Presented at the 7th Annual Summer Sleep Workshop Multi-Site Training Program for Basic Sleep Research September 16 - 21, 1994, Lake Arrowhead, California
Periodicity of Standardized EEG Spectral Measures across the Waking DayDavid A. Kaiser and M.B. Sterman
Veterans Administration Medical Center, Sepulveda, CA, and School of Medicine, UCLA
In the course of conducting numerous quantitative studies of the EEG correlates of human attention and cognitive performance we have amassed a substantial amount of topographic data from several standardized baseline conditions. The most reliable of these, with regard to assessment of background arousal, is the eyes open baseline control. For this test the subject stares at a blank or static video screen for 2-3 minutes while EEG data are collected. Due to the varied nature of these studies and the circumstances affecting subject availability, data were acquired at different times throughout the waking phase of the circadian cycle. It was possible, therefore, to examine these data across a 12 hour time period and to evaluate potential ultradian influences on the neural substrates of EEG rhythm generation.
Quantitative topographic EEG data were collected from 130
subjects during a baseline "eyes open" test condition. Both male
(85%) and female (15%) subjects were studied. Mean age was
32, with a range of 19 to 43. Using a standard questionnaire it
was determined that none of these subjects had chronic or acute
health problems or used any prescription medications. None had
used alcohol or other non-proprietary drugs during the preceding
24 hours, although tobacco use was not always assessed. All
subjects reviewed and signed an approved institutional consent
Referential EEG recordings were obtained using a standardized,
pre-measured electrode cap with 17 recording sites estimating
placements specified by the International 10/20 System. Each site
was referenced to linked earlobes. A ground lead was placed just
anterior of FZ. Impedance readings below 5K were required before
recording was initiated.
EEG files were then exported to custom, seamless FFT
software. This software applied a series of 1 sec. tapered
windows, each overlapping the next by 75%, for FFT calculations.
This method maintained edge protection while preserving an
updated temporal resolution of 0.25 sec. Resulting data were
transported to spreadsheet files for sorting according to site,
frequency band, and time of collection.
Analysis was based on spectral magnitude values in six 2 Hz frequency bands from 3 to 15 Hz. Data were expressed both as absolute magnitude in each band and as percentage magnitude across bands for each site in each subject. Percentage data were used to eliminate individual differences in spectral magnitudes, and to evaluate the relative modulation of activity in these frequency bands across time. An ANOVA and sequential round-robin planned comparison test were performed to determine the statistical significance of this modulation.
Different results were obtained for the absolute vs. relative spectral magnitude analyses. For absolute magnitudes both main effects and interactions (site, frequency, time of day based on 2 hr. intervals) were significant (p = <.05). A topographically generalized modulation was obtained in all frequencies with a dominant peak at approximately 2 PM. An analysis of this modulation in the parietal area (Figure 1) confirmed a significant trough between 10 and 12 AM and a peak at 2 PM in all frequency bands (p = <.05). Visual inspection of these data suggests an additional but weak ultradian modulation with a cycle of approximately 90-120 min.
Analysis of relative band differences (percent contribution to total magnitude) presented a quite different picture. The lowest frequency bands (3-5, 5-7, and 7-9 Hz) were consistent in showing little variation over time (Figure 2). Conversely, the higher frequency bands were clearly modulated, and this modulation appeared to differ among bands. Thus, relative activity in the 9-11 Hz band showed an ultradian periodicity with a variable cycle approximating 120 min. at all parietal sites. However, in the 11-13 Hz band temporal modulation was both slower and stronger, with peaks at approximately 9AM, 1 PM, and 5PM. The peak at 1-2 PM was dominant in this 4 hr. cycle. Modulation at 13-15 Hz was similar but attenuated.
The preliminary analysis of these data has disclosed several
interesting findings. First, spectral magnitudes during a
standardized eyes open test condition appeared to be clearly
modulated throughout the waking day. Further, this modulation
was consistent across a broad frequency range. Rhythmic activity
was reliably suppressed during during mid-morning and early
evening hours, and increased from 2-3 PM. Secondly, when the
relative contribution of different EEG frequency bands within the
3-15 Hz spectrum was evaluated it was found that certain
frequency components were expressed as a stable percentage of
total frequency across time, while others were clearly modulated
in this regard. Thus, frequencies, below 9 Hz were stable and
those above 9 Hz were modulated, and this modulation was
greatest but different for the 9-11 and 11-13 Hz bands.