Modality-specific organization for cutaneous and proprioceptive sense in human primary sensory cortex studied by chronic epicortical recording

Modality specificity of human primary somatosensory cortex was studied by recording somatosensory evoked potentials (SEPs) from subdural electrodes in a patient with intractable focal motor seizure. A newly developed device was used for selectively activating proprioception. The spatial and temporal distributions of proprioception-related SEPs elicited by brisk passive flexion movement at the proximal interphalangeal (PIP) joint of the middle finger (4 degrees in 25 ms) were quite different from those to cutaneous sense evoked by electric stimulation of the digital nerve at the same site. It was for the first time demonstrated that proprioception-related SEPs following passive finger movement do not originate in area 3b, which was clearly activated by cutaneous stimulation, and that other sites at the sensorimotor cortex such as areas 2, 3a and 4 possibly contribute to the cortical processing of proprioception.     

Peri-rolandic and fronto-parietal components of scalp-recorded giant SEPs in cortical myoclonus

Scalp topography of giant SEPs to median nerve stimulation was studied in 4 patients with cortical myoclonus of various etiology. The positive peak (P30) at the contralateral parietal area was simultaneously accompanied by a negative peak at the frontal area (N30), and at least one of these two peaks was enhanced in 2 patients. Another positive peak (P25) and a negative peak (N35) were also identified at the peri-rolandic area with different latency from P30 and N30, respectively, in all patients. N35 was enhanced in 3 patients, and P25 in 2 patients. It is concluded that, as seen in normal subjects, tangential (P30-N30) and radial (P25 and N35) components of SEPs are most likely distinguishable in giant SEPs, and that either one or both of those components is enhanced in different ways depending on the patients.     

Nerve,spinal cord and brain somatosensory evoked responses: a comparative study during electrical and magnetic peripheral nerve stimulation

Somatosensory evoked potentials (SEPs) and compound nerve action potentials (cNAPs) have been recorded in 15 subjects during electrical and magnetic nerve stimulation. Peripheral records were gathered at Erb's point and on nerve trunks at the elbow during median and ulnar nerve stimulation at the wrist. Erb responses to electrical stimulation were larger in amplitude and shorter in duration than the magnetic ones when ‘electrical’ and ‘magnetic’ compound muscle action potentials (cMAPs) of comparable amplitudes were elicited. SEPs were recorded respectively at Cv7 and on the somatosensory scalp areas contra- and ipsilateral to the stimulated side. SEPs showed a statistically significant difference in amplitude only for the brachial plexus response and for the ‘cortical’ N20-P25 complex; differences were not found between the magnetic and electrical central conduction times (CCTs) or for the peripheral nerve response latencies. Magnetic stimulation preferentially excited the motor and proprioceptive fibres when the nerve trunks were stimulated at motor threshold intensities.     

Acute posthypoxic myoclonus after cardiopulmonary resuscitation

ABSTRACT: BACKGROUND: Acute posthypoxic myoclonus (PHM) can occur in patients admitted after cardiopulmonary resuscitation (CPR) and is considered to have a poor prognosis. The origin can be cortical and/or subcortical and this might be an important determinant for treatment options and prognosis. The aim of the study was to investigate whether acute PHM originates from cortical or subcortical structures, using somatosensory evoked potential (SEP) and electroencephalogram (EEG). METHODS: Patients with acute PHM (focal myoclonus or status myoclonus) within 72 hours after CPR were retrospectively selected from a multicenter cohort study. All patients were treated with hypothermia. Criteria for cortical origin of the myoclonus were: giant SEP potentials; or epileptic activity, status epilepticus, or generalized periodic discharges on the EEG (no back-averaging was used). Good outcome was defined as good recovery or moderate disability after 6 months. RESULTS: Acute PHM was reported in 79/391 patients (20%). SEPs were available in 51/79 patients and in 27 of them (53%) N20 potentials were present. Giant potentials were seen in 3 patients. EEGs were available in 36/79 patients with 23/36 (64%) patients fulfilling criteria for a cortical origin. Nine patients (12%) had a good outcome. A broad variety of drugs was used for treatment. CONCLUSIONS: The results of this study show that acute PHM originates from subcortical, as well as cortical structures. Outcome of patients admitted after CPR who develop acute PHM in this cohort was better than previously reported in literature. The broad variety of drugs used for treatment shows the existing uncertainty about optimal treatment.     

Wave form decomposition of ‘giant SEP’ and its computer model for scalp topography

By using the decomposition technique developed by ourselves to investigate the scalp topography of evoked potentials, a computer model for the scalp topography of giant SEPs was computed from 5 patients with progressive myoclonic epilepsy and was compated with those obtained from 6 normal subjects. Components of giant SEPs were similar to those of normal SEPs with respect to various parameters, although the former were much larger than the latter. An experimental enlargement of some of the early cortical components of the normal SEP model gave rise to a wave from closely resembling that of the giant SEP. These findings support our previous conclusion, derived from study of the scalp topography of the original SEP wave form, that the giant SEP results from a pathological enhancement of certain early cortical components of the normal SEP. The underlying neuronal hyperexcitability seems to involve more than one subunit of the sensorimotor cortex.     

Topography of somatosensory evoked potentials to median nerve stimulation in patients with cerebral lesions

Scalp distributions and topographies of early cortical somatosensory evoked potentials (SEPs) to median nerve stimulation were studied in 22 patients with 5 different types of cerebral lesion due to cerebrovascular disease or tumor (thalamic, postcentral subcortical, precentral subcortical, diffuse subcortical and parieto-occipital lesions) in order to investigate the origins of frontal (P20, N24) and central-parietal SEPs (N20, P22, P23).In 2 patients with thalamic syndrome, N16 was delayed in latency and N20/P20 were not recorded. No early SEP except for N16 was recorded in 2 patients with pure hemisensory loss due to postcentral subcortical lesion. In all 11 patients with pure hemiparesis or hemiplegia due to precentral subcortical lesion N20/P20 and P22, P23/N24 components were of normal peak latencies. The amplitude of N24 was significantly decreased in all 3 patients with complete hemiplegia. These findings support the hypothesis that N20/P20 are generated as a horizontal dipole in the central sulcus (3b), whereas P23/N24 are a reflection of multiple generators in pre- and post-rolandic fissures. P22 was very localized in the central area contralateral to the stimulation.Topographical studies of early cortical SEPs are useful for detecting each component in abnormal SEPs     

Median nerve somatosensory evoked potentials in profound hypothermia for ascending aorta repair

Median nerve somatosensory evoked potentials (SEPs) were recorded in 9 patients undergoing profound hypothermia for surgical repair of the aortic arch. In addition to the known increase in peak latencies, hypothermia gave rise to the appearance of peaks (‘P13,’ ‘N14’) inconsistently recognized at normothermia; moreover, profound hypothermia is associated with the disappearance of cortical activities around 20°, of subcortical waves at lower temperatures. The practical implications of the results are 3-fold: firstly, they suggest that the ‘P13’ and P14 should both be intracranially generated, at a pre- and postsynaptic level with respect to the cuneate nucleus, respectively; secondly, they show that some discrepancies between previous papers dealing with SEPs and hypothermia can be explained by differences in the choice of the reference; thirdly, they bring some suggestions on a better use of SEPs to monitor patients undergoing aortic arch surgery.     

Steady-state analysis of somatosensory evoked potentials

We report the development of a new method for frequency domain analysis of steady-state somatosensory evoked potentials (SEPs) to amplitude-modulated electrical stimulation, which can be recorded in significantly less time than traditional SEPs. Resampling techniques were used to compare the steady-state SEP to traditional SEP recordings, which are based on signal averaging in the time domain of cortical responses to repetitive transient stimulation and take 1–2 min or more to obtain a satisfactory signal/noise ratio. Median nerves of 3 subjects were stimulated continuously with electrical alternating current at several modulation frequencies from 7 to 41 Hz. Amplitude modulation was used to concentrate the power in higher frequencies, away from the modulation frequency, to reduce the amount of stimulus artifact recorded. Data were tested for signal detectability in the frequency domain using the T_circ² statistic. A reliable steady-state response can be recorded from scalp electrodes overlying somatosensory cortex in only a few seconds. In contrast, no signal was statistically discriminable from noise in the transient SEP from as much as 20 s of data. This dramatic time savings accompanying steady-state somatosensory stimulation may prove useful for monitoring in the operating room or intensive care unit.     

Laser-evoked brain potentials in patients with dissociated loss of pain and temperature sensibility

Brief heat stimuli, elicited by a CO₂ laser (10.6 μm wave length), activate the most superficial cutaneous nerve terminals of the thin myelinated Aδ and unmyelinated C fibres which mediate heat and pain sensations. This paper investigates late cerebral potentials (SEPs) in response to laser pulses in comparison with those to conventional electrical stimulation in 18 patients with a dissociated sensory deficit (intact mechanosensibility and disturbed temperature and pain sensation). Patients were stimulated in the most disturbed limb (affected area) and in a corresponding control area.In all 18 patients the SEPs elicited by laser stimuli were able to identify the body site with heaviest disturbances in pain and thermosensibility: the SEPs from the affected area were reduced or delayed, compared to the control area. In contrast, no alterations in SEPs could be observed after conventional electrical nerve stimulation, in agreement with the normal mechanosensibility. However, the degree of SEP modulation in response to cutaneous heat stimuli did not correspond to the severity of the subjectively reported sensory deficit. Highest correlations between sensory deficits and abnormal SEPs were found in all those patients in whom computer tomography or MR imaging documented a localized destructive process in the CNS. All patients with the smallest SEP modulations despite a considerable sensory deficit had an inflammatory aetiology. Preliminary criteria to define a laser-evoked SEP as pathological are discussed.     

Ipsilateral median somatosensory evoked potentials recorded from human somatosensory cortex

Somatosensory evoked potentials (SEP) to ipsilateral and contralateral median nerve stimulations were recorded from subdural electrode grids over the perirolandic areas in 41 patients with medically refractory focal epilepsies who underwent evaluation for epilepsy surgery. All patients showed clearly defined, high-amplitude contralateral median SEPs. In addition, four patients showed ipsilateral SEPs. Compared with the contralateral SEPs, ipsilateral SEPs were very localized, had a different spatial distribution, were of considerably lower amplitude, had a longer latency (1.2–17.8 ms), did not show an initial negativity, and were markedly attenuated during sleep. Stimulation of the subdural electrodes overlying the sensory hand area was associated with contralateral hand paresthesias, but no ipsilateral hand paresthesias occurred. It was concluded that subdurally recorded cortical SEPs to ipsilateral stimulation of the median nerve (M) reflect unconscious sensory input from the hand possibly serving fast bimanual hand control. The anatomical pathway of these ipsilateral short-latency MSEPs is not yet known. Transcallosal transmission seems unlikely because of the short delay between the ipsilateral and contralateral responses in selected cases. The infrequent occurrence of ipsilateral subdurally recorded SEPs and their low amplitude and limited distribution suggest that they contribute very little to the short-latency ipsilateral median SEPs recorded on the scalp.     

Somatosensory evoked potentials in neonates and infants: developmental and normative data

Fifty-two sets of cortical somatosensory evoked potentials (SEPs) were recorded from 23 normal children between the ages of 1 day and 13 weeks with median nerve stimulation. Two bandpass settings 5–1500 Hz and 30–3000 Hz were used; rate of stimulation was 1.1 Hz and sweep-time was 200 msec. The state of wakefulness was documented, but SEPs were obtained and evaluated independently of the child being awake or asleep during the recording. SEPs were present in every recording. The bandpass 30–3000 Hz best differentiated positive and negative early potentials. The bandpass 5–1500 Hz was helpful in some cases, as late slow waves were recorded with this setting. Normative data were established. Mean values were calculated for 3 age groups: 0–2 weeks, 2–6 weeks and 7–13 weeks. P15 and N20 were the first components seen in the newborn, with the P22 becoming the major component by 2–3 weeks of age. The study indicates that maturation of the somatosensory system is fastest during the first 3 weeks of life.     

The assessment of severe head injury by short-latency somatosensory and brain-stem auditory evoked potentials

The relative prognostic value of short-latency somatosensory evoked potentials (SEPs) and brain-stem auditory evoked potentials (BAEPs) was assessed in 35 patients with post-traumatic coma. Analysis of the evoked potentials was restricted to those recorded within the first 4 days following head injury. Abnormal SEPs were defined as an increase in central somatosensory conduction time or an absence of the initial cortical potential following stimulation of either median nerve. Abnormal BAEPs were classified as an increase in the wave I–V interval or the loss of any or all of its 3 most stable components (waves I, III and V) following stimulation of either ear. SEPs reliably both good and bad outcomes. All 17 patients in whom SEPs were graded as normal had a favourable outcome and 15 of 18 patients in whom SEPs were abnormal had an unfavourable outcome. Although abnormal BAEPs were associated with an unfavourable outcome in almost all patients (6 of 7), only 19 of 28 patients with normal BAEPs had a favourable outcome. The finding of normal BAEPs was therefore of little prognostic significance. These results confirm the superiority and greater sensitivity of the SEP in detecting abnormalities of brain function shortly after severe head trauma.     

Somatotopy of human hand somatosensory cortex as studied in scalp EEG

We recorded somatosensory evoked potentials (SEPs) in scalp EEGs during stimulation of the median nerve, the ulnar nerve and the individual digits in 3 normal subjects and in 1 epilepsy patients. In this patient we also measured SEPs from chronically indwelling subdural grid electrodes during electrocorticography (ECoG). We applied dipole modelling techniques to study the 3-dimensional intracerebral locations and time activities of the neuronal sources underlying stimulation of different peripheral receptive fields. The sources underlying median nerve SEPs were located an average of 10.8 mm lateral inferior to those underlying ulnar nerve SEPs. Digit SEP sources showed a somatotopic arrangement from lateral inferior to medial superior in the order thumb, index finger, middle finger, ring finger and little finger, with some overlap or reversal for adjacent digits. The average distance between thumb and little finger was 12.5 mm. Thumb, index finger and middle finger were clustered around median nerve cortical representation, whereas ring finger and little finger were arranged around ulnar nerve cortex. In the epilepsy patient, the source localizations obtained in scalp EEGs showed good agreement with those on ECoGs. We conclude that SEPs recorded in scalp EEGs can be used to study functional topography of human somatosensory cortex non-invasively.     

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 NI 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 Nl-N2 between foot and hand stimulation. It is therefore concluded that NI of the foot pain SEPs is generated mainly from the foot area of SI. The variable scalp distribution of the N7 component of the foot pain SEPs is likely due to an anatomical variability among subjects and even between sides.     

Somatosensory evoked potentials in the ventrolateral thalamus

Within the target area (VL) used for the stereotactic treatment of parkinsonian tremor and spasmodic torticollis, electrical stimulation as well as recording of somatosensory evoked potential (SEP) was performed. The effects of stimulation in the target area are facilitation of muscle tone showing some degree of somatotopic distribution. The recorded SEPs indicate a projection of an afferent system (probably of muscle afferents) to the target area. We assume that the target area is a relay station involved in the control of muscle tone. The interruption of muscle afferents in combination with the correct somatotopic localization of the lesion is important for the therapeutic efficacy in parkinsonian tremor and spasmodic torticollis.     

Scalp topography and distribution of cortical somatosensory evoked potentials to median nerve stimulation

Topographies and distributions of cortical SEPs to median nerve stimulation were studied in 8 normal adults and 5 neurological patients. SEPs recorded from C4, P4, Pz, T6-A1A2 derivations to left median nerve stimulation were composed of 2 early negative (N16, N20) and 2 positive components (P12, P23), whereas those recorded from frontal electrodes (Fz, Fp1, Fp2) disclosed 2 early negativities (N16, N24) and 2 early positivities (P12, P20). N20 and P20, and P23 and N24, reversed across the rolandic fissure with no significant difference in their peak latencies. P23 was of slightly shorter latency at C4 than at more posterior electrodes (P4, T6, Pz).In 3 patients with complete hemiplegia but normal sensation, all the early SEP components were normal in scalp distribution and peak latencies except for a decrease of N24 amplitude. In 2 patients with complete hemiplegia and sensory loss no early cortical SEPs were seen. These findings suggest that N20 and P20 are generated as a single horizontal dipole in the central fissure, whereas P23 and N24 are a reflection of multiple generators in pre- and postrolandic regions.     

The perception of painful and nonpainful stimuli during voluntary motor activity in man

Previous studies have shown that voluntary movement diminishes the transmission of cutaneous afferent input through the dorsal column-medial lemniscal system, and also raises the threshold for detecting nonpainful, cutaneous stimuli (electrical shocks). Although there is some evidence that pain elicited by electrical stimulation is diminished during movement, no studies have tested the effect of movement on the perception of pain produced by natural stimulation. For this reason, we tested the effects of voluntary motor activity on the perception of noxious thermal stimuli in human volunteers. We first developed a motor paradigm in which the thermal stimulation could be applied to the immobile limb (isometric elbow flexion-extension). Both isometric and isotonic muscle contractions about the elbow increased the threshold for detecting weak cutaneous stimuli (electrical shocks) applied to the forearm, and to a lesser extent the detection of stimuli applied to the dorsum of the hand. Afterwards, noxious and innocuous heat stimuli were applied to the forearm during isometric contractions and at rest. Magnitude estimates for the intensity of the pain, as well as latency measures of the onset of pain, were recorded. We found no evidence that isometric motor activity diminished either the threshold for pain or the subjective intensity of the noxious and innocuous thermal stimuli. Thus, motor activity decreases the ability to detect weak low-threshold cutaneous inputs, but has no effect on the perception of warmth and heat pain.     

Giant central N20-P22 with normal area 3b N20-P20: an argument in favour of an area 3a generator of early median nerve cortical SEPs?

Generators of early cortical somatosensory evoked potentials (SEPs) still remain to be precisely localised. This gap in knowledge has often resulted in unclear and contrasting SEPs localisation in patients with focal hemispheric lesions. We recorded SEPs to median nerve stimulation in a patient with right frontal astrocytoma, using a 19-channel recording technique. After stimulation of the left median nerve, N20 amplitude was normal when recorded by the parietal electrode contralateral to the stimulation, while it was abnormally enhanced in traces obtained by the contralateral central electrode. The amplitude of the frontal P20 response was within normal limits. This finding suggests that two dipolar sources, tangential and radial to the scalp surface, respectively, contribute concomitantly to N20 generation. The possible location of the N20 radial source in area 3a is discussed. The P22 potential was also recorded with increased amplitude by the central electrode contralateral to the stimulation, while N30 amplitude was normal in frontal and central traces. We propose that the radial dipolar source of P22 response is independent from both N20 and N30 generators and can be located either in 3a or in area 4. This report illustrates the usefulness of multichannel recordings in diagnosing dysfunction of the sensorimotor cortex in focal cortical lesions.     

Vegetative versus Minimally Conscious States: A Study Using TMS-EEG,Sensory and Event-Related Potentials

Differential diagnoses between vegetative and minimally conscious states (VS and MCS, respectively) are frequently incorrect. Hence, further research is necessary to improve the diagnostic accuracy at the bedside. The main neuropathological feature of VS is the diffuse damage of cortical and subcortical connections. Starting with this premise, we used electroencephalography (EEG) recordings to evaluate the cortical reactivity and effective connectivity during transcranial magnetic stimulation (TMS) in chronic VS or MCS patients. Moreover, the TMS-EEG data were compared with the results from standard somatosensory-evoked potentials (SEPs) and event-related potentials (ERPs). Thirteen patients with chronic consciousness disorders were examined at their bedsides. A group of healthy volunteers served as the control group. The amplitudes (reactivity) and scalp distributions (connectivity) of the cortical potentials evoked by TMS (TEPs) of the primary motor cortex were measured. Short-latency median nerve SEPs and auditory ERPs were also recorded. Reproducible TEPs were present in all control subjects in both the ipsilateral and the contralateral hemispheres relative to the site of the TMS. The amplitudes of the ipsilateral and contralateral TEPs were reduced in four of the five MCS patients, and the TEPs were bilaterally absent in one MCS patient. Among the VS patients, five did not manifest ipsilateral or contralateral TEPs, and three of the patients exhibited only ipsilateral TEPs with reduced amplitudes. The SEPs were altered in five VS and two MCS patients but did not correlate with the clinical diagnosis. The ERPs were impaired in all patients and did not correlate with the clinical diagnosis. These TEP results suggest that cortical reactivity and connectivity are severely impaired in all VS patients, whereas in most MCS patients, the TEPs are preserved but with abnormal features. Therefore, TEPs may add valuable information to the current clinical and neurophysiological assessment of chronic consciousness disorders.     

Cortical potentials evoked by epidural stimulation of the cervical and thoracic spinal cord in man

Scalp somatosensory evoked potentials (SEPs) were recorded after electrical stimulation of the spinal cord in humans. Stimulating electrodes were placed at different vertebral levels of the epidural space over the midline of the posterior aspect of the spinal cord. The wave form of the response differed according to the level of the stimulating epidural electrodes. Cervical stimulation elicited an SEP very similar to that produced by stimulation of upper extremity nerves, e.g., bilateral median nerve SEP, but with a shorter latency. Epidural stimulation of the lower thoracic cord elicited an SEP similar to that produced by stimulation of lower extremity nerves. The results of upper thoracic stimulation appeared as a mixed upper and lower extremity type of SEP. The overall amplitudes of SEPs elicited by the epidural stimulation were higher than SEPs elicited by peripheral nerve stimulation. In 4 patients the CV along the spinal cord was calculated from the difference in latencies of the cortical responses to stimulation at two different vertebral levels. The CVs were in the range of 45–65 m/sec. The method was shown to be promising for future study of spinal cord dysfunctions.