1,721,044 research outputs found
Characterizing somatosensory evoked potential sources with dipole models: advantages and limitations
No full textSeveral 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
Pain-related modulation of the human motor cortex
No full textPain is a complex multi-dimensional phenomenon that influences a wide variety of nervous system functions, including sensory--discriminative, affective--motivational and cognitive--evaluative components. So far, these components have been studied in both patients with chronic pain and in normal subjects in whom pain was induced experimentally. The interaction between pain and motor function is not fully understood, although from everyday life it is known that pain affects movements. The effects of pain on motor control are typically seen as a limited or impaired ability to perform movements. Most studies have dealt with the effects of pain on the spinal cord reflexes, but in recent years, several lines of evidence suggest that the interaction between motor and pain systems in conditions of pain induced experimentally, rather than a simple spinal reflex, is a more complex process that involves also supraspinal brain areas. Although pain-motor interaction shows different features and time course depending on different pain variables, such as duration (tonic versus phasic pain), submodalities (deep versus superficial pain) and location (distal versus proximal pain), a common finding is that pain is able to inhibit the motor cortex. This motor cortex inhibition may act as a sort of motor 'decerebration' so as to allow the spinal motor system to freely develop protective responses to noxious stimulation. Further studies are required to assess the effects of pain on the motor system in patients suffering from chronic pain, in order to develop innovative rational therapeutic strategies to reduce both pain and motor disability
Selective abnormality of the N13 spinal SEP to dermatomal stimulation in patients with cervical monoradiculopathy
No full textScalp somatosensory evoked potentials (SEP) to dermatomal stimulation have so far proved to be only partially useful in the diagnosis of monoradiculopathy, mostly in cases without motor impairment. The aim of our study was to test the sensitivity of the spinal N13 potential in uncovering lesions of single cervical roots. We studied five patients suffering from cervical monoradiculopathy, using a recording technique allowing specific recording of the genuine N13 potential which is probably generated by dorsal horn cells. No patient showed signs of muscle impairment and needle EMG was always normal. In four patients, the N13 SEP was, absent following stimulation of the dermatome corresponding to the damaged root, while both the lemniscal P14 and the cortical N20 components were normal. SEP recorded after stimulation of upper limb troncular nerves showed no abnormality in all patients. Our findings suggest that the N13 potential, the loss of which after dermatomal stimulation could be due to deafferentation of dorsal horn neurones, is particularly sensitive to initial root compression. Therefore, our montage allowing analysis of the genuine N13 SEP can improve the sensitivity of dermatomal SEP recording in patients with cervical monoradiculopathies
Dipolar source modeling of somatosensory evoked potentials to painful and nonpainful median nerve stimulation
No full textdipolar source modeling might help in clarifying whether somatosensory evoked potentials (SEPs) after electrical stimulation at painful intensity contain any information related to the nociceptive processing. SEPs were recorded after left median nerve stimulation at three different intensities: intense but nonpainful (intensity 2); slightly painful (pain threshold; intensity 4); and moderately painful (intensity 6), scalp SEPs at intensities 2, 4, and 6 were fitted by a five-dipole model. when the strength modifications of the source activities up to 40 ms were examined across the different stimulus intensities, no significant difference was found. In the later epoch (40-200 ms), a posterior parietal dipole and two bilateral sources probably located in the second somatosensory (SII) areas increased significantly their dipole moments when the stimulus was increased from 2 to 4 and became painful. since no difference was found when the stimulus intensity was increased from 4 to 6, the observed increase of the dipolar strengths is probably related to a variation of the stimulus quality (nonpainful vs, painful), rather than of the stimulus intensity per se. our findings lead us to conclude that a large convergence of nociceptive and non-nociceptive afferents probably occurs bilaterally in the SII areas, (C
Different contribution of joint and cutaneous inputs to early scalp somatosensory evoked potentials
No full textTo elucidate whether the frontal components of scalp somatosensory evoked potentials (SEPs) depend on the type of peripheral input, we compared scalp SEPs in response to electrical stimuli applied to: (i) the proximal phalanx of the thumb, involving both deep and cutaneous afferents; and (ii) the distal phalanx of the thumb, involving cutaneous afferents, but excluding joint inputs coming from the interphalangeal articulation. We applied the same dipolar model that we built to explain the scalp SEP distribution to median nerve stimulation in previous investigations. Cortical SEPs after proximal stimulation were generated by three dipolar sources, one of which was likely to account for the frontal scalp N30. When we analyzed SEPs for distal (purely cutaneous) stimulation, the frontal and central recordings showed a clear reduction in amplitude of the negative responses having a latency of about 30 ms. Moreover, when applying the dipole model derived from analysis of responses to proximal stimulation to SEPs to distal stimulation, the source corresponding to the N30 distribution showed no activity, suggesting a strong relationship between joint and tendinous inputs and the activity of the N30 generator. (C) 1999 John Wiley & Sons, Inc
Ultrasound guidance increases diagnostic yield of needle EMG in plegic muscle
No full textObjectives: To increase the specificity of motor unit potential (MUPs) detection by using ultrasound guided electromyography (USG-EMG) in patients with muscle plegia due to traumatic nerve lesions. Methods: Forty-six patients with recent nerve trauma underwent baseline standard EMG (ST-EMG) evaluation with evidence of absent MUPs. In 41 of them, ST-EMG was repeated after 2–3 months (T1) and the patients were accordingly divided in two groups: ST-EMG+ (if MUPs were detected) or ST-EMG- (MUPs not detected). Then, ST-EMG- patients underwent muscle ultrasound evaluation (M-US) and, if isles of muscular contractility were found, they also had USG-EMG. The same protocol was repeated 4–6 months after baseline (T2). Results: At T1, 22/41 patients were ST-EMG+. While 19/41 were ST-EMG-; 9 of these patients had M-US consistent with residual muscular activity, for that reasons underwent USG-EMG with 7 of 9 demonstrating MUPs (at T2 all of these 7 patients resulted ST-EMG). In the other 2 patients, we found no MUPs at T1 but they became ST-EMG+ or USG-EMG positive at T2. The remaining 10 ST-EMG- patients had no EMG or US evidence of muscle contraction at T1, but at T2 2 of 10 became ST-EMG+ and 2 had USG-EMG showing MUPs. In the remaining 6 patients still M-US negative at T2, complete denervation was diagnosed. Concerning the 22 patients who were ST-EMG+ at T1, all but one showed increase of MUPs at T2. Conclusions: In this study, we demonstrated the utility of US guidance when performing EMG evaluation in locating isles of muscular contractility in patients who have no detectable MUPs on EMG after nerve trauma. Significance: USG-EMG significantly increases the specificity of needle EMG allowing earlier detection of MUPs
Learning potentiates neurophysiological and behavioral placebo analgesic responses
No full textExpectation and conditioning are supposed to be the two main psychological mechanisms for inducing a placebo response. Here, we further investigate the effects of both expectation, which was induced by verbal suggestion alone, and conditioning at the level of N1 and N2-P2 components of CO(2) laser-evoked potentials (LEPs) and subjective pain reports. Forty-four healthy volunteers were pseudorandomly assigned to one of three experimental groups: Group 1 was tested with verbal suggestion alone, Group 2 was tested with a conditioning procedure, whereby the intensity of painful stimulation was reduced surreptitiously, so as to make the volunteers believe that the treatment was effective, Group 3 was a control group that allowed us to rule out phenomena of sensitization and/or habituation. Pain perception was assessed according to a Numerical Rating Scale (NRS) ranging from 0=no pain sensation to 10=maximum imaginable pain. Both verbal suggestions (Group 1) and conditioning (Group 2) modified the N2-P2 complex, but not the N1 component of LEPs. However, the suggestion-induced LEP changes occurred without subjective perception of pain decrease. Conversely, the N2-P2 amplitude changes that were induced by the conditioning procedure were associated with the subjective perception of pain reduction. Compared to natural history, conditioning produced more robust reductions of LEP amplitudes than verbal suggestions alone. Overall, these findings indicate that prior positive experience plays a key role in maximizing both behavioral and neurophysiological placebo responses, emphasizing that the placebo effect is a learning phenomenon which affects the early central nociceptive processing
Functional changes of the primary somatosensory cortex in patients with unilateral cerebellar lesions
No full textalthough cerebellar lesions do not cause evident sensory deficits, it has been suggested recently that the cerebellum might play a role in sensory acquisition and discrimination. to determine whether the cerebellum influences the early phases of cortical somatosensory processing, we recorded cortical somatosensory evoked potentials after median nerve stimulation in five patients with unilateral cerebellar damage. We also performed a dipolar source analysis of traces by means of brain electrical source analysis. In all patients, the amplitude of the frontal N24 and parietal P24 components, as well as the strength of the corresponding dipolar sources, were significantly smaller after stimulation of the symptomatic side. these neurophysiological findings indicate that the primary somatosensory cortical processing Is altered after contralateral cerebellar damage. they represent the first indication of a possible substrate for the reduction in cerebral blood flow observed in the parietal cortex after cerebellar lesion, furthermore, the present data allow characterization of the functional influence of the cerebellar input to the primary somatosensory cortex as specifically acting over the inhibitory components of somatosensory processing
SOMATOSENSORY EVOKED POTENTIALS AFTER MULTISEGMENTAL LOWER LIMB STIMULATION IN FOCAL LESIONS OF THE LUMBOSACRAL SPINAL CORD
No full textOBJECTIVES:
Recording techniques permit the separate analysis of the response from cauda equina roots and the spinal potential that is probably generated by the activation of dorsal horn cells. To improve the functional assessment of focal lesions of the lumbosacral cord, lower limb somatosensory evoked potentials (SEPs) were measured by multisegmental stimulation.
METHODS:
Common peroneal and tibial nerves SEPs were recorded in 14 patients in whom MRI demonstrated compressive cord damage ranging from T9 to L1 levels. SEPs were recorded in each patient at the lumbar level (cauda equina response), lower thoracic level (spinal response), and from the scalp (cortical response).
RESULTS:
Abnormalities in spinal response occurred in 50% and 70% of tibial and common peroneal nerve SEPs respectively; these findings were well explained by the radiological compression level, involving in most of the patients lumbar rather than sacral myelomeres. The SEPs were often more effective than the clinical examination in showing the actual extension of damage.
CONCLUSIONS:
The recording of spinal SEPs after multisegmental lower limb stimulation proved useful in assessing cord dysfunction and determining the cord levels mainly involved by the compression
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