 in children were found to correspond to P9, P11, P13, P14, N18 and N20 in adults, respectively. The features of SEPs in children showed great maturational changes until adolescence when they showed adult patterns. The values of the latencies of P1 and P2 divided by body height, which may be regarded as a maturational index mainly in the peripheral part of the sensory pathway, decreased with age, reaching the adult range at 3-4 years of age. The value of the N1-P3 interpeak latency divided by head circumference, which may be regarded as a maturational index mainly in the central part of the sensory pathway, also decreased with age, reaching the adult range at school age. These indices may be useful in practice for developmental evaluation of the nervous system in children.

Somatosensory evoked potentials from posterior tibial nerve and lumbo-sacral dermatomes

Techniques for recording the somatosensory evoked potential following stimulation of the skin of L5 and S1 dermatomes are described and validated. Normal data and their range are given for 54 subjects (108 legs). The latency of the peak of the first positive wave (P40) can be predicted from the subject's height from the regression formulae: P40 latency in msec = height in metres X 23.7 + 8.6 for the L5 dermatome. P40 latency in msec = height in metres X 24.5 + 8.7 for the S1 dermatome. P40 latency in msec = height in metres X 15.0 + 14.6 for the posterior tibial nerve. The standard deviations are 2.90 for L5; 2.95 for S1 and 1.60 for the posterior tibial nerve. Age and sex of the subjects had no significant effect. The data will have value when dermatomal somatosensory evoked potentials are used to investigate radiculopathies.

Neural generator of P14 far-field somatosensory evoked potential studied in a patient with a pontine lesion

Somatosensory evoked potentials (SEPs) to electrical stimulation of the right and the left median nerves were studied in a patient with a pontine lesion. At first there was mainly right medial lemniscus involvement. Four months later the left medial lemniscus was found to be also involved. SEPs to stimulation of the right median nerve had normal wave forms and latencies while N20 was lacking and P14 was abnormal after stimulation of the left median nerve in the first SEP record. N20 and P14 were absent with preservation of P9 and P11 after stimulation of both left and right median nerves in the second SEP record. Therefore the P14 component has been found abnormal, then absent, in a patient with a pontine lesion.

Somatosensory evoked potentials after removal of somatosensory cortex in man

Somatosensory evoked potentials (SEPs) to median nerve, ulnar nerve, thumb, middle finger, and posterior tibial nerve stimulation were recorded in a patient with a discrete resection of part of the postcentral somatosensory cortex as a treatment for focal epilepsy. Comparison of the different stimulation sites confirmed electrophysiologically the restricted locus of the lesion. The results strongly suggest that the early negative component (N20) and subsequent components recorded postcentrally are of cortical origin and depend upon postcentral gyrus cytoarchitectonic areas 3, 2, and 1. Moreover, these postcentral SEPs are distinct from precentrally recorded activity.

Developmental change of short latency somatosensory evoked potential waves between P3 and N1 components in children

We attempted to isolate and identify the negative waves between the conventional P3 and N1 components of short latency somatosensory evoked potentials (S-SEPs) in children. Twenty normal children ranging in age from 3 months to 12 years, and 11 adolescents and adults were studied. The median nerve was stimulated and recordings from F (F3 or F4) and C' (1-2 cm posterior to C3 or C4) contralateral to the stimulation site were simultaneously obtained in order to identify each negative wave in the C' recording. The wave of C' coincident with that of F was regarded as the far-field potential, and the wave of C' that diverged from F as the near-field potential at the scalp. There were one negative wave (N1) in the far-field potential and 2 negative waves (N2 and N1(3)) in the near-field potential, after the P3 component. "N1(3)" was conventional N1 and "N2" was the negative wave just before N1(3). In infants and younger children, the interpeak latencies of P3-N1(3) and P3-N2 were markedly elongated in comparison with that in older children. These interpeak latencies may be useful as an index of cerebral maturation.

The dissociation of early SEP components in lesions of the cervico-medullary junction: a cue for routine interpretation of abnormal cervical responses to median nerve stimulation

In non-cephalic reference records the lesions of upper cervical cord and medulla dissociate SEPs to median nerve stimulation, cervical N11 and N13 potentials being preserved whereas later components, generated above the foramen magnum, are absent or desynchronized and delayed. This was observed in 4 patients with space occupying lesions of the cervico-medullary junction. In three of them serial postoperative SEP records demonstrated a progressive normalization of the responses following decompression. During normalization delayed cortical components could reappear on the scalp before the far-field P14 positivity. Before surgery the widespread N18 potential was absent, at least on one side, in the 4 patients and was never found to be dissociated from the P14 component. The reversibility of early SEP dissociations after surgery allowed a documented study of the abnormal patterns of cervical to Fz responses that may be observed with various lesions of the cervical cord, including demyelination. When the P14 component is delayed because of conduction slowing it is injected as an abnormal 'N14' in the Cv6-Fz response; in this situation the use of a non-cephalic reference is necessary to make the distinction between the cervical N13 potential and brain-stem 'N14' negativity.

Erb's and cervical somatosensory evoked potentials: correlations with body size

Median and ulnar Erb's (N9) and cervical (N13) somatosensory evoked potentials were correlated with height, arm span, arm length, and Erb's length. All body measurements had high correlations with the N9 and N13 latencies. The highest correlation was obtained between the N9 latency and Erb's length. The N13 latency was correlated higher with height than with arm length. The data suggest that height can be used to construct normogram correlating with the N13 latency.

Somatosensory evoked potentials in patients with thalamic lesions

Somatosensory evoked potentials (SEPs) were recorded in 20 patients with thalamic lesions confirmed by CT (10 with infarction, 10 with haemorrhage). The changes in SEP configuration are discussed in their relationship to clinical symptoms. Four types of SEP abnormality produced by thalamic lesion are distinguished: (1) "FF" type, (2) "N20/P23 dissociation" type, (3) "N18/N20 false shift" type, and (4) "reduced early component" type. It was shown that clinically similar lesions might produce different SEP patterns.

Topographic analysis of somatosensory evoked potentials in patients with well-localized thalamic infarctions

After stimulation of the median nerve, three major negative peaks (NI, NII and NIII) of somatosensory evoked potentials (SEP) have different latencies along the longitudinal array of scalp electrodes: NI and NII at frontal electrodes have the shortest latency (N17, N29). There is a progressive delay toward the central (N19, N32) and parietal areas (N20, N34). NIII (N60) latency differs from one electrode to another but without consistent anterior-posterior latency shift. A variety of SEP abnormalities was observed in 17 patients with non-hemorrhagic thalamic lesions reflecting a disturbance of the complex intra-thalamic and afferent and efferent thalamic pathways. When the lesions were classified into 5 groups according to the presumed vascular territories involved, there were general but not specific characteristics of SEP abnormalities in each group of patients. In four patients with lesions involving primary sensory nuclei and presenting with the thalamic syndrome or the loss of all modalities of sensation, all SEP components after P14 were absent when the affected arm was stimulated. NI peaks were intact in two patients with thalamic sensory lacunes but NII was delayed. NI was present but delayed in three patients with anterior thalamic lesions not involving primary sensory nuclei. In patients with medial thalamic lesions, delayed central NIII was a common finding. In patients with posterior capsule or lateral thalamus lesions either NII and NIII or NIII alone were affected. NI was also affected in some with sparing of the frontal component (N17). These complex relationships between the types of SEP abnormalities and thalamic lesions are compatible with the presence of multiple, at least partially independent, thalamic generators and thalamocortical projection systems mediating regionally specific SEP components.

Somatosensory evoked potentials in minor cerebral ischaemia: diagnostic significance and changes in serial records

Frontal, central and parietal short and middle latency somatosensory evoked potentials (SEPs) arising after stimulation of the contralateral median nerve were studied in 10 normal adults. Stable SEPs were recorded: a frontal P21-N30 complex and an N20-P23-P28-N35-P42 complex in the centro-parietal region. The use of a chin reference electrode allowed identification of (the thalamic) P15 and N18. SEP studies of 20 patients with unilateral cerebral ischaemia were also performed, about 4 and 18 days after the stroke. In 13 out of 18 patients with a minor stroke (TIA, RIND and PNS) abnormalities of the frontal and/or parietal SEPs were demonstrated. Improvement in these SEPs occurred in 5 cases. In two patients who suffered from a major ischaemic deficit, the SEPs were highly abnormal and did not show any change in the course of time. SEP studies may be useful for the diagnosis of minor cerebral ischaemia as well as quantification of recovery; an even more important indication for this neurophysiological method might be detection of subclinical lesions in patients who have suffered from transient cerebral ischaemia even weeks before the SEP studies are carried out.

Critical neuromonitoring at spinal and brainstem levels by somatosensory evoked potentials

Electronic averaging makes it possible to analyze somatosensory evoked potentials (SEP) recorded noninvasively from the body surface in man. With noncephalic reference recording, the SEP discloses a series of components that are volume-conducted from distinct open-field generators with a geometry adequate to produce external potential gradients over the head. Farfields are brief positive dips with widespread distribution that present stationary onset and peak latencies all over. They reflect the propagated afferent volley in axons bundles, thus in brachial plexus (P9), dorsal column (P11), and medial lemniscus (P14). Somehow unexpectedly, SEP traces also disclose a widespread prolonged farfield N18 of negative polarity that reflects neural generators in the brainstem below thalamus. Nearfields can be positive or negative, and they reflect neural generators located less than about 50 mm from the electrode. They are influenced to a greater extent by the position of the recording electrodes. For example, neck electrodes can follow the upward propagation of the dorsal column volley (N11), whereas scalp electrodes can map out the distinct contralateral parietal (N20, P27) or frontal (P22, N30) cortical generators. Electrodes around the neck also disclose the posterior N13 and anterior P13 responses that reflect the two sides of the same dorsal horn generator with a horizontal axis. Bit-mapped topographic color imaging of potential fields provides detailed data on time and spatial features of the different SEP neural generators. SEP neuromonitoring can use these results to titrate input to spinal cord (nerve potentials or P9 farfield), spinal generators (N11 nearfield or N13-P13 nearfield in posterior-to-anterior neck montages), brainstem generators (P14 farfield and N18 response), or cortical generators (parietal N20-P27 or frontal P22-N30). The central somatosensory conduction time can be titrated from the spinal entry and cortical arrival times measured in neck and scalp recordings.

Bit-mapped colour imaging of the potential fields of propagated and segmental subcortical components of somatosensory evoked potentials in man

Early somatosensory evoked potentials (SEPs) to median or ulnar nerve stimulation were recorded with non-cephalic reference from neck, oesophagus and scalp in normal young adults. Transit times and durations were estimated for different components. SEP fields were mapped with up to 24 skin electrodes around the neck or along the midline neck and scalp, and projected onto a 2-dimensional plane for bit-mapped colour imaging. The posterior neck N11 is a near-field potential propagated caudorostrally in the dorsal column, associated with a positive P11 far field beyond termination of the cuneate bundle. A true phase reversal of the posterior neck N13 into an anterior neck P13 is substantiated, identifying a segmental generator with horizontal axis in dorsal horn. The N13-P13 represents postsynaptic excitatory potentials in interneurones of layers IV-V of the dorsal horn. It is not reflected in any scalp far field. The duration and onset latency of N13-P13 are in line with this interpretation. A new montage of posterior-to-anterior neck can enhance this component without introducing extraneous potentials. The P14 far field does not extend below the inion and presents distinct features. Neck-to-front scalp montages confound the SEP components generated in the spinal cord and above the foramen magnum respectively, but may serve to estimate N11 onset latency.

Subcortical, thalamic and cortical somatosensory evoked potentials to median nerve stimulation

Subcortical and cortical somatosensory evoked potentials (SEPs) to median nerve stimulation were studied in 16 normal controls, 3 patients with Parkinson's disease, and 2 patients with thalamic lesions. Multiple electrodes were placed over the scalp and cervical spines and connected with a hand electrode or Fz in grid II. Thalamic SEPs were recorded directly from the Vim nucleus in 3 patients with Parkinson's disease during the stereotaxic operation. SEPs recorded from the scalp-hand derivations were composed of 4 negative (N9, N11, N16, N18) and 3 positive potentials (P8, P10, P12), whereas, at the scalp-Fz electrode, only one negative peak was identified (N20). N18 is of higher amplitude on the parieto-occipital areas and corresponds to N20. On the other hand, N16 can be identified more clearly on the fronto-central areas at scalp-hand derivations because of intermixture with N18 on the posterior head areas and disappears at scalp-scalp derivation. This suggests that N16 represents a subcortical component that is picked up as a far-field potential at the scalp electrodes. Components preceding N16 at scalp-hand derivations are also interpreted as far-field potentials because of their short latency and wide distribution over the scalp. The peak latency of N16 is not significantly different from the major negativity recorded directly from the thalamus, whereas N18 (N20) occurs significantly later. From this we conclude that N16 is most probably generated in the thalamus with the potentials of shorter latency originating caudal to the thalamus and N18 rostral to the thalamus.

Neuropsychological impairment after carotid endarterectomy correlates with intraoperative ischemia

Detailed neuropsychological assessments were performed before and shortly after carotid endarterectomy in thirty-four patients. The degree of intraoperative ischemia was assessed by monitoring the somatosensory evoked cortical potential change upon carotid clamping. Changed neuropsychological performance was found to be related to intraoperative ischemia most clearly in patients with a history of previous stroke and in those with more severe vascular disease. In such patients greater SSEP change was correlated with greater neuropsychological change postoperatively.