Laminar analysis of the origin of the various components of evoked potentials in slices of rat sensorimotor cortex

In slices of rat sensorimotor cortex, extracellular field potentials evoked by electrical stimulation of the white matter were recorded at various cortical depths. In order to determine the nature of the various components, experiments were performed in 3 situations: in a control perfusion medium, in a solution in which calcium ions have been replaced by magnesium ions to block synaptic transmission, and in cortices in which the pyramidal neurons of layer V had been previously induced to degenerate.In the control situation, the response at or near the surface was a positive-negative wave. From a depth of about 150 μm downwards, the evoked response consisted usually of 6 successive components, 3 positive-going, P11, P3 and P6 and 3 negative-going, N2, N4 and N5. P1 and N4 were apparent in superficial layers only. The amplitude of the remaining waves variable in the cortex but all diminished near the white matter.The early part of the surface positive wave arises from a non-synaptic activation of superficial elements, probably apical dendrites. The late part of the surface positive wave and the negative wave are due to the synaptic activation of neurons located probably in layer III.The large negative wave N2 represents principally the antidromic activation of cell bodies and possibly of proximal dendrites of neurons situated in layers III, IV and V, through the compound action potentials of afferent and efferent fibers may contribute to a reduced part to its generation.The late components N4 to P6 are post-synaptic responses. The negative component N5, the amplitude of which is largest in layers III and IV, represents excitatory responses of neurons located at various depths in the cortex. The nature of the positive component P6 is less clear, although the underlying mechanism might be inhibitory synaptic potentials.     

Hemispheric asymetry of the evoked electrical activity of the cerebral cortex to letter and non-verbal stimuli]

Average evoked potentials (AEP) were recorded in practically healthy subjects to "meaningless" figures and letters, presented to different halves of the visual field. Analysis of the amplitudes of AEP late components to verbal and non-verbal stimuli reveals hemispheric asymmetry. A higher amplitude of the late positive evoked response (P300) to a "direct" stimulation both by verbal and non-verbal stimuli (in the contralateral field of vision) is recorded in the left hemisphere than in the right one. Similar stimulation of the right hemisphere does not reveal sucha difference. In the left hemisphere the P300 wave is of a clearly greater amplitude to a "direct" stimulation (contralateral visual field) than to an "indirect" one (ipsilateral visual field), regardless of the nature of the stimulus. No such difference is observed in the right hemisphere. The magnitude of the late negative wave (component N200) to non-verbal stimuli is greater in the right hemisphere both in response to "direct" and "indirect" stimulations. No intrahemispheric difference has been found in the amplitude of late evoked responses of the cerebral cortex to verbal and non-verbal stimuli.     

Auditory brain stem responses to monoaural stimulation registered in symmetrical areas of cat's brain cortex

In five anaesthetized cats (Nembutal 35 mg/kg) with 14 chronically implanted recording epidural electrodes the auditory brain stem responses (ABR) to monoaural stimulation (click) in symmetrical areas of the brain cortex were recorded. Each ABR to acoustic stimulus of sufficient intensity is formed by a complex of alternating five positive (P1-P5) and four negative (N1-N4) peaks; two further small peaks often follow on this complex. The amplitude of ABR peaks N3, P4, N4 and P5 to monoaural stimulation in symmetrical areas of cat's cortex was always higher in records from the hemisphere contralateral to the stimulated ear than in records from the ipsilateral one. The amplitude of P3 ABR peak behaved to the contrary--it was higher on ipsilateral hemisphere. On the other hand the amplitude of ABR peaks P1, N1, P2 and N2 to monoaural stimulation in symmetrical areas of the brain cortex showed no degree of lateralization in our experimental animals. The present findings support indirectly the presumption that each peak of the ABR is generated by a particular acoustic brain stem structure.     

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.     

Early components of transcallosal responses in acute ischaemia of the corpus callosum

In 8 baboons maintained under propofol anaesthesia, transcallosal evoked responses were recorded from the primary motor cortex following electrical stimulation of the contralateral homotopic cortical surface. The corpus callosum was made ischaemic by transorbital occlusion of the common anterior cerebral artery; blood flow (measured by the hydrogen clearance method) in the stimulating and recording regions was not significantly affected by this procedure. The transcallosal responses from the normally perfused brain contained early positive (P1) and negative (N1) components. As stepwise ischaemia was produced in the corpus callosum, the amplitude of P1 initially increased up to 150% of control and the peak latency of P1 was significantly prolonged. At flows below 8 ml/100/g/min the amplitude rapidly decreased. Wave form changes and flow threshold of N1 were similar to those of P1. These results suggest that measurement of early transcallosal responses could be useful clinically as monitors of the ischaemic level in anterior cerebral artery territory.     

Dose-dependent tazepam modulation of amplitude-temporal characteristics of thalamocortical responses and the constant potential of the sensorimotor cortex in rabbits at eye opening

The pronounced benzodiazepine (antiphobic) modulation of the amplitude-temporal parameters of different components of the thalamocortical responses (TCR) of the sensorimotor cortex is observed in rabbits in their early postnatal ontogeny. This modulation is of a dose-dependent character and is registered not after the injection of tazepam in a concentration of the "therapeutic tranquilizing window" but also in the psychotoxic plasma range. A gradual increase in blood tazepam concentration in a young rabbit pup is accompanied by the wave-like and differential decrease in the amplitude of the second and third positive (P2 and P3) and third negative (N3) TCR components, while the second negative (N2) and fourth positive (P4) components tend to a wave-like increase. The dose-dependent dynamics of tazepam modulation of the P2, P3, and N3 latencies is characterized by a wave-like and differential increase. The latency of P4 decreases slightly and that of the N2 increases with a low degree of significance. The selective dynamics of benzodiazepine modulation appears to be related with peculiarities of the electrogenesis of each of the components. The dose-dependent modulation of the level of cortical DC potential is of the same character as the respective amplitude changes in P2, P3, and N3, but its fluctiatuons are more pronounced.     

VEP's in normal full-term and premature neonates: longitudinal versus cross-sectional data

VEPs have been shown to change with CNS maturation in children, yet few studies had documented maturational changes in the premature infant. Using LED goggles, VEPs were studied in 75 neurologically normal infants of 22–42 weeks gestational age (GA) within the first 3 days of life. Twenty of these (22–32 weeks GA) were also followed longitudinally.The 22–23 week GA neonates had no identifiable waves. In all infants >24 weeks a large negative wave is seen with a latency around 300 msec (N300). After 27 weeks GA a late positive wave was present (P400), but with more variable latency and morphology. Between 30 and 35 weeks GA a small positive wave (P200) was seen in over one-third of the neonates; this component was present in all infants > 36 weeks GA.The consistency of the N300 across the ages studied suggests that it might arise from the basilar dendrites in the visual cortex, which are well developed by 24 weeks GA and undergo relatively little further development between 24 weeks and term. The P200 is suggested to arise from the apical dendrites which develop in the last trimester, explaining the emergence of P200 after 30 weeks GA.The infants followed longitudinally showed the same components, emerging in the same order, but with more rapid development (particularly of P200) compared to the cross-sectional studies. These data suggest that there are differences in the maturation of the visual system in the extrauterine versus intrauterine environment.     

Comparison of human transcallosal responses evoked by magnetic coil and electrical stimulation

Human transcallosal responses (TCRs) were elicited by focal magnetic oil (MC) stimulation of homologous sites in contralateral frontal cortex and compared with those to focal anodic stimulation. With MC stimulation, the TCR consisted of an initially positive wave with an onset latency of 8.8–12.2 msec, a duration of 7–15 msec, and an amplitude which reached up to 20 μV, sometimes followed by a broad low amplitude negative wave. With anodic stimulation, a similar response was obtained in which the positive wave was similar in latency and maximum amplitude, but had a greater duration. With anodic stimulation, not only was the TCR threshold below that for contralateral movement, but it reached substantial size at intensities below motor threshold. With MC stimulation, contralateral arm movement and scalp corticomotor potentials were observed when the MC was displaced posteriorly towards the central sulcus. Unlike with anodic stimulation, the MC evoked TCR was usually not preceded by a prominent EMG potential from temporalis muscle and was not associated with subject discomfort.The TCR provides unique information concerning the functional integrity of callosal projection neurons, their axons and transsynaptic processes in recipient cortex. This information may prove useful in the evaluation of intrinsic cerebral mechanisms and in establishing cortical viability.     

Interactions of human cord dorsum potential.

The slow positive wave (P2 wave) of the evoked spinal electrogram was recorded from the posterior epidural space in wakeful man, and studied by applying several modes of peripheral nerve stimulation. With graded stimulation the P2 wave amplitude rapidly reached the maximum at weaker stimulation than that required for the initially positive spikes (P1) and the preceding negative (N1) wave. The "second" component of the P2 appeared during stronger stimulation or during excitemenpt of the subjects. With prolonged repetitive stimulation the P2 wave increased its duration with several summits on the decaying phase. Two interactions were observed between the P2 waves produced by conditioning and testing stimulations in the same or different nerves: inhibition or occlusion by strong stimulation and faciliation by weak stimulation. Thus, the characteristic of the P2 wave in man was similar in part to that of the positive wave observed in decerebrate animals, and differnt in other ways presumably due to influences from supraspinal structures or species differences.     

Evoked potential correlates of early conditioning in the rat ontogeny.

In rats aged 2-8 weeks cortical EP to CS (20 flash - tone combinations, 0,9/sec, reinforced since the 10th application by electric shocks to the hind leg) were studied within different kinds of behavioral responses during avoidance learning and extinguishing. In contrast to our results in freely moving rats no developmental trend was found in this kind of avoidance (lifting of the hind leg). Average EP within reinforced trials (with escape or no reactions) differed in isolated application of CS from those when both CS and US were acting together. In younger animals the EP to CS combined with US were characterized by an evident late negative wave which shifted later (5-6 weeks toward the early negative complex. The EP changes in the auditory cortex were more pronounced, whereas visual EP with CS-US combination were rather decreased. In the youngest animals (2 weeks) the auditory EP within trials with avoidance were characterized by a distinct short latency deflection of the first positive wave, whereas in EP to extinguished CS the second deflection of the first positive wave prevailed. Also in these phenomena, the typical changes were clearly revealed in the auditory cortex. At later developmental stages (starting the 3rd, more prominently the 4th and 5th week) the wave following primary positive - negative complex was shifted toward the negativity if the animal responded by an avoidance; on the contrary an ample positive, often a double-peak wave arose if the response was extinguished. The stimulus and reaction dependence in the cortical EP showed the role of not yet fully mature cerebral cortex in avoidance learning. Both, fast as well as with some delay running processes participated in the observed phenomena during the ontogenetical development.     

Reflection of the Emotional Significance of Visual Stimuli in the Characteristics of Evoked EEG Potentials

Healthy subjects (n = 88) were asked to passively visualize positive and passive emotiogenic visual stimuli and also stimuli with a neutral emotional content. Images of the International Affective Picture System (IAPS) were used. Amplitude/time characteristics of the components of evoked EEG potentials (EPs), P1, N1, P2, N2, and P3 and topographic distribution of the latter components were analyzed. The latencies, amplitudes, and topography of the EP waves induced by presentation of positive and negative stimuli were found to be different from the respective values for the EPs induced by neutral stimuli. The level and pattern of these differences typical of different EP components were dissimilar and depended on the sign of the emotions. Specificities related to the valency of an identified stimulus were observed within nearly all stages of processing of visual signals, for the negative stimuli, beginning from an early stage of sensory analysis corresponding to the development of wave Р1. The latencies of components Р1 in the case of presentation of emotiogenic negative stimuli and those of components N1, N2, and Р3 in the case of presentation of the stimuli of both valencies were shorter than the latencies observed at neutral stimuli. The amplitude of component N2 at perception of positive stimuli was, on average, lower, while the Р3 amplitude at perception of all emotiogenic stimuli was higher than in the case of presentation of neutral stimuli. The time dynamics of topographic peculiarities of processing of emotiogenic information were complicated. Activation of the left hemisphere was observed during the earliest stages of perception, while the right hemisphere was activated within the intermediate stages. Generalized activation of the cortex after the action of negative signals and dominance of the left hemisphere under conditions of presentation of positive stimuli were observed only within the final stages. As is supposed, emotiogenic stimuli possess a greater biological significance than neutral ones, and this is why the former attract visual attention first; they more intensely activate the respective cortical zones, and the corresponding visual information is processed more rapidly. The observed effects were more clearly expressed in the case of action of negative stimuli; these effects involved more extensive cortical zones. These facts are indicative of the higher intensity of activating influences of negative emotiogenic stimuli on neutral systems of the higher CNS structures.     

Intracranial recording of short latency somatosensory evoked potentials in man: Identification of origin of each component

In normal subjects the short latency SEPs generally consisted of 3 positive waves (P9, P11, P14) and a succeeding negative wave (N20). To determine the origins of these waves we have made intracranial records from 17 patients, which suggest the following results. P9 originates in stimulated median nerve peripheral to the dorsal roots such as brachial plexus, P11 in the dorsal column of the cervical cord, P14 in the cuneate nucleus and medial lemniscal pathway, and N20 in the cerebral cortex. On the basis of intracranial and intraspinal records, the onset of P11 indicates the arrival of the afferent volley at the cord entry and the peak latency of P11 its arrival time at the C1–2 level dorsal column. The onset latency of P14 indicates the onset of postsynaptic events in cuneate nucleus neurons and the peak latency of P14 arrival at the midbrain.From the ventral surface of the brain-stem 3 positive waves (P′9, P′11, P′14) like the initial positive components of the scalp short latency SEPs (P9, P11, P14) were recorded. The amplitude of P′14 was large compared to that of P14. The peak latencies of P′14 recorded at the medulla and the pons were shorter than that of P14 by 0.7–0.8 msec and 0.2–0.5 msec, respectively. The peak latency of P′14 at the midbrain was almost the same as that of P14. By measuring the distance between the recording electrodes in the brain-stem and the peak latency difference of P′14, the fastest lemniscal conduction velocity was estimated as 56 m/sec.     

Stationary pattern adaptation and the early components in human visual evoked potentials

Pattern-onset visual evoked potentials were elicited from humans by sinusoidal gratings of 0.5, 1, 2 and 4 cpd (cycles/degree) following adaptation to a blank field or one of the gratings. The wave forms recorded after blank field adaptation showed an early positive component, P0, which decreased in amplitude with spatial frequency, whereas the immediately succeeding negative component, N1, increased in amplitude with spatial frequency. P0 and N1 components of comparable size were recorded at 1 cpd. Stationary pattern adaptation to a grating of the same spatial frequency as the test grating significantly reduced N1 amplitude at 4, 2 and 1 cpd. The N1 component elicited at 4 cpd was attenuated in log-linear fashion as the spatial frequency of the adaptation grating increased. P0, on the other hand, was unaffected by stationary pattern adaptation at all combinations of test and adapting spatial frequencies, although P0 amplitude is known to be attenuated by adaptation to a drifting grating. Since N1, but not P0, was significantly attenuated following adaptation and testing at 1 cpd, it was concluded that the neurons generating these components are functionally distinct. The use of a common adaptation grating discounted the possibility that N1, but not P0, was affected due to a difference in the rates of retinal image modulation caused by eye movements made while viewing adaptation gratings of different spatial frequencies. The neurons generating N1 were adapted at a lower rate of retinal image modulation than that apparently required for adaptation of the neurons generating P0, which suggests a difference between these neurons in the rate of stimulus modulation necessary for activation.     

Change in P wave morphology after convergent atrial fibrillation ablation

Convergent atrial fibrillation ablation involves extensive epicardial as well as endocardial ablation of the left atrium. We examined whether it changes the morphology of the surface P wave. We reviewed electrocardiograms of 29 patients who underwent convergent ablation for atrial fibrillation. In leads V₁, II and III, we measured P wave duration, area and amplitude before ablation, and at 1, 3 and 6 months from ablation.After ablation, there were no significant changes in P wave amplitude, area, or duration in leads II and III. There was a significant reduction in the area of the terminal negative deflection of the P wave in V₁ from 0.38 mm² to 0.13 mm² (p = 0.03). There is also an acute increase in the amplitude and duration of the positive component of the P wave in V₁ followed by a reduction in both by 6 months. Before ablation, 62.5% of the patients had biphasic P waves in V₁. In 6 months, only 39.2% of them had biphasic P waves.Hybrid ablation causes a reduction of the terminal negative deflection of the P wave in V₁ as well as temporal changes in the duration and amplitude of the positive component of the P wave in V₁. This likely reflects the reduced electrical contribution of the posterior left atrium after ablation as well as anatomical and autonomic remodeling. Recognition of this altered sinus P wave morphology is useful in the diagnosis of atrial arrhythmias in this patient population.     

SEPs in two patients with localized lesions of the postcentral gyrus

Scalp distributions of median nerve SEPs were studied in normal controls and 2 patients with localized lesions of the postcentral gyrus. In controls, parieto-occipital electrodes registered N20-P27 while frontal electrodes registered P20-N27. Other small components, parieto-occipital P22 and frontal N22, were recognized in about half of the control records. The wave forms at a frontal and a parieto-occipital electrode, both distant from the central region, formed exact mirror images of each other concerning N20-(P22)-P27 and P20-(N22)-N27. Electrodes near the central region contralateral to the stimulation registered cP22-cN30 (central P22 and central N30). When the postcentral gyrus was damaged, N20/P20-P27/N27 and cP22-cN30 were eliminated and the only remaining components were a frontal negative wave (frN) and a contralateral parieto-occipital positive wave (poP). Digital nerve stimulation also evoked poP and frN in both cases. In case 2, poP coincided with P22 of the non-affected side. The following generators were proposed; N20/P20-P27/N27: area 3b, cP22-cN30: areas 1 and 2, poP/early frN (= P22/N22): area 4 at the anterior wall of the central sulcus (due to direct thalamic inputs to motor cortex), late frN: uncertain (SMA?, SII?).     

Respiratory-related evoked potential elicited by expiratory occlusion.

Respiratory-related evoked potentials (RREPs) have been elicited by inspiratory loads in adults and children. The RREP was recorded over the somatosensory region of the cerebral cortex. It was hypothesized that a RREP could be recorded by using expiratory occlusion. Electroencephalographic activity was recorded in adults from 14 scalp locations, referenced to the linked earlobes. The occlusion was presented as an interruption of expiration. Epochs of electroencephalographic activity and mouth pressure were recorded for each expiratory occlusion presentation. There were two occlusion trials and a control trial of 100 presentations each. The epochs in each trial were averaged and examined for the presence of short-latency, occlusion-related peaks. RREP peaks were observed bilaterally with expiratory occlusion and were absent in control unoccluded averages. A positive peak, P(34), was observed at central and postcentral sites. A negative peak, N(53), was observed at frontal and central sites. A second positive peak, P(95), was observed at frontal and central sites. These results demonstrate that expiratory occlusion elicits a RREP. This suggests that expiratory occlusion-related sensory information activates the cerebral cortex similar to that for inspiratory loads.     

Comparison of evoked potentials to tones and clicks recorded simulataneously by multiple electrodes from the auditory cortex in unanesthetized cats

Evoked potentials to tones and clicks were recorded simultaneously from seven points of the auditory cortex and one or two points of the somatosensory cortex in unanesthetized cats. Comparison of evoked potentials to tones of equal loudness in the 250–7000 Hz band showed no common pattern of cortical tonotopic distribution. However, an individual dependence of the components of the evoked potential on pitch and on localization of the recording point exists for each animal. With a change in stimulus intensity the absolute and relative values of these components of the evoked potential vary. The initial positive waves are the most variable; besides the two waves already known a third, intermediate wave, particulary sensitive to loudness, was discovered. The negative wave of the primary response increases proportionally to loudness. Evoked potentials to clicks are more uniform over the auditory cortex and more stable than those to tones. Responses appeared in the somatosensory cortex to loud stimuli, more regularly to clicks than to tones. It is concluded that the parameter of pitch is reflected in the cat cortex as a complex spatially-individual distribution of the amplitude and time parameters of the evoked potentials.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 7, No. 2, pp. 115–125, March–April, 1975.     

Effects of Q-switched Nd:YAG laser irradiation on neural impulse propagation: I. Spinal cord.

We studied the effect of irradiation with Q-switched Nd:YAG laser light (1064 nm) on spinal cord dorsal column and dorsolateral white matter in anesthetized rats. To evoke a synchronous sensory input, the sciatic nerve was stimulated electrically and the resulting evoked spinal cord potential (SCP) recorded from the dorsal columns of the ipsilateral side. The waveshape of the SCP showed three components: an early positive peak (P1), representing the responses of the most rapidly conducting fibers, followed by two negative peaks (N1 and N2), which are mainly due to synaptic effects of the volley on dorsal horn cells located in dorsal grey matter. Laser irradiation at 50 mJ/pulse and above resulted in severe reduction in the amplitudes of N1 and N2. In contrast, there was either no reduction at all or only a slight decrease in the amplitude of P1. The selective loss of the synaptic field might be mediated by impairment of synaptic transmission or by loss of high threshold fiber input to dorsal horn neurones. In either event it is likely that the mechanism of the differential effects of laser irradiation on the components of the electrically evoked SCP is at least in part photothermally mediated, since intracord temperatures during laser application greatly exceeded the physiological range.     

Visual evoked potentials produced by monocular flash stimuli in the cerebral cortex of the rabbit. I. Typography

The visually evoked potentials in the hemisphere contralateral to the stimulated eye in rabbit, can be described topographically as follows. While a positive wave (P1) begins forming in the anterior zones and in the V I binocular zone, the N0 wave, at times very large, is produced in a more occipital zone, which corresponds to the visual streak. Immediately afterwards, the positivity, P1, practically invades the whole of the hemisphere. After this, the N1 wave which is produced in the most posterior parts of the V I, begins forming. The whole phenomenon comes to an end when the P2 wave is generated in the most occipital zones.     

Effect of electric stimulation of the locus coeruleus on transcallosal response in parietal associative cortex of the cat brain

The influence of conditioning locus coeruleus (LC) stimulation on various components of transcallosal field response was investigated in the parietal cortex of the cat brain. Conditioning LC simulation caused a decrease in fast positive wave amplitude and facilitated slow negative wave. It is concluded that LC suppresses excitatory and facilitates inhibitor processes evoked in the parietal cortex by transcallosal stimulation.