The structures of the ultradian variations among the cognitive task performance and the EEG activities were studied on eight male students. Referential central (C3, C4) and parietal (P3, P4) EEGs were recorded every 15 min of the eye closed rest period, when the subjects were asked to estimate a 120 sec period and to keep eyes closed for this period. Artifact free EEG data were analyzed for one minute by the FFT. EEG parameters used in this study are the amplitude (root power)

EEG laterality index and interhemispheric coherence. The EEG data were divided into five frequency bands (δ, θ, α, β1, β2).

After the time estimation task, subjects were asked to draw a vertical or horizontal 20 cm line and to point the midpoint of these lines.

The least square spectral method was applied on the time series of five task performance and EEG variables. About 100 min ultradian variations were identified in all of the variables, with the exception of the midpoint judgment which showed more shorter period, about 60 min cycles. Principal component analysis were examined on the task performance EEG variables. The results of PCA revealed that the EEG indicies of the interhemispheric activity were more closely related to the task performance than the single EEG amplitude. The EEG latelality indicies were highly correlated to the vertical line drawing, while the interhemispheric coherence were highly correlated to the horizontal line drawing. Anisotropy of space were observed in the daytime behavioral ultradian variations.