Role Of High-frequency Oscillations Research Articles

Role of High Frequency Oscillations of Somatosensory Evoked Potentials in Deciphering Pathophysiology of Migraine.

Background Habituation deficit is considered as a neurophysiological abnormality among migraineurs in the interictal period. For clear comprehension and clarity about the mechanism underlying habituation in migraine, a sophisticated method, i.e., high frequency oscillations (HFOs) evoked potentials, have been utilized. However, studies pertaining to this in the Indian context are rare. Objective The aim of the study is to determine the utility of HFO of somatosensory evoked potential (SSEP) in deciphering the pathophysiology of migraine. Materials and Methods Sixty subjects including 30 migraineurs in the interictal period and 30 healthy controls were considered for the study. Median nerve SSEP was recorded in patients and controls by standard protocols. HFO was extracted offline using the Digital zero-phase shift band-pass filtering at 450 and 750 Hz. The early and late HFOs were determined with respect to the N20 peak and were compared between the groups. Results Of total 30 migraineurs, 18 had hemicranial headache and 12 had holocranial headache. N20 latency, P25 latency, N20 onset to peak amplitude, and N20 onset to P25 amplitude were comparable in migraineurs and controls. The intraburst frequency of early HFOs in migraineurs was significantly higher ( p = 0.04), whereas the peak-to-peak amplitude was significantly lower ( p = 0.001). Conclusion Early HFOs on SSEP represent the thalamocortical excitatory drive in migraineurs. Overall, the study reports that reduced amplitude of early HFOs in the interictal period suggest reduced thalamocortical drive in migraineurs.

ANALYSIS OF HIGH-RESOLUTION MICROELECTRODE EEG RECORDINGS IN AN ANIMAL MODEL OF SPONTANEOUS LIMBIC SEIZURES

We perform a systematic data analysis on high resolution (0.5–12 kHz) multiarray microelectrode recordings from an animal model of spontaneous limbic epilepsy, to investigate the role of high frequency oscillations and the occurrence of early precursors for seizures. Results of spectral analysis confirm the importance of very high frequency oscillations (even greater than 600 Hz) in normal (healthy) and abnormal (epileptic) hippocampus. Furthermore, we show that the measures of Recurrence Quantification Analysis (RQA) and Recurrence Time Statistics (RTS) are successful in indicating, rather uniquely, the onset of ictal state and the occurrence of some warnings/precursors during the pre-ictal state, in contrast to the linear measures investigated.

Presidential Symposium

1 Gregory L. Holmes, 2 Jermoe Engel, 3 Yehezkel Ben‐Ari, 4 Robert Muller, and 5 John Huguenard ( 1 Dartmouth Medical School, Lebanon, NH ; 2 Department of Neurology, UCLA School of Medicine, Los Angeles, CA ; 3 Inmed Institute, Marseille, France ; 4 SUNDY Downstate Medical Center, Brooklyn, NY ; and 5 Department of Neurology, Stanford University, Stanford, CA ,)The purpose of this symposium is to provide clinicians, basic scientists, and professions in health care with an understanding of the importance of brain rhythms in normal and abnormal brain function. Different brain rhythms, consisting of both low and fast frequencies, are grouped within complex wave‐sequences. These rhythms serve as powerful synchronizing forces and play important roles in all aspects of higher cortical function. During this symposium a review of the normal inherent network properties of brain excitatory and inhibitory circuits will be reviewed along with the cellular and molecular mechanisms responsible for these rhythms. The relationship between intrinsic hippocampal rhythms and learning and memory will be discussed. Aberrant rhythms may contribute to pathological processes such as memory impairment. There is also increasing evidence that high frequency oscillations may precede epilepsy in both the immature and mature brain, having an important role in epileptogenesis.Dr. Jerome Engle will review the physiological basis of brain rhythms and the role of high frequency oscillations in temporal lobe epilepsy. Dr.Yehezkel Ben‐Ari will discuss the role of high frequency stimulations in the mirror focus in immature animals. Dr. Robert Muller will provide information about the relationship between single cell firing patterns, hippocampal rhythms, and learning. Dr. John Huguenard will discuss oscillations in the thalamocortical system and the mechanisms of epileptic synchrony.This session will emphasize that epilepsy and its related morbidities are caused by neuronal network abnormalities. The brain's rhythms are critical in determining whether these networks perform normally or abnormally.

The role of high frequency oscillations in the penetration of plasma clouds across magnetic boundaries

Experiments are reported where a collissionfree plasma cloud penetrates a magnetic barrier by self-polarization. Three closely related effects, all fundamental for the penetration mechanism, are studied quantitatively: (1) anomalous fast magnetic field penetration (two orders of magnitude faster than classical), (2) anomalous fast electron transport (three orders of magnitude faster than classical and two orders of magnitude faster than Bohm diffusion), and (3) the ion energy budget as ions enter the potential structure set up by the self-polarized plasma cloud. It is concluded that all three phenomena are closely related and that they are mediated by highly nonlinear oscillations in the lower hybrid range, driven by a strong diamagnetic current loop which is set up in the plasma in the penetration process. The fast magnetic field penetration occurs as a consequence of the anomalous resistivity caused by the wave field and the fast electron transport across magnetic field lines is caused by the correlation between electric field and density oscillations in the wave field. It is also found that ions do not lose energy in proportion to the potential “hill” they have to climb, rather they are transported against the dc potential structure by the same correlation that is responsible for the electron transport. The results obtained through direct measurements are compared to particle in cell simulations that reproduce most aspects of the high frequency wave field.

The penetration of plasma clouds across magnetic boundaries: The role of high frequency oscillations

Experiments are reported where a collision-free plasma cloud penetrates a magnetic barrier by self-polarization. Two closely related effects, both fundamental for the penetration mechanism, are studied quantitatively: anomalous fast magnetic field penetration (two orders of magnitude faster than classical), and anomalous fast electron transport (three orders of magnitude faster than classical and two orders of magnitude faster than Bohm diffusion). It is concluded that they are both mediated by highly nonlinear oscillations in the lower hybrid range, driven by a strong diamagnetic current loop which is set up in the plasma in the penetration process.