Dissociation induced by voluntary movement between two different components of the centro-parietal P40 SEP to tibial nerve stimulation

Topographic distribution of the tibial somatosensory evoked potential using coherence

The objective of the present study was to determine the adequate cortical regions based on the signal-to-noise ratio (SNR) for somatosensory evoked potential (SEP) recording. This investigation was carried out using magnitude-squared coherence (MSC), a frequency domain objective response detection technique. Electroencephalographic signals were collected (International 10-20 System) from 38 volunteers, without history of neurological pathology, during somatosensory stimulation. Stimuli were applied to the right posterior tibial nerve at the rate of 5 Hz and intensity slightly above the motor threshold. Response detection was based on rejecting the null hypothesis of response absence (significance level alpha= 0.05 and M = 500 epochs). The best detection rates (maximum percentage of volunteers for whom the response was detected for the frequencies between 4.8 and 72 Hz) were obtained for the parietal and central leads mid-sagittal and ipsilateral to the stimulated leg: C4 (87%), P4 (82%), Cz (89%), and Pz (89%). The P37-N45 time-components of the SEP can also be observed in these leads. The other leads, including the central and parietal contralateral and the frontal and fronto-polar leads, presented low detection capacity. If only contralateral leads were considered, the centro-parietal region (C3 and P3) was among the best regions for response detection, presenting a correspondent well-defined N37; however, this was not observed in some volunteers. The results of the present study showed that the central and parietal regions, especially sagittal and ipsilateral to the stimuli, presented the best SNR in the gamma range. Furthermore, these findings suggest that the MSC can be a useful tool for monitoring purposes.

Unmasking of presynaptic and postsynaptic high-frequency oscillations in epidural cervical somatosensory evoked potentials during voluntary movement

OBJECTIVE

To investigate the effect of the voluntary movement on the amplitude of the somatosensory evoked potentials (SEPs) recorded by an epidural electrode at level of the cervical spinal cord (CSC).

METHODS

Fourteen patients underwent an epidural electrode implant at CSC level for pain relief. After the median nerve stimulation, SEPs were recorded from the epidural electrode and from 4 surface electrodes (in frontal and parietal regions contralateral to the stimulated side, over the 6th cervical vertebra, and on the Erb's point). SEPs were recorded at rest and during a voluntary flexo-extension movement of the stimulated wrist. Beyond the low-frequency SEPs, also the high-frequency oscillations (HFOs) were analysed.

RESULTS

The epidural electrode contacts recorded a triphasic potential (P1-N1-P2), whose negative peak showed the same latency as the cervical N13 response. The epidural potential amplitude was significantly decreased during the voluntary movement, as compared to the rest. Two main HFOs were identifiable: (1) the 1200 Hz HFO which was significantly lower in amplitude during movement than at rest, and (2) the 500 Hz HFO which was not modified by the voluntary movement.

CONCLUSIONS

The low-frequency cervical SEP component is subtended by HFOs probably generated by: (1) postsynaptic potentials in the dorsal horn neurones (1200 Hz), and (2) presynaptic ascending somatosensory inputs (500 Hz).

SIGNIFICANCE

Our findings show that the voluntary movement may affect the somatosensory input processing also at CSC level.

Reduction in amplitude of the subcortical low- and high-frequency somatosensory evoked potentials during voluntary movement: an intracerebral recording study

OBJECTIVE

To investigate whether the reduction of amplitude of the scalp somatosensory evoked potentials (SEPs) during movement (gating) is due to an attenuation of the afferent volley at subcortical level.

METHODS

Median nerve SEPs were recorded from 9 patients suffering from Parkinson's disease, who underwent implant of intracerebral (IC) electrodes in the subthalamic nucleus or in the globus pallidum. SEPs were recorded from Erb's point ipsilateral to stimulation, from the scalp surface and from the IC leads, at rest and during a voluntary flexo-extension movement of the stimulated wrist. The recorded IC traces were submitted to an off-line filtering by a 300-1500 bandpass to obtain the high-frequency SEP bursts.

RESULTS

IC leads recorded a triphasic component (P1-N1-P2) from 14 to 22 ms of latency. The amplitudes of the scalp N20, P20 and N30 potentials and of the IC triphasic component were significantly decreased during movement, while the peripheral N9 amplitude remained unchanged. Also the IC bursts, whose frequency was around 1000 Hz, were reduced in amplitude by the voluntary movement.

CONCLUSIONS

Since the IC triphasic component is probably generated by neurons of the thalamic ventro-postero-lateral nucleus, which receive the somatosensory afferent volley, the P1-N1 amplitude reduction during movement suggests that the gating phenomenon involves also the subcortical structures.

Differential gating of slow postsynaptic and high-frequency spike-like components in human somatosensory evoked potentials under isometric motor interference

Human cortical somatosensory evoked potentials (SEP) can be modified by concomitant motor tasks ('gating'), through peripheral occlusion and/or central mechanisms. The present study aimed (1) at refining earlier results concerning motor-gating of the primary cortical EPSP-related N20 response after electric median nerve stimulation, and (2) at providing first data on motor-gating of the 600 Hz SEP wavelet burst which occurs superimposed onto N20 and primarily reflects repetitive cerebral population spikes. In 12 healthy subjects median nerve SEP were elicited, using electrical stimuli with intensities below, at and above motor threshold, under either rest or an isometric fist clenching task. Amplitude and latency modifications were analysed for the peripheral compound action potential (CAP), low-frequency SEP components (N20, P25, N35 and P70) and the high-frequency burst. While the peripheral CAP remained unchanged, isometric motor innervation significantly attenuated N20, P25 and P70 amplitudes and shortened peak latencies progressively for all components after N20. In contrast, the high-frequency 600 Hz burst was modulated neither in amplitude nor in latency. Regular amplitude recruitment occurred for all components independent from the motor task, excluding channel saturation as an explanation for gating. We suggest that SEP gating under isometric motor innervation is a central process which selectively operates on specific SEP components and could partly reflect an "efference copy" mechanism.

Source generators of the early somatosensory evoked potentials to tibial nerve stimulation: an intracerebral and scalp recording study

OBJECTIVE

To investigate the location of the cerebral generators of the early scalp somatosensory evoked potentials (SEPs) after tibial nerve stimulation.

METHODS

Tibial nerve SEPs were recorded in 15 patients, suffering from Parkinson's disease, who underwent implantation of intracerebral (IC) electrodes in the subthalamic nucleus, in the globus pallidum or in the thalamic ventralis intermediate nucleus. SEPs were recorded both from the scalp surface and from the IC leads.

RESULTS

The lemniscal P30 response was recorded by all the electrodes. The IC waveforms included a negative N40IC response, followed by a positive (P50IC) and a negative (N60IC) potential. The N40IC, the P50IC and the N60IC potentials did not differ in latency from the P40, the N50 and the P60 responses recorded by the Cz electrode. In 6 patients, in which SEPs were recorded also during the voluntary movement of the stimulated foot (active gating), an amplitude reduction of the SEP components following the P30 potential was observed during movement at the vertex and in the IC traces. Instead, in the contralateral temporal traces the SEP components (N40temp and P50temp) were not modified by active gating, and in the ipsilateral parietal traces only the positive potentials at about 60ms of latency was decreased.

CONCLUSIONS

Two differently oriented generators are active in the contralateral hemisphere at both 40 and 50ms of latency after tibial nerve stimulation. One source is oriented perpendicularly to the mesial hemispheric surface and generates the potentials recorded by the contralateral temporal and the ipsilateral parietal leads; the other dipolar source is radial to the hemispheric convexity, and generates the potentials at the vertex and those recorded by the IC electrodes.

Characterizing somatosensory evoked potential sources with dipole models: Advantages and limitations

Several methods have been developed to investigate the cerebral generators of scalp somatosensory evoked potentials (SEPs), because simple visual inspection of the electroencephalographic signal does not allow for immediate identification of the active brain regions. When the neurons fired by the afferent inputs are closely grouped, as usually occurs in SEP generation, they can be represented as a dipole, that is, as a linear source with two opposite poles. Several techniques for dipolar source modeling, which use different algorithms, have been employed to build source models of early, middle-latency, and late cognitive SEPs. Modifications of SEP dipolar activities after experimental maneuvers or in pathological conditions have also been observed. Although the effectiveness of dipolar source analysis should not be overestimated due to the intrinsic limitations of the approach, dipole modeling provides a means to assess SEPs in terms of cerebral sources and voltage fields that they produce over the head.

Scalp distribution of the earliest cortical somatosensory evoked potential to tibial nerve stimulation: proposal of a new recording montage

OBJECTIVE

To investigate the most reliable method to record the earliest cortical somatosensory evoked potential (SEP) after tibial nerve stimulation. The 'gating' phenomenon was used to dissociate the overlapping cortical SEP components.

METHODS

In 11 subjects we recorded the scalp SEPs at rest, during the voluntary (active gating) and passive (passive gating) foot movement and during the isometric calf muscle contraction (isometric gating).

RESULTS

At the vertex the P40 amplitude was reduced in all the gating conditions. Instead, both the P40 response recorded in the parietal region ipsilateral to the stimulation (indicated as P40par) and the fronto-temporal N37 potential were reduced in amplitude only during the passive foot movement.

CONCLUSIONS

The same behaviour of the N37 and P40par potentials suggests that they can represent the opposite counterparts of the same dipolar generator. Instead, the real P40 amplitude, which is affected in all the gating conditions, is recorded at the vertex and might be generated by a different source. We conclude that the montage obtained by referring a temporal electrode contralateral to the stimulation to an ipsilateral parietal lead can reliably record the earliest cortical component (N37/P40par) after tibial nerve stimulation.

Sensory and motor interfering influences on somatosensory evoked potentials

The interfering influences by which the different components of the early somatosensory evoked potentials are modified are reviewed from both neurophysiologic and clinical perspectives. Special consideration is given to the specific differences between sensory and motor interferences. In this context, the specific effect of the mental movement simulation task on the frontal N30 component is discussed in relation to the involvement of this evoked wave as a physiologic index of the dopaminergic motor pathways. Relevant interfering approaches, including concurrent events ranging from tactile stimulation to locomotion, are reviewed and discussed insofar as these data provide insights into the neurophysiologic processes of interaction between competing internal models controlling motor acts and sensory information.

Neuroanatomic substrates of lower extremity somatosensory evoked potentials

After stimulation of the lower extremity nerve (tibial nerve), N21 and N23 are recorded from L4 and T12 spine respectively. The far-field potentials of P31 and N35 are registered from Fpz-C5s (fifth cervical spine) or CPi (ipsilateral with respect to the side of stimulation)-ear derivation. Additional far-field potentials of P17 and P24 may be recorded from the scalp when a noncephalic (knee) reference is used. The major positive peak, P40, is registered at the vertex and the CPi. Preceding P40, there is a small negative peak, N37, recorded at the contralateral (CPc) hemisphere. Neuroanatomic substrates of these somatosensory evoked potential (SSEP) components are less well clarified compared with those of upper extremity (median nerve) SSEPs, primarily because clinical application of lower extremity SSEPs is more difficult, and all of the aforementioned potentials but one (P40) are not obligatory components. The concept of "paradoxical lateralization" complicates the issue further. Accumulating evidence, however, suggests that the far-field potentials of P17 and P31 arise from the distal portion of the sacral plexus and brainstem respectively. These correspond to P9 and P14 of the median nerve SSEPs respectively. The spinal potential of N23 is equivalent to the N13 cervical potential of the median nerve SSEP. N35 recorded from the ipsilateral hemisphere is analogous to N18 of the median nerve. Paradoxically lateralized P40 has been thought to represent the positive end of a dipole field, reflected by the negativity at the mesial surface of the contralateral hemisphere, and has commonly been considered to be equivalent to the first cortical potentials (N20) of the median nerve SSEP. However, more recent evidence suggests that the primary positivity is at the mesial cortical surface, and it more likely corresponds to P26 of the median nerve SSEP. Thus the first cortical potential corresponding to N20 is probably a small and inconsistent N37 recorded on the contralateral hemisphere. These assumptions need to be verified further by more extensive clinical studies applied to various neurologic disorders.

Effect of movement on dipolar source activities of somatosensory evoked potentials

The early scalp somatosensory evoked potentials (SEPs) to median and tibial nerve stimulation were recorded at rest and during voluntary movement of the stimulated hand and foot, respectively. Both tibial and median nerve SEP distributions at rest could be explained by four-dipole models, in which one dipole was activated at the same latency as the subcortical far field and the three remaining dipolar sources were located in the perirolandic region contralateral to the stimulated side. Voluntary movement reduced all cortical dipoles in strength, while the subcortical one remained unchanged, suggesting that the effect of movement occurs above the cervicomedullary junction. In animals, cutaneous inputs are suppressed during movement and we therefore interpreted the depression of activity in the primary somatosensory cortex induced by movement as due to selective "gating" of cutaneous afferents. Because the reduction in strength of the cortical dipoles was generally lower during passive than active movement, both centrifugal and centripetal mechanisms probably contribute to the phenomenon of "gating."

Brain electrical source analysis of primary cortical components of the tibial nerve somatosensory evoked potential using regional sources

Tibial nerve somatosensory evoked potentials (SEPs) show higher amplitudes ipsilateral to the side of stimulation, whereas subdural recordings revealed a source in the foot area of the contralateral hemisphere. We now investigated this paradoxical lateralization by performing a brain electrical source analysis in the P40 time window (34-46 ms). The tibial nerve was stimulated behind the ankle (8 subjects). On each side, 2048 stimuli were applied twice. SEPs were recorded using 32 magnetic resonance imaging (MRI)-verified electrode positions (bandpass 0.5-500 Hz). In each case, the P40 amplitude was higher ipsilaterally (0.45 +/- 0.14 microV) than contralaterally (-0.49 +/- 0.16 microV). The best fitting regional source, however, was always located in the contralateral hemisphere with a mean distance of 8.2 +/- 4.3 mm from the midline. The positivity pointed ipsilaterally shifting from a frontal orientation (P37) to a parietal direction (P40). The P40 dipole moment was 2.5 times stronger than the dipole moment of P37, which makes P40 most prominent in EEG recordings. However, with its oblique dipole orientation compared to the tangential P37 dipole, it is systematically underestimated in MEG. Dipole orientations explained interindividual variability of scalp potential distribution. SEP amplitudes were smaller when generated in the dominant (left) hemisphere. This is explained by deeper located sources (5.4 +/- 1.6 mm) with a more tangential orientation (delta theta = 17.5 +/- 2.3 degrees) in the left hemisphere.

Dipolar generators of the early scalp somatosensory evoked potentials to tibial nerve stimulation in human subjects

We performed the brain electrical source analysis (BESA) of the early scalp somatosensory evoked potentials (SEPs) to tibial nerve stimulation. A four-dipole model could well explain the scalp SEP distribution. One dipole showing a subcortical location was activated at the latency of the lemniscal P30. The three remaining dipoles were located near the sagittal fissure in the hemisphere contralateral to the stimulation. Two of these dipoles showed a biphasic activity and contributed to the signal evoked in the N37-P40 latency range. Also in the N50-P50 latency range two different source activities were involved in SEP building. This finding suggests that one of two possible N50 subcomponents represents the negative counterpart of the N50/P50 dipolar field and the other is originated by a radial source.