Vertex Electrodes Research Articles

Methodological considerations when measuring and analyzing auditory steady-state responses with multi-channel EEG.

The auditory steady-state response (ASSR) has been traditionally recorded with few electrodes and is often measured as the voltage difference between mastoid and vertex electrodes (vertical montage). As high-density EEG recording systems have gained popularity, multi-channel analysis methods have been developed to integrate the ASSR signal across channels. The phases of ASSR across electrodes can be affected by factors including the stimulus modulation rate and re-referencing strategy, which will in turn affect the estimated ASSR strength. To explore the relationship between the classical vertical-montage ASSR and whole-scalp ASSR, we applied these two techniques to the same data to estimate the strength of ASSRs evoked by tones with sinusoidal amplitude modulation rates of around 40, 100, and 200Hz. The whole-scalp methods evaluated in our study, with either linked-mastoid or common-average reference, included ones that assume equal phase across all channels, as well as ones that allow for different phase relationships. The performance of simple averaging was compared to that of more complex methods involving principal component analysis. Overall, the root-mean-square of the phase locking values (PLVs) across all channels provided the most efficient method to detect ASSR across the range of modulation rates tested here.

Neural Mechanisms of Attentional Switching Between Pain and a Visual Illusion Task: A Laser Evoked Potential Study

Previous studies demonstrated that pain induced by a noxious stimulus during a distraction task is affected by both stimulus-driven and goal-directed processes which interact and change over time. The purpose of this exploratory study was to analyse associations of aspects of subjective pain experience and engagement with the distracting task with attention-sensitive components of noxious laser-evoked potentials (LEPs) on a single-trial basis. A laser heat stimulus was applied to the dorsum of the left hand while subjects either viewed the Rubin vase-face illusion (RVI), or focused on their pain and associated somatosensory sensations occurring on their stimulated hand. Pain-related sensations occurring with every laser stimulus were evaluated using a set of visual analogue scales. Factor analysis was used to identify the principal dimensions of pain experience. LEPs were correlated with subjective aspects of pain experience on a single-trial basis using a multiple linear regression model. A positive LEP component at the vertex electrodes in the interval 294–351 ms (P2) was smaller during focusing on RVI than during focusing on the stimulated hand. Single-trial amplitude variations of the P2 component correlated with changes in Factor 1, representing essential aspects of pain, and inversely with both Factor 2, accounting for anticipated pain, and the number of RVI figure reversals. A source dipole located in the posterior region of the cingulate cortex was the strongest contributor to the attention-related single-trial variations of the P2 component. Instantaneous amplitude variations of the P2 LEP component during switching attention towards pain in the presence of a distracting task are related to the strength of pain experience, engagement with the task, and the level of anticipated pain. Results provide neurophysiological underpinning for the use of distraction analgesia acute pain relief.

Benign Neonatal Sleep Myoclonus Evokes Somatosensory Responses.

Benign neonatal sleep myoclonus is a common nonepileptic condition occurring in neurologically normal full-term newborns. During jerks, EEG has always been described as normal. The aim of this study was to describe EEG changes associated with the myoclonic jerks. Polygraphic video-EEG recordings of four full-term neonates presenting benign neonatal sleep myoclonus were studied. Myoclonic jerks were analyzed regarding their topography, frequency, propagation pattern, and reflex component. EEG averaging time-locked to myoclonic jerks and to somatosensory stimuli (realized by tapping on palms and feet) was performed to study eventual EEG correlates of myoclonus and to asses somatosensory evoked responses-for the latter, two control newborns were added. Visual analysis of the EEG disclosed theta band slow waves on central and vertex electrodes concomitant to myoclonic jerks and jerk-locked back-averaging disclosed a sequence of deflections, not preceding, but following the myoclonus. This response predominated on the vertex electrode (CZ) and consisted of five components (N1, P1, N2, P2, and N3), with only the three later components being constantly present (at 110, 200, and 350-500 ms, respectively). Back-averaging locked to the tactile stimuli in four subjects and two control newborns showed similar components and were comparable to those described in the literature as late somatosensory evoked responses in full-term newborns. Myoclonic jerks in benign neonatal sleep myoclonus can evoke visually identifiable EEG potentials on vertex electrodes corresponding to somatosensory responses. This EEG aspect may be misleading and could give rise to an anti-seizure treatment that mostly worsens the condition.

P382 Neural mechanisms of attentional switching during distraction analgesia: A single-trial laser evoked potential study

Objectives Noxious stimuli capture attention and at the same time, pain is attenuated by a concurrent cognitive task. The present study aimed to elucidate the cortical mechanisms underlying diminution of pain during attentional distraction using single-trial electrocortical and subjective responses. Methods In 24 healthy participants, a laser heat stimulus was applied to the dorsum of the left hand while subjects either viewed Rubin vase-face illusion (RVI), or focused on their pain and associated somatosensory sensations occurring on their stimulated hand. Pain-related sensations occurring on every laser stimulus were evaluated using a set of visual analogue scales. Factor analysis was used to identify the principal dimensions of pain experience. Laser evoked potentials (LEPs) were correlated with subjective aspects of pain experience on a single-trial basis using a multivariate regression model. Results Subjects anticipated and perceived smaller pain whilst focusing on RVI than on their pain-related sensations. A positive LEP component at the vertex electrodes in the interval 294–351 ms was smaller during focusing on RVI than during focusing on the stimulated hand. The attention-related amplitude changes in LEPs were accounted for by an equivalent source dipole located in posterior cingulate cortex (PCC). Single-trial variations of source activity in PCC correlated with changes in Factor 1, representing essential aspects of pain, and inversely with both Factor 2, accounting for anticipated pain, and with the number of RVI figure reversals. Discussion Single-trial analysis of LEPs and subjective responses to a noxious laser stimulus point to the possibility that the attentional switching during a distraction analgesia operates in PCC via three channels: 1. The pain modulation channel which affects the pain experience during the distraction by neural and functional connections of the posterior cingulate cortex with the anterior cingulate cortex. 2. The task engagement channel represented by reduction of the LEP potential in PCC in trials with a large number of RVIs. 3. The anticipation channel representing a contextual, top-down modulation of the distraction effect. Conclusions PCC appears to mediate the attentional switching towards the pain during concurrent, distracting task. Switching between pain and non-pain task is modulated by the strength of engagement in a cognitive-perceptual task, and by the level of anticipated pain. Significance Our findings shed light on the factors underlying distraction analgesia. The level of engagement in the non-pain task and the role of anticipation are the factors which can be boosted to strengthen the analgesia in clinical applications.

Bursts of 15–30 Hz oscillations following noxious laser stimulus originate in posterior cingulate cortex

Previous EEG studies reported the presence of synchronised 15–30 Hz oscillations in vertex electrodes following innocuous somatosensory stimulation and noxious laser stimulation. The purpose of the present study was to analyse the sources of poststimulus increases of 15–30 Hz oscillations during noxious laser stimulation of four different body regions and to compare the sources of the poststimulus synchronisation with the sources of the N2 component of laser-evoked potential (LEP). In 10 healthy subjects, moderately painful laser stimuli were applied to the dorsum of the right hand, dorsum of the right foot, right groin, and right side of the face. EEG data, recorded from 111 scalp sites, were analysed using event-related desynchronisation method and source dipole analysis. A profound amplitude increase of 15–30 Hz oscillations peaking 1–2 s after noxious laser stimulation was found during stimulation of each body part. The sources of these oscillations were located in the dorsal posterior cingulate cortex and showed no somatotopic arrangement. The sources of the N2–LEP component were located in the anterior mid-cingulate cortex 25–30 mm rostral to the sources of 15–30 Hz oscillations. The amplitude of the poststimulus synchronisation of 15–30 Hz oscillations correlated ( P < 0.05) with the amplitude of N2–LEP component. Results show that noxious laser stimuli induce bursts of 15–30 Hz oscillations in the posterior cingulate cortex. The poststimulus increases of 15–30 Hz oscillations may stand for transient cortical inhibition possibly aiding temporary suppression of motor programs that have been primed by noxious stimulation.

Burst suppression during propofol anaesthesia recorded from scalp and subthalamic electrodes: report of three cases

Measurement of slow EEG activity and burst suppression are the main tasks in monitoring the effects of anaesthestics with EEG, which is often done with commercial univariate indexes such as BIS. The aim of this study was to describe the characteristics of burst suppression EEG during propofol anaesthesia using scalp electrodes and depth electrodes in the subthalamic nucleus. Specifically, we describe the electrical fields of the three EEG patterns we have previously described: the sharp wave, the burst and the spindle. We recorded the EEG of three Parkinson patients during propofol anaesthesia from the scalp electrodes and the depth electrode implanted in the subthalamic nucleus for treating parkinsonism. (1) The slow waves of bursts recorded from all surface electrodes on scalp or neck with depth electrode reference are positive and have the highest amplitude in frontal electrodes, suggesting synchronous generation in the whole cerebral cortex. (2) The sharp wave and spindles have the highest amplitude at vertex. They are opposite in polarity in vertex and depth electrodes when referred to the neck electrode, suggesting generation in the sensorimotor cortex. Recording simultaneously EEG from the depth and scalp electrodes shows that bursts and their slow wave oscillations are synchronous in the whole cortex while spindles and sharp waves are produced by the sensorimotor cortex. The amplitude of slow waves recorded with surface electrodes is equal to the difference of the wave at two electrodes and therefore only a small part of that generated by the cortex.

Characterization of neonatal seizures by conventional EEG and single-channel EEG

Objective To perform a detailed, contemporary temporal–spatial characterization of neonatal seizures (NS) and to compare conventional EEG (CEEG) to single-channel EEG for NS detection. Methods Digitally recorded CEEGs were reviewed for NS characteristics (quantity, duration, location of onset, peak-to-peak amplitude). The presence and characteristics of each NS were simultaneously noted in a single, derived EEG channel (C 3 → C 4). Results Eight hundred fifty-one seizures from 125 CEEGs recorded were analyzed. Mean seizure rate was 7.0 NS/h (range: 0.5–21). Mean seizure burden (percent time CEEG showed NS at any location) was 24.8% (range: 0.7–86.9). Seizure rate was only moderately correlated with seizure burden (Spearman coefficient = 0.58). Eighty-one percent of NS originated from central–temporal or midline vertex electrodes. Seventy-eight percent of NS appeared in the C 3 → C 4 channel. Conclusions Accurate measurement of NS burden requires detailed temporal–spatial characterization. The theoretical ceiling of sensitivity for NS detection in the single EEG channel C 3 → C 4 is high. However, further processing the raw EEG in limited electrode arrays may reduce the sensitivity of NS detection. Significance CEEG is the gold standard for NS detection. However, reduced montage EEG techniques are increasingly available. This detailed contemporary temporal–spatial characterization of NS evaluates the potential limitations of reduced montage techniques.

Discharge rate of the auditory nerve during noise revealed by electrocochlear stimulation

The purpose of this study was to evaluate the average discharge rate of all fibres in the whole auditory nerve ( R wn) when a broad-band noise with steady-state effects is applied to the ear. We assessed the R wn parameter by detecting the state of refractoriness of the nerve during noise stimulation using an electric stimulus (ES) as a probe. The technique, applied in awake pre-implanted guinea pigs (Charlet de Sauvage et al., 1994), made it possible to obtain electro-acoustic responses (EARs), from which an estimate of the R wn parameter could be deduced. Negative current pulses of 100 μs duration, each followed by an identical pulse of positive polarity for charge balance, were applied between round window and indifferent vertex electrodes at intervals of 160 ms. The 120 ms wide-band noise masker started 92 ms before every other negative ES. The signal on the stimulating electrodes was averaged over a 5.12 ms window in synchrony with the negative pulse. EARs were obtained by alternately subtracting recordings during noise from those during silence. The R wn parameter was determined by comparing experimental and computed EAR patterns. For this purpose, a model of unit response incorporating changes in amplitude and conduction velocity during the relative refractory period was designed. The recovery function of the firing probability in response to ES was evaluated. Fibres were classified in different categories according to their background discharge rates. The probability of response of single fibres to ES in each category was calculated on the basis of their interval histograms during silence and noise. Individual spikes were combined accordingly to obtain the computed EAR waveform. R wn was determined by adjusting the EAR amplitude of the model in relation to that of the experimental EAR. R wn generally increases in a linear fashion with respect to noise intensity expressed in dB, thus following the increase in loudness perception estimated by Weber’s law. At the highest noise levels, R wn tends to saturate. The estimated saturation rate was found to be about 380 spikes/s.

Electrical and physiological changes during short-term and chronic electrical stimulation of the normal cochlea

In the electrical stimulation (ES) of auditory pathways, the type of stimulus and the electrode/tissue interface are critical parameters for the safety and efficacy of the protocol. In this study the influence of alternate pulses, applied between round window and vertex electrodes in chronically implanted guinea pigs, and maintained during 1 and 25 daily periods of 2 h (short-term and long-term experiments, respectively), was investigated. ES consisted of monophasic current pulses of ±70 μA and 300 μs in duration at a rate of 167/s, with alternate polarity. Compound Action Potential (CAP) audiograms, amplitudes and latencies of click-evoked CAPs, amplitudes and latencies of electrically-evoked auditory responses (EARs), and electrode impedances, were measured periodically outside or during the ES periods. Short-term ES induced no change in CAP thresholds, amplitude and latency in response to clicks at 80 dB above normal threshold, but induced a slight latency increase and amplitude decrease of the EAR, correlated with an exponential decay of the electrode impedance. On a long-term basis, CAP audiograms and latencies did not change significantly, whereas CAP amplitudes and electrode impedances increased, in correlation with each other. In control guinea pigs receiving no ES, the same CAP amplitude and impedance changes were observed over the same long-term period. The EAR and CAP changes can be explained by a variation of the electrical impedance of the electrode/tissue interface. This is possibly due to a change in electrotytes around the electrode under the influence of the ES for the short-term variation, and to an electrode encapsulation by fibrous tissue independent of the ES for the long-term change. In itself, and under the conditions of this experiment, the ES demonstrated no adverse effects on the auditory function and can be safely used for inner-ear exploration.

Differential sensitivity to rotation measured on potentials evoked by electrical stimulation of the guinea-pig ear

Responses to electrical stimulation of the ear applied between round-window and vertex electrodes were recorded in awake guinea-pigs from the same electrodes or from separate vertex/mastoid subdermal needle electrodes. They were averaged during opposite phases of sinusoidal rotation or before and after constant velocity rotation. In both cases the responses were subtracted from each other and yielded differential per- or post-rotatory “electrovestibular” responses. For comparison, responses were also recorded in the same animals and conditions of electrical stimulation during silence and during presentation of a broad-band noise. The difference yielded “electroacoustic” responses. In round-window records, electrovestibular and electroacoustic responses presented typical compound nerve action potential patterns. Electrovestibular responses could be recorded for head angular velocities as low as 3° sec −1 at 0.1 Hz. Response amplitude showed a logarithmic relation to head velocity. Changes in amplitude, as a function of time after rotation, were comparable to those reported for vestibular nerve fibre responses. In vertex/mastoid records, electroacoustic responses presented a sequence of peaks similar to the click-evoked auditory brain-stem responses, and electrovestibular responses presented two peaks, presumably representing contributions of central vestibular structures. Such “electrovestibulography” permits the study of an individual ear and makes available to the investigator a large range of vestibular stimulation conditions.

Differential sensitivity to rotation measured on potentials evoked by electrical stimulation of the guinea-pig ear**

Responses to electrical stimulation of the ear applied between round-window and vertex electrodes were recorded in awake guinea-pigs from the same electrodes or from separate vertex/mastoid subdermal needle electrodes. They were averaged during opposite phases of sinusoidal rotation or before and after constant velocity rotation. In both cases the responses were subtracted from each other and yielded differential per- or post-rotatory “electrovestibular” responses. For comparison, responses were also recorded in the same animals and conditions of electrical stimulation during silence and during presentation of a broad-band noise. The difference yielded “electroacoustic” responses. In round-window records, electrovestibular and electroacoustic responses presented typical compound nerve action potential patterns. Electrovestibular responses could be recorded for head angular velocities as low as 3° sec−1 at 0.1 Hz. Response amplitude showed a logarithmic relation to head velocity. Changes in amplitude, as a function of time after rotation, were comparable to those reported for vestibular nerve fibre responses. In vertex/mastoid records, electroacoustic responses presented a sequence of peaks similar to the click-evoked auditory brain-stem responses, and electrovestibular responses presented two peaks, presumably representing contributions of central vestibular structures. Such “electrovestibulography” permits the study of an individual ear and makes available to the investigator a large range of vestibular stimulation conditions.

Malformation and degeneration in the inner ear of mos transgenic mice.

Transgenic mice carrying the mos proto-oncogene linked to a retroviral transcriptional control sequence display behavioral abnormalities including circling, hyperactivity, head tilt, and bobbing. Axonal degeneration, neuronal chromatolysis, spongiform encephalopathy, gliosis, and inflammatory infiltrates are reportedly found in the central nervous systems of all mutants with the behavioral traits. Hearing was tested by means of broadband free-field rarefaction clicks with auditory brain stem response recorded between vertex and mouth electrodes. No detectable auditory response was elicited in transgenic animals, in contrast to five distinct positive peaks observed in littermate control animals. Light microscopic survey of temporal bone histopathology in mutants revealed extensive degeneration of the organ of Corti with loss of hair cells in all cochlear turns and loss of supporting cells and atrophy of spiral ganglion cells. The spiral limbus was deformed, with replacement of the usual convexity of the superior surface by a flattened trough configuration. Hair cells of the vestibular end organs appeared normal. Pathologic alteration in levels of mos transgene RNA appears to have a direct effect on the structural integrity of the inner ear.

Midline foci of epileptiform activity in children and neonates.

We report the clinical and electroencephalographic (EEG) correlates of focal potentially epileptogenic discharge arising from midline sagittal vertices (Fz, Cz, Pz) in 21 children prospectively identified from among 7,051 consecutive EEGs recorded during a 27-month period. The patients range in age from neonates to 13 years. EEGs were obtained during evaluations for solitary or recurrent seizures (17/21), attention deficit disorder (2/21), psychomotor retardation (1/21), and headache (1/21). Clinical seizure types were diverse and included simple partial (5/17), complex partial (2/17), generalized tonic-clonic (4/17), mixed (2/17), and neonatal (4/17). The majority (13/21) of patients had an identifiable etiology for their disorder; CT scans verified mass lesions in two patients. Midline epileptogenic foci were present during wakefulness in 14 of 17 older children and restricted to sleep in the others. Sleep states were indeterminate in the four neonates. Midline foci were exquisitely confined to Fz, Cz, and/or Pz in 6 of 17 older children and would have been entirely missed by a recording montage that did not include vertex electrodes. In five other children, midline foci spread preferentially to the adjacent central-parietal regions and closely resembled the appearance of benign rolandic foci in the longitudinal EEG montages, a potentially serious cause of EEG misinterpretation in view of the high incidence of neuropathology in this patient group.

Do static magnetic fields of NMR influence biological signals?

When measuring auditory evoked potentials on test persons we found that an early signal, measurable between the mastoid and vertex electrodes and possibly caused by capacitive or/and inductive coupling from the electrodynamic headphone, was phase-shifted when the person was exposed to a static 0.35 T field. This shift only slowly (greater than 15 min) returned to zero after termination of the magnetic exposure. Using a circuit stimulating the EEG measurement we now show that this shift is caused by a change in the total capacity of the electrode impedances. Therefore, the magnetically induced shift may be explained by changes in electric capacities of the magnetically exposed biological system.

Evoked responses from peripheral stimulation of the facial nerve

Evoked responses to electrical stimulation of the facial nerve were recorded in dogs with vertex electrodes placed both inside and outside the skull. Three waveforms were recorded. The first two waveforms appeared to be related to electrical activity carried by antidromic propagation within fibers of the facial nerve. The third waveform appeared to be caused by the muscular response that occurred as a result of orthograde conduction in the nerve. The effects of changing the stimulation site, voltage, duration, and frequency were recorded. It appears that, by means of far-field techniques, it is possible to reliably record evoked responses that reflect antigrade conduction via fibers of the facial nerve.

Frequency-following responses in the cat

The frequency-following response was studied in five cats aged 1 week to 3 years. Animals studied had either monaural or binaural hearing. Responses were recorded differentially from vertex and circumaural electrodes to tonebursts at octave intervals from 0.5 to 4 kHz. Thresholds and dynamics of the responses with and without white-noise masking were investigated. Evidence was found to support the view that the cochlear microphonic makes a major contribution to the frequency-following response, while the double frequency-following response to the 0.5 kHz stimulus is neural.

Remotely recorded cochlear microphonic in the cat.

The cochlear microphonic (CM) was recorded differentially with subcutaneous electrodes at the vertex and a variety of positions on the head and neck of the cat, and the phase and amplitude were compared with simultaneous recordings from vertex and contralateral ear electrodes. Recordings were made in bilaterally hearing animals, both with and without ear canal occlusion, following unilateral labyrinthectomy, and in white-coat cats with hereditary unilateral deafness, and in different age-groups. The cochlear microphonic was in phase with the contralateral recording at all electrode sites except those in the immediate vicinity of the stimulus ear. The amplitude was less at the circumaural positions and greater at cervical locations. The results were consistent in all age-groups and in bilaterally and unilaterally hearing animals. Precise electrode placement in the near-field conditions around the stimulus ear is therefore critical, especially for phase, while far-field conditions would appear to predominate at the remaining electrode sites.

Clinical and electroencephalographic characteristics of midline parasagittal foci.

We report the clinical and electroencephalographic (EEG) characteristics of 14 patients with midline parasagittal epileptogenic foci. Montages not including the vertex electrodes demonstrated the abnormal activity poorly or not at all. The clinical manifestations were diverse and included tonic-clonic, atypical absence, and partial seizures. In the majority, no cause could be established. The interictal EEG abnormality did not predict the pattern of clinical attack. Half the patients had evidence of structural brain disease. Four patients had no history of seizures.

The source along the basilar membrane of the cochlear microphonic potential recorded by surface electrodes in man

In order to determine the region along the basilar membrane which contributes to the recorded CM, an attempt was made to record CM responses in subjects with normal hearing and in subjects with high frequency cochlear hearing losses (e.g. acoustic trauma). In such cases of high frequency hearing loss, one may assume that damage has occurred to the hair cells in the basal turn of the cochlea. The CM, elicited by one cycle sinusoidal stimuli of various frequencies, intensities, phases, etc., was recorded by means of earlobe and scalp vertex electrodes. The CM recorded in subjects with normal hearing to high frequency sounds showed (besides the previously reported properties of the CM) large amplitudes and short latencies in response to low frequency stimuli. On the other hand, the CM recorded in subjects with high frequency hearing loss were either prolonged in latency and small in amplitude or completely absent. A plot of the relationship between CM latency to high intensity 500 Hz stimuli and audiometric hearing loss in 66 ears shows clearly that in those cases in which hearing was normal at frequencies up to about 8 kHz, CM latency was very short. On the other hand, in those cases in which the high frequency hearing loss progressed to include more and more lower frequencies, CM latency was more prolonged and smaller in amplitude until CM could no longer be observed. These results indicate that the CM recorded in normally hearing subjects from skin electrodes in response to low frequency (500 Hz) stimuli is generated in the basal turn of the cochlea. This finding is probably a consequence of the form of the mechanical response of the basilar membrane (travelling wave) to low frequency stimuli.

Normal short-latency electrophysiological filtered click responses recorded from vertex and external auditory meatus

We recorded normal electrophysiological responses to third-octave filtered clicks from external auditory meatus (EAM) and vertex electrodes referred to coupled earlobe electrodes (forehead ground). From both vertex and EAM, polarity-sensitive responses predominated at low frequencies and exhibited characteristics of both phase-locked neural responses (frequency-following response or FFR) and cochlear microphonics (CM). The FFR-like response predominated at the vertex site and the CM-like response predominated at EAM. At high frequencies, polarity-insensitive responses closely resembled rectangular-pulse click action potentials and brainstem evoked potentials, with clearly defined N1 and V peaks recorded from EAM and vertex, respectively. As frequency was lowered, the N1 and V peak latencies increased, the peaks broadened, and the latency-intensity curves steepened with greater prolongation occurring at lower click intensities. Lowering click frequency also shortened the N1-V interval and caused the plot of N1-V interval versus click intensity to become steeper. Plots of polarity-insensitive response amplitudes and thresholds against frequency revealed a high frequency bias for both N1 and V, but the V "frequency response" was flatter. A possible explanation of the shortened N1-V interval at low click frequencies based on this flatter V "Frequency response" is presented.