Continuous high‐frequency activity in mesial temporal lobe structures

<jats:title>Summary</jats:title><jats:p><jats:bold>Purpose: </jats:bold> Many recent studies have reported the importance of high‐frequency oscillations (HFOs) in the intracerebral electroencephalography (EEG) of patients with epilepsy. These HFOs have been defined as events that stand out from the background. We have noticed that this background often consists itself of high‐frequency rhythmic activity. The purpose of this study is to perform a first evaluation of the characteristics of high‐frequency continuous or semicontinuous background activity.</jats:p><jats:p><jats:bold>Methods: </jats:bold> Because the continuous high‐frequency pattern was noted mainly in mesial temporal structures, we reviewed the EEG studies from these structures in 24 unselected patients with electrodes implanted in these regions. Sections of background away from interictal spikes were marked visually during periods of slow‐wave sleep and wakefulness. They were then high‐passed filtered at 80 Hz and categorized as having high‐frequency rhythmic activity in one of three patterns: continuous/semicontinuous, irregular, sporadic. Wavelet entropy, which measures the degree of rhythmicity of a signal, was calculated for the marked background sections.</jats:p><jats:p><jats:bold>Key Findings: </jats:bold> Ninety‐six bipolar channels were analyzed. The continuous/semicontinuous pattern was found frequently (29/96 channels during wake and 34/96 during sleep). The different patterns were consistent between sleep and wakefulness. The continuous/semicontinuous pattern was found significantly more often in the hippocampus than in the parahippocampal gyrus and was rarely found in the amygdala. The types of pattern were not influenced by whether a channel was within the seizure‐onset zone, or whether it was a lesional channel. The continuous/semicontinuous pattern was associated with a higher frequency of spikes and with high rates of ripples and fast ripples.</jats:p><jats:p><jats:bold>Significance: </jats:bold> It appears that high‐frequency activity (above 80 Hz) does not appear only in the form of brief paroxysmal events but also in the form of continuous rhythmic activity or very long bursts. In this study limited to mesial temporal structures, we found a clear anatomic preference for the hippocampus. Although associated with spikes and with distinct HFOs, this pattern was not clearly associated with the seizure‐onset zone. Future studies will need to evaluate systematically the presence of this pattern, as it may have a pathophysiologic significance and it will also have an important influence on the very definition of HFOs.</jats:p>