Dipolar sources of the early scalp somatosensory evoked potentials to upper limb stimulation Effect of increasing stimulus rates

Brain electrical source analysis (BESA) of the scalp electroencephalographic activity is well adapted to distinguish neighbouring cerebral generators precisely. Therefore, we performed dipolar source modelling in scalp medium nerve somatosensory evoked potentials (SEPs) recorded at 1.5-Hz stimulation rate, where all the early components should be identifiable. We built a four-dipole model, which was issued from the grand average, and applied it also to recordings from single individuals. Our model included a dipole at the base of the skull and three other perirolandic dipoles. The first of the latter dipoles was tangentially oriented and was active at the same latencies as the N20/P20 potential and, with opposite polarity, the P24/N24 response. The second perirolandic dipole showed an initial peak of activity slightly earlier than that of the N20/P20 dipolar source and, later, it was active at the same latency as the central P22 potential. Lastly, the third perirolandic dipole exaplaining the fronto-central N30 potential scalp distribution was constantly more posterior than the first one. In order to evaluate the effect of an increasing repetition frequency on the activity of SEP dipolar sources, we applied the model built from 1.5-Hz SEPs to traces recorded at 3-Hz and 10-Hz repetition rates. We found that the 10-Hz stimulus frequency reduced selectively the later of the two activity phases of the first perirolandic dipole. The decrement in strength of this dipolar source can be explained if we assume that: (a) the later activity of the first perirolandic dipole can represent the inhibitory phase of a “primary response”; (b) two different clusters of cells generate the opposite activities of the tangential perirolandic dipole. An additional finding in our model was that two different perirolandic dipoles contribute to the centro-parietal N20 potential generation. Received: 5 August 1997 / Accepted: 26 November 1997     

Finger movement is associated with attenuated cutaneous reflexes recorded from human first dorsal interosseous muscle

Cutaneomuscular reflexes (CMR) have been recorded from the first dorsal interosseous muscle (1DI) of the preferred hand, somatosensory evoked potentials (SEP) were recorded from the contralateral sensory cortex and the sensory nerve action potential (SNAP) was recorded from the median nerve of 15 adult subjects whilst electrically stimulating the digital nerves of the index finger. Subjects performed the following tasks (a) a sustained abduction of the index finger against resistance at 10–20 % maximum voluntary contraction (MVC), and (b) abduction of the index finger as in (a) whilst performing self paced low amplitude tapping of the (i) index finger, (ii) thumb, (iii) middle finger, (iv) little finger and (v) ipsilateral foot. The E2 component of the CMR and the N20/P25 components of the SEP were significantly reduced during finger tapping ( P < 0.05). This reduction was independent of which finger was tapping ( P > 0.05). There was a significant (qualitative) relationship between the decrease in the size of the E2 component of the CMR and the N20/P25 components of the SEP (χ² test; P < 0.05). There were no significant changes in E1 and I1 ( P > 0.05). The size of the SNAP was independent of task ( P > 0.05). The size of the E1, I1, E2 components of the CMR, and the N20/P25 components of the SEP were unaltered during foot tapping ( P > 0.05, n = 4). We conclude that the decrease in size of the E2 component associated with finger tapping results from gating of the digital nerve input.     

Altered cortical integration of dual somatosensory input following the cessation of a 20 min period of repetitive muscle activity

The adult human central nervous system (CNS) retains its ability to reorganize itself in response to altered afferent input. Intracortical inhibition is thought to play an important role in central motor reorganization. However, the mechanisms responsible for altered cortical sensory maps remain more elusive. The aim of the current study was to investigate changes in the intrinsic inhibitory interactions within the somatosensory system subsequent to a period of repetitive contractions. To achieve this, the dual peripheral nerve stimulation somatosensory evoked potential (SEP) ratio technique was utilized in 14 subjects. SEPs were recorded following median and ulnar nerve stimulation at the wrist (1 ms square wave pulse, 2.47 Hz, 1× motor threshold). SEP ratios were calculated for the N9, N11, N13, P14–18, N20–P25 and P22–N30 peak complexes from SEP amplitudes obtained from simultaneous median and ulnar (MU) stimulation divided by the arithmetic sum of SEPs obtained from individual stimulation of the median (M) and ulnar (U) nerves. There was a significant increase in the MU/M + U ratio for both cortical SEP components following the 20 min repetitive contraction task, i.e. the N20–P25 complex, and the P22–N30 SEP complex. These cortical ratio changes appear to be due to a reduced ability to suppress the dual input, as there was also a significant increase in the amplitude of the MU recordings for the same two cortical SEP peaks (N20–P25 and P22–N30) following the typing task. No changes were observed following a control intervention. The N20 (S1) changes may reflect the mechanism responsible for altering the boundaries of cortical sensory maps, changing the way the CNS perceives and processes information from adjacent body parts. The N30 changes may be related to the intracortical inhibitory changes shown previously with both single and paired pulse TMS. These findings may have implications for understanding the role of the cortex in the initiation of overuse injuries.     

椎基底动脉供血不足家兔模型体感诱发电位分析

目的：研究椎基底动脉供血不足(VBI)时体感诱发电位(SEP)的动态变化及其诊断意义。方法：选择成年家兔10只，通过结扎家兔一侧椎动脉人为造成VBI模型，以N27、P40峰潜期及峰间期为指数，动态检测其右侧胫后神经SEP的变化，研究其变化规律。结果：VBI后SEPN27，p40的峰潜伏期明显延长，峰间期无明显变化。结论：SEP对VBI发作期感觉功能的变化有诊断价值，并可评价发作的严重程度。     

Variation of frontal P20 potential due to rotation of the N20-P20 dipole moment of SEPs

Summary We have applied the artificial neural network method to estimate the N20-P20 dipole from scalp SEP potentials, and have investigated the rotation of the dipole moment caused by the compression effect of a brain tumor (para-Rolandic tumor) adjacent to the central sulcus. The variation of the frontal P20 potential was demonstrated by the rotation of the N20-P20 dipole moment in 8 cases of para-Rolandic tumor. By estimation of the rotation of the dipole moment, it may be possible to obtain preoperative information regarding the relation between the central sulcus and the tumor.     

Spatial mapping of SEP in comatose patients: Improved outcome prediction by combined parietal N20 and frontal N30 analysis

Summary The aim of this study is to evaluate whether SEP spatial mapping can improve outcome prediction in comparison to the conventional SEP recordings. Twenty patients comatose as a result of head injury or cerebral vascular disorders were submitted to 19-channel SEP mapping from median nerve stimulation. SEP recording were performed within the 4th hospital day in 18 cases and over one month from the insult in the remaining two. Nine patients (45%) showed a good recovery or a mild disability, 3 (15%) a severe disability and the rest (40%) died or remained in a vegetative state. Five patients (28%) had bilaterally normal SEP, 5 (28%) the absence of both parietal N20 and frontal N30, while the others (44%) had a dissociation N20/N30 (namely, preserved N20 with absent N30). The SEP mapping was significantly related to the outcome (P=0.0087) and improved the outcome prediction in comparison to the conventional SEP recordings, allowing to check the presence of frontal N30: in patients with bilaterally present N20 the outcome appeared to depend upon the N30. SEP mapping proved to be a far superior prognostic indicator than the Glasgow Coma Scale. In 3 patients with midline shift on CT scan an abnormal spatial distribution of N20 was disclosed by SEP mapping. Our preliminary results suggest that SEP mapping may improve the assessment of comatose patients in comparison to the use of parietal derivations only.     

78例儿童孤独症患者体感诱发电位分析

目的:通过观察和分析儿童孤独症患者体感诱发电位(SEP)的改变,探讨其在儿童孤独症诊断中的临床意义。方法:对1岁零8个月至8岁的78例孤独症儿童进行SEP检查,观察皮层第一个电位P40潜伏期及P40、N50、P60、N75波幅的改变。结果:78例孤独症患儿皮层电位P40潜伏期均有延长,其中双侧延长55例,单侧延长23例,并波幅分化差或左右不对称18例。结论:孤独症患者体感传导通路皮层电位潜伏期延长和波幅的改变,提示皮层下(包括脑干、丘脑)和相关皮层功能受损的可能,结合影像学检查,对评价神经系统功能及预后有一定的价值,对孤独症患儿临床诊断有帮助。     

体感诱发电位磁共振对脊髓炎的定位诊断价值对比研究

目的：比较体感诱发电位(SEP)和脊髓磁共振(MRI)检查对视神经脊髓炎(NMO)和急性横贯性脊髓炎(ATM)病人进行定位诊断的价值。方法：对21例临床确诊为NMO和36例ATM病人的SEP和MRI检查结果进行回顾性分析，比较其定位符合率。结果：①ATM组病人MRI以胸髓损害为主，NMO组病人以颈胸段脊髓联合损害为主；②ATM组病人SEP主要表现为N20正常、P40异常，而NMO组病人则表现为N20和P40均异常；当病变位于颈髓时，NMO组SEP与MRI的定位符合率为94．1％，ATM组为78．6％；当病变位于胸髓时，ATM组为100％，NMO组为66．7％。结论：在NMO和ATM病人中，SEP提示的神经电生理异常部位基本与MRI所证实的病变部位相吻合。     

High frequency vibration induced gating of subcortical and cortical median nerve somatosensory evoked potentials: different effects on the cervical N13 and on the P13 and P14 far-field SEP components.

Subcortical and cortical somatosensory evoked potentials (SEP) to median nerve stimulation were recorded before, during and after high frequency (270 Hz) vibration of the fingers 1-3 in 8 healthy subjects. A marked decrease of the amplitude of all potentials was observed. The attenuation of the sensory nerve action potential (SNAP) of the median nerve and the attenuation of SEP components N9, N11 and N13 showed no differences, while the attenuation of the subcortical P14 component was significantly higher. This is in accordance with a generator of the cervical N13 in the interneurons beside the lemniscal pathway. The cortical N20 (post-rolandic) was significantly more decreased in amplitude than P14 while P22 (pre-rolandic) remained reduced in amplitude like P14. An increased latency of the far-field subcortical P14 was observed, while P13 recorded in the same montage remained unchanged in latency. These findings suggest different generators of these peaks. A generator of P14 above the nucleus cuneatus is confirmed. A presynaptic generator of P13 is suspected.     

胫后神经体感诱发电位对脊髓病变的定位诊断

目的：研究胫后神经体感诱发电位对脊髓病变的定位诊断价值。方法：通过经皮恒流电刺激胫后神经，采用髂棘、脐部和头皮Fpz作参考电极，分别于第4腰椎棘突、第12胸椎棘突和头皮Cz’置记录电极，依次记录CE、N24和P40电位之峰潜伏期。共检测15例健康成人和29例经核磁共振证实的腰骶神经根或脊髓病变患者。结果：①CE、N24和P40电位均异常，见于腰骶神经根病变；②CE正常，N24和P40异常，见于腰骶髓病变；③CE、N24正常，P40异常，见于颈胸髓病变。结论：胫后神经体感诱发电位能够鉴别腰骶神经根或不同节段的脊髓病变，临床上具有定位诊断价值。但对于轻度的腰骶神经根或脊髓腹侧病变，结果可出现“假阴性”。     

肝硬化失代偿期病人体感诱发电位观察

目的 :探讨肝硬化失代偿期病人的神经功能状态。方法 :对 30例肝硬化失代偿期病人进行体感诱发电位 (SEP)检查 ,刺激部位均为右腕正中神经。结果 :肝硬化失代偿期病人的SEP有着不同程度的改变 ,其中P15、N2 0 、P2 5、N3 5的潜伏期延长和N13 -P15的波间期延长分别占本组异常数的 85 %、40 %、35 %、2 5 %和 2 0 %。这些值与正常人组相比较 ,其两组间差异均有极显著性意义 (P <0 0 1)。结论 :SEP可为研究亚临床肝性脑病提供电生理学的客观依据。     

Dynamic Brain Topography of Somatosensory Evoked Potentials and Equivalent Dipoles in Response to Graded Painful Skin and Muscle Stimulation

The differential effects of painful stimulation of skin vs. muscle on the cerebral electrophysiology have been poorly described. This study examined the somatosensory evoked potentials (SEPs) and the associated dipole models of non-painful and graded painful electrical stimulation applied to the skin and muscle in 20 healthy subjects. With the psychophysical stimulus-response functions determined, the skin stimulation showed a steeper slope than muscle stimulation. For both types of stimulation, the SEPs indicated a similar temporo-spatial activation sequence: F4/N90-P4/P95, Fc2/N135, Cz/P250, Cz/P300, and Cz/N460. The SEP amplitudes increased significantly with the stimulus intensities in these components. The peak SEP latencies of skin stimulation were in general shorter than that of muscle stimulation. The SEP amplitudes to skin stimulation were significantly larger than those caused by muscle stimulation at every stimulus intensity level, except the early mid-latency component. In this case, muscle stimulation caused higher amplitudes over the contralateral parietal-frontal sites. For both types of stimulation, the topographic maps were quite similar. Equivalent dipole modeling revealed identical site parameters (<1.0 cm) between skin and muscle stimulation. However, the electrical skin stimulation did not correlate with the pain intensity. Pain intensity, in contrast, was uniquely associated with the Cz/P250 amplitudes for the muscle stimulation. It is concluded that non-nociceptive and nociceptive electrical stimuli applied to skin and muscle are processed in the common cerebral areas, but exhibit differential SEP effects.     

Multichannel derived median nerve SEP compared to EEG in patients with vascular cerebral lesions.

In order to compare the sensitivity of multichannel derived median nerve SEP with EEG in vascular cerebral lesions we examined 22 normals and 23 patients. SEP components within the first 50 ms could be divided into main waveform patterns: (1) a W-shaped parietal pattern consisting of N20, P25, N35 and P45 in most cases. (2) a frontal pattern with P20 and N30 as well as possibly detectible N24, P28, P33, N40 and P50. (3) a central P22. Two younger normals showed a V-shaped parietal pattern with N20 and P35, a frontal pattern with P20 and N36, and central P22 with a remarkably long latency. All components could be analysed sufficiently by means of three representative electrode positions (stimulation right/left): P3/P4, C3/C4, and F3/F4, which reduces the expense of recording and analysing considerably. 21 patients (91.3%) showed abnormal results in SEP, whereas 14 patients (60.9%) in EEG. A three channel electrode array can increase the usefulness of SEP and detect cerebral dysfunctions in cerebral lesions in spite of normal EEG under routine examination conditions. Analysis of multichannel derived SEPs during treatment and recovery after stroke and search for the prognostic value in the acute stage of the disease should be done in future.     

痉挛性斜颈体感诱发电位的研究

目的：探讨痉挛性斜颈患者大脑皮层功能的变化。方法：对30例痉挛性斜颈患者刺激正中神经后体感诱发电位（SEP）的P22、N30波潜伏期及P22、N30波幅进行比较分析,30例正常对照组仅在颈部主动向右侧扭转时对双侧P22、N30波幅进行比较分析。结果：病例组SEPP22、N30潜伏期正常,双侧比较差异无统计学意义,头部扭转方向的对侧大脑半球前中央区的P22-N30波幅比明显高于对侧,差异有统计学意义。正常对照组前中央区记录的双侧P22N30波幅比较差异无统计学意义。结论：SEP P22、N30潜伏期正常提示传导通路结构完整,头部扭转方向的对侧大脑半球前中央区的P22-N30波幅比明显高于对侧,提示患者对侧大脑皮层前中央区电活动存在异常的兴奋及抑制,即抑制性减弱,兴奋性增高,N30记录的是刺激正中神经SEP中长潜伏成分,可能来源于运动辅助区,进一步提示患者存在感觉一运动整合功能异常。     

Changes in muscle and cutaneous cerebral potentials during standing

Summary The cerebral potentials produced by electrical stimulation of mechanoreceptive afferents from the foot were recorded in the sitting and standing postures to determine whether transmission to cortex was altered by the postural change. The latencies of the early components of the cerebral potentials produced by muscle afferents (posterior tibial nerve) and cutaneous afferents (sural nerve) did not change with posture. Standing was associated with an approximately 25–35% decline in amplitude of the earliest components of the posterior tibial cerebral potential (N38-P40, P40-N50) for a stimulus intensity associated with a submaximal afferent volley. The amplitude of the equivalent N38-P40 and P40-N50 components produced by sural afferents also declined during quiet stance. In most experiments the subcortical component (P32-N38) was not reduced by stance so that the amplitude attenuation probably occurs in part at cortical level. Qualitatively similar changes in the cerebral potentials were documented for a range of stimulus intensities, including those which evoked a maximal initial component in the nerve volley. For a similar reduction in the initial (N38-P40) component of the cerebral potential, voluntary plantar flexion in the sitting position produced less attenuation in subsequent components than did standing. Thus, attenuation of the cerebral potential during standing may involve specific posture-related factors in addition to those related to volition.     

Source analysis of median nerve and finger stimulated somatosensory evoked potentials: Multichannel simultaneous recording of electric and magnetic fields combined with 3d-MR tomography

Summary At the current state of technology, multichannel simultaneous recording of combined electric potentials and magnetic fields should constitute the most powerful tool for separation and localization of focal brain activity. We performed an explorative study of multichannel simultaneous electric SEPs and magnetically recorded SEFs. MEG only sees tangentially oriented sources, while EEG signals include the entire activity of the brain. These characteristics were found to be very useful in separating multiple sources with overlap of activity in time. The electrically recorded SEPs were adequately modelled by three equivalent dipoles located: (1) in the region of the brainstem, modelling the P14 peak at the scalp, (2) a tangentially oriented dipole, modelling the N20-P20 and N30-P30 peaks, and part of the P45, and (3) a radially oriented dipole, modelling the P22 peak and part of the P45, both located in the region of the somatosensory cortex. Magnetically recorded SEFs were adequately modelled by a single equivalent dipole, modelling the N20-P20 and N30-P30 peaks, located close to the posterior bank of the central sulcus, in area 3b (mean deviation: 3 mm). The tangential sources in the electrical data were located 6 mm on average from the area 3b. MEG and EEG was able to locate the sources of finger stimulated SEFs in accordance with the somatotopic arrangement along the central fissure. A combined analysis demonstrated that MEG can provide constraints to the orientation and location of sources and helps to stabilize the inverse solution in a multiple-source model of the EEG.     

Median sep and blink reflex in thyroid diseases

Pathological disturbances of thyroid hormones is associated with central and peripheral nervous system disturbances. The aim of this study is to evaluate median nerve stimulated somatosensory evoked potential (SEP) and blink reflex of thyroid patients (hypo and hyperthyroidism). Median SEP was performed in 40 patients (21 with hyperthyroidism and 19 with hypothyroidism). We evaluated the latencies of N9, N11, N13, P9, P11, P14, N20 and P25 waves and the N9-N20, N9-N13, N13-N20 and P14-N20 interpeak latencies. We compared the results of patients with the control group (26 persons). We found that the N20 latency was longer in patients with hyperthyroidism than in the control group and the difference was statistically significant. There was not any statistically significant difference regarding the N9, N11, N13, P9, P11, P14, N20 and P25 latencies and the N9-N20, N9-N13, N13-N20 and P14-N20 interpeak latencies between hypothyroid patients and controls. We performed the blink reflex study in 28 of 40 patients (14 patients with hyperthyroidism and 14 patients with hypothyroidism). Comparing the R1, R2, CR2 (contralateral R2) latencies and durations of the patients and controls, we found that R2 and CR2 duration was shorter in patients with hyperthyroidism. This difference was statistically significant.     

Trajectories of shifting dipole sources of visual evoked potentials across the human brain

Visual evoked potentials in response to images of a set of horizontal and vertical lines or crosses were recorded from the brains of 18 human subjects in 34 leads. Inverse EEG analyses were used for the dynamic location of the dipole current sources of the N1, P1, and N2 waves using a two-dipole spherical model with a 1-msec step. The occipital lobes of all subjects showed significant displacement of the dipoles of evoked potential waves along predominantly arc-shaped trajectories (75.8% of cases). Trajectory durations (average about 25 msec) were characterized by insignificant interindividual variability and were independent of the type of stimulus and the phase of the evoked potential. A characteristic (occurring in 85% of cases) “jump” in the coordinates of the dipole, which constituted a rapid, sharp, and significant medial displacement, was seen between the first and second trajectories of the equivalent current dipoles (at 110–120 msec after stimulus onset). The possible significance of these data for understanding the dynamics and kinetics of processing of local image features in the human visual cortex is discussed. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 57, No. 6, pp. 673–683, November–December, 2007.     

The effect of changing stimulus intensities on median nerve somatosensory-evoked potentials

We investigated median nerve somatosensory-evoked potentials (SEP) in 31 healthy volunteers to test the hypothesis that 1) increasing stimulus intensity influences SEP components in both amplitude and latency 2) SEP components respond differently to changing intensities. Cluster analysis and analysis of variance were used for statistical testing. Three groups of components could be found according to latency changes in response to increasing stimulus intensities: N13, and P15, the primary cortical response (N19, P22) and the components over 30 ms. In general, SEP components below 30 ms significantly shortened in latency and increased in amplitude with subsequent saturation. In contrast, in components over 30 ms latencies decreased linearly and amplitudes changed inhomogeneously. The clear effect of stimulus intensity on most median nerve SEP components makes it necessary to maintain comparable stimulus intensities when comparing intra- and interindividual registrations.