Abstracts

Rationale: The conventional range of EEG analysis usually involves frequencies below 40Hz. Studies over the last decade suggest that high-frequency oscillations (HFOs) in the range of 40 Hz to greater than 80 Hz may have an essential role in normal and pathologic brain function. HFOs have been reported to be associated with seizure onset zones, epileptic spasms and in areas surrounding cortical tubers in tuberous sclerosis. However, to our knowledge no study has systematically investigated the presence of HFOs in physiologically normal volunteers. This is needed to determine the specificity and sensitivity of HFOs in relation to epilepsy. The objective of this study is therefore to preliminarily characterize HFOs in physiologically normal volunteers and to correlate their presence to those patterns seen in the routine EEG using standard settings. Methods: Routine EEGs were recorded in the standard manner using the international 10-20 system for at least 20 minutes per subject. No photic stimulation or hyperventilation was included in the procedure. To see the HFOs, the sweep speed of the recording was changed from 30 mm/sec to 240 mm/sec, the sensitivity from 7 V/mm to 1 V/mm, and the low frequency filter from 1 Hz to 80 Hz as described in the literature. After the HFOs were noted with these parameters, the latter were subsequently returned to standard settings to determine what patterns on the standard EEG caused the HFOs.Results: Five physiologically normal volunteers were examined (4 females and one male), age range 14-21 years. A total of 151 HFOs were recorded, with an average of 30.2 per volunteer (range 11-52). 149 HFOs (98.7%) were associated with muscle artefact on the original recording. 150 HFOs (99.3%) were recorded during quiet wakefulness and 1 HFO (0.7%) during stage I sleep. Two of the HFOs (1.3%) were not associated with muscle artefact, but in one instance were associated with a 50 Hz rhythm and in the second instance with a 35 Hz rhythm. In 5 instances of noting the presence of muscle artefact on the original recording, no HFOs were seen in the same locations.Conclusions: A low rate of HFO presence was noted in physiologically normal volunteers that were not associated with movement. However these HFOs are associated with 35-50 Hz rhythms, which may be significant when an attempt is made to correlate HFOs with epileptic foci.