High frequency oscillations in early cortical somatosensory evoked potentials

OBJECTIVE: To evaluate the characteristics of high frequency (HF) components of the early cortical somatosensory evoked potentials (SEPs). METHODS: We recorded 8-channel SEPs from the frontal and left centro-parietal scalp after right median nerve stimulation with a wide band-pass (0.5-2000 Hz) and digitized at 40 kHz sampling rate in 12 healthy subjects. HF components were analyzed after digital band-pass filtering (300-1000 Hz). The power spectrum was obtained by a maximum entropy method. RESULTS: HF oscillations (maximum power at 600-800 Hz) consisting of 5 to 8 peaks were discriminated from the preceding P14 far-field in all cases and their phases were reversed between the frontal and contralateral parietal regions. In addition, in subjects with a high amplitude central P22 potential in original wide-band recordings, a single HF oscillation with a maximum at the central region was present. Furthermore, this component showed no phase reversal over the centro-parietal area. CONCLUSION: We therefore conclude that HF oscillations are superimposed not only on the tangential N20-P20 but on the radial P22 potential, and are generated from both tangential (area 3b) and radial (area 1) current sources.     

One-week test-retest reliability of spinal and cortical somatosensory evoked potentials by tibial nerve stimulation

Spinal (Th12) and cortical somatosensory evoked potentials by right and left posterior tibial nerve stimulation at the ankle were performed in 20 healthy volunteers (10 females and 10 males) aged 23-50 years. The procedure was repeated after one week to assess the reliability of the parameters and to establish upper normal variability limits. Reliability was measured by the intraclass correlation coefficient and was excellent for all absolute latencies and at least good for amplitudes and for the spinal-cortical conduction time. Upper variability limits were calculated using a method based on the within-subject mean square, which can be also applied in the case of more than two repetitions.     

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

Previous studies have shown that characteristics of posttraumatic temporomandibular disorders (pTMD) differ considerably from those of nontraumatic or idiopathic temporomandibular disorders (iTMD). Both the rate of recovery and the amount of treatment required appear to be different for both groups. In this blinded study, 14 patients with iTMD and 13 patients with pTMD were examined. Patients submitted to a variety of reaction-time tests and neuropsychologic assessments to test their ability to cope with simple and more complex tasks with and without a variety of cognitive interferences. Clinical examination was used to assess signs of TMD. Eleven of the subjects (six iTMD, five pTMD) consented to a second phase of the investigation, whereby the patients were studied with single-photon emission computerized tomography (SPECT) using 99mTc-hexamethylpropyleneamineoxime (HMPAO). For simple and complex reaction-time tests, the pTMD group was significantly slower than the iTMD group (P < .05 to P < .001). Other neuropsychologic assessment tools such as the Consonant Trigram Test and the California Verbal Learning Test indicated that pTMD patients were more affected by both proactive and retroactive interferences and were more likely to perseverate on a single thought. In clinical examination, pTMD patients demonstrated greater reaction to muscle palpation than did iTMD patients (P < .05). The SPECT results suggested that there were mild differences between the two populations, and further ther studies are required to confirm this finding. The results lend support to the concept that there are differences between pTMD and iTMD populations. It is suggested that although patients with pTMD may have some similarities to those with iTMD, the former population may benefit from being handled somewhat differently and should be assessed and treated using a more broad, multidisciplinary treatment paradigm. These results must be confirmed in studies of larger populations.     

Multimodality (auditory, visual and somatosensory) evoked potentials in the sand rat, Psammomys obesus

Multimodality (auditory, visual and somatosensory) evoked potentials were recorded in the sand rat (Psammomys obesus) and compared to those obtained in albino rats, under almost identical conditions. Clear responses could be obtained from each species. The evoked potentials from the sand rat were qualitatively similar in waveform, latency and amplitude to those recorded in albino rats. Since there is a strain of sand rat which develops diabetes when on standard chow and since the sand rat has adapted to the desert environment, further study should involve evoked potential recordings in sand rats during hyper- and hypothermia and in diabetic sand rats.     

The effects of cortical stimulation, anesthesia and recording site on somatosensory evoked potentials in the rat

The purpose of this study was to standardize the method of spinal cord monitoring with evoked potentials in the rat. Seventeen male Wistar rats were anesthetized with alpha-chloralose and urethane. Somatosensory evoked potential (SEP) and cerebellar evoked potential (CEP) following sciatic nerve stimulation were mapped at different time points after induction of anesthesia. SEP peaks at latencies of 13-18 ms (P13, N18) were localized to an extremely small area over the sensory cortex. In contrasts, a negative peak of the SEP at 11 ms (N11) and the CEP were widely distributed over the cerebral or cerebellar surface. Anesthesia significantly influenced the cortical components of the SEP. In 10 rats, MEP or posterior fossa evoked potential (PFEP) following stimulation of the sensorimotor or cerebellar cortices respectively, were recorded at T9. Stimulation of different points produced little change on the waveforms of the MEP or PFEP. Successive recordings of MEP and SEP revealed that the P13-N18 complex of the SEP was markedly suppressed after MEP recordings were made. In conclusion, this study identified several factors which alter SEP waveforms in the rat including location of recording, anesthesia and sequence with respect to MEP recording. MEP by stimulation of the same sensory cortex as SEP recordings should not be used for concurrent monitoring, since cortical stimulation will change the waveforms of the SEP.     

P300 potentials evoked by visual stimulation

INTRODUCTION: The P300 potential is a long-latency endogenous component of the event-related potentials to low-probability target stimuli. The same stimulus delivered without cognitive process does not provoke endogenous components; moreover event-related potentials are not dependent of the sensory pathways used for stimulating the subjects. P300 potential is a biological parameter used in scientific investigations in Clinical Neurophysiology, Neurology, Psychophysiology and Psychiatry. The most frequent methodology for obtaining P300 event-related potential is based on the 'odd-ball' paradigm using auditory stimulation. In this revision we analyzed the difficulties of this methodology and we propose to use the visual stimulation in order to obtain well defined P300 potential, based on a better signal to noise ratio and minor overlapping of exogenous and endogenous components of the evoked potentials. The improvement in the quality of the results obtained when comparing with auditory stimulation, it is supposed to facilitate that the use of P300 potential overflows the field of the investigation and permits their extensive use in the clinical practice.     

Responses of neurons of the first and second somatosensory zones of the cortex to peripheral, thalamic and cortical stimulation

Experiments were performed on cats immobilized with d-tubocurarine or myorelaxin. Neuronal responses were studied in the first somatosensory cortex (SI) to the second somatosensory cortex (SII), ventroposterior nucleus (VP) and contralateral forepaw stimulation. Besides, neuronal responses in SII to SI, VP and contralateral forepaw stimulation were also studied. It was shown that in SII the percentage of neurons excited by afferent volley with two or more synaptic change-overs in the cerebral cortex was larger than in SI. Neurons of SI and SII responded to cortical stimulation ortho- and antidromically, thus confirming the existence of bilateral cortico-cortical connections. Both in SI and SII, PSPs to cortical stimulation were similar in character to PSPs in the same neurons to VP stimulation. In 50.0% of SI neurons and 37.1 of SII neurons the difference in latencies of orthodromic spike potentials to VP and cortical stimulation was less than 1.0 ms. In 19.6% of SI neurons and 41.4% of SII neurons the latency of the response to cortical stimulation was 1.6-4.7 ms shorter than that of the response in the same neuron to VP stimulation. It is supposed that impulses from SI participate significantly in afferent activation of SII neurons.     

Responses of visual, somatosensory, and auditory neurones in the golden hamster's superior colliculus

1. The response characteristics of visual, somatosensory, and auditory neurones in the golden hamster's superior colliculus were investigated.2. As has been noted for other mammalian species, a distinct difference between the functional organizations of the superficial and deeper layers of the superior colliculus was observed.3. Neurones in the superficial layers were exclusively visual, with small receptive-fields, and generally did not show response decrements with repeated stimulation. The sizes of the receptive-fields did not vary appreciably as a function of retinal eccentricity.4. In the deeper layers, visual receptive-fields were large, or could not be accurately delimited, and response habituation was often evident. In addition, many cells in the deeper layers of the colliculus responded only to somatosensory stimuli. Far fewer cells, which appeared to be confined to the caudal portions of the colliculus, responded to auditory stimuli. Polymodal cells were also encountered.5. Selectivity to opposing directions of movement was tested for ninety-four visual cells. Using a ;null' criterion, 27.7% of these cells were judged to be directionally selective. A distribution of the preferred directions of these cells showed a significant preference for movement with an upper-nasal component. With a statistical criterion, 60.6% of these cells were considered to show a significant asymmetry in responding to movement in opposing directions.6. Directional selectivity was also tested for ninety-two cells following acute, unilateral, lesions of the visual cortex. For the eighty cells recorded, homolateral to the ablated cortex, 27.5% were judged as directionally selective using the statistical criterion, while 12.5% were selective with the ;null' criterion. Of the twelve cells isolated in the colliculus, contralateral to the lesions, seven were judged as directionally selective with the statistical, and three with the ;null' criterion.7. The effects of visual cortical lesions upon directional selectivity appeared to be confined to cells in the superficial layers of the colliculus. It was suggested that directional selectivity of many cells in the superficial layers of the tectum of the hamster is organized cortically.8. A clear spatial correspondence was observed for the receptive-fields of visual, somatosensory, and auditory neurones.9. As has been suggested for other species, the hamster's superior colliculus appears to play an important role in orienting the animal toward visual, somatosensory, and auditory stimuli.     

Nonlinear analysis of visual evoked potentials elicited by color stimulation

The relationship between chromatically modulated stimuli and visual evoked potentials (VEPs) was considered. VEPs of normal subjects elicited by chromatically modulated stimuli were measured under several color adaptations, and their binary kernels were estimated. Up to the second-order, binary kernels obtained from VEPs were so characteristic that the VEP-chromatic modulation system showed second-order nonlinearity. First-order binary kernels depended on the color of the stimulus and adaptation, whereas second-order kernels showed almost no difference. This result indicates that the waveforms of first-order binary kernels reflect perceived color (hue). This supports the suggestion that kernels of VEPs include color responses, and could be used as a probe with which to examine the color visual system.     

The cortical representation of somatosensory evoked potentials of the phrenic nerve

Parasitic third-stage larvae of the sheep abomasal nematode Haemonchus contortus develop and molt in vitro to the fourth stage (L4) in 48-72 hr, at which time they begin feeding. Coincident with the third molt, larvae begin to secrete significant amounts of protein into culture fluids, including a zinc metalloproteinase. This culture-derived zinc metalloproteinase differs from a previously described metalloproteinase from infective third-stage larvae (L3[2M]), which mediates the ecdysis process. These differences include time of expression, molecular mass, and substrate specificity. The purified proteinase, from cultures of L4, has a molecular weight of approximately 46 kDa, functions as an endopeptidase, and digests several native proteins of host origin including fibrinogen and fibronectin.     

Effects of postinatal stress on visual and auditory evoked potentials

Maturation of visual and auditory evoked potentials (mainly the P3 wave) of 10 controls and of 10 infants exposed to recurrent postnatal stressful events (crying spells lacking organic basis) have been compared. The sourse of maturation of P3 waves may serve as indicator of growth and nature of cognitive processes (including perception). Recurrent srying spells seemed to delay appearance of visual P3 wave, prolonged its latency, and delayed appearance of visible differences in the shape of visual P3 wave upon exposure of the subjects to different visual experiences. A compensatory increase of maturation of the auditory P3 wave appeared. The results suggest that recurrent exposure to stressful events during the early postnatal period may delay the ability of the memory banks of auditory engrams with unusual contents (including memory traces of stresses), a potential basis for future auditory hallucinations.     

Electroacupuncture and laser stimulation treatment: evaluated by somatosensory evoked potential in conscious rabbits

Tooth pulp generated somatosensory evoked potential (TPSEP) recordings were used to evaluate analgesic responses to Electroacupuncture (EA) and laser stimulation (Ls) treatments in Dutch-hybrid male rabbits. TPSEP recorded 100 trials at 30 minute intervals for 120 minutes. Three groups, EA, Ls and control were analyzed. The EA group received intermittent 4 Hz, intensity 2-10 volts electro-stimulation for twenty minutes. In the Ls group, a gallium aluminium arsenide (GaAlAs) laser diode with wavelength 780 nm, switched pulse frequency 9720 Hz, energy density 0.6 J/point was employed. Both EA and Ls groups received stimulation of the points Ho-ku (LI-4) and Tzu-San-Li (ST-36). Results showed tooth pulp generated noxious stimulation produced a consistent late near-field SEP waveform, which is similar to those recorded in humans and correlated to the sensation of pains, thus confirming that tooth pulp stimulation is a reliable dolormetric index. EA and Ls TPSEP recordings showed decreased peak-wave amplitude in late near-field components. These decreases correlate to analgesia. This technique provides an objective and reliable dolormetric index.     

Restoration of sensory gating of auditory evoked response by nicotine in fimbria-fornix lesioned rats

Recordings of auditory evoked potentials in the CA3 region of the hippocampus reveal a decrement in the N40 wave after the presentation of the second of closely paired auditory stimuli (interstimulus interval of 500 ms), a phenomenon known as sensory gating. Previous experiments have suggested the involvement of nicotinic cholinergic systems in auditory sensory processing. The present study examined the effects of lesioning the fimbria-fornix on auditory sensory processing in the hippocampus. Fimbria-fornix lesions resulted in a failure to decrement the N40 wave in the auditory evoked response to the second tone. When nicotine was administered to rats with fimbria-fornix lesions the drug was able to reinstate the normal suppression of the second auditory evoked response. These data support the involvement of nicotinic cholinergic afferents in auditory sensory modulation in the hippocampus.     

Anatomic organization of evoked potentials in rat parietotemporal cortex: somatosensory and auditory responses

1. Two 8 x 8-channel microelectrode arrays were used to map epicortical field potentials from a 3.5 x 3.5-mm2 area in homologous regions of right and left parietotemporal cortex of four rats. Potentials were evoked with bilaterally presented click stimuli and with bilateral tactile stimulation of the 25 major vibrissae. The spatial distribution of temporal components of the somatosensory evoked potential (SEP) and auditory evoked potential (AEP) complex were compared directly with cytochrome oxidase-stained sections of the recorded region. 2. Epicortical responses in both hemispheres to bilateral vibrissal stimuli consisted of a biphasic sharp wave (P1a-N1) constrained to the vibrissa/barrel granular region of primary somatosensory cortex (SmI). A slightly later sharp positive wave (P1b) was localized to secondary somatosensory cortex (SmII) and to perigranular cortex medial to the vibrissa/barrel field. The SEP complex ended with a biphasic slow wave (P2-N2). The P2 was centered on SmI and spread to dysgranular lateral cortex, caudal to but excluding SmII. The N2 was centered on SmII and spread to dysgranular cortex caudal to but excluding SmI. 3. The anatomic organization of the AEP in many ways approximated that of the SEP in the same animals. The timing and morphology of the AEP were nearly identical to the SEP. The AEP consisted of a P1a-N1 sharp wave constrained to the estimated region of primary auditory cortex (AI) in the lateral parietotemporal region, a later P1b localized to secondary auditory cortex (AII), and subsequent slow waves (P2 and N2) that were centered on AI and AII, respectively, and spread to dysgranular regions overlapping the distributions of the P2 and N2 of the SEP complex. 4. These data suggest that the basic neural generators for the SEP and AEP in parietotemporal cortex are quite similar, and provide evidence for the functional anatomy of each temporal component of the sensory evoked potential complex. It is concluded that the early fast waves of the SEP and AEP are modality specific and may represent the parallel activation of primary and secondary sensory cortex through established parallel afferent projections from lateral and medial thalamic nuclei. The later slow waves of the SEP and AEP appear to selectively involve primary and secondary sensory cortex but are more widely distributed, possibly reflecting a less modality-specific level of information processing in dysgranular cortex.     

Immediate and simultaneous sensory reorganization at cortical and subcortical levels of the somatosensory system

The occurrence of cortical plasticity during adulthood has been demonstrated using many experimental paradigms. Whether this phenomenon is generated exclusively by changes in intrinsic cortical circuitry, or whether it involves concomitant cortical and subcortical reorganization, remains controversial. Here, we addressed this issue by simultaneously recording the extracellular activity of up to 135 neurons in the primary somatosensory cortex, ventral posterior medial nucleus of the thalamus, and trigeminal brainstem complex of adult rats, before and after a reversible sensory deactivation was produced by subcutaneous injections of lidocaine. Following the onset of the deactivation, immediate and simultaneous sensory reorganization was observed at all levels of the somatosensory system. No statistical difference was observed when the overall spatial extent of the cortical (9.1 +/- 1.2 whiskers, mean +/- SE) and the thalamic (6.1 +/- 1.6 whiskers) reorganization was compared. Likewise, no significant difference was found in the percentage of cortical (71.1 +/- 5.2%) and thalamic (66. 4 +/- 10.7%) neurons exhibiting unmasked sensory responses. Although unmasked cortical responses occurred at significantly higher latencies (19.6 +/- 0.3 ms, mean +/- SE) than thalamic responses (13. 1 +/- 0.6 ms), variations in neuronal latency induced by the sensory deafferentation occurred as often in the thalamus as in the cortex. These data clearly demonstrate that peripheral sensory deafferentation triggers a system-wide reorganization, and strongly suggest that the spatiotemporal attributes of cortical plasticity are paralleled by subcortical reorganization.     

The stabilization of cortical evoked potentials during selective visual attention

The event-related potentials (ERP) to lateralized moving visual stimuli (administered to the right and to the left of the subject) were recorded in six derivations in eight healthy subjects during passive viewing and selective attention to one of the stimuli (the relevant one), which demanded precise and rapid motor reaction. It was shown that during the selective voluntary attention correlation between successive ERP was substantially higher than that under conditions of passive (involuntary) attention, i.e. the ERP in the occipital, parietal, and frontal derivations during voluntary attention were more stable. The less was correlation between the ERP at passive viewing, the greater was the difference between the values of correlation coefficients during selective and passive attention. It is suggested that role of selected attention consists in stabilization of activity of the cortical structures, which are involved in solving the behavioral task.     

Modulation of voltage-activated calcium currents by mechanical stimulation in rat sensory neurons

We examined the effects of mechanical stress, induced by a stream of bath solution, on evoked action potentials, electrical excitability, and Ca2+ currents in rat dorsal root ganglion neurons in culture with the use of the whole cell patch-clamp technique. Action-potential duration was altered reversibly by flow in 39% of the 51 neurons tested, but membrane potential and excitability were unaffected. The flow-induced increases and decreases in action-potential duration were consistent with the different effects of flow on two types of Ca2+ channel, determined by voltage-clamp recordings of Ba2+ currents. Current through omega-conotoxin-sensitive (N-type) Ca2+ channels increased by an estimated 74% with flow, corresponding to 23% increase in the total high voltage-activated current, whereas current through low-threshold voltage-activated (T-type) channels decreased by 14%. We conclude that modulation of voltage-activated Ca2+ currents constitutes a route by which mechanical events can regulate Ca2+ influx in sensory neurons.