Effects on median nerve SEPs of tactile stimulation applied to adjacent and remote areas of the body surface

Study of the influence of continuous tactile stimulation on somatosensory evoked potentials (SEPs) following electrical stimulation of the median nerve revealed an effect due to interfering input from both adjacent and remote regions of the body surface. The distribution of the effect was demonstrated by subtracting the 'interference' from the 'control' response to derive a 'difference' wave form. Tactile stimulation of the thumb ipsilateral to the stimulated median nerve produced a difference wave form in which a marked phase reversal was apparent between pre- and post-central areas for 2 complexes, at latencies of approximately 20 and 30 msec. It is proposed that this may have been due to partial 'saturation' of a generator in the hand region of area 3b in the primary somatosensory cortex (SI), which was then unable to respond fully to the median nerve impulse. A similar effect was observed when the interfering stimulus was applied to the ipsilateral little finger, possibly reflecting a process of 'surround inhibition.' Tactile stimulation of more remote regions (principally the face and contralateral hand) resulted in consistent difference wave forms in which the early components (less than 30 msec latency) had scalp distributions differing from one another but consistent with influence on generators in the face or hand region of the second somatosensory cortex (SII). Later potentials consistently identifiable in the difference wave forms were similar for all locations of the interfering stimulus apart from the ipsilateral thumb and were distributed in accordance with a proposed generator in the parietal 'association' cortex.     

Comparison of cervical SEPs on median, radial and ulnar nerve stimulation

Cervical responses (SEPs) to stimulation of the median, radial and ulnar nerves were studied in 9 healthy subjects. In recordings from e Cv₇ electrode referenced to a scalp electrode P11 presented a bilobed (P11a+P11b) profile for all three nerves whereas from Cv₂ only P11b appeared as a rule. P11a and P11b were more distinct in the ulnar than in the median and radial nerves. The P11_onset-P13_onset interval was virtually the same for the radial and median nerves and approximately 0.4 msec longer for the ulnar nerve. This difference probably represents the Cv₈ to Cv₆ intramedullary conduction time. An exact evaluation of P11_onset is possible only in low cervical recordings, though the P11_peak may be a useful landmark when recording from Cv₂. P11a would appear to originate at (or near) spinal entry, P11b high in the cervical cord and P13 at supraspinal level.

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Sommario In nove soggetti, sono state studiate le risposte cervicali da stimolazione dei nervi mediano, radiale ed ulnare. Utilizzando un riferimento cefalico, P11 appare bilobata, registrando da Cv₇, per tutti e tre i nervi, mentre solo la seconda delle due subcomponenti (P11a e P11b, rispettivamente) viene registrata generalmente da Cv₂. P11a e P11b sono meglio distinte fra di loro nel caso del nervo ulnare, che non del radiale o mediano. L'intervallo P11_onset-P13_onset è sovrapponibile fra mediano e radiale e circa 0.45 msec più lungo per il nervo ulnare; questa differenza dovrebbe essere dovuta ad un tempo di conduzione intramidollare fra Cv₈ e Cv₆. Una esatta valutazione di P11_onset è possibile solo a livello cervicale inferiore, mentre il picco P11b può essere utilizzato nel computo delle latenze solo da Cv₂. P11a dovrebbe originare all'ingresso spinale, P11b nel tratto cervicale superiore, P13 a livello sopraspinale.

     

Frontal SEPs and parietal SEPs following median nerve stimulation--clinical correlation and consideration of their generation sites

In order to know the characteristics of frontal and parietal SEP components following median nerve stimulation, 25 patients with unilateral cerebral lesions above the thalamus were examined, and their SSEPs were carefully compared with the clinical and radiological findings. In 10 normal subjects, there were three cortical components of the frontal SEPs (P 20-N 28-P 44) and four those components of the parietal SEPs (N 18-P 22-N 26-P 42). In patient's group, central conduction times (CCTs) between components P 13 and each cortical component were measured and the latency differences between normal side and affected side were calculated. When the latency differences increased over 3 S.D. from the mean of the control values or the some cortical components disappeared, they were regarded as abnormal. According to the combination of the abnormalities in frontal and parietal SEPs, three groups were classified as follows: group 1; frontal and parietal SEPs were normal (n = 10), group 2; frontal and parietal SEPs were both affected (n = 10), group 3; parietal SEPs were affected but frontal components were preserved in normal range (n = 5). CT scan showed that the region from internal capsule to cortex around the central sulcus remained intact in the patients of group 1, while this region was involved in various degrees in all cases of the group 2. In patients of group 3, frontal or parietal regions were variously affected. Both the motor and sensory functions were mainly intact in group 1, and disturbed in group 2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serial median nerve SEPs and SSEPs in patients with West syndrome

We studied serial median nerve somatosensory evoked potentials (SEPs) and short latency somatosensory evoked potentials (SSEPs) in 17 patients with West syndrome. Four of the 7 patients with absent SEPs in the initial examination showed recognizable SEPs in the follow-up studies, associated with improvement of electroencephalogram (EEG). This indicated that SEPs were variable with condition of epilepsy and lack of initial SEPs was not always a poor prognostic factor for seizure control and developmental outcome. Persistent lack of SEPs, however, indicated poor outcome of seizures, EEG and development. Central conduction time in SSEPs did not correlate with seizure or developmental outcome.     

Dipole modelling of median nerve SEPs in normal subjects and patients with small subcortical infarcts.

Somatosensory evoked potentials (SEPs) to median nerve stimulation were analyzed by means of spatio-temporal dipole modelling in 6 normal subjects and 8 patients with small infarcts in the thalamo-cortical radiation or thalamus. The SEPs could be modelled by a tangentially and a radially oriented equivalent dipole in the region of the contralateral cortical hand area and an equivalent dipole located in the region of the brain-stem. In 3 patients with absence or reduction of amplitudes of cortically generated SEP components, the activity of both cortical dipoles was lost or reduced. In 2 patients the frontally recorded SEP component P20 was lost; in one of them the activity of mainly the tangential dipole was reduced.     

Temporal processing of visuotactile and tactile stimuli in writer's cramp

Neurophysiological studies in animals show that basal ganglia are involved not only in motor and nonmotor timing functions but also in integrating tactile and visual signals delivered in the peripersonal space. We tested temporal discrimination of cross-modal and unimodal stimuli in 13 controls and 14 patients with writer's cramp, a disorder supposedly linked to dysfunction of basal ganglia. Subjects were asked to discriminate whether pairs of visual, tactile, or visuotactile stimuli were simultaneous or sequential (temporal discrimination threshold) and which stimulus preceded the other (temporal order judgment). Patients were impaired in temporal processing of tactile and cross-modal stimuli. A significant positive correlation between temporal deficits and the severity of disability was detected for both affected and unaffected sides. Findings suggest that multimodal and not only modality-specific temporal processing is defective in focal hand dystonia. Deficits of temporal processing of stimuli delivered to the unaffected side may represent a behavioral index of the susceptibility to develop dystonia and thus have remarkable practical and theoretical implications.     

Somatosensory evoked potentials (SEPs) and cortical single unit responses elicited by mechanical tactile stimuli in awake monkeys

The origins of surface recorded evoked potentials have been investigated by combining recordings of single unit responses and somatosensory evoked potentials (SEPs) from the postcentral gyrus of 4 alert macaque monkeys. Responses were elicited by mechanical tactile stimuli (airpuffs) which selectively activate rapidly adapting cutaneous mechanoreceptors, and permit patterned stimulation of a restricted area of skin. Epidurally recorded SEPs consisted of an early positive complex, beginning 8-10 msec after airpuff onset, with two prominent positive peaks (P15 and P25), succeeded by a large negative potential (N43) lasting 30 msec, and a late slow positivity (P70). SEPs, while consistent in wave form, varied slightly between monkeys. The amplitude of the early positive complex was enhanced by increasing the number of stimulated points, or by placing the airpuffs in the receptive fields of cortical neurons located beneath the SEP recording electrode. SEP amplitude was depressed when preceded 20-40 msec earlier by a conditioning stimulus to the same skin area. Single unit responses in areas 3b and 1 of primary somatosensory (SI) cortex consisted of a burst of impulses, beginning 11-12 msec after the airpuff onset, and lasting another 15-20 msec. Peak unitary activity occurred at 12-15 msec, corresponding to the P15 wave in the SEP. No peak in SI unit responses occurred in conjunction with the P25 wave. Although SI neurons fired at lower rates during P25, the lack of any peak in SI unit responses suggests that activity in other cortical areas, such as SII cortex, contributes to this wave. Most unit activity in SI cortex ceased by the onset of N43, and was replaced by a period of profound response depression, in which unit responses to additional tactile stimuli were reduced. We propose that the N43 wave reflects IPSPs in cortical neurons previously depolarized and excited by the airpuff stimulus. Late positive potentials (P70) in the SEP had no apparent counterpart in SI unit activity, suggesting generation at other cortical loci.     

Effects of passive tactile co-activation on median ulnar nerve representation in human SI

In animals simple passive co-activation causes a fusion and expansion of the involved cortical representations. We used passive tactile finger co-activation for 40 min to investigate cortical representational changes in the human somatosensory cortex. Magnetic source imaging revealed that the euclidean distance between median and ulnar nerve somatosensory evoked fields (SEF) was significantly reduced after application of 600 synchronous airpuff stimuli to the fingertips of four fingers. In the control experiment without co-activation no significant change in distance was observed. Perception threshold and spatial two-point discrimination were not affected by the synchronous stimulation. This is in contrast to blind three-finger Braille readers who frequently mislocalize stimuli applied to the reading fingers. This points to a lack of behavioural relevance or the short duration of co-activation.     

Thalamo-cortical processing of near-threshold somatosensory stimuli in humans

Somatosensory stimuli elicit complex cortical responses that are discernible as somatosensory evoked potentials (SEPs) in scalp electroencephalographic recordings. Whereas earlier SEP components, occurring up to 100 ms after stimulus delivery, have been labeled 'preconscious', later responses have been associated with stimulus awareness. To date, how far these processes are primarily cortical or comprise additional subcortical operations remains open. Therefore, we recorded thalamic and scalp SEPs evoked by perceived as well as unperceived median nerve stimulation in neurosurgical patients with electrodes implanted into the ventral intermediate nucleus of the thalamus for deep brain stimulation. At stimulation intensities below perceptual threshold, only thalamic SEP components appeared consistently during the first 75 ms after stimulus delivery. Stimulation that was perceived by the patients elicited cortical as well as thalamic SEPs that lasted longer than 75 ms. These results indicate that the thalamus remains active after the primary propagation of a sensory signal to the cortex, and suggest that the transition from elementary to higher-order somatosensory processing is based on thalamo-cortical interactions.     

Parietal generators of low- and high-frequency MN (median nerve) SEPs: data from intracortical human recordings.

OBJECTIVE: To identify low and high-frequency median nerve (MN) somatosensory evoked potential (SEP) generators by means of chronically implanted electrodes in the parietal lobe (SI and neighbouring areas) of two epileptic patients. METHODS: Wide-pass short-latency and long-latency SEPs to electrical MN stimulation were recorded in two epileptic patients by stereotactically chronically implanted electrodes in the parietal lobe (SI and neighbouring areas). To study high-frequency responses (HFOs) an off-line digital filtering of depth short-latency SEPs was performed (500-800 Hz, 24 dB roll-off). Spectral analysis was performed by fast Fourier transform. RESULTS: In both patients we recorded a N20/P30 potential followed by a biphasic N50/P70 response. A little negative response in the 100 ms latency range was the last detectable wide-pass SEP in both patients. Two HFOs components (called iP1 and iP2) were detected by mere visual analysis and spectral analysis, and were supposed to be originated within the parietal cortex. CONCLUSIONS: This was the very first study that recorded wide bandpass and high frequency SEPs by electrodes, exploring both the lateral and the mesial part of the parietal lobe and particularly that of the post-central gyrus.     

Cortical processing of near-threshold tactile stimuli in a paired-stimulus paradigm--an MEG study

In the present magnetoencephalography study, we applied a paired‐stimulus paradigm to study the weak cortical responses evoked by near‐threshold tactile prime stimuli by means of their attenuating effect on the cortical responses evoked by subsequently applied above‐threshold test stimuli. In stimulus pairs with adequate interstimulus intervals (ISIs), the extent of test stimulus response attenuation is related to the amplitude of prime stimulus responses, and the duration of the attenuating effect indicates how long memory traces of a prime stimulus reside in cortical areas. We hypothesized that the attenuation of test stimulus responses, studied for ISIs of 30, 60 and 150 ms, would provide insight into the temporal dynamics of near‐threshold stimulus processing in primary (SI) and secondary somatosensory cortex (SII), and reveal differences in response amplitude due to conscious perception. Attenuation of test stimulus responses in SI was observed for ISIs up to 60 ms, whereas in SII the effect outlasted the ISI of 150 ms. Differences due to conscious perception of the near‐threshold stimuli were only observed in SII with stronger attenuation for perceived than for missed near‐threshold stimuli. Applying this indirect approach to near‐threshold stimulus processing, we could show that the extent and duration of response attenuation is related to prime stimulus processing and differential temporal and functional characteristics of near‐threshold stimulus information processing in SI and SII: transient processing of basic stimulus information not sufficient for conscious perception in SI and long‐lasting activations involving conscious perception in SII.     

N10 component in median nerve somatosensory evoked potentials (SEPs) is not an antidromic motor potential.

OBJECTIVE: To test the hypothesis that the N10 far-field potential in median nerve somatosensory evoked potentials is generated by the motor axons by examining patients with amyotrophic lateral sclerosis (ALS). METHODS: Subjects were 5 ALS patients showing pronounced or complete denervation of median-innervated small hand muscles. We evaluated N10 over scalp, and proximal plexus volleys (PPVs) at lateral or anterior cervical electrode. RESULTS: N10 and PPVs were definitely preserved for every ALS subject. N10 amplitudes of ALS subjects were even significantly larger than control subjects. In one ALS patient completely lacking motor axons, N10 was larger than the largest one among control subjects. CONCLUSIONS: Present results clearly indicate that N10 is not predominantly generated by motor axons but by the whole median nerve dominated by sensory axons. We propose a theory that N10 is a junctional potential generated by the entrance of the median nerve into bone at the intervertebral foramen, producing a positive pole at the non-cephalic reference electrode. Significantly larger N10 in ALS subjects may be due to the lack of cancellation by slower motor axons. SIGNIFICANCE: The hypothesis that N10 is generated by motor axons is refuted, and a new theory of its generation is presented.     

Contribution of cutaneous and muscle afferent fibres to cortical SEPs following median and radial nerve stimulation in man

Somatosensory evoked potentials were recorded after stimulation of motor and cutaneous nerves in the upper limb. Stimulation of the thenar motor branch of the median nerve and the deep motor branch of the radial nerve produced only broad, ill-defined and small-amplitude scalp-recorded responses. In contrast, stimulation of purely cutaneous nerves (digital and the superficial radial) gave responses of large amplitude. The cortical responses following combined deep and superficial radial nerve stimulation were of smaller amplitude than the two individual responses combined. These findings suggest that, contrary to an earlier report (Gandevia et al. 1984), muscle afferents do not make a major positive contribution to the scalp-recorded cortical responses produced by electrical stimulation of mixed nerves in the upper limb.     

Serial recording of median nerve stimulated subcortical somatosensory evoked potentials (SEPs) in developing brain death

Subcortical somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded serially in 35 patients during the evolution towards brain death and in brain death. Neuropathological alterations of the central nervous system down to the C1/C2 spinal cord segment in brain death are well known. SEP components supposed to be generated above this level should be lost in brain death, while components generated below should not be altered. Erb's point, scalp and neck potentials were recorded at C3/4, or over the spinous process C7, using an Fz reference. In 10 patients additional montages, including spinous process C2-Fz, a non-cephalic reference (Fz-contralateral shoulder) and a posterior to anterior neck montage (spinous process C7-jugulum) were used. The cephalic referenced N9 and N11 peaks remained unchanged until brain death. N9 and N11 decreased in parallel in amplitude and increased in latency after systemic effects like hypoxia or hypothermia occurred. The cephalic referenced 'N14' decreased in amplitude and increased in latency after the clinical brain death syndrome was observed, while N13 in the posterior to anterior neck montage remained unchanged. The alteration of 'N14' went parallel to the decrease of the P14 amplitude. The subcortical SEPs in the cephalic referenced lead are supposed to be a peak composed by a horizontally orientated dorsal horn generated N13 and a rostrally orientated P14 arising at the level of the foramen magnum. The deterioration of the non-cephalic referenced P14 and of its cephalic referenced reflection 'N14' seems to provide an additional objective criterion for the diagnosis of brain death.     

Changes in median nerve somatosensory transmission and motor output following transient deafferentation of the radial nerve in humans.

OBJECTIVE: To determine if transient anaesthetic deafferentation of the radial nerve would lead to alterations in processing of early somatosensory evoked potentials (SEPs) from the median nerve or alter cortico-motor output to the median nerve innervated abductor pollicis brevis (APB) muscle. METHODS: Spinal, brainstem, and cortical SEPs to median nerve stimulation were recorded before, during and after ipsilateral radial nerve block with local anaesthesia. Motor evoked potentials (MEPs) and motor cortex output maps were recorded from the APB muscle. RESULTS: There were no significant changes to most early SEP peaks. The N30 peak, however, showed a significant increase in amplitude, which remained elevated throughout the anaesthetic period, returning to baseline once the anaesthetic had completely worn off. MEP amplitude of the median nerve innervated APB muscle was significantly decreased during the radial nerve blockade. There was also a significant alteration in the APB optimal site location, and a small but significant decrease in the silent period during the radial nerve blockade. CONCLUSIONS: Transient anaesthetic deafferentation of the radial nerve at the elbow leads to a rapid modulation of cortical processing of median nerve input and output. These changes suggest an overall decrease in motor cortex output to a median nerve innervated muscle not affected by the radial nerve block, occurring concomitantly with an increased amplitude of the median nerve generated N30 SEP peak, thought to represent processing in the supplementary motor area (SMA). Independent subcortical connections to the SMA are thought to contribute to the N30 response observed in this study. Unmasking of pre-existing but latent cortico-cortical and/or thalamo-cortical connections may be the mechanism underlying the cortical SEP increases observed following radial nerve deafferentation. SIGNIFICANCE: Transient deafferentation of the radial nerve, which supplies wrist and hand extensor muscles, has been shown to alter sensory processing from and motor output to the median nerve innervated thenar muscles.