Comparison of somatosensory evoked high-frequency oscillations after posterior tibial and median nerve stimulation

Subcortical Hyperexcitability in Migraineurs: A High-Frequency Oscillation Study

Objective:

An abnormal central nervous system excitability level was found in patients with migraine. Whether it is hyper- or hypo-excitable is still debated. This study aimed to compare the somatosensory high-frequency oscillations (HFOs), which reflected subcortical excitability (early phase) and intracortical inhibition (late phase), between patients with migraine and control subjects.

Methods:

HFOs were recorded from C3'-Fz, using a 500-1000 Hz frequency filter after stimulation at right median nerves at the wrists, and divided into early and late phases based on the N20 peak. Fifty-nine untreated patients (n=24 during ictal period; n=35, interictal) and 22 controls finished the study.

Results:

In early HFOs, patients both during ictal and interictal periods had higher maximal amplitudes (p =0.039) and area-under-curve (p =0.029) than those of the controls. Regarding the late HFOs, there were no significant differences among these groups.

Conclusion:

Our study suggests a hyper-excitable state in the subcortical regions in patients with migraine both during interictal and ictal periods.

High-frequency oscillations in epileptic brain

PURPOSE OF REVIEW

It has been 10 years since pathological high-frequency oscillations (pHFOs) were described in the brain of epileptic animals and patients. This review summarizes progress in research on mechanisms of their generation and potential clinical applications over that period.

RECENT FINDINGS

Initially, pHFOs were recorded with microelectrodes in the hippocampus of rodents and patients with mesial temporal lobe epilepsy (MTLE), but recently pHFOs have also been recorded with clinical depth and grid electrodes in multiple brain areas including the hippocampus and neocortex of patients with different types of epilepsy. One hypothesis is that pHFOs reflect fields of hypersynchronized action potentials (bursts of population spikes) within small discrete neuronal clusters responsible for seizure generation. Studies suggest that pHFOs can be used as a reliable biomarker for epileptogenesis, epileptogenicity, and the delineation of the epileptogenic region.

SUMMARY

Recording of pHFOs with clinical electrodes provides a means for further investigation of their functional role in the epileptic brain and as a potential biomarker of epileptogenesis and epileptogenicity and for presurgical mapping.

High-frequency oscillations: what is normal and what is not?

High-frequency oscillations (HFOs) in the 80-200 Hz range can be recorded from normal hippocampus and parahippocampal structures of humans and animals. They are believed to reflect inhibitory field potentials, which facilitate information transfer by synchronizing neuronal activity over long distances. HFOs in the range of 250-600 Hz (fast ripples, FRs) are pathologic and are readily recorded from hippocampus and parahippocampal structures of patients with mesial temporal lobe epilepsy, as well as rodent models of this disorder. These oscillations, and similar HFOs recorded from neocortex of patients, appear to identify brain tissue capable of spontaneous ictogenesis and are believed to reflect the neuronal substrates of epileptogenesis and epileptogenicity. The distinction between normal and pathologic HFOs (pHFOs), however, cannot be made on the basis of frequency alone, as oscillations in the FR frequency range can be recorded from some areas of normal neocortex, whereas oscillations in the ripple frequency range are present in epileptic dentate gyrus where normal ripples never occur and, therefore, appear to be pathologic. The suggestion that FRs may be harmonics of normal ripples is unlikely, because of their spatially distinct generators, and evidence that FRs reflect synchronized firing of abnormally bursting neurons rather than inhibitory field potentials. These synchronous population spikes, however, can fire at ripple frequencies, and their harmonics appear to give rise to FRs. Investigations into the fundamental neuronal processes responsible for pHFOs could provide insights into basic mechanisms of epilepsy. The potential for pHFOs to act as biomarkers for epileptogenesis and epileptogenicity is also discussed.

Low-frequency oscillations in human tibial somatosensory evoked potentials

Oscillatory cerebral electric activity has been related to sensorial and perceptual-cognitive functions. The aim of this work is to investigate low frequency oscillations (<300 Hz), particularly within the gamma band (30-110 Hz), during tibial stimulation. Twenty-one volunteers were subjected to 5 Hz stimulation by current pulses of 0.2 ms duration and the minimum intensity to provoke involuntary twitch. EEG signals without (spontaneously) and during stimulation were recorded at primary somatosensory area. A time-frequency analysis indicated the effect of the stimulus artifact in the somatosensory evoked potential (SEP) frequencies up to 5 ms after the stimulus. The oscillations up to 100 Hz presented the highest relative power contribution (approximately 99%) for the SEP and showed difference (p<0.01) from the frequencies of the spontaneously EEG average. Moreover, the range 30-58 Hz was identified as the band with the highest contribution for the tibial SEP morphology (p<0.0001).

Specific somatosensory processing in somatosensory area 3b for human thumb: a neuromagnetic study

OBJECTIVES

We examined the relation between somatosensory N20m primary responses and high-frequency oscillations (HFOs) after thumb and middle finger stimulation.

METHODS

Somatosensory evoked fields (SEFs) from 12 subjects were measured following electric stimulation of the thumb and middle finger. SEFs were recorded with a wide bandpass (3-2000 Hz) and then N20m and HFOs were separated by subsequent 3-300 and 300-900 Hz bandpass filtering.

RESULTS

The N20m peak-to-peak amplitude did not differ significantly between thumb and middle finger SEFs. In contrast, HFOs had a significantly larger number of peaks and were higher in the maximum amplitude and the total amplitude after thumb stimulation than after middle finger stimulation.

CONCLUSIONS

Our present data demonstrate a different relation between N20m and HFOs after thumb and middle finger stimulation. In view of the fact that the human thumb has uniquely evolved functionally and morphologically, the somatosensory information from the thumb will be processed differently for a fine motor control. We speculate that HFOs are generated by inhibitory interneurons in layer 4 in area 3b. Thus, enhanced activity of interneurons reflected by high amplitude HFOs exerts stronger inhibition on downstream pyramidal cells in area 3b for thumb stimulation.

High-frequency oscillations in human posterior tibial somatosensory evoked potentials are enhanced in patients with Parkinson's disease and multiple system atrophy

High-frequency oscillations (HFOs) and the underlying P37 primary somatosensory response evoked by posterior tibial nerve stimulation were recorded in patients with Parkinson's disease (PD) and in those with multiple system atrophy (MSA), as well as in normal controls. In order to increase the signal-to-noise ratio, we averaged a large number of responses (9998 epochs) with a high sampling rate (20 kHz per channel). HFOs were extracted by filtering the wide band-pass recording of the P37 potential with a 600-900 Hz band-pass filter. High-amplitude HFOs were observed in both the PD and MSA patients. Furthermore, the duration of illness was positively correlated with the amplitude of HFOs. The results suggest that HFOs are enhanced by dysfunction of the basal ganglia.

High-frequency oscillations in human somatosensory evoked potentials are enhanced in school children

High-frequency oscillations (HFOs) in the range of 300-900Hz have been shown to occur simultaneously with the primary response (N20) of the human somatosensory cortex following median nerve stimulation. We studied the effects of development on somatosensory evoked HFOs. Somatosensory evoked potentials (SEPs) were recorded with a filter set at 10-2000Hz following median nerve stimulation in 15 normal adult subjects and 15 normal children (6-12years). The HFOs were obtained by digitally filtering the wide-band SEPs (10-2000Hz) using a band-pass of 300-900Hz. In children, the HFOs were significantly higher in amplitude, longer in duration and larger in number of negative peaks than adult subjects. In addition, the interpeak latency of HFOs in children was longer than that of adult subjects. These results suggest that the HFOs are influenced by development.

High-frequency oscillations of somatosensory evoked potentials and fields

A short review of previous studies is presented on somatic, evoked high-frequency oscillations. Also described briefly is recent data on the detection of high-frequency oscillations to posterior tibial nerve stimulation, and also on both tangential (area 3b) and radial (area 1) dipoles to median nerve stimulation. The findings show that high-frequency oscillations are not specific to median nerve stimulation but represent ubiquitous activity in the primary somatosensory cortex. Modulation of high-frequency oscillations versus electric and magnetic N20, N20 (m), primary response by a wake-sleep cycle, by attention or interference, by aging, and in central nervous system diseases such as Parkinson's disease and myoclonus epilepsy are also presented. Finally, a gamma-aminobutyric acid inhibitory interneuron hypothesis is presented for high-frequency oscillations based primarily on the findings regarding reciprocal modulation of the high-frequency oscillations and the underlying magnetic N20 (N20m) by a wake-sleep cycle and by attention or interference.

The later part of high-frequency oscillations in human somatosensory evoked potentials is enhanced in aged subjects

High-frequency oscillations (HFOs) in the range of 300-900 Hz have been shown to occur simultaneously with the primary response (N20) of the human somatosensory cortex following median nerve stimulation. We studied the effects of aging on somatosensory evoked HFOs. Somatosensory evoked potentials (SEPs) were recorded with a filter set at 10-2000 Hz following right median nerve stimulation in 15 normal young subjects and 15 normal aged subjects. The HFOs were obtained by digitally filtering the wide-band SEPs (10-2000 Hz) using a band-pass of 300-900 Hz. In the aged subjects, the number of negative peaks, amplitudes, duration and area of the HFOs between N20 peak and HFO endpoint were significantly larger than those of young subjects. However, those between N20 onset and N20 peak did not differ between the groups. These results suggest that the later part of HFOs is associated with aging.

Reciprocal modulation of somatosensory evoked N20m primary response and high-frequency oscillations by interference stimulation

OBJECTIVES

We examined whether the inverse relation between somatic evoked N20m primary response and high-frequency oscillations during a wake-sleep cycle (Hashimoto, I., Mashiko, T., Imada, T., Somatic evoked high-frequency magnetic oscillations reflect activity of inhibitory interneurons in the human somatosensory cortex, Electroenceph clin Neurophysiol 1996;100:189-203) holds for interference stimulation.

METHODS

Somatosensory evoked fields (SEFs) from 14 subjects were measured following electric median nerve stimulation at the wrist with, and without, concurrent brushing of the palm and fingers. SEFs were recorded with a wide bandpass (0.1-1200 Hz) and then N20m and high-frequency oscillations were separated by subsequent low-pass (< 300 Hz) and high-pass (> 300 Hz) filtering.

RESULTS

The N20m decreased dramatically in amplitude during interference stimulation. In contrast, the high-frequency oscillations moderately increased in number of peaks.

CONCLUSIONS

These results demonstrate the presence of an inverse relation between N20m and high-frequency oscillations for interference stimulation. We speculate that the high-frequency oscillations represent a localized activity of GABAergic inhibitory interneurons of layer 4, characterized by a high-frequency spike burst (200-1000 Hz) without adaptation, and that the continuous interference stimulation induces tonic excitation of the interneurons, leading to a facilitation of responses to the coherent afferent volley elicited by the median nerve stimulation (bottom-up mechanism). On the other hand, refractoriness of the pyramidal neurons caused directly by interference stimulation along with an enhanced feed-forward inhibition from the interneurons will lead to a decrease of N20m amplitude.