Paradoxical lateralization of cortical potentials evoked by stimulation of posterior tibial nerve

Imagined paralysis alters somatosensory evoked-potentials

Recent studies employing body illusions have shown that multisensory conflict can alter body representations and modulate low-level sensory processing. One defining feature of these body illusions is that they are sensory driven and thus passive on behalf of the participant. Thus, it remained to establish whether explicit alteration of own-body representations modulates low-level sensory processing. We investigated whether tibial nerve somatosensory-evoked potentials were modulated when participants imagined paralysis of their legs and arms. Imagined paralysis of the legs decreased P40 amplitude, but not imagined paralysis of the arms. These results show modulation of early somatosensory processing via explicit, top-down alteration to the internal representation of the body. Interestingly, P40 suppression positively correlated with bodily awareness scores whereas it negatively correlated with body dissociation scores. This suggests that the ability to actively alter own-body representation and its corresponding sensory processing depends upon dispositions to attend to and focus on bodily sensations.

SEP Montage Variability Comparison during Intraoperative Neurophysiologic Monitoring

Intraoperative monitoring is performed to provide real-time assessment of the neural structures that can be at risk during spinal surgery. Somatosensory evoked potentials (SEPs) are the most commonly used modality for intraoperative monitoring. SEP stability can be affected by many factors during the surgery. This study is a prospective review of SEP recordings obtained during intraoperative monitoring of instrumented spinal surgeries that were performed for chronic underlying neurologic and neuromuscular conditions, such as scoliosis, myelopathy, and spinal stenosis. We analyzed multiple montages at the baseline, and then followed their development throughout the procedure. Our intention was to examine the stability of the SEP recordings throughout the surgical procedure on multiple montages of cortical SEP recordings, with the goal of identifying the appropriate combination of the least number of montages that gives the highest yield of monitorable surgeries. Our study shows that it is necessary to have multiple montages for SEP recordings, as it reduces the number of non-monitorable cases, improves IOM reliability, and therefore could reduce false positives warnings to the surgeons. Out of all the typical montages available for use, our study has shown that the recording montage Cz-C4/Cz-C3 (Cz-Cc) is the most reliable and stable throughout the procedure and should be the preferred montage followed throughout the surgery.

P37 latency mismatch between lateral and midline potentials is influenced by transversal afference

SUMMARY

P37 cortical peak latency registered on Ci' (lateral) is usually approximately 1 millisecond shorter than Cz' (midline) in lower limb somatosensory evoked potential. At the present time, the underlying mechanism that leads to this mismatch remains unknown. Superficial peroneal nerve, posttibial nerve, and sural nerve somatosensory evoked potentials were obtained from 26 anesthetized individuals by using Ci'-Cc', Cc'-Fpz, and Cz'-Fpz recording montages. P37 latency mismatches between the lateral (Ci'-Cc') and midline (Cz'-Fpz) potentials (P < 0.001) were recorded in superficial peroneal nerve, posttibial nerve, and sural nerve somatosensory evoked potentials; all showed shorter Ci'-Cc' P37 latency (1-1.7 milliseconds). However, in individuals who had minimal or no N37 potential on Cc' recording, the mean P37 latencies of Ci'-Cc' and Cz'-Fpz equalized with the P37 latency of Ci'-Cc' approaching to default Cz'-Fpz value. The data showed that N37 seemed to potentiate the P37 latency difference between Ci-Cc' and Cz'-Fpz recordings. We postulate that N37 may preferentially reflect the dipoles of transversal afference; lack of it thereof suggests poor dipole sources primarily perpendicular to the mesial hemisphere.

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.

Changes in Excitability of the Cortical Projections to the Human Tibialis Anterior After Paired Associative Stimulation

Paired associative stimulation (PAS) based on Hebb's law of association can induce plastic changes in the intact human. The optimal interstimulus interval (ISI) between the peripheral nerve and transcranial magnetic stimulus is not known for muscles of the lower leg. The aims of this study were to investigate the effect of PAS for a variety of ISIs and to explore the efficacy of PAS when applied during dynamic activation of the target muscle. PAS was applied at 0.2 Hz for 30 min with the tibialis anterior (TA) at rest. The ISI was varied randomly in seven sessions ( n = 5). Subsequently, PAS was applied ( n = 14, ISI = 55 ms) with the TA relaxed or dorsi-flexing. Finally, an optimized ISI based on the subject somatosensory evoked potential (SEP) latency plus a central processing delay (6 ms) was used ( n = 13). Motor-evoked potentials (MEPs) were elicited in the TA before and after the intervention, and the size of the TA MEP was extracted. ISIs of 45, 50, and 55 ms increased and 40 ms decreased TA MEP significantly ( P = 0.01). PAS during dorsi-flexion increased TA MEP size by 92% ( P = 0.001). PAS delivered at rest resulted in a nonsignificant increase; however, when the ISI was optimized from SEP latency recordings, all subjects showed significant increases ( P = 0.002). No changes in MEP size occurred in the antagonist. Results confirm that the excitability of the corticospinal projections to the TA but not the antagonist can be increased after PAS. This is strongly dependent on the individualized ISI and on the activation state of the muscle.

A comparison between derivation optimization and Cz′–FPz for posterior tibial P37 somatosensory evoked potential intraoperative monitoring

OBJECTIVE

To compare P37 derivation optimization to Cz'-FPz.

METHODS

After induction in 120 patients, monitoring derivations optimized by mapping FPz, Cz, Cz', Pz, C4', C2', C1' and C3'-mastoid to determine the P37 and N37 maximums for use as inputs 1 and 2 were compared to Cz'-FPz. This was repeated later in 35 surgeries.

RESULTS

Eleven optimal derivations occurred and usually differed between sides. Input 1 was Cz', Pz, Cz, iCi', or Ci' and input 2 was Cc', FPz, Ci' or Pz. Even the most frequent Cz'-Cc' derivation was optimal for both sides of an individual in only 17% and this was true for Cz'-FPz in only 4%. Optimization produced higher amplitudes than Cz'-FPz (P<0.001). The ratio was [squareroot of 2] : 1 in 61% of patients and > or =2:1 in 28%, approximately halving or quartering averaging times. Optimization assessed decussation, disclosing non-decussation in one patient while Cz'-FPz did not. Alterations of P37 topography that reduced initially optimal derivation amplitude and made a different derivation optimal were demonstrated by repeat optimization in 13 of 35 patients, preventing misinterpretation in one. While also affected, Cz'-FPz neither detected nor adjusted for potentially misleading topographic changes.

CONCLUSIONS

Higher amplitudes, decussation assessment and topographic adjustment make P37 derivation optimization superior to Cz'-FPz for monitoring this highly variable potential.

Intraoperative neurophysiologic discovery of uncrossed sensory and motor pathways in a patient with horizontal gaze palsy and scoliosis

OBJECTIVE

To report the intraoperative neurophysiologic discovery of clinically unsuspected non-decussation of the somatosensory and motor pathways.

METHODS

We performed somatosensory evoked potential (SEP) and transcranial electric stimulation (TES) muscle motor evoked potential (MEP) monitoring during scoliosis surgery for a 16 year old patient with familial horizontal gaze palsy and progressive scoliosis. Our routine procedures included optimizing tibial cortical SEP monitoring derivations through saggital and coronal (C4', C2', Cz', C1', C3'-mastoid) P37 mapping, which surprisingly indicated non-decussation. Consequently, we also obtained coronal median nerve SEPs and simultaneous bilateral muscle recordings to lateralized TES (C3-Cz, C4-Cz) intraoperatively and focal hand area transcranial magnetic stimulation (TMS) postoperatively.

RESULTS

For each nerve, tibial P37/N37 distribution was contralateral/ipsilateral and median N20 ipsilateral. For each hemisphere, ipsilateral TES MEPs had lower thresholds and TMS MEPs were exclusively ipsilateral. Accurate monitoring required reversed montages. Reevaluation of an MRI (previously reported normal) disclosed a ventral midline cleft of the medulla.

CONCLUSIONS

The results indicate uncrossed dorsal column-medial lemniscal and corticospinal pathways due to brain-stem malformation with absent internal arcuate and pyramidal decussations.

SIGNIFICANCE

Simultaneous bilateral recording to unilateral stimulation demonstrates SEP/MEP hemispheric origin and is important for accurate interpretation and monitoring because decussation anomalies exist.

Fundamental principles of somatosensory evoked potentials

Studies of SSEP provide unique opportunities for investigating physioanatomic substrates of sensory pathway and cognitive functions of the sensory system. Progress of clinical investigation and application of SSEP have been stalled in more recent years, although SSEP still remain a useful tool for diagnosis of various neurologic disorders and for the monitoring of spinal cord function during surgery. Reflecting complex sensory system in human, scalp-recorded SSEP consists of multiple waves, having different distribution, amplitude, and latencies among different electrodes. The physioanatomic significance of these multiple waves, especially the late components, is largely unknown. These should be explored further, especially in relation to cognitive function.

Paradoxical lateralization of parasagittal spikes revealed by back averaging of EEG and MEG in a case with epilepsia partialis continua

Our aim was to localize the generator site of parasagittal epileptiform discharges in a patient with epilepsia partialis continua (EPC) in the right leg. We examined a 32-year-old woman with EPC whose conventional EEG did not show any epileptic discharge. We performed the jerk-locked back averaging (JLA) of EEG and magnetoencephalography (MEG) to localize the dipole source of sharp transients. The myoclonic discharges in the right soleus muscle were used as a trigger pulse. JLA revealed consistent EEG and MEG sharp transients that coincided consistently and constantly preceded the myoclonic jerks. JLA of EEG demonstrated sharp waves paradoxically distributed over the vertex and right hemisphere. However, the estimated dipoles of MEG were localized in a restricted area in the primary leg motor area in the left hemisphere, which was closely located in the abnormal lesion on the brain MRI. JLA of MEG is considered to be a useful non-invasive method for localizing the epileptogenic area in EPC even when paradoxical lateralization of electroencephalographic discharges was noted.

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.

Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring

This investigation sought the optimal (highest amplitude) derivation for monitoring the posterior tibial P37 for each side in each individual, and determined whether this may change intraoperatively. Fifty monitored patients were studied using a partial P37 map consisting of FPz, Fz, Cz, Cz', Pz, POz, C4', and C3' to a noncephalic reference. From this, the highest amplitude scalp derivation was determined for each side. Of 100 tibial nerves, the initial optimal input 1 was Cz' in 52%, Pz in 28%, and Cz or iC' in 10%, and optimal input 2 was cC' in 69% and FPz in 31%. The optimal derivation was the same for each side in 34% of patients and different in 66%. Of 31 patients with at least one subsequent trial later during surgery, P37 topography changed in 14 and affected optimal inputs in 12. This occurred regularly during sitting-position posterior fossa surgery because of intracranial air, but sometimes occurred during other surgeries as well. The most common change consisted of FPz replacing cC' as optimal input 2. Input 1 changes were predominantly in an anterior or posterior sagittal direction. The results demonstrate great inter- and intraindividual P37 variability, and document intraoperative topographic changes. Both phenomena can be addressed by a practical method to refine intraoperative monitoring by individually optimizing scalp derivations and identifying topographic P37 changes during surgery.

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.

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.

Comparison of somatosensory evoked high-frequency oscillations after posterior tibial and median nerve stimulation

OBJECTIVE

We compared the high-frequency oscillations (HFOs) evoked by posterior tibial nerve (PTN) and median nerve (MN) stimulation.

METHODS

Somatosensory evoked potentials (SEPs) were recorded with a filter set at 10-2000 Hz to right PTN and to right MN stimulation in 10 healthy subjects. The HFOs were obtained by digitally filtering the wide-band SEPs with a band-pass of 300-900 Hz.

RESULTS

HFOs were recorded in 8 of the 10 subjects for PTN, and in all subjects for MN stimulation. The HFOs after both PTN and MN stimulation started approximately at or after the onset of the primary cortical response (P37 and N20) and ended around the middle of the second slope. HFO amplitudes and area after PTN stimulation were significantly smaller than those after MN stimulation. HFO duration after PTN stimulation was markedly longer than that after MN stimulation. However, HFO interpeak latencies did not differ between the two nerves.

CONCLUSIONS

The present findings suggest that the HFOs after PTN and MN stimulation reflect a neural mechanism common to the hand and foot somatosensory cortex.

Evidence for an abnormal cortical sensory processing in dystonia: selective enhancement of lower limb P37-N50 somatosensory evoked potential

We evaluated brain stem P30, contralateral frontal N37, and the vertex-ipsilateral central P37, N50 somatosensory evoked potentials (SEPs) obtained in response to stimulation of the tibial nerve in 10 patients with idiopathic dystonia. Results were compared with those obtained in 10 healthy subjects matched for age and sex. The amplitude of the brain stem P30 potential and of the contralateral frontal N37 response in dystonic patients was not significantly different from that recorded in normal subjects. The vertex- ipsilateral central P37-N50 complex, which is thought to originate in the pre-rolandic cortex, was significantly enhanced in patients compared with the control group. These results suggest the enhancement of the vertex-ipsilateral central P37-N50 complex might reflect an abnormal response to somatosensory inputs of a precentral cortex which is excessively activated because of a disorder of the basal ganglia. Such inefficient sensory processing in motor areas might contribute to motor impairment in dystonia.

Parkinson's disease and lower limb somatosensory evoked potentials: apomorphine-induced relief of the akinetic-rigid syndrome and vertex P37-N50 potentials

We evaluated brainstem P30, vertex-central P37-N50 and contralateral frontal N37 somatosensory evoked potentials (SEPs) from the tibial nerve in 14 patients affected by Parkinson's disease (PD) with akinetic-rigid syndrome. In seven patients SEPs were recorded after administration of apomorphine. The cortical P37-N50 complex was either absent (five patients, eight tested sides) or significantly smaller in patients as compared to the control group (n = 18). There was a relationship between abnormalities of early vertex potentials and degree of motor impairment. Administration of apomorphine was followed by an increase in amplitude of P37-N50 response, which was maximal after 15-30 min and then progressively returned to basal values in parallel with clinical improvement. Amplitude of brainstem P30 and frontal N37 responses was within normal values and did not vary following drug administration. These results suggest that the P37-N50 complex arises from independent cortical generators, probably located in the pre-rolandic cortex, which may be selectively affected by basal ganglia dysfunction. Amplitude decrease of the P37-50 complex may reflect an abnormal processing of somatosensory inputs within the pre-central cortex due to defective modulation exerted by basal ganglia circuitry on cortical excitability. SEP potentiation following apomorphine, besides indicating that this dysfunction is partly reversible, might suggest objective method to measure therapeutic efficacy.

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.

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

Whether the two earliest cortical somatosensory evoked potentials (SEPs) to tibial nerve stimulation (N37 and P40) are generated by the same dipolar source or, instead, originate from different neuronal populations is still a debated problem. We recorded the early scalp SEPs to tibial nerve stimulation in 10 healthy subjects at rest and during voluntary movement of the stimulated foot. We found that the P40, which reached its highest amplitude on the vertex at rest, changed its topography during movement, since its amplitude was reduced much more in the central than in the parietal traces. These findings suggest that two different components contribute to the centro-parietal positivity at rest: (1) the P37 response, which is parietally distributed and is not modified by movement, and (2) the 'real' P40 SEP, which is focused on the vertex and is reduced in amplitude during voluntary movement. Since, also, the N37 response did not vary its amplitude under interference condition, it is possible that the N37 and P37 potentials are generated by the same dipolar source. Other later components, namely P50 and N50 were significantly reduced in amplitude during foot movement. Lastly, the subcortical P30 far-field remained unchanged and this suggests that the phenomenon of amplitude reduction during movement (i.e. gating) occurs above the cervico-medullary junction.

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.

Three-dimensional localization of dipoles for potentials evoked by posterior tibial nerve stimulation in the monkey

Somatosensory evoked potentials (SEPs) elicited by right posterior tibial nerve stimulation were simultaneously recorded from 21-27 epidural electrodes in three monkeys. N23-P40 was recorded anterior to the left central sulcus, and P23-N40 was recorded on the parietal midline and the middle portion of the right hemisphere. These potentials were thought to be the primary cortical responses elicited by posterior tibial nerve stimulation in the monkey, since a topographical map made of them corresponded to the paradoxical lateralization of the primary cortical components in human posterior tibial nerve SEPs. Current source generators (dipoles) of these potentials were 3-dimensionally identified dipoles located in the left side of the mesial wall of the anterior parietal cortex, and oriented obliquely toward the right hemisphere by a dipole tracing (DT) method in which the 3-dimensional localization of dipoles in the brain were estimated and superimposed on magnetic resonance imaging (MRI) images.

Median and tibial nerve somatosensory evoked potentials: middle-latency components from the vicinity of the secondary somatosensory cortex in humans

The topography of the middle-latency N110 after radial nerve stimulation suggested a generator in SII. To support this hypothesis, we have tried to identify a homologous component in the tibial nerve SEP (somatosensory evoked potential). Evoked potentials following tibial nerve stimulation (motor + sensory threshold) were recorded with 29 electrodes (bandpass 0.5-500 Hz, sampling rate 1000 Hz). For comparison, the median nerve was stimulated at the wrist. Components were identified as peaks in the global field power (GFP). Map series were generated around GFP peaks and amplitudes were measured from electrodes near map maxima. With median nerve stimulation, we recorded a negativity with a maximum in temporal electrode positions and 106 +/- 12 ms peak latency (mean +/- SD), comparable to the N110 following radial nerve stimulation. After tibial nerve stimulation the latency of a component with the same topography was 131 +/- 11 ms (N130). Both N110 and N130 were present ipsi- as well as contralaterally. Amplitudes were significantly higher on the contralateral than the ipsilateral scalp for both median (3.1 +/- 2.4 microV vs. 1.7 +/- 1.6 microV) and tibial nerve (1.9 +/- 1.2 microV vs. 0.6 + 1 microV). The topography of the N130 can be explained by a generator in the vicinity of SII. The latency difference between median and tibial nerve stimulation is related to the longer conduction distance (cf. N20 and P40). The smaller ipsilateral N130 is consistent with the bilateral body representation in SII.

Selective gating of lower limb cortical somatosensory evoked potentials (SEPs) during passive and active foot movements

We evaluated subcortical and cortical somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation in 4 experimental conditions of foot movement and compared them with the baseline condition of full relaxation. The experimental conditions were: (a) active flexion-extension of the stimulated foot; (b) active flexion-extension of the non-stimulated foot; (c) passive flexion-extension of the stimulated foot in complete relaxation; (d) tonic active flexion of the stimulated foot. We analyzed latencies and amplitudes of the subcortical P30 potential, of the contralateral pre-rolandic N37 and P50 responses and of the P37, N50 and P60 potentials recorded over the vertex. Latencies did not vary in any of the paradigms. The amplitude of subcortical P30 potential did not change during any of the paradigms. Among the cortical waves, P37, N50 and P60 amplitudes were significantly attenuated in all conditions except active movement of the non-stimulated foot (b). This attenuation was less during passive (c) than during active movements of the stimulated foot (a and d). The contralateral pre-rolandic waves N37 and P50 showed no significant decrease during any of the paradigms. These results suggest that gating occurs rostrally to the cervico-medullary junction, probably at cortical level. The different behavior of N37, P50 and P37, N50 cortical responses during movement of the stimulated foot provides evidence suggestive of a highly localized gating process occurring at cortical level. These potentials could reflect activation of separate, functionally distinct generators.

Magnetoencephalographic study of intracerebral interactions caused by bilateral posterior tibial nerve stimulation in man

We studied somatosensory evoked magnetic fields (SEFs) following stimulation of bilateral posterior tibial nerves ('bilateral' waveform) in normal subjects to determine the inter- and intra-hemispheric interference effects caused by activation of sensory areas in bilateral hemispheres. Activated areas in the primary and second sensory cortices (SI and SII) in each hemisphere following bilateral stimulation were clearly identified by estimation of the double best-fitted equivalent current dipoles (ECD) using the spherical head model, and the large inter-individual differences were identified. SEFs following the right posterior tibial nerve stimulation and those following the left stimulation were summated ('summated' waveform). The 'difference' waveform was induced by a subtraction of 'bilateral' waveforms from the 'summated' waveform. Short-latency deflections showed no consistent changes between the 'summated' and 'bilateral' waveforms, but the long-latency deflection, the N100m-P100m, in the 'bilateral' waveform was significantly (P < 0.02) reduced in amplitude as compared with the 'summated' waveform. The differences were clearly identified in the 'difference' waveform, in which the main deflections, U100m-D100m, were found. The ECDs of the short-latency deflections were located in SI contralateral to the stimulated nerve, but the ECDs of the N100m-P100m were located in bilateral SII which are considered to receive ascending signals from the body bilaterally. Therefore, some inhibitory interactions might take place in SII by receiving inputs from the body bilaterally.

Amplitude changes of tibial nerve cortical somatosensory evoked potentials when the ipsilateral or contralateral ear is used as reference

We performed topographical mapping of somatosensory evoked potentials (SEPs) to the posterior tibial nerve using earlobe references both ipsilateral and contralateral to the stimulation side. The voltage of the frontal contralateral N37 and P50 components was enhanced, while the voltage of the parietal ipsilateral P37 and N50 components was reduced when the contralateral earlobe was substituted by the ipsilateral earlobe reference. Maps of the same data documented concomitant changes in negative and positive potential fields, showing an expansion of the pre-Rolandic N37 toward the centrotemporal contralateral regions, and a tendency of the parietal ipsilateral P37, N50 components to be more focally distributed at the vertex. SEPs recorded at each earlobe (Cv6 reference) provided an explanation of these results: The contralateral earlobe detected a negative potential corresponding to the N37 potential recorded over the scalp, followed by a P50 potential that attenuated the contralateral responses and enhanced the ipsilateral ones. The ipsilateral earlobe had no significant effects on scalp SEPs, since it detected only a large N33 negativity. Current source density (CSD) maps were, of course, not influenced by the ear used as reference. Our results suggest that the ipsilateral ear reference is better than the contralateral one for recording "genuine" cortical SEPs. Therefore, it can be recommended in the clinical domain for mapping studies of lower-limb cortical SEPs.

Posterior tibial somatosensory evoked potentials in very preterm infants

Cortical posterior tibial somatosensory (SSEP) responses were reliably recorded from 67 infants of 33 weeks gestation or less who had normal neurological outcome at 24 months corrected age. Cross-sectional and longitudinal data did not show a change in waveform morphology with advancing gestation or postnatal age. The latency of the first cortical component shortened as maturation increased. This study provides normative data for the peak component waveforms of the response in very preterm infants. The role of the posterior tibial SSEP in the prediction of functional brain injury in this high risk population can now be determined.

Functional localization of sensorimotor cortex by somatosensory evoked potentials produced by femoral nerve stimulation

Cortical somatosensory evoked potentials (SSEPs) can be used to localize the central sulcus during a craniotomy. In particular, contralateral median nerve stimulation producing SSEPs can disclose the location of the central sulcus around the sensorimotor hand representation area. However, the median nerve cannot be stimulated in patients who undergo craniotomy at locations other than the hand representation area. The present study attempts to localize the central sulcus in the lateral surface of the brain near the interhemispheric fissure by stimulating the contralateral femoral nerve to produce SSEPs. Somatosensory evoked potentials were recorded between the superior lip of the interhemispheric fissure and 1.5 to 2 cm laterally in the cortex. Only seven of the 12 patients studied showed a phase reversal of the initial component across the central sulcus. The polarity was negative in the postcentral gyrus and positive in the precentral gyrus. The other five patients did not show a phase reversal of the initial component across the central sulcus. The amplitude was highest in the postcentral gyrus and the polarity was positive. Based on these results, the authors hypothesize that stimulating the contralateral femoral nerve to produce SSEPs and then analyzing the distribution of the SSEPs may provide a method for functional localization of the sensorimotor cortex around the interhemispheric fissure during craniotomy.

Differentiation of receptive fields in the sensory cortex following stimulation of various nerves of the lower limb in humans: a magnetoencephalographic study

The authors investigated magnetoencephalography following stimulation of the posterior tibial (PT) and sural (SU) nerves at the ankle, the peroneal nerve (PE) at the knee, and the femoral nerve (FE) overlying the inguinal ligament in seven normal subjects (14 limbs) and confirmed its usefulness in clarifying the detailed differentiation of the receptive fields in the lower limb area of the primary sensory cortex in humans. The results were summarized as follows: 1) the equivalent current dipoles (ECDs) estimated by the magnetic fields following stimulation of the PT and SU were located very close to each other, along the interhemispheric fissure in all 14 limbs. They were directed horizontally to the hemisphere ipsilateral to the stimulated nerve. 2) The ECD following stimulation of the FE was clearly different from that seen in the other nerves, in terms of the location and/or direction, in all 14 limbs. The ECDs of 14 limbs were classified into two types according to the distance of ECD location between PT and FE; Type 1 (> 1 cm, nine limbs) and Type 2 (< 1 cm, five limbs). The ECD following FE stimulation was located on the crown of the postcentral gyrus or at the edge of the interhemispheric fissure in Type 1 and was close to the ECDs following PT and SU stimulation along the interhemispheric fissure in Type 2. 3) The ECD following PE stimulation was located along the interhemispheric fissure in all 14 limbs as for PT and SU. Its location was slightly but significantly higher than that of PT and SU in Type 1 and was close to ECDs following PT and SU stimulation in Type 2. The present findings indicated that approximately 65% (nine of 14) of the limbs showed the particular receptive fields compatible with the homunculus. Large inter- and the intraindividual (left-right) differences found in the present study indicated a significant anatomical variation in the area of the lower limb in the sensory cortex of humans.

Effect of stimulus rate on the cortical posterior tibial nerve SEPs: a topographic study

We performed topographical mapping of somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation delivered at 2, 5 and 7.5 Hz in 15 healthy subjects. P37 was significantly attenuated at 5 and 7.5 Hz and the N50 component attenuated only at 5 Hz, its amplitude remaining stable for further increases in stimulus frequency. Frontal N37 and P50 potentials showed no significant decrease when the stimulus repetition frequency was changed from 2 to 7.5 Hz. P60 showed an attenuation of the amplitude only at 7.5 Hz. Latency and scalp topographies of all cortical components examined remained unchanged for the 3 stimulus rates tested. The optimal stimulus rate for mapping of tibial nerve SEPs was lower than 5 Hz. The distinct recovery function of the contralateral. N37-P50 and ipsilateral P37-N50 responses suggests that these potentials arise from separate generators.

Localization of functional regions of human mesial cortex by somatosensory evoked potential recording and by cortical stimulation

We describe methods of localizing functional regions of the mesial wall, based on 47 patients studied intraoperatively or following chronic implantation of subdural electrodes. Somatosensory evoked potentials were recorded to stimulation of posterior tibial, dorsal pudendal, median, and trigeminal nerves. Bipolar cortical stimulation was performed, and in 4 cases movement-related potentials were recorded. The cingulate and marginal sulci formed the inferior and posterior borders of the sensorimotor areas and the supplementary motor area (SMA). The foot sensory area occupied the posterior paracentral lobule, while the genitalia were represented anterior to the foot sensory area, near the cingulate sulcus. The foot motor area was interior and superior to the sensory areas, but there was overlap in these representations. There was a rough somatotopic organization within the SMA, with the face represented anterior to the hand. However, there was little evidence of the "pre-SMA" region described in monkeys. Complex movements involving more than one extremity were elicited by stimulation of much of the SMA. The region comprising the supplementary sensory area was not clearly identified, but may involve much of the precuneus. Movement-related potentials did not provide additional localizing information, although in some recordings readiness potentials were recorded from the SMA that appeared to be locally generated.

Cortical and subcortical SEPs following posterior tibial nerve stimulation

Intra-operative cortical and subcortical SEPs from the cerebral convexity and from the inter-hemispheric fissure were recorded following posterior tibial nerve (PTN) stimulation. Cortical and subcortical SEPs from the cerebral convexity after contra-lateral PTN stimulation consisted of N38 and P46, and their polarity reversed when the ipsi-lateral site was stimulated. On the other hand, cortical SEPs from the inter-hemispheric fissure always showed P38 and N46, whether the right or the left PTN was stimulated. Cortical and subcortical SEPs from the inter-hemispheric fissure showed clear cut polarity reversals. These findings provide good evidence for the existence of a tangential dipole oriented perpendicular to the inter-hemispheric fissure in the foot sensory area of the primary sensory cortex. SEPs recorded from the superficial part of the inter-hemispheric fissure showed smaller amplitudes and longer latencies than those of SEPs from the deeper regions. These findings suggest the existence of another dipole responsible for the generation of SEPs after PTN stimulation.

Topographic analyses of somatosensory evoked potentials following stimulation of tibial, sural and lateral femoral cutaneous nerves

Using topographic maps, we studied the scalp field distribution of somatosensory evoked potentials (SEPs) in response to the stimulation of the tibial (TN), sural (SN) and lateral femoral cutaneous (LFCN) nerves in 24 normal volunteers. Cortical peaks, i.e., N35, P40, N50 and P60 were generally dominant in the contralateral hemisphere for the LFCN-SEP, whereas all peaks except N35 had dominance in the ipsilateral hemisphere to TN- and SN-SEPs. The findings imply that ipsilateral or contralateral peak dominance for the lower extremity SEP is determined by where the cortical leg representation occurs. As a result, mesial hemisphere representation results in peak dominance projected to the hemisphere ipsilateral to stimulation. Representations at the superior lip of the interhemispheric fissure or lateral convexity lead to midline or contralateral peak dominance. These findings also suggest that the paradoxically lateralized P40 is not the result of a positive field dipole shadow generated by the primary negative wave in the mesial hemisphere, but is the primary positive wave, analogous to P26 of the median nerve SEP. Accordingly, contralaterally dominant N35 is likely equivalent to the first cortical potential of N20 in the median nerve SEP. The difference in vector directions of potential fields between N35 and P40 may account for the opposite hemispheric dominance for these peaks in TN- and SN-SEPs.

Topography of somatosensory evoked magnetic fields following posterior tibial nerve stimulation

The topography of somatosensory evoked magnetic fields (SEFs) following stimulation of the right and left posterior tibial nerves was investigated in 5 normal subjects (10 nerves). The main deflections N37m-P45m-N60m-P75m and their counterparts P37m-N45m-P60m-N75m were identified in the hemisphere contralateral to the stimulated nerve. Their equivalent current dipoles (ECDs) were located in the foot area of the primary sensory cortex (SI), probably in area 3b. Restricted minor deflections, P40m-N40m and N50m-P50m, were considered to be generated in area 1 in SI. As the generator sources of P37m-N37m, P40m-N40m and N45m-P45m were temporarily changed and interfered with each other, the direction of ECDs appeared to be rotated with the passage of time. Small middle-latency deflections, N100m-P100m, were clearly identified in 2 subjects. ECDs of these deflections were found in the second sensory cortex (SII), in both hemispheres, although they were clearer in the hemisphere contralateral to the stimulated nerve. In conclusion, short- and middle-latency SEFs are mainly generated in area 3b in SI contralateral to the stimulated nerve, and responses generated in area 1 of SI and SII affect the SEFs to some degree, but interindividual differences are large compared with SEFs evoked by upper limb stimulation.

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 N1 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 N1-N2 between foot and hand stimulation. It is therefore concluded that N1 of the foot pain SEPs is generated mainly from the foot area of SI. The variable scalp distribution of the N1 component of the foot pain SEPs is likely due to an anatomical variability among subjects and even between sides.

Posterior tibial and sural nerve somatosensory evoked potentials: a study in spastic paraparesis and spinal cord lesions

In two groups of patients posterior tibial nerve (PTN) and sural nerve (SN) somatosensory evoked potentials (SEPs) were compared to each other and related to classified neurological signs. Group A consisted of 7 patients with hereditary spastic paraparesis (HSP) and 8 with primary lateral sclerosis (PLS), with solely or primarily motor deficits. Group B consisted of 12 patients with different spinal cord diseases causing variable mixed sensory and motor impairments. Normal values were derived from 39 controls. A clear trend towards more frequently prolonged PTN SEP than SN SEP latencies was found in both groups and appears to make PTN SEPs more useful for clinical application than SN SEPs. No significant differences were found in SEP abnormalities when the two patient groups were compared to each other. No relationships were found between SEP abnormalities and spasticity, weakness or any single sensory modality, making the two SEPs questionable as a quantitative test for neurological deficits in our patients.

Current source density mapping of somatosensory evoked responses following median and tibial nerve stimulation

Potential recording of brain activities always encounters the problem resulting from the activation of reference electrodes. Current source density (CSD) computation does not take reference sites into account and consequently may better localize the generator sources. In the past, several attempts have been made to record CSDs of the somatosensory evoked responses (SERs) following median nerve stimulation. In order to compare the generating mechanisms of SERs following median nerve and tibial nerve stimulation, the scalp CSD distributions of the median nerve SER and the tibial nerve SER were compared in 5 normal subjects. In the median nerve SER, far-field potentials such as P14 and N16 were abolished in the CSD records. N20, P25 and N35 showed almost identical CSD distributions, albeit P25 had a reversed polarity. By contrast, the tibial nerve SER showed similar distributions for P40 and P60 CSDs, but N50 had a different distribution from the others. In the potential records, P40 and P60 were distributed predominantly ipsilateral to the stimulus (paradoxical lateralization), whereas the P40 and P60 CSDs formed a dipole localized over the contralateral foot somatosensory area. N50 disclosed the same tendency, although it had a slightly different CSD pattern from that of P40 and P60. These findings suggest that the median nerve and tibial nerve SER components are not necessarily comparable and that under certain circumstances CSDs are better indicators of local electrical events than the corresponding potentials.

Magnetic stimulation of muscle evokes cerebral potentials

Somatosensory evoked potentials (SEPs) were recorded from the scalp in man to magnetic stimulation of various skeletal muscles. The potentials consisted of several components, the earliest of which decreased in latency as the stimulated site moved rostrally, ranging from 46 msec for stimulation of the gastrocnemius, to 14 msec for stimulation of the deltoid. Experiments were performed to distinguish the mechanisms by which magnetic stimulation of the muscle was effective in evoking these cerebral potentials. For the gastrocnemius, the intensity of the magnetic stimulus needed for evoking cerebral potentials was less than that required for activating mixed or sensory nerves in proximity to the muscle belly (eg, posterior tibial nerve in the popliteal fossa, sural nerve at the ankle). Vibration of the muscle or passive lengthening of the muscle, procedures which activate muscle spindles, were accompanied by a significant attenuation of the potentials evoked by magnetic stimulation of the muscle. Anesthesia of the skin underlying the stimulating coil had no effect on the latency or amplitude of the early components of the magnetically evoked potentials, whereas electrically evoked potentials from skin electrodes were abolished. Thus, the cerebral potentials accompanying magnetic stimulation of the muscle appear to be due to activation of muscle afferents. We suggest that magnetic stimulation of muscle can provide a relatively simple method for quantifying the function of muscle afferents originating from a wide variety of skeletal musculature.

A prospective 1 year follow-up study with somatosensory potentials evoked by stimulation of the posterior tibial nerve in patients with supratentorial cerebral infarction

Somatosensory potentials evoked by stimulation of the posterior tibial nerve (tibial nerve SEPs) were studied in 40 patients with supratentorial non-haemorrhagic cerebral infarction and in 25 control subjects, SEPs were recorded twice in 39 patients and thrice in 35 patients. The first examination was carried out 4-19 days after the onset of the symptoms, the second examination 56-100 days after the stroke, and the third examination 348-393 days after the stroke. Increased side-to-side differences in the P57 and N75 peak latencies and absence of the P40 peak were the most frequent abnormal findings. The latency abnormalities were associated with involvement of the subcortical white matter of the rolandic region. The absence of the P40 peak was, in contrast, closely related to the extension of the infarcted area into the cortical gray matter of the rolandic region. When all SEP abnormalities were taken into account 55% of patients showed at least one abnormality in the tibial nerve SEP during the acute stage, 51% of patients had abnormal SEPs in the second examination and 43% of patients in the third examination. A nearly significant decrease was observed in the number of latency abnormalities, but the number of amplitude abnormalities, including absent responses, did not change during the 1 year follow-up period.

Correlation of tibial nerve SEPs with the development of seizures in patients with supratentorial cerebral infarcts

In 40 patients with supratentorial non-haemorrhagic cerebral infarct, the findings in the tibial nerve SEPs recorded during the acute stage correlated significantly with the development of seizures during a 1 year follow-up period. Abnormality in the side-to-side difference of the P40-N48 amplitude was the finding with the highest correlation with the development of seizures: 87.5% of the patients were classified correctly as to the risk of seizures.

Diagnostic significance of tibial nerve somatosensory evoked potentials (spinal and cortical components) with spinal cord lesions

Forty normal subjects were investigated with the somatosensory evoked potential technique following stimulation of the posterior tibial nerve in order to yield information upon normal values, and especially the origin of the cervical potential. Subsequently an electrophysiologic localization of tumours along the spinal cord was attempted in 18 patients with spinal tumours. In C2 recordings with Fz reference 3 components, N28, N30, and N34, are visible. Simultaneous cortical and cervical recordings with mastoid, knee, and neck references suggest a cranio-cervical origin of N28, a subcortical origin of N30 and a thalamic or thalamo-cortical origin of N34. In patients, the lumbar spinal cord potentials regularly yielded a useful additional information with respect to the tumour site. The cervical potentials added in 5 out of 18 patients additional information localizing the tumour within the spinal cord or brain-stem.

Maturation of the cortical evoked response to posterior-nerve stimulation in the preterm neonate

Short-latency somatosensory evoked potentials elicited by stimulation of the posterior tibial nerve at the ankle were studied in 75 preterm infants. Normative data for the latency of the first cortical component (P1) were obtained for infants from 27 weeks gestation to term, and showed a linear decrease with increasing gestational age. As the pathway of this response traverses areas of the brain likely to be affected by ischaemic and haemorrhagic lesions, abnormalities in the response might indicate later motor disorder.

Projection of low-threshold afferents from human intercostal muscles to the cerebral cortex

Low-threshold afferents from human limb muscles are known to project to the sensorimotor cortex and to contribute to proprioception. However, there are few data on the cortical projection of afferents from human respiratory muscles. The present study employed evoked-potential techniques to determine whether low-threshold muscle afferents from the chest wall project to cortical levels in conscious human subjects. In four subjects intramuscular afferents of the second parasternal and fifth lateral intercostal muscles were selectively stimulated through an insulated microelectrode inserted percutaneously at the respective motor point. Evoked potentials were recorded and averaged from eight scalp sites. The initial cortical component of the cerebral response to intramuscular stimulation of the second and fifth interspaces was a negative potential commencing at 19.2 +/- 2.1 msec and 20.7 +/- 1.1 msec respectively. The dominant early cortical potential was largest at the vertex, and was comparable in amplitude (0.58 +/- 0.23 microV) to that for individual muscles of the upper and lower limbs. The cortical focus was distributed differently from that for cutaneous afferents of the chest wall and for both muscle and cutaneous afferents from the upper and lower limbs. This study provides direct evidence for a short-latency projection from intercostal muscle afferents (group I and/or II) to the human cerebral cortex.