Selective attention effects on somatosensory evoked potentials

Short and long latency somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded over the contralateral hemisphere. Simultaneously, signals evoked by the same stimulus were monitored at Erb's point. Recordings were made during three conditions which were presented in a different random order to each of the subjects tested. During the control condition the subjects were instructed to attend to and count the number of electrical pulses delivered to the median nerve of their right hand. During the two task conditions, in addition to the pulses, the subjects received stimulation on the dorsal surface of one of their hands. This consisted in drawing circles lightly for the duration of SEP recording using a cotton swab (Q-tip). During the trial, the Q-tip was momentarily withdrawn 15 to 20 times and the subject's task was to ignore the pulses, attend to this cutaneous stimulation and count the number of times the cutaneous stimulation was interrupted. SEPs to the pulse were significantly greater in amplitude when cutaneous stimulation was delivered to the same hand as the pulse (the right hand) than when it was delivered to the left hand. This effect was confined to the long-latency SEPs and did not appear in either the Erb's point response or the short latency SEPs. These data indicate that selective attention to peripheral stimulation is a relatively late process mediated by cortical mechanisms and argue against the notion of early suppression of irrelevant stimulus channels in subcortical centers or in the periphery.(ABSTRACT TRUNCATED AT 250 WORDS)     

Long-latency somatosensory evoked potentials

Theoretically, long-latency somatosensory evoked potentials (SEPs) provide information on the function of somatosensory associative cortical structures. Their potential role in clinical studies and research has been hampered by the lack of standardized methodology in the use of these SEPs. Other factors, such as drugs, simultaneous stimuli, and state of consciousness, also have far-reaching influences on the various parameters of long-latency SEPs. The knowledge of the origin of most SEP components is at best fragmentary; studies on clinical-electrophysiological correlations seem to be hopeful in this respect. As yet, clinical applications of long-latency SEPs are limited; for future research, studies of disturbances of SEPs are most promising, mainly with regard to diseases of the gray matter, the influence of drugs on the cerebral function, and psychopathology.     

Three-dimensional human somatosensory evoked potentials

Median nerve somatosensory evoked potentials were recorded from 30 normal adults using conventional scalp derivations and an orthogonal bipolar surface electrode montage. This allowed the determination of the spatial orientation of the hypothetical centrally located equivalent dipole derived from the evoked response recorded in 3-dimensional voltage space. The 3-dimensional voltage trajectory describing changes in equivalent dipole orientation and magnitude revealed 4 major apices between 5 and 25 msec, 3 of which corresponded to the traditional P14, N20 and P25 peaks. A fourth apex at 17 msec was not as evident in the conventional recordings and signaled a transition from a vertical P14-N18 generator process to a horizontal N20 generator process. The normal within- and between-subject variability of trajectory apices, segments and planes are described, along with the theoretical and practical implications of this recording technique.     

Early somatosensory evoked potentials.

The early somatosensory evoked potential secondary to median nerve stimulation in the human had an onset latency of 9--12 msec when recorded from scalp electrodes at vertex-to-mastoid, vertex-to-inion or at the base of the skull. Similar latencies were observed from responses recorded over the cervical dorsal columns during neurologic surgery. A latency difference of 1.5 msec was observed between the early response and the responses recorded from the junction of medial lemniscus and nucleus ventralis posterior lateralis of the thalamus during human stereotaxic surgery. Cervical cord transections and transection at the midpontine levels of the monkey showed that the evoked potential was due to generators between these levels. Depth recording of the monkey indicate that the early evoked potential originates in the region of dorsal column nuclei, while the later components are secondary to generators in cerebral cortex.     

Segmentally specific somatosensory evoked potentials

The recording of segmentally specific somatosensory evoked potentials (SEPs) is time-consuming and the findings have generally been clinically unhelpful. This article critically evaluates the role of these SEPs in patients with disorders of the spinal cord or nerve roots.     

Pain-Related somatosensory evoked potentials.

The authors reviewed basic and clinical reports of pain-related somatosensory evoked potentials (SSEP) after high-intensity electrical stimulation [pain SSEP(E)] and painful laser stimulation [pain SSEP(L)]. The conduction velocity of peripheral nerves for both pain SSEP(E) and pain SSEP(L) is approximately 10 to 15 m/second, in a range of Adelta fibers. The generator sources are considered to be the secondary somatosensory cortex and insula, and the limbic system, including the cingulate cortex, amygdala, or hippocampus of the bilateral hemispheres. The latencies and amplitudes are clearly affected by vigilance, attention-distraction, and various kinds of stimulation applied simultaneously with pain. Abnormalities of pain SSEP(L) reflect an impairment of pain-temperature sensation, probably relating to dysfunction of A5 fibers of the peripheral nerve and spinothalamic tract. In contrast, conventional SSEP after nonpainful electrical stimulation reflects an impairment of tactile, vibratory, and deep sensation, probably relating to dysfunction of Aalpha or Abeta fibers of the peripheral nerve and dorsal column. Therefore, combining the study of pain SSEP(L) and conventional SSEP is useful to detect physiologic abnormalities, and sometimes subclinical abnormalities, of patients with peripheral and central nervous system lesions.     

Effect of diazepam on somatosensory evoked potentials

Somatosensory evoked potentials (SEP) were recorded in 15 patients with extrapyramidal disturbances after intravenous administration of 10 mg of diazepam (Relanium, Polfa). Cortical SEP were recorded before and after operations and thalamic SEP were recorded during stereotaxic interventions on thalamic nuclei VL and Vim. The results demonstrated that diazepam had a significant positive influence on the recording of SEP, eliminating muscular artifacts caused by increased muscular tonus and extrapyramidal tremor. Diazepam had no evident effect on the SEP and caused no changes of the short-latency specific cortical SEP. This effect was, however, evident on the later components of the SEP and it seemed to affect mainly the N63 wave whose amplitude was always reduced or absent. In the postoperative investigations the cortical SEP in the hemisphere operated on (contralateral to the site of stimulation) were reduced in the phase of short-latency components. In the ipsilateral hemisphere the cortical SEP remained similar to the normal ones, and after diazepam their amplitude was markedly reduced. Intraoperatively recorded thalamic SEP showed after diazepam a considerable decrease of the amplitude of all components, sometimes the curve was completely flat, including the specific phase of the potential. On the other hand, diazepam failed to extinguish cortical SEP which showed even a greater amplitude of short-latency and long-latency components than in the records obtained without this drug, with the exception of disappearing N63 component.     

The effects of a supratentorial mass lesion on brain-stem auditory evoked potentials and short latency somatosensory evoked potentials

Nineteen patients with unilateral supratentorial mass lesion and without any evident clinical signs of transtentorial herniation were studied with Computed Tomography (CT), brain-stem evoked potentials (BAEPs) and central conduction time (CCT) of short latency somatosensory evoked potentials (SEPs). Sixteen had tumours, two had intracranial haematoma and one had chronic subdural haematoma. CT detected the initial signs of transtentorial herniation in every case. Preoperative I-V interpeak latency (IPL) was significantly (M+2SD) prolonged in 26% of cases on the lesion side and in 21% of cases on the opposite side. The mean I-V IPL was significantly prolonged both on the lesion side and the opposite side (P < 0.01, P < 0.02, respectively). Suppression of Wave V (M-2SD) was seen only in two cases, however, the mean amplitude of Wave V was significantly decreased both on the lesion side and on the opposite side (P < 0.001, P < 0.01, respectively). CCT of SEPs was significantly (M+2SD) prolonged in 33% of cases on the lesion side and in only 13% on the opposite side. The mean CCT was, however, significantly prolonged both on the lesion and on the opposite side (P < 0.001, P < 0.02, respectively). Postoperative I-V IPL was significantly prolonged in only 11% of cases while the mean I-V IPL was still significantly prolonged (P < 0.07) and the mean amplitude of Wave V was still suppressed (P < 0.001) on the lesion side. On the other hand, there was neither abnormality of I-V IPL nor suppression of Wave V on the opposite side. Postoperative CCT was significantly prolonged in 43% of cases and the mean CCT was also significantly (P < 0.01) prolonged on the lesion side. However, there was no prolongation of CCT on the opposite side. Preoperative findings of both BAEPs and SEPs show the abnormality due to the supratentorial lesion and postoperative findings of these potentials indicate both the effects of surgical decompression and the residual abnormalities due to the supratentorial mass lesion.     

The effects of sleep on median nerve short latency somatosensory evoked potentials

The effects of sleep on median nerve short latency somatosensory evoked potentials were studied in 7 subjects made up of 6 patients being evaluated for seizure disorders by all-night electroencephalograms and 1 normal healthy volunteer. The median nerve was stimulated at the wrist, and the peripheral (N9), subcortical (P13) and early cortical (N1, P2) evoked potentials were recorded during full wakefulness and natural night-time sleep. Sleep-wake state was monitored by the simultaneously obtained polysomnogram. The latencies of the cortical responses were prolonged during non-rapid eye movement (NREM) sleep. In 3 of the subjects P2 was consistently bifid during NREM sleep only. The second component of the bifid potential, 3-4 msec longer in latency than the first, appeared to be selectively enhanced during NREM sleep whereas the first component tended to become less prominent or even disappear. This suggests that the 2 peaks have different generators that are affected differently by NREM sleep. These are clinically relevant findings for interpretation of routine clinical studies.     

Retrograde effects of digital nerve severance on somatosensory evoked potentials in man

To determine if retrograde conduction changes might occur long after injury of the most distal peripheral nerves, short-latency somatosensory evoked potentials (SEPs) to median or ulnar nerve stimulation at the wrist were studied in 10 subjects who had sustained traumatic digit amputation 4 months to 15 years previously. SEPs were recorded from Erb's point (N9), the cervical region (N13), and the contralateral scalp hand area (N20). While N9 latency was slightly delayed or not affected, the amplitude was either markedly reduced or undetectable. For N13 and N20 components, latency prolongation and amplitude reduction were mild to moderate, but the central conduction time (N13–N20) remained normal. The present data indicate that even the most distal nerve injury may have profound long-term retrograde effects on parental nerve function which are presumed mainly due to an axonopathy. © 1994 John Wiley & Sons, Inc.     

The effects of antiepileptic drugs on high-frequency oscillations in somatosensory evoked potentials

ObjectivesHigh frequency oscillations (HFOs) of Somatosensory evoked potentials (SEPs) reflect the activity of thalamo-cortical and cortical neurons from the sensory pathway. Antiepileptic-drugs (AEDs) reduce seizures acting on the balance between excitation and inhibition. We aimed to study the effect of AED mono and polytherapy on SEP-HFO’s components.MethodsTwenty-five patients with focal epilepsy were enrolled for the purpose of this study. Patients were divided in 3 groups according to the number of AEDs (1, 2 or 3 AEDs). Patients in group 1 underwent SEP-HFOs recording in drug naïve condition and at 1 month after AED titration. HFOs were compared in duration, amplitude and latency among the three groups.ResultsThe amplitude and duration of late HFOs of the affected hemisphere (AH) are different between groups and inversely correlated with the number of AEDs. In naïve patients monotherapy reverts the asymmetry in totHFOs (total HFOs) duration.ConclusionOur results demonstrate that SEP-HFOs are sensitive to the action of AEDs on cortical excitability. This effect seems to affect mainly the cortical component of HFOs in the AH and it is related to the number of AEDs taken.SignificanceSEP-HFOs might be a viable tool to probe cortical excitability changes induced by AEDs.     

Anesthesia and temperature effects on somatosensory evoked potentials produced by train stimuli

OBJECTIVE: To determine the effect of anesthesia, temperature, and stimulus characteristics on the response of upper extremity somatosensory evoked responses (SSEP) to repetitive stimulation. METHODS: Pairs and trains of electrical stimuli were used to elicit the upper extremity SSEP, and the amplitudes of the N20-P22, N13, and Erb's point potentials produced by each stimulus were measured. The ratio of the amplitude of the response to each stimulus to that produced by the first stimulus in a given train was computed. Recordings were also made directly from the cortical surface in a single patient. RESULTS: There were only minimal effects of anesthesia, temperature, or stimulus characteristics on the amplitude ratios for the N13 and Erb's point potentials. The N20-P22 amplitude ratio demonstrated facilitation with interstimulus intervals in the 20-100 ms range and was markedly reduced with hypothermia. The degree of facilitation was strongly dependent on isoflurane and nitrous oxide concentrations. In response to stimulation with trains of four stimuli, increased amplitudes of the N20-P22 complex were seen maximally with the second response while the third and fourth responses were of lower amplitude. CONCLUSIONS: There are strong effects of anesthesia and temperature on the cortical responses to repetitive stimulation of the upper extremity. Speculations on the origin of these effects were made.     

Topographical displays of somatosensory evoked potentials

The distribution of somatosensory evoked potentials (SEPs) after stimulation of the median nerve at the wrist was examined in 10 normal subjects using isopotential maps. The latencies of continuous negative and positive peaks were measured in each lead. The differences of the potentials at these latencies were measured in all the leads and the isopotential maps were constructed. The distribution of P0-NI was all similar. The latencies of P0 were almost the same in all the leads at about 13 msec. The distribution of NI-PI-NII was divided into three types--N16-P20-N28 localized in the frontal region, N17-P22-N30 localized in the central region and N19-P25-N33 distributed in the parieto-occipito-temporal regions. The distributions of NII-PII and PII-NIII were all similar, with high amplitudes in the central region. The latencies of PII and NIII were almost the same in all the leads at about 45 msec and 68 msec.     

Maturation of cerebral somatosensory evoked potentials

Cerebral somatosensory evoked potentials (SEPs) were elicited by stimulation of the median nerve and/or posterior tibial nerve in 117 children of 1 day to 16 years old. A major negative wave (N) was consistently recorded from the parietal region of the scalp when the arm was stimulated. The peak latency, the onset latency, the rising time and the duration of H wave are closely correlated with age and body length. The latencies are shortest in the subjects of 1-3 years old. SEPs to lower extremity stimulation were inconstant in the infants before the age of one. The major positive wave (P) has a variable topographic distribution along the middle line, over the scalp. The latencies are also very variable in the different subjects of the same age as well as in the same subject with different locations of active electrode. Among the parameters studied as for N wave, only the rising time of P wave is significantly correlated with age. The latencies of P wave have the shortest value in the subjects of 1-3 years old. The comparison of SEPs to upper and to lower limb stimulations shows that there is no relationship between them in respect to their morphology and amplitude. The minimum value of the latencies of N and P waves was observed at the same age but the difference between the peak latencies of P and N waves in the same subject increases considerably after 2 years of age and reaches the adult value after 5 years of age. These resultats indicate that the maturation of the peripheral somatosensory pathways proceeds at a higher rate than that of the central somatosensory pathways, that the maturation of the somatosensory pathways of the upper limb precedes that of the lower limb, and that the rising time of N or P waves is a good index of cortical maturation. The clinical utility of these SEPs in pediatrics is discussed.     

Cerebral evoked potentials and somatosensory perception

In normal subjects, somatosensory evoked potentials (SEPs) were produced by increases or decreases of the load on the biceps muscle during voluntary contraction. The stimuli lasted only 20 msec and caused less than 2 degrees of elbow flexion or extension. When the stimulus was applied during voluntary movement of the elbow, the SEP was attenuated, and the subject was less able to discriminate between loading and unloading pulses. In each of eight subjects, there was a positive correlation between the percentage of correct responses and the size of the SEP. The measurement of both SEPs and perceptual accuracy under various test conditions provides a refined technique for studying the relations between electrical events and sensory processes.     

Intersession stability of somatosensory evoked potentials

Upper and lower limb nerve somatosensory evoked potentials (SEPs) were recorded from a group of normal adults in order to assess their intersession stability. Median, peroneal and sural nerve SEPs were recorded from each subject on 3 successive occasions at weekly intervals. For median nerve SEPs, the intrinsic variability was more pronounced for interpeak latencies (N9-N20, N13-N20) than for the latencies of individual components. Their inherent fluctuation was not, however, notably greater for the primary cortical response (N20) than for the more caudally generated components (N9 and N13). These relationships were not so apparent for lower limb SEPs where intersession variability was markedly greater overall than for upper limb SEPs.     

The effects of halothane on somatosensory and flash visual evoked potentials during operations

Abstract

Intraoperative use of somatosensory evoked potentials (SEP’s) to monitor intracranial aneurysm surgery and flash visual evoked potentials (F-VEP’s) for parasellar surgery have been routinely employed in our clinic. We found that both EP modalities are sensitive to the changing concentration of our standard hypotensive agent, halothane. The prolongation of the N14-N20 interpeak latency to median nerve stimulation at the wrist, and prolongation of P100 latency with altered configuration of early VEP components to flash light stimulation, appear to be the results of direct pharmacological effects of the agent and not an effect of secondary hypotension. VEP is found easily abolished by halothane at a concentration of 2.0%, while the SEP is more resistant. Halothane is not ideal however when monitoring intraoperative VEP.