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The Journal of Neurophysiology Vol. 88 No. 4 October 2002, pp. 1743-1752
Copyright ©2002 by the American Physiological Society
Departments of 1Neurobiology, 2Neurology, and 3Neurosurgery and 4The Brain Research Institute, David Geffen School of Medicine at UCLA, Los Angeles, California 90095
Staba, Richard J.,
Charles L. Wilson,
Anatol Bragin,
Itzhak Fried, and
Jerome Engel Jr.
Quantitative Analysis of High-Frequency Oscillations (80-500 Hz)
Recorded in Human Epileptic Hippocampus and Entorhinal Cortex. J. Neurophysiol. 88: 1743-1752, 2002. High-frequency oscillations (100-200 Hz), termed ripples, have been
identified in hippocampal (Hip) and entorhinal cortical (EC) areas of
rodents and humans. In contrast, higher-frequency oscillations
(250-500 Hz), termed fast ripples (FR), have been described in
seizure-generating limbic areas of rodents made epileptic by
intrahippocampal injection of kainic acid and observed in humans ipsilateral to areas of seizure initiation. However, quantitative studies supporting the existence of two spectrally distinct oscillatory events have not been carried out in humans nor has the preferential appearance of FR within seizure generating areas received statistical evaluation based on analysis of a large sample of oscillatory events.
Interictal oscillations within the bandwidth of 80-500 Hz were
detected in Hip and EC areas of patients with mesial temporal lobe
epilepsy using wideband EEG recorded during non-rapid eye-movement sleep from chronically implanted depth electrodes. Power spectral analysis showed that oscillations detected from Hip and EC areas were
composed of two spectrally distinct groups. The lower-frequency ripple
group was defined by a frequency of 96 ± 14 Hz (median ± width), while the higher-frequency FR group had a frequency of
262 ± 59 Hz. FR oscillations were significantly shorter in duration compared with ripple oscillations (P < 0.0001). In regard to the occurrence of FR and ripples in epileptic Hip
and EC, the mean ratio of the number of FR to ripples generated in
areas ipsilateral to seizure onset was significantly higher compared
with the mean ratio of FR to ripple generation from contralateral areas
(P = 0.008). Furthermore, sites ipsilateral to seizure
onset with hippocampal atrophy had significantly higher ratios compared
with sites contralateral to both seizure onset and hippocampal atrophy
(P = 0.001). These data provide compelling quantitative
and statistical evidence for the existence of two spectrally distinct
groups of limbic oscillations that have frequency and duration
characteristics similar to those previously described in epileptic rat
and human Hip and EC. The strong association between FR and regions of
seizure initiation supports the view that FR reflects pathological
hypersynchronous events crucially associated with seizure genesis.
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