Cortical potentials evoked by epidural stimulation of the cervical and thoracic spinal cord in man

Scalp somatosensory evoked potentials (SEPs) were recorded after electrical stimulation of the spinal cord in humans. Stimulating electrodes were placed at different vertebral levels of the epidural space over the midline of the posterior aspect of the spinal cord. The wave form of the response differed according to the level of the stimulating epidural electrodes. Cervical stimulation elicited an SEP very similar to that produced by stimulation of upper extremity nerves, e.g., bilateral median nerve SEP, but with a shorter latency. Epidural stimulation of the lower thoracic cord elicited an SEP similar to that produced by stimulation of lower extremity nerves. The results of upper thoracic stimulation appeared as a mixed upper and lower extremity type of SEP. The overall amplitudes of SEPs elicited by the epidural stimulation were higher than SEPs elicited by peripheral nerve stimulation. In 4 patients the CV along the spinal cord was calculated from the difference in latencies of the cortical responses to stimulation at two different vertebral levels. The CVs were in the range of 45–65 m/sec. The method was shown to be promising for future study of spinal cord dysfunctions.     

Electric and CO2 laser SEPs in a patient with asymptomatic syringomyelia

We recorded electrically stimulated somatosensory evoked potentials (electric SEPs) and pain-related SEPs following CO₂ laser stimulation (CO₂ laser SEPs) from a 17-year-old patient affected by myotonic dystrophy whose MRI disclosed a large syrinx extending from spinal level C2 to S3. Careful clinical and electromyographic examinations revealed no motor or sensory disturbances, apart from myotonia. The only abnormality noted in median and ulnar nerve short-latency electric SEPs (recorded with a non-cephalic reference electrode) was the absence of cervical component N13, the other SEP responses (N9, N10, N11, P14, N20) being normal. The cutaneous pain threshold and CO₂ laser SEPs (both obtained by a CO₂ laser beam applied to the back of the hand) were normal. Thus cervical component N13 appears to be highly sensitive to the effects of central cord lesions, even when these are asymptomatic.     

Identification of pain,intensity and P300 components in the pain evoked potential

This study examined the relationships among 3 components of the somatosensory evoked potential (SEP) to painful stimuli. Painful stimuli were produced using intracutaneous electrical stimulation of a fingertip and two levels of non-painful stimuli were produced by superficial electrical stimulation of a neighboring fingertip. SEPs were recorded from Cz-A1 and Pz-A1, and difference waves were computed for 3 components: (1) a pain component (the difference between SEPs to painful vs. strong but non-painful stimuli); (2) an intensity component that is not related to pain (the difference between SEPs to strong non-painful vs. mild non-painful stimuli); and (3) a P300 component (the difference between SEPs to the same stimuli under Target instructions vs. Standard instructions).The positive peaks in the 3 types of difference waves differed in both latency and topography, although with latency and topography overlap. The intensity component had an earlier positive peak than the pain component, and the pain component had an earlier positive peak than the P300 component. The pain and intensity components were larger at Cz than Pz, whereas the P300 component was larger at Pz than Cz. Under certain conditions, the pain evoked SEP consists of a weighted combination of the 3 components, complicating interpretation of the positive peaks in the recorded wave forms.     

Maps of somatosensory evoked potentials (SEPs) to mechanical (tapping) stimuli: comparison with P14, N20, P22, N30 of electrically elicited SEPs

Bit mapped color imaging of SEPs was recorded from 19 derivations in 11 healthy volunteers after electrical stimulation of the median nerve at the wrist, index finger digital nerve stimulation, and mechanical stimulation of the index fingertips by an electromechanically driven vibrating thin metallic plate. The latencies of SEP components increased for the various stimulation modalities, being shortestafter median nerve stimulation at the wrist and longest after mechanical stimulation of the index fingertips.The scalp distribution of SEPs to mechanical stimuli was, however, the same as other SEPs, independently of the stimulation employed, and components corresponding to N20 and P22 were recorded only contralaterally to the stimulated side.     

Somatosensory evoked potentials and magnetic resonance imaging in syringomyelia

Somatosensory evoked potentials (SEPs) to median and posterior tibial stimulation were obtained in 22 patients with syringomyelia. All patients had magnetic resonance imaging (MR) which defined the maximum transverse diameter of the syrinx as well as its longitudinal extension. SEP was abnormal in 16 (72%) patients. Median and posterior tibial SEPs were abnormal in 11 and 15 patients respectively. Both tests were abnormal in 10 patients. Ten patients showed absence of one or more central potentials (P/N13, N20, N22) and 7 patients demonstrated increased conduction times (N9–N20, P/N13–N20, N22–P40). The mean maximum transverse diameter of the syrinx was 7.5 mm in patients with normal SEPs and 16.2 mm in patients with abnormal SEPs. Abnormal SEP was observed in all 5 patients with loss of position sense, in 9 of 13 (69%) with loss of superficial pain and temperature, and 1 of 2 patients with motor deficit only. Central SEP abnormalities were observed in 3 of 5 patients with sensory deficits indistinguishable from a peripheral neuropathy and in 2 patients in the asymptomatic extremity. Three of 4 patients with syringomyelia and Chiari malformation had a normal SEP.     

Ipsilateral median somatosensory evoked potentials recorded from human somatosensory cortex

Somatosensory evoked potentials (SEP) to ipsilateral and contralateral median nerve stimulations were recorded from subdural electrode grids over the perirolandic areas in 41 patients with medically refractory focal epilepsies who underwent evaluation for epilepsy surgery. All patients showed clearly defined, high-amplitude contralateral median SEPs. In addition, four patients showed ipsilateral SEPs. Compared with the contralateral SEPs, ipsilateral SEPs were very localized, had a different spatial distribution, were of considerably lower amplitude, had a longer latency (1.2–17.8 ms), did not show an initial negativity, and were markedly attenuated during sleep. Stimulation of the subdural electrodes overlying the sensory hand area was associated with contralateral hand paresthesias, but no ipsilateral hand paresthesias occurred. It was concluded that subdurally recorded cortical SEPs to ipsilateral stimulation of the median nerve (M) reflect unconscious sensory input from the hand possibly serving fast bimanual hand control. The anatomical pathway of these ipsilateral short-latency MSEPs is not yet known. Transcallosal transmission seems unlikely because of the short delay between the ipsilateral and contralateral responses in selected cases. The infrequent occurrence of ipsilateral subdurally recorded SEPs and their low amplitude and limited distribution suggest that they contribute very little to the short-latency ipsilateral median SEPs recorded on the scalp.     

Pain-related somatosensory evoked potentials following CO2 laser stimulation of foot in man

Since our previous study of pain somatosensory evoked potentials (SEPs) following CO₂ laser stimulation of the hand dorsum could not clarify whether the early cortical component NI was generated from the primary somatosensory cortex (SI) or the secondary somatosensory cortex (SII) or both, the scalp topography of SEPs following CO₂ laser stimulation of the foot dorsum was studied in 10 normal subjects and was compared with that of the hand pain SEPs and the conventional SEPs following electrical stimulation of the posterior tibial nerve recorded in 8 and 6 of the 10 subjects, respectively. Three components (N1, N2 and P2) were recorded for both foot and hand pain SEPs. N1 of the foot pain SEPs was maximal at the midline electrodes (Cz or CPz) in all data where that potential was recognized, but the potential field distribution was variable among subjects and even between two sides within the same subject. N1 of the hand pain SEPs was maximal at the contralateral central or midtemporal electrode. The scalp distribution of N2 and P2, however, was not different between the foot and hand pain SEPs. The mean peak latency of N1 following stimulation of foot and hand was found to be 191 msec and 150 msec, respectively, but there was no significant difference in the interpeak latency of Nl-N2 between foot and hand stimulation. It is therefore concluded that NI of the foot pain SEPs is generated mainly from the foot area of SI. The variable scalp distribution of the N7 component of the foot pain SEPs is likely due to an anatomical variability among subjects and even between sides.     

Influence of concurrent tactile stimulation on somatosensory evoked potentials following posterior tibial nerve stimulation in man

A topographical study was made of SEPs following stimulation of the right posterior tibial nerve at the ankle, with and without concurrent tactile stimulation of the soles of either foot or the palm of the right hand. Effects of the interfering stimulus were best demonstrated by subtracting the wave forms to derive ‘difference’ potentials.The majority of SEP components were significantly attenuated by tactile stimulation of the ipsilateral foot, and the difference wave form was of similar morphology to the control response. Components of opposite polarity peaking at 39 msec were consistent with the field of a cortical generator with dipolar properties, situated in the contralateral hemisphere just posterior to the vertex with the positive poles oriented towards the ipsilateral side. By analogy with median SEP findings, these potentials were believed to originate in the foot region of area 3b where neurones are mainly concerned with cutaneous sensory processing.When the tactile stimulus was applied to the contralateral foot, difference potentials maximally recorded just posterior to the vertex were of smaller amplitude but similar morphology to ipsilateral foot difference components. This suggested the possibility that input from the two lower extremities may converge at cortical or subcortical level, the effect being manifested in the response of certain neurones in area 3b. With both contralateral foot and ipsilateral hand stimulation, other difference potentials were present which suggested that there may be cortical regions responding to combinations of sensory stimuli applied to various parts of the body surface.     

Predictability of Painful Stimulation Modulates the Somatosensory-Evoked Potential in the Rat

Somatosensory-evoked potentials (SEPs) are used in humans and animals to increase knowledge about nociception and pain. Since the SEP in humans increases when noxious stimuli are administered unpredictably, predictability potentially influences the SEP in animals as well. To assess the effect of predictability on the SEP in animals, classical fear conditioning was applied to compare SEPs between rats receiving SEP-evoking electrical stimuli either predictably or unpredictably. As in humans, the rat’s SEP increased when SEP-evoking stimuli were administered unpredictably. These data support the hypothesis that the predictability of noxious stimuli plays a distinctive role in the processing of these stimuli in animals. The influence of predictability should be considered when studying nociception and pain in animals. Additionally, this finding suggests that animals confronted with (un)predictable noxious stimuli can be used to investigate the mechanisms underlying the influence of predictability on central processing of noxious stimuli.     

The perception of painful and nonpainful stimuli during voluntary motor activity in man

Previous studies have shown that voluntary movement diminishes the transmission of cutaneous afferent input through the dorsal column-medial lemniscal system, and also raises the threshold for detecting nonpainful, cutaneous stimuli (electrical shocks). Although there is some evidence that pain elicited by electrical stimulation is diminished during movement, no studies have tested the effect of movement on the perception of pain produced by natural stimulation. For this reason, we tested the effects of voluntary motor activity on the perception of noxious thermal stimuli in human volunteers. We first developed a motor paradigm in which the thermal stimulation could be applied to the immobile limb (isometric elbow flexion-extension). Both isometric and isotonic muscle contractions about the elbow increased the threshold for detecting weak cutaneous stimuli (electrical shocks) applied to the forearm, and to a lesser extent the detection of stimuli applied to the dorsum of the hand. Afterwards, noxious and innocuous heat stimuli were applied to the forearm during isometric contractions and at rest. Magnitude estimates for the intensity of the pain, as well as latency measures of the onset of pain, were recorded. We found no evidence that isometric motor activity diminished either the threshold for pain or the subjective intensity of the noxious and innocuous thermal stimuli. Thus, motor activity decreases the ability to detect weak low-threshold cutaneous inputs, but has no effect on the perception of warmth and heat pain.     

Pain-related somatosensory evoked potentials following CO2 laser stimulation in man

0ain-related somatosensory evoked potentials (SEPs) following CO₂ laser stimulation were analyzed in normal volunteers. Low power and long wavelength CO₂ laser stimuli to the hand induced a sharp pain which was associated with a large positive component, P320, recorded over the scalp. Amplitude decreased and latency increased with reduction in stimulus intensity and subjective pain feeling. P320 was maximal at the vertex but was distributed widely over the scalp. There were no topographic differences between left- and right-hand stimulation, or between hand and chest stimulation. Lidocaine injection to produce anesthetic nerve block resulted in loss of P320, but the potential was relatively preserved during ischemic nerve block. No potential corresponding to P320 could be recorded following electrical or mechanical tactile stimulation.We consider P320 to be generated by impulses arising from pain stimuli and ascending through Aδ fibers. We propose the thalamus as a generator source from considering its scalp topography, but pain-specific cognition or perception may also be involved in generating this potential.     

Somatosensory evoked potentials elicited by dorsal penile and posterior tibial nerve stimulation

SEPs were elicited by stimulation of the dorsal penile nerve (DPN) or posterior tibial nerve (PTN) under 3 conditions of stimulation: random and constant interstimulus intervals, and subject-initiated stimulation. Within these conditions, the effects of repeated stimulation were also examined. The latency of the N90 peak decreased with repeated stimulation. N90 amplitude decreased with increased foreknowledge as well as with repeated stimulation. Factors extracted by principal components analysis revealed similar effects. A difference between DPN and PTN stimulation was seen in a factor associated with the N90 peak, wherein the condition involving subject self-initiation of the stimulus reflected a significantly greater decrease in SEP amplitude when the DPN was stimulated. Morphological commonalities were observed in the SEPs elicited by DPN and PTN for a given subject.     

Monitoring of subcortical and cortical somatosensory evoked potentials during carotid endarterectomy: comparison with stump pressure levels

Monitoring of multichannel somatosensory evoked potentials (SEPs) has been performed in 40 cases of carotid endarterectomy (CEA). SEPs were obtained after median nerve stimulation at wrist, recording from 2nd cervical and from the scalp parietal (ipsi- and contralateral) and central (contralateral) positions. The reduction of CBF due to clamping of the carotid artery provoked SEP abnormalities in 10 of the 40 cases. None of the 30 patients with unmodified SEPs developed post-surgical neurological sequelae.SEP alterations were characterized exclusively by amplitude decrements and latency increases of the cortical components, the subcortical ones being unaffected. In 5 of these patients, SEPs returned to normal values before the end of the intervention and no neurological deficit was observed on awakening. In the remaining 5 cases SEPs retained their abnormalities and patients developed post-surgery neurological sequelae (4 immediately, 1 the day after).SEP alterations affected parietal and central components to a similar extent; however, in a few cases cerebral blood flow deficits provoked by carotid clamping modified differently the central P22 and the parietal N20–P25 waves.Comparisons with stump (back) pressure in the carotid artery revealed a higher sensitivity of the SEP technique in detecting vascularization problems due to carotid clamping.The time course of the appearance of SEP abnormalities seems to discriminate alterations secondary to collateral revascularization from those determined by embolization.     

Distribution of lumbar spinal evoked potentials and their correlation with stimulation-induced paresthesiae

In 7 awake patients with neuropathic lower extremity pain, spinal somatosensory evoked potentials (SEP) were elicited from the non-painful leg by electrical stimulation of the peroneal nerve and mechanical stimulation of the hallux ball. Recording was made epidurally in the thoraco-lumbar region by means of an electrode temporarily inserted for trial of pain-suppressing stimulation.In response to peroneal nerve stimulation, two major SEP complexes were found. The first complex consisted, as has been described earlier, of an initial positivity (P12), a spike-like negativity (N14), a slow negativity (N16) and a slow positivity (P23). The second complex consisted of a slow biphasic wave, conceivably mediated by a supraspinal loop. Both complexes had a similar longitudinal distribution with amplitude maxima at the T12 vertebral body.The SEP evoked by mechanical hallux ball stimulation had a relatively small amplitude, and there was no significant second complex. The relationship between stimulus intensity and SEP amplitude was negatively accelerating.The longitudinal distribution of spinal SEP was compated with the somatotopic distribution of paresthesiae induced by stimulation through the epidural electrode. It was found that stimulation applied at the level of maximal SEP generally induced paresthesiae in the corresponding peripheral region. Therefore, spinal SEP may be used as a guide for optimal positioning of a spinal electrode for therapeutic stimulation when implanted under general anesthesia.An attempt was made to record the antidromic potential in the peroneal nerve elicited from the dorsal columns by epidural stimulation. The antidromic response was, however, very sensitive to minimal changes of stimulus strength and body position of the patient, and was also contaminated by simultaneously evoked muscular reflex potentials.Thus, peripheral responses evoked by epidural stimulation appeared too unreliable to be useful for the permanent implantation of a spinal electrode for therapeutic stimulation.     

Validity of median nerve somatosensory evoked potentials in the diagnosis of supraclavicular brachial plexus lesions

Experience with median nerve SEPs in the diagnosis of brachial plexus lesions is analysed in 49 patients selected from a total material of 264 cases with brachial plexus problems tested by SEP techniques. Median nerve SEPs were always compared with the results of SEPs after stimulation of at least one other nerve relevant to the site of the lesion as suspected clinically and electromyographically. All patients presented with unilateral brachial plexus problems and all root lesions were verified by clinical presentation, EMG studies, myelogram or surgery. There were 19 brachial plexus injuries, 13 cases with cervical spondylopathic rediculopaties without myelopathy and 7 patients presented brachial plexopathy with systemic cancer. It was found that median nerve SEPs were always normal in injuries of upper trunk and root avulsions confined to one or two root levels. Median nerve SEPs were abnormal in multiple trunk lesions and multiple root avulsions. In patients with spondylopathic radiculopathies median nerve SEPs were normal apart from one case where involvement of multiple roots was present. Median nerve SEPs were useful in assessing patients presenting brachial plexus problems in the presence of systematic cancer apart from cases where lower trunk involvement was present.In general, median nerve SEPs are useful if they are combined with SEP testing of other nerves anatomically more closely related to the problem as outlined clinically and electromyographically.     

Nerve,spinal cord and brain somatosensory evoked responses: a comparative study during electrical and magnetic peripheral nerve stimulation

Somatosensory evoked potentials (SEPs) and compound nerve action potentials (cNAPs) have been recorded in 15 subjects during electrical and magnetic nerve stimulation. Peripheral records were gathered at Erb's point and on nerve trunks at the elbow during median and ulnar nerve stimulation at the wrist. Erb responses to electrical stimulation were larger in amplitude and shorter in duration than the magnetic ones when ‘electrical’ and ‘magnetic’ compound muscle action potentials (cMAPs) of comparable amplitudes were elicited. SEPs were recorded respectively at Cv7 and on the somatosensory scalp areas contra- and ipsilateral to the stimulated side. SEPs showed a statistically significant difference in amplitude only for the brachial plexus response and for the ‘cortical’ N20-P25 complex; differences were not found between the magnetic and electrical central conduction times (CCTs) or for the peripheral nerve response latencies. Magnetic stimulation preferentially excited the motor and proprioceptive fibres when the nerve trunks were stimulated at motor threshold intensities.     

Somatosensory evoked potentials recovery (SEP-R) in myoclonic patients

Recovery of somatosensory evoked potentials (SEPs) was studied by paired stimulation of the median nerve in patients with various kinds of myoclonus. This technique revealed the hyperexcitability of the central nervous system (CNS) which could not be detected by the conventional SEP technique using a single stimulus. This technique would be useful for studying the excitability of the CNS.     

Evaluation of reference sites for scalp potentials evoked by painful and non-painful sural nerve stimulation

The objective of this study was to evaluate reference sites for recording the middle- and long-latency scalp potentials elicited by painful and non-painful sural nerve stimulation. Somatosensory evoked potentials (SEPs) were recorded from the scalp, the mastoid, the earlobe, the neck, and the wrist. Each site was referenced to the sterno-vertebral (SV) electrode, which is a balanced non-cephalic reference with essentially no ECG contamination.There was little or no activity recorded between the wrist and SV, and the SV was located within a region extending from the rostral neck to the wrist where the potentials were stable over space. Hence, the SV reference is indifferent for the middle- and long-latency potentials evoked by painful and non-painful sural nerve stimulation. There was, however, significant activity recorded from the earlobe and mastoid, sites which are frequently used as references for the SEP. It is important that investigators using these cephalic references to study the middle- and long-latency peaks of the SEP be aware of this activity as it will distort SEPs recorded from single sites and the SEP scalp topography, distortions which could unnecessarily complicate their interpretation.     

Modality-specific organization for cutaneous and proprioceptive sense in human primary sensory cortex studied by chronic epicortical recording

Modality specificity of human primary somatosensory cortex was studied by recording somatosensory evoked potentials (SEPs) from subdural electrodes in a patient with intractable focal motor seizure. A newly developed device was used for selectively activating proprioception. The spatial and temporal distributions of proprioception-related SEPs elicited by brisk passive flexion movement at the proximal interphalangeal (PIP) joint of the middle finger (4 degrees in 25 ms) were quite different from those to cutaneous sense evoked by electric stimulation of the digital nerve at the same site. It was for the first time demonstrated that proprioception-related SEPs following passive finger movement do not originate in area 3b, which was clearly activated by cutaneous stimulation, and that other sites at the sensorimotor cortex such as areas 2, 3a and 4 possibly contribute to the cortical processing of proprioception.     

Estimation of conduction velocity of the spino-thalamic tract in man

This is the first report of estimating conduction velocity (CV) of the slowly conducting somatosensory spinal tracts or the spino-thalamic tract (STT) in man. The CV of the STT was measured by recording somatosensory evoked potentials (SEPs) following CO₂ laser stimulation of the hand and foot, which was previously shown to cause pain or heat sensation by activating cutaneous nociceptors and by its ascending signals through Aδ fibers and probably STT. When the CV of Aδ fibers was assumed to be 10–15 m/sec, the CV of STT was found to be approximately 8–10 m/sec in normal young subjects. It was slightly slower in subjects over 60 years of age. In contrast, the CV of the posterior column, which was calculated based on SEPs following electrical stimulation of the median and posterior tibial nerves, was approximately 50–60 m/sec.