Topography of middle-latency somatosensory evoked potentials following painful laser stimuli and non-painful electrical stimuli

The aim of this study was to compare cerebral evoked potentials following selective activation of Aβ and Aδ fibers. In 15 healthy subjects, Aβ fibers were activated by electrical stimulation of the left radial nerve at the wrist. Aδ fibers were activated by short painful radian heat pulses, applied to the dorsum of the left hand by a CO₂ laser. Evoked potentials were recorded with 15–27 scalp electrodes, evenly distributed over both hemispheres (bandpass 0.5–200 Hz). The laser-evoked potentials exhibited a component with a mean peak latency of 176 msec (N170). Its scalp topography showed a parieto-temporal maximum contralateral to the stimulus side. In contrast, the subsequent vertex negativity (N240), which appeared about 60 msec later, had a symmetrical scalp distribution. Electrically evoked potentials showed a component at 110 msec (N110), that had a topography similar to the laser-evoked N170. The topographies of the N170 and N110 suggest that they may both be generated in the secondary somatosensory cortex. There was no component in the electrically evoked potential that had a comparable interpeak latency to the following vertex potential: for N60 it was longer, for N110 it was shorter. On the other hand, in the laser-evoked potentials no component could be identified the topography of which corresponded to the primary cortical component N20 following electrical stimulation.     

Electroencephalographic brain dynamics following manually responded visual targets

Scalp-recorded electroencephalographic (EEG) signals produced by partial synchronization of cortical field activity mix locally synchronous electrical activities of many cortical areas. Analysis of event-related EEG signals typically assumes that poststimulus potentials emerge out of a flat baseline. Signals associated with a particular type of cognitive event are then assessed by averaging data from each scalp channel across trials, producing averaged event-related potentials (ERPs). ERP averaging, however, filters out much of the information about cortical dynamics available in the unaveraged data trials. Here, we studied the dynamics of cortical electrical activity while subjects detected and manually responded to visual targets, viewing signals retained in ERP averages not as responses of an otherwise silent system but as resulting from event-related alterations in ongoing EEG processes. We applied infomax independent component analysis to parse the dynamics of the unaveraged 31-channel EEG signals into maximally independent processes, then clustered the resulting processes across subjects by similarities in their scalp maps and activity power spectra, identifying nine classes of EEG processes with distinct spatial distributions and event-related dynamics. Coupled two-cycle postmotor theta bursts followed button presses in frontal midline and somatomotor clusters, while the broad postmotor "P300" positivity summed distinct contributions from several classes of frontal, parietal, and occipital processes. The observed event-related changes in local field activities, within and between cortical areas, may serve to modulate the strength of spike-based communication between cortical areas to update attention, expectancy, memory, and motor preparation during and after target recognition and speeded responding.     

Evaluation of reference sites for scalp potentials evoked by painful and non-painful sural nerve stimulation

The objective of this study was to evaluate reference sites for recording the middle- and long-latency scalp potentials elicited by painful and non-painful sural nerve stimulation. Somatosensory evoked potentials (SEPs) were recorded from the scalp, the mastoid, the earlobe, the neck, and the wrist. Each site was referenced to the sterno-vertebral (SV) electrode, which is a balanced non-cephalic reference with essentially no ECG contamination.There was little or no activity recorded between the wrist and SV, and the SV was located within a region extending from the rostral neck to the wrist where the potentials were stable over space. Hence, the SV reference is indifferent for the middle- and long-latency potentials evoked by painful and non-painful sural nerve stimulation. There was, however, significant activity recorded from the earlobe and mastoid, sites which are frequently used as references for the SEP. It is important that investigators using these cephalic references to study the middle- and long-latency peaks of the SEP be aware of this activity as it will distort SEPs recorded from single sites and the SEP scalp topography, distortions which could unnecessarily complicate their interpretation.     

Estimating the cortex and autonomic nervous activity during a mental arithmetic task

The cerebral cortex has massive connections with autonomic nervous system and then arouses cardiovascular events, but the coupling mechanism between brain and heart is not clear. In this study the heart rate variability (HRV) and directed transfer function (DTF) methods are used to investigate the cortico-cortical functional coupling and direction of information flow between brain and heart during a mental arithmetic (MA) task. Electroencephalogram (EEG) and ECG were used for measuring neural/cardiac activity. Forty-three healthy male subjects were voluntarily participated in the study. Our results showed compared with control, LF/HF and LFn significantly increased while HF, HFn and total power significantly decreased (P < 0.05) during MA task. HR (79 ± 1.7 beats/min) was also significantly higher compared with the control (71 ± 1.4 beats/min). Moreover, MA task trigger the neurons of pre-central and central areas and then information transmit from front to back, and finished information integration at parietal and occipital locations. Our findings suggested that MA task caused an increase of the coupling of brain regions and quickened heart rate by virtue of increasing sympathetic activity and decreasing parasympathetic activity. The regulation from post-central areas to heart as well as feedback regulation from heart to central areas exists in the MA task.     

Brain activity during the memorization of visual scenes from TV commercials: An application of high resolution EEG and steady state somatosensory evoked potentials technologies

The aim of this study was to elucidate if the TV commercials that were remembered by the subjects after their observation within a documentary elicited particular brain activity when compared to the activity generated during the observation of TV commercials that were forgotten. High resolution EEG recordings were performed in a group of 10 healthy subjects with the steady state somatosensory evoked potentials (SSSEPs) technique, in which a series of light electrical stimulation at the left wrist were delivered at the frequency of 20 Hz. The brain activity was indexed by the phase delay of the EEG spectral responses at 20 Hz with respect to the stimulus delivering and evaluated at the scalp level as well as at the cortical surface using several regions of interest coincident with the Brodmann areas (BAs). Results suggest that the cerebral processes involved during the observation of TV commercials that were remembered by the population examined (RMB dataset) are generated by the posterior parietal cortices and the prefrontal areas, rather bilaterally. These results are compatible with previously results obtained in literature by using MEG and fMRI devices during similar experimental tasks. High resolution EEG is able to summarize, with the use of SSSEPs methodologies, the behavior of the estimated cortical networks subserving the proposed memory tasks. It is likely that such tool could play a role in the next future for the investigation of the neural substrates of the human behavior in decision-making and recognition tasks.     

Median and tibial nerve somatosensory evoked potentials: middle-latency components from the vicinity of the secondary somatosensory cortex in humans

The topography of the middle-latency N110 after radial nerve stimulation suggested a generator in SII. To support this hypothesis, we have tried to identify a homologous component in the tibial nerve SEP (somatosensory evoked potential). Evoked potentials following tibial nerve stimulation (motor+sensory threshold) were recorded with 29 electrodes (bandpass 0.5–500 Hz, sampling rate 1000 Hz). For comparison, the median nerve was stimulated at the wrist. Components were identified as peaks in the global field power (GFP). Map series were generated around GFP peaks and amplitudes were measured from electrodes near map maxima. With median nerve stimulation, we recorded a negativity with a maximum in temporal electrode positions and 106±12 ms peak latency (mean±SD), comparable to the N110 following radial nerve stimulation. After tibial nerve stimulation the latency of a component with the same topography was 131±11 ms (N130). Both N110 and N130 were present ipsi- as well as contralaterally. Amplitudes were significantly higher on the contralateral than the ipsilateral scalp for both median (3.1±2.4 μV vs. 1.7±1.6 μV) and tibial nerve (1.9±1.2 μV vs. 0.6+1 μV). The topography of the N130 can be explained by a generator in the vicinity of SII. The latency difference between median and tibial nerve stimulation is related to the longer conduction distance (cf. N20 and P40). The smaller ipsilateral N130 is consistent with the bilateral body representation in SII.     

Laser-evoked potentials: exogenous and endogenous components

The aim of this study was to distinguish the exogenous component (related to the physical properties of the stimulus) and the endogenous component (reflecting event-related cognitive processing) of the laser-evoked potential (LEP). Short painful radiant heat pulses generated by a CO₂-laser were applied to the dorsum of the right and left foot. LEPs were recorded with 5 scalp electrodes in the midline versus linked earlobes in 26 healthy subjects. In order to identify the exogenous component, the LEP was recorded during a standardised distraction task (reading a short story). To identify the endogenous component P3 for the LEP, a 2-stimulus oddball paradigm was used (20% probability of targets). When the task of the oddball paradigm consisted of pressing a button, a movement-related long-latency negativity (N1200) was recorded in frontal leads that was absent in a counting task. The LEP of targets, frequent non-targets and during distraction was dominated by a single large positivity. The amplitude of this positivity was task-dependent and increased the more attention the subject payed to the laser stimuli (distraction < neutral < non-target < target). The laser-evoked positivity during distraction had a peak latency of about 400 msec (P400) and a maximum amplitude at the vertex, which was independent of inter-stimulus interval. The P3 following laser stimulation had a significantly later peak at about 570 msec (P570) and a different scalp topography with a parietal maximum. Its amplitude decreased when the interstimulus interval was reduced from 10 to 6 sec. Under neutral instructions, the LEP positivity consisted of a superposition of both the exogenous P400 and the endogenous P570.     

Pain-related somatosensory evoked potentials following CO2 laser stimulation in man

0ain-related somatosensory evoked potentials (SEPs) following CO₂ laser stimulation were analyzed in normal volunteers. Low power and long wavelength CO₂ laser stimuli to the hand induced a sharp pain which was associated with a large positive component, P320, recorded over the scalp. Amplitude decreased and latency increased with reduction in stimulus intensity and subjective pain feeling. P320 was maximal at the vertex but was distributed widely over the scalp. There were no topographic differences between left- and right-hand stimulation, or between hand and chest stimulation. Lidocaine injection to produce anesthetic nerve block resulted in loss of P320, but the potential was relatively preserved during ischemic nerve block. No potential corresponding to P320 could be recorded following electrical or mechanical tactile stimulation.We consider P320 to be generated by impulses arising from pain stimuli and ascending through Aδ fibers. We propose the thalamus as a generator source from considering its scalp topography, but pain-specific cognition or perception may also be involved in generating this potential.     

Effect of swinging on EEG of rats of juvenile age in the wakefulness state

Simultaneous recording of the EEG activity of superficial cortical and deep (caudate nucleus, dorsal hippocampus, anterior hypothalamus) brain parts has been performed for the first time after a 2-h swinging of frequency of 0.2 Hz in Wistar rats of juvenile age. Swinging was produced on a 4-bar parallel swing. Using a Neuron-Spectre electroencephalograph and a Diana program, normalized power spectra of wave EEG components, synchronization coefficients, and coefficients of cross-correlation between bioelectrical potentials of various brain structures were determined. After a 2-h swinging, the mean value of normalized power of slow waves of δ-diapason in hypothalamus and hippocampus was found to increase statistically significantly, while normalized power of fast waves of α-and β₁-diapasons in hippocampus decreased (p < 0.05). A statistically significant increase of synchronization coefficient was observed in hypothalamus and hippocampus. Changes of coefficients of cross-correlation between hypothalamus and hippocampus and other brain strictures were of the oppositely directed, individual character. In the parietal occipital brain cortex and in caudate nucleus, the changes of the EEG spectral composition also were of individual character. The obtained results on the whole correspond to data about an enhancement of the EEG low-frequency rhythms at swinging and agree with the resonance hypothesis of motion sickness.     

Representation of collective electrical behavior of cardiac cell sheets

The electrocardiogram (ECG) is a measure of the collective electrical behavior of the heart based on body surface measurements. With computational models or tissue preparations, various methods have been used to compute the pseudo-ECG (pECG) of bipolar and unipolar leads that can be given clinical interpretation. When spatial maps of transmembrane potential (V_m) are available, pECG can be derived from a weighted sum of the spatial gradients of V_m. The concept of a lead field can be used to define sensitivity curves for different bipolar and unipolar leads and to determine an effective operating height for the bipolar lead position for a two-dimensional sheet of heart cells. The pseudo-vectorcardiogram (pVCG) is computed from orthogonal bipolar lead voltages, which are derived in this study from optical voltage maps of cultured monolayers of cardiac cells. Rate and propagation direction for paced activity, rotation frequency for reentrant activity, direction of the common pathway for figure-eight reentry, and transitions from paced activity to reentry can all be distinguished using the pVCG. In contrast, the unipolar pECG does not clearly distinguish among many of the different types of electrical activity. We also show that pECG can be rapidly computed by two geometrically weighted sums of V_m, one that is summed over the area of the cell sheet and the other over the perimeter of the cell sheet. Our results are compared with those of an ad hoc difference method used in the past that consists of a simple difference of the sum of transmembrane potentials on one side of a tissue sheet and that of the other.     

Influence of informational oversaturation on the quality of creative activity and spatial organization of EEG

The features of the spatial organization of bioelectric potentials in the cases of successful and unsuccessful creative imagination (refusal or poor quality of the product) under conditions of informational oversaturation. Two groups of subjects were compared: professionals (23 healthy students of the Department of Graphic Arts) and non-professionals (34 persons whose specialties were not linked with systematic visual imagination). During the experiments, the subjects were asked to mentally create a visual image based on two simple graphic elements, a right angle and a diagonal; after recording the EEG, they had to draw the image on paper and give it a title. The total number of elements exceeded 7 ± 2; hence, information processing at the conscious level was impossible, which caused the necessity of involving the mechanisms of unconscious information processing. The quality of the created product was evaluated with regard to the degree of success of performing the task and the features of spatial EEG organization. The EEG was recorded from 24 scalp sites of a subject using a SIT-EEG portable telemetric device. In the case of the successful performance of the task by professionals, the parameters of spatial organization of bioelectric potentials, i.e., spatial synchronization (linear processes) and spatial disorder (non-linear process) were strengthened compared to the base-line level in the frontal temporal areas of the right hemisphere and parietal occipital areas of the left hemisphere. Conversely, in the nonprofessionals, these parameters increased in the frontal temporal areas of the left hemisphere and parietal occipital areas of the right hemisphere. In the case a task was not successfully performed by the professionals, the spatial disorder of bioelectric potentials increased in all the cortical areas; in the nonprofessionals, only weak changes were observed. In all situations, the groups differed also in the parameters of coherence (Coh), spectral power (Sp) of bioelectric potentials, and informational energy, which reflects the level of expenditure of the brain’s energy for information processing. The evidence obtained is interpreted in terms of electroencephalographic correlates of successful task performance by professionals and nonprofessionals, i.e., their creative imagination under the conditions of informational over-saturation.     

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.     

Ion Levels and Membrane Potential in Chick Heart Tissue and Cultured Cells

Intracellular concentrations of sodium and potassium as well as resting potentials and overshoots have been determined in heart tissue from chick embryos aged 2–18 days. Intracellular potassium declined from 167 mM at day 2 to 117–119 mM at days 14–18. Intracellular sodium remained nearly constant at 30–35 mM during the same period. The mean resting potential increased from -61.8 mV at day 3 to about -80 mV at days 14–18. The mean overshoot during the same period increased from 12 to 30 mV. P_Na/P_K calculated from the ion data and resting potentials declined from 0.08 at day 3 to 0.01 at days 14–18. Thus, the development of embryonic chick heart during days 2–14 is characterized by a declining intracellular potassium concentration and an increasing resting potential and overshoot. Heart cells from 7- to 8-day embryos, cultured either in monolayer or reassociated into aggregates, were compared with intact tissue of the same age. The intracellular concentrations of sodium and potassium were similar in the three preparations and cultured cells responded to incubation in low potassium medium or treatment with ouabain in a manner similar to that of intact tissue. Resting potentials and overshoots were also similar in the three preparations.     

Mapping somatosensory evoked potentials to finger stimulation at intervals of 450 to 4000 msec and the issue of habituation when assessing early cognitive components

Somatosensory evoked potentials (SEPs) to mild electric stimulation of two fingers of the left hand were studied at regular interstimulus intervals (ISIs) of 450, 800, 1400, 2500 and 4000 msec. Habituation was evaluated while the subject was reading a novel so as to virtually ignore the finger stimuli while maintaining steady vigilance levels. Brain SEPs recorded from 25 scalp electrodes were assessed by scatter displays, electronic subtraction, bit-mapped potentials fields, and by calculating theZ estimator and dilation factor. Similar results were obtained with randomly varying ISIs. The P14 farfield and cortical N20 did not change with ISIs. The parietal P27–P45 decreased at ISIs of 800 and 450 msec, but showed no significant habituation at ISIs of 1400, 2500 or 4000 msec. This validated the control conditions used for assessing the early cognitive P30 and P40 to attended target stimuli. The frontal N30 also decremented at the shorter ISIs but still habituated up to ISIs of 2500 msec. The clear dissociation between frontal N30 and parietal P27 at the larger ISIs suggests that they involve at least in part distinct neural generators.     

3D Finite Element Electrical Model of Larval Zebrafish ECG Signals

Assessment of heart function in zebrafish larvae using electrocardiography (ECG) is a potentially useful tool in developing cardiac treatments and the assessment of drug therapies. In order to better understand how a measured ECG waveform is related to the structure of the heart, its position within the larva and the position of the electrodes, a 3D model of a 3 days post fertilisation (dpf) larval zebrafish was developed to simulate cardiac electrical activity and investigate the voltage distribution throughout the body. The geometry consisted of two main components; the zebrafish body was modelled as a homogeneous volume, while the heart was split into five distinct regions (sinoatrial region, atrial wall, atrioventricular band, ventricular wall and heart chambers). Similarly, the electrical model consisted of two parts with the body described by Laplace’s equation and the heart using a bidomain ionic model based upon the Fitzhugh-Nagumo equations. Each region of the heart was differentiated by action potential (AP) parameters and activation wave conduction velocities, which were fitted and scaled based on previously published experimental results. ECG measurements in vivo at different electrode recording positions were then compared to the model results. The model was able to simulate action potentials, wave propagation and all the major features (P wave, R wave, T wave) of the ECG, as well as polarity of the peaks observed at each position. This model was based upon our current understanding of the structure of the normal zebrafish larval heart. Further development would enable us to incorporate features associated with the diseased heart and hence assist in the interpretation of larval zebrafish ECGs in these conditions.     

Changes in P300 following alternate nostril yoga breathing and breath awareness

This study assessed the effect of alternate nostril yoga breathing (nadisuddhi pranayama) on P300 auditory evoked potentials compared to a session of breath awareness of equal duration, in 20 male adult volunteers who had an experience of yoga breathing practices for more than three months. Peak amplitudes and peak latencies of the P300 were assessed before and after the respective sessions. There was a significant increase in the P300 peak amplitudes at Fz, Cz, and Pz and a significant decrease in the peak latency at Fz alone following alternate nostril yoga breathing. Following breath awareness there was a significant increase in the peak amplitude of P300 at Cz. This suggests that alternate nostril yoga breathing positively influences cognitive processes which are required for sustained attention at different scalp sites (frontal, vertex and parietal), whereas breath awareness brings about changes at the vertex alone.     

The neurophysiological basis of the marijuana experience

Experiments were done with 75 healthy young adults to explore the neurophysiological basis of the acute marijuana intoxication state. Tests included recording the scalp EEG, visual and auditory cerebral evoked-potentials, the CNV, cerebral slow potentials related to certainty of response correctness in auditory discrimination tasks, heart rate, respiration and the galvanic skin response. All variables were recorded over 45 minutes before and 45 minutes after smoking a marijuana cigarette containing either 4.8, 9.1 or less than 0.01 mg. Δ⁹-THC.High doses of marijuana induced a significant decrease in the peak power of the alpha rhythm and an increase in auditory evoked-response latency. The CNV increased in ampiitude after smoking marijuana in low doses and sequential CNVs showed changes consistent with sustained attention but decreased certainty about performance following either low or high dose. Marijuana interfered significantly with performance of the discrimination task itself.     

Mapping P300 waves onto inhibition: Go/NoGo discrimination

Subjects viewed letters presented at 2 sec intervals and prepared a fast button press whenever an “O” appeared. If the next letter was an “X” the button press was executed (Go signal), but if the letter was a non-X character (T, H, Z) suppression of the response was required (NoGo cue). NoGo signals elicited a P300-like wave that was larger at central and frontal scalp sites contralateral to the prepared movement, compared to P300s elicited by Go cues which were symmetric about the sagittal midline and dominant at parietal sites. Subtraction of preparatory CNVs from the NoGo P300 did not remove differences in scalp topography, or reduce the amplitude of the NoGo P300 to that seen following control letters that required perceptual identification but did not call for suppression of prepared motor responses. Principal components analysis identified a middle positive wave following X-alone control stimuli whose topography resembled the NoGo P300. These findings suggest that the source of augmented NoGo P300s is a generator involved with sensorimotor inhibition. We discuss the mechanism of P300 waves and evidence linking these waves with inhibition in other task arrangements.     

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.     

The effects of simulated microgravity on characteristics of slow presaccadic potentials

The parameters of saccades and presaccadic slow potentials were studied in seven right-handed male volunteers with a dominant right eye before and after exposure to 6-day dry immersion. Visual stimuli were presented using three light diodes, which were located in the center of the visual field (the central fixation stimulus) and 10° to the right and left of it (peripheral stimuli (PSs)). The subjects performed a test with simple saccades to a PS and a test with antisaccades to the point located symmetrically in the opposite visual field. The EEG (19 monopolar leads) and electrooculogram were recorded. To isolate slow potentials, backward EEG averaging was performed, with the moment of switching on the PS serving as a trigger for the averaging. It was found that the characteristics of saccadic eye movements did not substantially change after exposure to immersion. However, both tests revealed a change in topography and a decrease in the amplitude of presaccadic slow negative potentials (PSNPs) during immersion. Characteristically, the focus of presaccadic negativity shifted to the right hemisphere so that the PSNP amplitude sharply decreased in the left and increased in the right hemisphere. A significant decrease in the PSNP amplitude on day 6 of immersion was found in the midline and left-hemispheric frontal and parietal leads. It may be suggested that, because of support unloading and a decrease in proprioceptive input, exposure to microgravity causes a decrease in the activity of the left hemisphere and prefrontal and parietal cortices, initially involved in preparation and realization of motor responses. The activation of the right hemisphere could be of compensatory character.