Event-related potentials recorded from the scalp and nasopharynx. I. N1 and P2

Event-related potentials were recorded from the scalp and nasopharynx during a signal-detection task. These responses were evaluated with respect to the effects of interstimulus interval, intensity, frequency, attention and modality. Our results indicate the existence of at least 4 distinct processes occurring in the 75–150 msec latency range following auditory stimuli. The first process is indexed in the vertex-N1b/temporal-N1a component. This component does not reverse in polarity below the sylvian fissure and is not seen in nasopharyngeal recordings. The location of its generator is not known. Probably there are two or more sources active at this latency. The second process finds reflection in the N1c/PgP120 component. This component is recorded with maximum amplitude on the side contralateral to the ear of delivery. A source in the lateral surface of the temporal lobe is a likely generator. The third process corresponds to Wolpaw and Penry's (1975) Ta positivity. How much of this represents the underside of a vertically oriented dipole and how much a surface positivity is unknown. Finally, during attention, a lateralized processing negativity seems to overlap these components at the scalp, but not at the nasopharynx. The auditory vertex N1 peak is quite distinct from the visual N1 which is more posteriorly recorded on the scalp and which is associated with a definite nasopharyngeal positive wave. The P2 peak is quite similar across auditory and visual modalities. In both modalities it is maximally recorded from the vertex and has no nasopharyngeal concomitant.     

Comparison in man of short latency averaged evoked potentials recorded in thalamic and scalp hand zones of representation

Recordings were performed in the thalamus of 13 patients suffering from either abnormal movements or intractable pain, with the aim of delimiting the region to be destroyed or stimulated in order to diminish the syndrome. In 11 of these patients averaged evoked potentials were recorded simultaneously from the scalp and specific thalamus (VP) hand area levels following median nerve stimulation. These recordings were done during the operation or afterwards when an electrode was left in place for a program of stimulation.The latencies of onsets and peaks on the scalp ‘P15’ were compared with those of the VP wave; a clear correspondence was found. Moreover, when increased stimulation was used, both waves began to develop in parallel. Thus in the contralateral ‘P15’ a component exists due to the field produced by the thalamic response. To explain the presence of an ipsilateral scalp ‘P15’ wave, we propose that a second wave having the same latency and a slightly shorter peak exists on the scalp due to a field produced by a brain-stem response. This double origin of ‘P15’ is also shown by the different changes which the ipsilateral and contralateral waves present during changes in alertness.The scalp ‘N18–N20’ is also composed of at least 2 components. The first peak appears on the scalp with a latency shorter than that of the negativity which develops in the thalamus. The N wave, moreover, increases in latency with rapid stimulus repetition. We propose with others that ‘N18’ is a cortical event reflecting the arrival of the thalamo-cortical volley. The second component, ‘N20,’ has a peak latency closely correlated to that of the thalamic negativity. This component was present alone in ‘N’ when rapid stimulation (> 4/sec) was used, which did not change the thalamic response. It must be a field produced by the thalamic negativity.     

Nasopharyngeal recordings of somatosensory evoked potentials document the medullary origin of the N18 far-field

Because the nasopharyngeal electrode provides non-invasive access to the ventral brain-stem at the medullo-pontine level we used it for recording somatosensory evoked potentials (SEPs) to median nerve stimulation (non-cephalic reference). After the P9 and P11 far-fields, the nasopharyngeal SEPs disclosed a negative-going component which was interpreted as the near-field equivalent of the P14 scalp far-field generated in the caudal part of the medial lemniscus. Nasopharyngeal SEPs also revealed a large N18 with voltage and features strikingly similar to those of the scalp-recorded N18 far-field. These results suggest that N18 is generated in the medulla and not more rostrally in the brain-stem. The use of a nasopharyngeal electrode as reference for topographic brain mapping is discussed. The paper documents the feasibility and relevance of nasopharyngeal recordings for non-invasive analysis of short-latency SEPs.     

Differences in semantic event-related potentials in learning-disabled, normal, and gifted children.

Cortical event-related potentials (ERP) were recorded over FZ, CZ, and PZ scalp sites in 15 learning-disabled (LD), 14 gifted (G), and 13 normal control (N) children of ages 8-12. The common stimulus consisted of nouns presented 80 percent of the time; the target stimulus of animal names presented 20 per cent of the time. ERPs were averaged over subjects from 180 msec pre-stimulus to 900 msec post-stimulus. Principal components analysis was used to determine if there were amplitude differences at different post-stimulus latencies as a function of condition. Differences in ERP's between groups (LD, gifted, and controls), scalp locations, and common versus target stimuli were analyzed by ANOVAs. P3, Late, P2, and N1 components represented by four factors were identified. Significant differences between G and LD and the N and LD groups were found target stimulus at all central locations for the P3 component. Differences were found centrally between G and LD, G and N, and N and LD groups for the P2 component centrally. Other differences were found for the N1 and late components. These differences could be interpreted as a deficit in either attentional mechanisms or information processing for the LD group.     

Neural generators of the somatosensory evoked potentials: Recording from the cuneate nucleus in man and monkeys

The neural generators of the somatosensory evoked potentials (SEPs) elicited by electrical stimulation of the median nerve were studied in man and in rhesus monkeys. Recordings from the cuneate nucleus were compared to the far-field potentials recorded from electrodes placed on the scalp. It was found that the shape of the response from the surface of the human cuneate nucleus to stimulation of the median nerve is similar to that of the response recorded more caudally in the dorsal column, i.e., an initially small positivity followed by a negative wave that is in turn followed by a slow positive wave. The beginning of the negative wave coincides in time with the N14 peak in the SEP recorded from the scalp, and its latency is 13 msec. The response from the cuneate nucleus in the rhesus monkey has a similar shape and its negative peak appears with the same latency as the positive peak in the vertex response that has a latency of 4.5 msec; the peak negativity has a latency of about 6 msec and thus coincides with P6.2 in the vertex recording. Depth recordings from the cuneate nucleus and antidromic stimulation of the dorsal column fibers in the monkey provide evidence that the early components of the response from the surface of the cuneate nucleus are generated by the dorsal column fibers that terminate in the nucleus.The results support the hypothesis that the P14 peak in the human SEP is generated by the termination of the dorsal column fibers and that the cuneate nucleus itself contributes little to the far-field potentials.     

Functionally independent components of early event-related potentials in a visual spatial attention task.

Spatial visual attention modulates the first negative-going deflection in the human averaged event-related potential (ERP) in response to visual target and non-target stimuli (the N1 complex). Here we demonstrate a decomposition of N1 into functionally independent subcomponents with functionally distinct relations to task and stimulus conditions. ERPs were collected from 20 subjects in response to visual target and non-target stimuli presented at five attended and non-attended screen locations. Independent component analysis, a new method for blind source separation, was trained simultaneously on 500 ms grand average responses from all 25 stimulus-attention conditions and decomposed the non-target N1 complexes into five spatially fixed, temporally independent and physiologically plausible components. Activity of an early, laterally symmetrical component pair (N1aR and N1aL) was evoked by the left and right visual field stimuli, respectively. Component N1aR peaked ca. 9 ms earlier than N1aL. Central stimuli evoked both components with the same peak latency difference, producing a bilateral scalp distribution. The amplitudes of these components were no reliably augmented by spatial attention. Stimuli in the right visual field evoked activity in a spatio-temporally overlapping bilateral component (N1b) that peaked at ca. 180 ms and was strongly enhanced by attention. Stimuli presented at unattended locations evoked a fourth component (P2a) peaking near 240 ms. A fifth component (P3f) was evoked only by targets presented in either visual field. The distinct response patterns of these components across the array of stimulus and attention conditions suggest that they reflect activity in functionally independent brain systems involved in processing attended and unattended visuospatial events.     

Scalp-recorded short and middle latency peroneal somatosensory evoked potentials in normals: comparison with peroneal and median nerve SEPs in patients with unilateral hemispheric lesions

Peroneal somatosensory evoked potentials (SEPs) were performed on 23 normal subjects and 9 selected patients with unilateral hemispheric lesions involving somatosensory pathways.Recording obtained from right and left peroneal nerve (PN) stimulations were compared in all subjects, using open and restricted frequency bandpass filters. Restricted filter (100–3000 Hz) and linked ear reference (A1–A2) enhanced the detection of short latency potentials (P1, P2, N1 with mean peak latency of 17.72, 21.07, 24.09) recorded from scalp electrodes over primary sensory cortex regions. Patients with lesions in the parietal cortex and adjacent subcortical areas demonstrated low amplitude and poorly formed short latency peroneal potentials, and absence of components beyond P3 peak with mean latency of 28.06 msec. In these patients, recordings to right and left median nerve (MN) stimulation showed absence or distorted components subsequent to N1 (N18) potential.These observations suggest that components subsequent to P3 potential in response to PN stimulation, and subsequent to N18 potential in response to MN stimulation, are generated in the parietal cortical regions.     

Mismatch negativity of ERP in cross-modal attention

Event-related potentials were measured in 12 healthy youth subjects aged 19-22 using the paradigm "cross-modal and delayed response" which is able to improve unattended purity and to avoid the effect of task target on the deviant components of ERP. The experiment included two conditions: (i) Attend visual modality, ignore auditory modality; (ii) attend auditory modality, ignore visual modality. The stimuli under the two conditions were the same. The difference wave was obtained by subtracting ERPs of the standard stimuli from that of the deviant stim-uli. The present results showed that mismatch negativity (MMN), N2b and P3 components can be produced in the auditory and visual modalities under attention condition. However, only MMN was observed in the two modalities un-der inattention condition. Auditory and visual MMN have some features in common: their largest MMN wave peaks were distributed respectively over their primary sensory projection areas of the scalp under attention condition, but over front     

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.     

Electrophysiological correlates preceding and following the movement onset in man

The cortical potential changes associated with unilateral voluntary self-paced hand movements were detected over the surface of the scalp by the summation method of EEG activity in 20 young subjects. A typical complex wave form of average movement potential (AMP): N1, P1, N2, P2, were discerned in all subjects in our records. This paper presents the results of the topographical distribution of the second potential of the AMP (Premotion Positivity, P1) and the last potential of the AMP (Positive Postmovement onset Potential, P2). Our results indicate a bilateral symmetrical presence of both positive components precentrally and parietally. They also indicate that both these potentials are bilaterally large posterior to the rolandic fissure, and laterality effects in amplitudes occurred only in the second positive wave parietally during right-hand responses in right handers.     

The scalp to earlobe montage as standard in routine SEP recording. Comparison with the non-cephalic reference in patients with lesions of the upper cervical cord

We compared scalp somatosensory evoked potential (SEP) recordings by non-cephalic and earlobe reference in 14 healthy subjects and in 5 patients with lesions of the upper cervical cord. In healthy subjects, the scalp to earlobe montage tended to cancel all far-field potentials preceding the scalp P14. On the contrary, the P14 far-field was more difficult to identify in scalp to non-cephalic recordings, because in 12/14 cases it followed another far-field (P13), which was very close in latency to the P14. In 4 patients, the scalp to non-cephalic traces showed a single positive wave (P13/P14 complex) in the P14 latency range. If this complex had been labelled as P14, the somatosensory dysfunction would have been localised above the foramen magnum. On the other hand, the scalp to earlobe recording allowed correct localisation of the lesion since it showed the `real' and delayed P14 in two patients and no far-field response in the remaining two. Therefore, we propose the use of the scalp to earlobe montage as standard in routine examinations.     

小波能量评价EEG的不同成分对癫痫发作预报的价值

癫痫是一种严重危害人类健康的常见疾病,对癫痫发作进行预报具有重要的重要意义。通过对3例部分性继发全身性发作的癫痫病人在发作最长约30min的8导EEG进行小波分解,将EEG中的棘波、尖波成分与慢波成分分别突出到不同的尺度上,并计算相应尺度上这些成分的能量,考察这些不同成分在发作前的变化趋势。发现在发作前的若干分钟,8导EEG的慢波能量都有显著增大,而与棘波／尖波有关的快波能量基本上没有什么变化趋势,说明EEG慢波成分的增大对部分性继发全身性发作的预报具有重要价值,EEG的“慢波过大”可能是癫痫从发作间状态转变为发作的重要因素。     

Direct recording of somatosensory evoked potentials in the vicinity of the dorsal column nuclei in man: their generator mechanisms and contribution to the scalp far-field potentials

Somatosensory evoked potentials (SEPs) in the vicinity of the dorsal column nuclei in response to electrical stimulation of the median nerve (MN) and posterior tibial nerve (PTN) were studied by analyzing the wave forms, topographical distribution, effects of higher rates of stimulation and correlation with components of the scalp-recorded SEPs. Recordings were done on 4 patients with spasmodic torticollis during neurosurgical operations for microvascular decompression of the eleventh nerve. The dorsal column SEPs to MN stimulation (MN-SEPs) were characterized by a major negative wave (N1; 13 msec in mean latency), preceded by a small positivity (P1) and followed by a large positive wave (P2). Similar wave forms (P1′-N1′-P2′) were obtained with stimulation of PTN (PTN-SEPs), with a mean latency of N1′ being 28 msec. Maximal potentials of MN-SEPs and PTN-SEPs were located in the vicinity of the ipsilateral cuneate and gracile nuclei, respectively, at a level slightly caudal to the nuclei. The latencies of P1 and N1 increased progressively at more rostral cervical cord segments and medulla, but that of P2 did not. A higher rate of stimulation (16 Hz) caused no effects on P1 and N1, while it markedly attenuated the P2 component. These findings suggest that P1 and N1 of MN-SEPs, as well as P1′ and N1′ of PTN-SEPs, are generated by the dorsal column fibers, and P2 and P2′ are possibly of postsynaptic origin in the respective dorsal column nuclei.The peak latency of N1 recorded on the cuneate nucleus was identical with the scalp-recorded far-field potential of P13–14 in all patients, while no scalp components were found which corresponded to P2. These findings support the previous assumption that the scalp-recorded P13–14 is generated by the presynaptic activities of the dorsal column fibers at their terminals in the cuneate nucleus.     

Pain-related and cognitive components of somatosensory evoked potentials following CO2 laser stimulation in man

We recorded cortical potentials evoked by painful CO₂ laser stimulation (pain SEP), employing an oddball paradigm in an effort to demonstrate event-related potentials (ERP) associated with pain. In 12 healthy subjects, frequent (standard) pain stimuli (probability 0.8) were delivered to one side of the dorsum of the left hand while rare (target) pain stimuli (probability 0.2) were delivered to the other side of the same hand. Subjects were instructed to perform either a mental count or button press in response to the target stimuli. Two early components (N2 and P2) of the pain SEP demonstrated a Cz maximal distribution, and showed no difference in latency, amplitude or scalp topography between the oddball conditions or between response tasks. In addition, another positive component (P3) following the P2 was recorded maximally at Pz only in response to the target stimuli with a peak latency of 593 msec for the count task and 560 msec for the button press task. Its scalp topography was the same as that for electric and auditory P3. The longer latency of pain P3 can be explained not only by its slower impulse conduction but also by the effects of task difficulty in the oddball paradigm employing the pain stimulus compared with electric and auditory stimulus paradigms. It is concluded that the P3 for the pain modality is mainly related to a cognitive process and corresponds to the P3 of electric and auditory evoked responses, whereas both N2 and P2 are mainly pain-related components.     

P300 in young and elderly subjects: Auditory frequency and intensity effects

Auditory event-related potentials (ERPs) were assessed in young and elderly subjects when stimulus intensity (40 vs. 60 dB SL) and standard/target tone frequency (250/500 Hz and 1000/2000 Hz) were manipulated to study the effects of these variables on the P3(00) and N1, P2 and N2 components. Auditory thresholds for each stimulus type were obtained, and the stimulus intensity was adjusted to effect perceptually equal intensities across conditions for each subject. Younger subjects demonstrated larger P3 amplitudes and shorter latencies than elderly subjects. The low frequency stimuli produced larger P3 amplitude and shorter latencies than the high frequency stimuli. Low intensity stimuli yielded somewhat smaller P3 amplitudes and longer peak latencies than high intensity stimulus tones. Although additional stimulus intensity and frequency effects were obtained for the N1, P2 and N2 components, these generally differed relatively little with subject age. The findings suggest that auditory stimulus parameters contribute to P3 measures, which are different for young compared to elderly subjects.     

Human gamma oscillations during slow wave sleep

Neocortical local field potentials have shown that gamma oscillations occur spontaneously during slow-wave sleep (SWS). At the macroscopic EEG level in the human brain, no evidences were reported so far. In this study, by using simultaneous scalp and intracranial EEG recordings in 20 epileptic subjects, we examined gamma oscillations in cerebral cortex during SWS. We report that gamma oscillations in low (30-50 Hz) and high (60-120 Hz) frequency bands recurrently emerged in all investigated regions and their amplitudes coincided with specific phases of the cortical slow wave. In most of the cases, multiple oscillatory bursts in different frequency bands from 30 to 120 Hz were correlated with positive peaks of scalp slow waves ("IN-phase" pattern), confirming previous animal findings. In addition, we report another gamma pattern that appears preferentially during the negative phase of the slow wave ("ANTI-phase" pattern). This new pattern presented dominant peaks in the high gamma range and was preferentially expressed in the temporal cortex. Finally, we found that the spatial coherence between cortical sites exhibiting gamma activities was local and fell off quickly when computed between distant sites. Overall, these results provide the first human evidences that gamma oscillations can be observed in macroscopic EEG recordings during sleep. They support the concept that these high-frequency activities might be associated with phasic increases of neural activity during slow oscillations. Such patterned activity in the sleeping brain could play a role in off-line processing of cortical networks.     

Scalp topography and dipolar source modelling of potentials evoked by CO2 laser stimulation of the hand

CO₂ laser evoked potentials to hand stimulation recorded using a scalp 19-channel montage in 11 normal subjects consistently showed early N1/P1 dipolar field distribution peaking at a mean latency of 159 ms. The N1 negativity was distributed in the temporoparietal region contralateral to stimulation and the P1 positivity in the frontal region. The N1/P1 response was followed by 3 distinct components: (1) N2a reaching its maximal amplitude at the vertex and ipsilaterally to the stimulated hand, (2) N2b mostly distributed in the frontal region, and (3) P2 with a mid-central topography. Brain electrical source analysis showed that this sequence was explained, with a residual variance below 5%, by a model including two dipoles in the upper bank of the Sylvian fissure of each hemisphere, a frontal dipole close to the midline, and two anterior medial temporal dipoles, thus suggesting a sequential activation of the two second somatosensory areas, anterior cingulate gyrus and the amygdalar nuclei or the hippocampal formations, respectively. This model fitted well with the scalp field topography of grand average responses to stimulation of left and right hand obtained across all subjects as well as when applied to individual data. Our findings suggest that the second somatosensory area contralateral to the stimulation is the first involved in the building of pain-related responses, followed by ipsilateral second somatosensory area and limbic areas receiving noxious inputs from the periphery.     

High Resolution Topography of Age-Related Changes in Non-Rapid Eye Movement Sleep Electroencephalography

Sleeping brain activity reflects brain anatomy and physiology. The aim of this study was to use high density (256 channel) electroencephalography (EEG) during sleep to characterize topographic changes in sleep EEG power across normal aging, with high spatial resolution. Sleep was evaluated in 92 healthy adults aged 18–65 years old using full polysomnography and high density EEG. After artifact removal, spectral power density was calculated for standard frequency bands for all channels, averaged across the NREM periods of the first 3 sleep cycles. To quantify topographic changes with age, maps were generated of the Pearson’s coefficient of the correlation between power and age at each electrode. Significant correlations were determined by statistical non-parametric mapping. Absolute slow wave power declined significantly with increasing age across the entire scalp, whereas declines in theta and sigma power were significant only in frontal regions. Power in fast spindle frequencies declined significantly with increasing age frontally, whereas absolute power of slow spindle frequencies showed no significant change with age. When EEG power was normalized across the scalp, a left centro-parietal region showed significantly less age-related decline in power than the rest of the scalp. This partial preservation was particularly significant in the slow wave and sigma bands. The effect of age on sleep EEG varies substantially by region and frequency band. This non-uniformity should inform the design of future investigations of aging and sleep. This study provides normative data on the effect of age on sleep EEG topography, and provides a basis from which to explore the mechanisms of normal aging as well as neurodegenerative disorders for which age is a risk factor.     

Origin and distribution of P13 and P14 far-field potentials after median nerve stimulation. Scalp,nasopharyngeal and neck recording in healthy subjects and in patients with cervical and cervico-medullary lesions

We studied median nerve SEPs in 10 healthy subjects, by means of simultaneous recording over the scalp, around the neck and near the ventral surface of the medulla using a nasopharyngeal (NP) electrode. This recording technique enabled us to clearly differentiate P13 and P14 potentials. The former was always found in NP records, while the latter was more evident in scalp traces. The same technique was used to study 9 patients with various lesions of the cervical cord or cervico-medullary junction. Patients with high cervical lesions demonstrated abnormalities of both P13 and P14 potentials, while patients with lesions of the cervico-medullary junction demonstrated a clear dissociation between normal P13 in scalp and NP traces, and abnormal scalp P14. Patients with lower cervical lesions, selectively involving the central grey matter, showed normal P13 and P14 potentials, in spite of abnormal N13 cervical responses. Our findings strongly suggest that both scalp and NP P13 have the same generators in higher segments of the cervical cord, and that NP more than scalp records are effective in analyzing the P13 response. We suggest that the selective recording of the P13 potential could be useful in the assessment of focal lesions of the higher cervical cord or of the cervico-medullary junction.     

Auditory event-related potentials in adult patients with Gilles de la Tourette's syndrome in the oddball paradigm

In 20 Tourette patients and 20 control subjects auditory event-related potentials evoked in an oddball paradigm were studied in 2 conditions: a non-motor condition (NMC) in which subjects had to attend tones, and a motor condition (MC) in which they had to press a microswitch to deviant tones. In the NMC patients had a reduced P2 in response to the standards. The deviant-standard subtraction wave forms of the NMC showed a discernible MMN-P165-N2b-P3 complex in the controls, whereas in the patients only the P3 was well developed. In the MC patients had a reduced N1 to the standards. Both groups showed in the deviant-standard subtraction wave forms a clear MMN-P165-N2b-P3 complex, N2b being reduced in the patients. In the patients the P2 amplitude and latency to the standards and in the controls the N2b amplitude in the deviant-standard subtraction wave form were larger in the MC than in the NMC. Both groups also showed a larger P3 and a larger parietal slow positive wave in the MC than in the NMC.The results are discussed in relation to behavioural and neuropsychological disturbances found in Gilles de la Tourette's syndrome.