Evoked Vertex Potentials Research Articles

Trigeminal small-fibre function assessed with contact heat evoked potentials in humans

Contact heat stimuli have been reported to excite mechano-thermal nociceptors and to evoke brain potentials (CHEPs) from the limbs. We investigated whether contact heat evokes reproducible CHEPs from the trigeminal territory and may prove a reliable diagnostic tool in facial neuropathic pain. We applied contact heat stimuli to the perioral and supraorbital regions; CHEPs were recorded from the vertex in 20 controls and 2 patients with facial neuropathic pains, and reflex responses from the orbicularis oculi and masticatory muscles in 5 controls. We studied the correlation between CHEP data and perceptive ratings, site of stimulation, and age. Finally, we compared CHEPs with laser evoked potentials (LEPs). Contact heat stimuli at 51 °C evoked vertex potentials consisting of an NP complex similar to that elicited by laser pulses, though with a latency some 100-ms longer. Perioral stimulation yielded higher pain intensity ratings, shorter latency and larger amplitude CHEPs than supraorbital stimulation. CHEP data correlated significantly with age. Contact heat stimuli at 53 °C evoked a blink-like response in the relaxed orbicularis oculi muscle and a silent period in the contracted masseter muscle. In patients with facial neuropathic pain the CHEP abnormalities paralleled those seen with LEPs. We were unable to achieve reproducible signals related to C-receptor stimulation by contact heat stimuli at 41 °C in the ten subjects in whom they were tested. Contact heat stimulation, as well as laser stimulation, easily yields large-amplitude brain potentials and nociceptive reflexes, both related to the Aδ input. However CHEPs are not suitable for C-fibres potentials recording.

A comparative magnetoencephalographic study of cortical activations evoked by noxious and innocuous somatosensory stimulations

We recorded somatosensory-evoked magnetic fields and potentials produced by painful intra-epidermal stimulation (ES) and non-painful transcutaneous electrical stimulation (TS) applied to the left hand in 12 healthy volunteers to compare cortical responses to noxious and innocuous somatosensory stimulations. Our results revealed that cortical processing following noxious and innocuous stimulations was strikingly similar except that the former was delayed approximately 60 ms relative to the latter, which was well explained by a difference in peripheral conduction velocity mediating noxious (Aδ fiber) and innocuous (Aβ fiber) inputs. The first cortical activity evoked by both ES and TS was in the primary somatosensory cortex (SI) in the hemisphere contralateral to the stimulated side. The following activities were in the bilateral secondary somatosensory cortex (SII), insular cortex, cingulate cortex, anterior medial temporal area and ipsilateral SI. The source locations did not differ between the two stimulus modalities except that the dipole for insular activity following ES was located more anterior to that following TS. Both ES and TS evoked vertex potentials consisting of a negativity followed by a positivity at a latency of 202 and 304 ms, and 134 and 243 ms, respectively. The time course of the vertex potential corresponded to that of the activity of the medial temporal area. Our results suggested that cortical processing was similar between noxious and innocuous stimulation in SI and SII, but different in insular cortex. Our data also implied that activities in the amygdala/hippocampal formation represented common effects of noxious and tactile stimulations.

Trigeminal responses evoked by painful electrical stimulation.

Trigeminal sensory and motor responses to high-intensity electrical stimuli were studied in 10 healthy male subjects. During contraction at 10% of maximal bite force, late exteroceptive suppression periods assessed by EMG and bite force from masseter muscles were recorded together with evoked vertex potentials. Electrical stimuli were either delivered as 1 ms single pulses or as 21 ms (5 x 1 ms, 200 Hz) train pulses at two different intensities above the pain threshold. Three different perioral regions were stimulated in random order. The duration of the masseter suppression period was significantly longer and its latency significantly shorter at higher stimulus intensities whereas the amplitude of the evoked vertex potentials remained unchanged. Both the evoked potentials and the exteroceptive suppression period in the masseter muscle differed significantly between stimulation at the three perioral regions. The present results from combined measurements of sensory and motor responses in the trigeminal system may be explained by the function of trigeminal polysynaptic reflex circuits and the central cerebral processing of afferent inputs.

Laser-evoked cerebral potentials and sensory function in patients with central pain

Central pain syndromes (CPS) could be caused by disinhibition of spinothalamic excitability or by other central nervous system (CNS) changes caused by reduced spinothalamic function. To examine these possibilities, we studied 11 patients (ages 51–82 years) with unilateral central pain and with reproducible cerebral evoked vertex potentials in response to cutaneous stimulation of the normal side with pulses from an infra-red CO 2 laser. All patients had normal tactile and kinesthetic sensation; one had slightly decreased vibratory sense bilaterally. All showed, from the unaffected (asymptomatic) side, laser evoked potentials (LEPs) with negative (N) components ranging from 208 to 280 msec peak latency (av: 240 ± 6 SE msec) and peak amplitudes of 1–7 μV (av: 2.9 ± 0.5 SE μV), followed, in all but 1 patient, by positive (P) potentials ranging from 288 to 370 msec peak latency (av: 319 ± 7.7 SE msec) with peak amplitudes of 1–7 μV (2.8 ± 0.5 SE μV). Laser stimulation of the affected (symptomatic) side in 5 patients evoked LEPs with N-P interpeak amplitudes that were within 20% of those evoked from the normal side. All but one of these patients had thresholds for warm, heat pain, and deep pain that were normal in comparison with the unaffected side. The excepted patient had the largest N-P interpeak amplitude asymmetry (18.5%) of this group. Ratings of laser pulse intensity were either symmetrical (n = 2) or increased on the affected side (n = 3) in these patients. In contrast, laser stimulation of the affected side failed to evoke either N or P potentials in 6 patients, all of whom had lateralized increased thresholds for warm, heat pain, or deep pain, or reduced ratings of laser pulse sensation. Although 1 patient had increased ratings of laser pulse sensation, the amplitude of the LEP was always reduced on the side of increased pain or heat threshold in these CPS patients (Fisher exact test: P = 0.015). These results reflect primarily a deficit in spinothalamic tract function and do not suggest excessive CNS responses to synchronous activation of cutaneous heat nociceptors in patients with CPS.

Analgesic effect in humans of subanaesthetic isoflurane concentrations evaluated by evoked potentials

The aim of this study was to see if an analgesic effect of subanaesthetic concentrations of isoflurane could be detected with evoked potentials elicited by nociceptive stimuli. We studied 10 healthy volunteers breathing three steady-state subanaesthetic concentrations of isoflurane (0.08, 0.16 and 0.24 vol% end-tidal). Reaction time, subjective pain intensities and evoked vertex potentials to laser (LEP) and electrical (SEP) stimuli were recorded and compared with auditory evoked potentials (AEP). Compared with baseline, the subanaesthetic concentrations of isoflurane did not change the latencies of the evoked potentials, but caused a significant reduction in the amplitudes of the LEP and SEP at 0.16 and 0.24 vol% and of the AEP at all three concentrations. There were no changes in perceived pain intensity, and isoflurane produced similar reductions in evoked potentials elicited by both nociceptive and non-nociceptive stimuli. The reaction time was increased significantly at 0.24 vol% isoflurane. The results demonstrated that subanaesthetic isoflurane concentrations caused similar changes in evoked potentials with both painful and non-painful stimuli, with no effect on perceived pain intensity.

An explorative evaluation of the local anaesthetic activity of dimethindene

The study investigated the local anaesthetic efficacy and its time course of dimethindene. The pilot-study was carried out with 8 healthy volunteers in a placebo-controlled, randomized, crossover design with four gel formulations [Dimethindene, non-occlusive (DNO) and occlusive (DO); Lidocaine (L); Placebo (P)]. Selective thermo-nociceptive stimulation (Aδ-and C-fibers) was induced by a CO2-laser. Somatosensory evoked vertex potentials (SEPs) from an EEG record were simultaneously recorded. The effect of the active treatments was concentrated on the peripheral N1-component of the somatosensory evoked potentials. The overall means of the N1-amplitude were reduced to an extent between 12.6% (DNO) and 23.5% (DO) compared to placebo treatment. The N1-latencies were increased between 4% (DNO) and 9.4% (DO). Thus all treatments showed a local anaesthetic effect compared to placebo at all assessment times.

Analgesic Effect of Acetylsalicylic Acid (ASA) Versus a Lithium-ASA combination: An Evoked Potential Study Employing Radiant Heat Stimulation with a CO2Laser

Objective experimental algesimetry was used to assess quantitative differences in analgesic properties between acetylsalicylic acid (ASA, 750 mg) and a special combination of ASA, trilithium citrate and quinine-2 HCl (750 mg, 126 mg, 4.5 mg) in a placebo-controlled double-blind crossover study on nine healthy subjects. Radiant heat stimulation was applied with a CO2 laser and somatosensory evoked vertex potentials (LSEP) were recorded while the subjects were simultaneously engaged in an adaptive pursuit tracking task in order to stabilize their vigilance. The N1 amplitude of the LSEP decreased under both verum medications; however, the drug combination was significantly more effective. The time course of this effect was attended by a marked intradiurnal variation of the LSEP amplitudes. The role of lithium in the combination with ASA and quinine-2 HCl (Togal) and an amplification of the analgesic potency of ASA are discussed.

Stimulus intensity and inter-stimulus interval effects on pain-related cerebral potentials

The inter-relationship between stimulus intensity and inter-stimulus interval (ISI) on pain-related evoked vertex potentials was studied. Sixteen subjects were tested with 4 stimulus intensities at 4 different ISIs forming 16 averaged event-related potentials for each subject. Data were analyzed in 2 ways: first by multiple regression analysis of peak-to-peak amplitudes and secondly by single-trial analysis for each subject based on a linear model employing principal component loadings as basis functions, from which were derived separate time-dependent functions describing the contributions of intensity and ISI. Peak-to-peak amplitudes of the averaged waves increased with increases in either intensity or ISI. There are no significant interactions. However, single-trial analysis revealed subtle, but consistent, differences in the peak latencies between stimulus and ISI components, suggesting that the components arise from distinct sources.

Temporary Changes of the Auditory System Due to Prolonged Exposure to Noise

One subject was exposed to an octave-band noise centered at 500 Hz for 48 h at 81.5 dB SPL and on a second occasion for 29.5 h at 92.5 dB SPL. Auditory sensitivity was measured before, during, and following the exposure as was temporal auditory summation, loudness, frequency discrimination, and evoked vertex potentials. After 8–12 h of exposure, the growth of temporary threshold shift (TTS) was asymptotic. Recovery from asymptotic TTS required 2–4 days even though the magnitudes of TTS were only about 10 dB for the 81.5 dB exposure and 27.5 dB for the 92.5 dB exposure. Other test results showed that during and following asymptotic TTS, the time constant of temporal auditory summation was decreased, and recruitment of loudness was delayed and incomplete; however, frequency discrimination and the slope of the input-output function of the N1−P2 potential were unaffected.

Registration of acoustically evoked vertex potentials with unconventional averaging technique.

The recording of acoustically evoked EEG potentials is normally considered to be dependent on an electronic average computer. The present paper describes a measuring set—up for such registrations without use of a conventional averaging technique. The repetitive signals are displayed on an oscilloscope with a camera attachment supplied with a special lens system. The image obtained by superposition of the individually displayed traces will under appropriate conditions disclose the true average of the received evoked potentials. Some examples are given to illustrate the clinical applicability of the method.

Amplitude of Auditory Evoked Vertex Potentials as a Function of Interstimulus Interval During Slow Repetitive Stimulation

Averaged auditory evoked potentials were recorded from human vertex in response to repetitive tone bursts presented at different interstimulus intervals. Interstimulus intervals were varied in discrete steps from 0.25 to 10 sec with a counterbalanced order between subjects. Tone bursts were 20-msec pulsed pure tones with 20-msec rise and decay times. Frequencies used were 500, 1000, and 2000 Hz. Although large variance exists in averaged evoked response amplitudes (N1−P2 peaks) within and between subjects, group means show N1−P2 amplitude to be an increasing function of interstimulus interval: as interstimulus intervals are changed from 0.25 sec to about 2.0 sec, response amplitude increases sharply to a partial asymptote between interstimulus intervals of 2.0 and 3.0 sec, then from about 3.0-sec through at least 10-sec interstimulus intervals, response amplitude continues to increase but the slope of the function is greatly reduced.

The variability of single evoked vertex potentials in man

The variability of single (not averaged) auditory evoked responses was studied in a subject with uniquely high voltage vertex potentials. Her N 1-P 2 amplitude averaged about 100 μV at 80 dB hearing level (ISO) with 10 sec inter-stimulus intervals. The voltages of single responses varied widely in approximately Gaussian distributions. No cyclical pattern of variation in amplitude was found. With shorter inter-stimulus intervals or with weaker stimuli the average responses were smaller, the distributions were smaller. This combination suggests the interaction of a random process with the parameters such as intensity and inter-stimulus interval that determine the amplitude of each response.