Guy Cheron

Central somatosensory conduction in man: Neural generators and interpeak latencies of the far-field components recorded from neck and right or left scalp and earlobes

Early somatosensory evoked potential (SEP) components to median nerve or finger stimulation were recorded with non-cephalic references in normal young adults. Detailed topographic data over scalp and neck were related to anatomical observations on the actual conduction distances in dorsal column, medial lemniscus and thalamo-cortical parts of the somatosensory pathway. The extrapolation of afferent conduction velocity (CV) measured from sensory nerve potentials along the peripheral nerve to the C6-C7 spinal segments identified the spinal entry time with the onset of the neck N11 or scalp P11 (far field 2 or FF2). The first far field (FF1) is generated in the nerve proximal to axilla. The definite latency shift of the spinal negativity along the neck indicates a CV of 58 m/sec. Data about the maximal diameter of lemniscal axons in man were used to calculate a CV of 40.5 m/sec. Consideration of transit times from spinal entry to cortex and of synaptic delays clarified the arrival times of the afferent volley at various relay nuclei, and also suggested a thalamo-cortical CV of about 33 m/sec. Interpeak and onset-to-peak measures on scalp far fields suggest that FF3-FF4 are generated in medial lemniscus rather than above the thalamus. Consistent differences in amplitude, but not in wave form, were recorded at right and left earlobes for FF2 (larger ipsilaterally) and FF3-FF4 (larger contralaterally). The scalp topography of far fields was analysed in detail.

Non-cephalic reference recording of early somatosensory potentials to finger stimulation in adult or aging normal: differentiation of widespread N18 and contralateral N20 from the prerolandic p22 and N30 components

Prerolandic and parietal SEPs to electrical stimulation of fingers or median nerve were studied with non-cephalic reference in 40 normal young adults and in 35 healthy octogenarians. Limb temperatures were 36-37 degrees C. Intersubject variations of SEP components were analysed. A new widespread component N18 was identified and shown to be generated below the cortex. This N18 is about the only early component recorded at the parietal ipsilateral region after the positive far-field potentials P9, P11 and P13-P14. Transit times along the central somatosensory pathway were replicated and discussed as well as other evidence about the sequential activation of the various neural structures involved. The N20 potential representing the earliest cortical response is recorded from the contralateral parietal region, but is absent ipsilaterally. The prerolandic potential is related to distinct generators and is elicited by a separate thalamocortical pathway rather than by corticocortical connections from areas 2 and 5 in parietal cortex. The changes associated with normal aging have been confirmed and extended.

Prevertebral (oesophageal) recording of subcortical somatosensory evoked potentials in man: The spinal P13 component and the dual nature of the spinal generators

Abstract Short-latency somatosensory evoked potential (SEP) components to median nerve or finger stimulation were recorded in 12 normal young adults with 14 channels, using a non-cephalic reference. Several electrodes were placed along the posterior neck, earlobes and scalp while oesophageal probes provided 2–6 recording sites at known levels in front of vertebrae C2 to Th3 (Fig. 1). The oesophageal electrodes provided a new non-invasive point of entry that secured important data about the generator sources of the spinal SEP components. All cephalic electrodes recorded a P 9 far field which is greater over the scalp, earlobes and rostral cervical spine than over the caudal cervical spine. P 9 is the volume-conducted peripheral nerve volley coursing between axilla and spinal cord. The direct recording of arrival times of the peripheral volley along the median nerve and brachial plexus (Erbs point) served to estimate the spinal entry time which coincided with the onset of the posterior neck N 11 (at levels C6-C7) and of the scalp P 11 far field (Fig.3). The latency shift of the onset of N 11 from lower to upper neck has been replicated (Fig. 8C, D) and N 11 is interpreted as a presynaptic volley ascending the dorsal column (central branch of the primary neurone) at a velocity of 58 m/sec (Desmedt and Cheron 1980a). At oesophageal electrodes in front of the C7-Th3 vertebrae, the first negative component starts before the spinal entry time and it corresponds to action potentials in the spinal roots (Figs. 2 and 3). The most remarkable feature of oesophageal recordings in front of the C3 to Th2 vertebrae is the positive P 13 component which represented a phase reversal of the second negativity N 13 recorded from posterior neck (Fig. 5). The P 13 -N 13 phenomenon was recorded in all subjects and presented a stable latency throughout the cervical cord (Fig. 6C). It was clearly differentiated from the scalp far-field P 14 which is related to the ascending volley in the median lemniscus (Figs. 5 and 8; Tables I and II). The spinal P 13 did not extend above C2 and it was obviously generated below the foramen magnum. P 13 is interpreted as a dorsal horn postsynaptic potential which is elicited by collateral branches of the (bifurcated) primary afferent fibre. The use of 14 simultaneous recordings including a series of pre- and post-vertebral electrodes clearly identified two distinct generators for the spinal SEP components to median nerve stimulation: a presynaptic generator which ascends the dorsal column (N 11 ) and a postsynaptic fixed generator in the dorsal horn of the cervical spinal cord (N 13 -P 13 ).

Somatosensory evoked potentials to finger stimulation in healthy octogenarians and in young adults: Wave forms, scalp topography and transit times of pariental and frontal components☆

Abstract Frontal and parietal SEPs to electrical stimulation of fingers were studied in conjunction with the spinal SEP and sensory nerve action potentials in 25 young adults and 19 healthy octogenarians. Limb temperatures were 36–37°C. Intersubject variations of SEP wave forms and detailed scalp topography helped delineate several genuine SEP components, namely the P25, N32 and N40, at precentral scalp sites. The parietal ‘W’ pattern (P30-N35-P45) was recorded in only 48% of the young, but 90% of the old subjects. All parietal components were significantly increased in size in the old, which contrasts with the reduced spinal SEP and sensory nerve potentials (Table II). The precentral SEP components were either maintained (P25) or reduced (N32). The primary afferent neurones appeared to age at a faster rate (CV reduced about 0.16 m/sec/year) than the second or third afferent neurones (no significant change in transit time from spinal entry to postcentral cortex or in mean central CV) (Table I). The scalp recorded SEPs were quite different in front of or behind the central fissure. Contrary to the ‘deep dipole’ hypothesis the SEPs recorded pre- or postcentrally were not mirror images and exhibited separate non-congruent intersubject variabilities (Figs. 1–5). The ‘travelling wave’ hypothesis was also rejected since the SEP components presented peak latencies that were stable within their scalp territory. It is suggested that the sequential activation of cortical modules in parietal and frontal areas, in part through demonstrated cortico-cortical axonal connections and possibly also through thalamo-cortical projections, could generate successive SEP components. The cortical generators of SEP components appeared to be similar in the octogenarians although their relative strengths changed and transit times between different components significantly increased (Table III).

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 Erbs 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.

Spinal and far-field components of human somatosensory evoked potentials to posterior tibial nerve stimulation analysed with oesophageal derivations and non-cephalic reference recording.

Somatosensory evoked potentials (SEPs) were elicited by stimulation of the right posterior tibial nerve at the ankle in 20 experiments on 18 normal adults. A non-cephalic reference on the left knee was used throughout (with triggering of averaging cycles from the ECG), except for recording the peripheral nerve potentials. The responses were recorded along the spine, from oesophageal probes and from the scalp. The peripheral nerve volley propagated at a mean maximum conduction velocity (CV) of 59.2 m/sec served to identify the spinal entry time (mean 19.7 msec) at spinal segments S1-S3, under the D12 spine. This entry time coincided with the onset of the N21 component which was interpreted as the dorsal column volley and considered equivalent to the neck N11 of the median nerve SEP. The large voltage of the spinal response at the D12 spine probably results from summation of N21 with a fixed latency N24 potential that phase reverses at oesophageal recording sites into a P24. The N24-P24 reflects a horizontal dipole in the dorsal horn and is equivalent to the N13-P13 of the neck SEP to median nerve stimulation. Spinal conduction between D12-C7 spines was spuriously overestimated because the true length of the dorsal spinal cord is shorter by about 13% than the distance measured on the skin over the dorsal convexity. This correction should be applied routinely and it leads to a mean maximum spinal CV of 57 m/sec. Several positive far fields with widespread scalp distribution and stationary latencies have been identified. The P17 (over spine and head) reflects the peripheral nerve volley at the upper buttock. The P21 is synchronous with the N21 at the D12 spine and reflects the initial volley in the dorsal column. No far-field equivalent has been found for the N24-P24, due to the horizontal axis of the corresponding dipole. The P26 far field reflects the ascending volley at spinal levels D10-D4. The P31 reflects the initial volley in the medial lemniscus. The P40 at Cz represents the cortical response of the foot projection. Average central CVs were calculated and discussed.

Gating of the early components of the frontal and parietal somatosensory evoked potentials in different sensory-motor interference modalities ☆

Three different interfering conditions were studied during the recording of pre- and postcentral somatosensory evoked potentials (SEPs) following median nerve stimulation at the wrist in 16 normal subjects: active finger movement (MVT), light superficial massage (LSM) and deep muscular massage (DMM) of the hand. Special attention was focused on selective effects on individual SEP components. The frontal N30 component showed the most significant amplitude reduction during the three interfering conditions (76.4% of reduction in MVT, 36.4% in DMM and 32.9% in LSM). In contrast the frontal N23 was not significantly changed and the preceding P22 component was only reduced in the MVT condition. Postcentral N20 was unchanged by the three conditions while P27 was clearly gated by movement but not significantly by LSM and DMM. The three interfering conditions enhanced the parietal N32 and had no significant effect on the parietal P45. An important point was the interindividual variability of these effects and it appeared that group average wave forms would therefore be confusing. The peak latency of some SEP components was changed during the interfering conditions. The most important effect was an increase of postcentral P45 latency which was found to be related to the amplitude enhancement of N32.

Performance of the Emotiv Epoc headset for P300-based applications

BackgroundFor two decades, EEG-based Brain-Computer Interface (BCI) systems have been widely studied in research labs. Now, researchers want to consider out-of-the-lab applications and make this technology available to everybody. However, medical-grade EEG recording devices are still much too expensive for end-users, especially disabled people. Therefore, several low-cost alternatives have appeared on the market. The Emotiv Epoc headset is one of them. Although some previous work showed this device could suit the customer’s needs in terms of performance, no quantitative classification-based assessments compared to a medical system are available.MethodsThis paper aims at statistically comparing a medical-grade system, the ANT device, and the Emotiv Epoc headset by determining their respective performances in a P300 BCI using the same electrodes. On top of that, a review of previous Emotiv studies and a discussion on practical considerations regarding both systems are proposed. Nine healthy subjects participated in this experiment during which the ANT and the Emotiv systems are used in two different conditions: sitting on a chair and walking on a treadmill at constant speed.ResultsThe Emotiv headset performs significantly worse than the medical device; observed effect sizes vary from medium to large. The Emotiv headset has higher relative operational and maintenance costs than its medical-grade competitor.ConclusionsAlthough this low-cost headset is able to record EEG data in a satisfying manner, it should only be chosen for non critical applications such as games, communication systems, etc. For rehabilitation or prosthesis control, this lack of reliability may lead to serious consequences. For research purposes, the medical system should be chosen except if a lot of trials are available or when the Signal-to-Noise Ratio is high. This also suggests that the design of a specific low-cost EEG recording system for critical applications and research is still required.

Does the coordination between posture and movement during human whole-body reaching ensure center of mass stabilization?

Abstract The whole-body center of mass (CoM) has been classically regarded as the stabilized reference value for human voluntary movements executed upon a fixed base of support. Axial synergies (opposing displacements of head and trunk with hip segments) are believed to minimize antero-posterior (A/P) CoM displacements during forward trunk movements. It is also widely accepted that anticipatory postural adjustments (APAs) create forces of inertia that counteract disturbances arising from the moving segment(s). In the present study, we investigated CoM stabilization by axial synergies and APAs during a whole-body reaching task. Subjects reached towards an object placed on the ground in front of them in their sagittal plane using a strategy of coordinated trunk, knee, and hip flexion. The reaching task imposed constraints on arm-trajectory formation and equilibrium maintenance. To manipulate equilibrium constraints, differing conditions of distance and speed were imposed. The comparison of distance conditions suggested that axial synergies were not entirely devoted to CoM stabilization: backward A/P hip displacements reduced as head and trunk forward A/P displacements increased. Analysis of upper- and lower-body centers of mass in relation to the CoM also showed no strict minimization of A/P CoM displacements. Mechanical analysis of the effects of APAs revealed that, rather than acting to stabilize the CoM, APAs created necessary conditions for forward CoM displacement within the base of support in each condition. The results have implications for the CoM as the primary stabilized reference for posture and movement coordination during whole-body reaching and for the central control of posture and voluntary movement.

Mental movement simulation affects the N30 frontal component of the somatosensory evoked potential

It is known that somatosensory evoked potentials can be influenced by several centripetal and centrifugal factors which modify their amplitude. The present study shows for the first time that the frontal waves N30 and to a lesser extent N23 are specifically attenuated during mental movement simulation (MMS) activity. This gating phenomenon, tested on 16 normal subjects, occurred when repeated fast finger movements on the stimulated side are mentally simulated. In contrast, no significant modification appeared when the subject performed the MMS activity with the hand contralateral to that receiving electrical stimuli or when the subject performed a mental operation unrelated to the MMS. The MMS was shown by Roland et al. (1980) to increase the regional blood flow exclusively in the supplementary motor area (SMA). Our experimental data therefore indicate that the SMA could play an important role in the generation of the frontal N30.