Performance on cognitive tasks cycled at ultradian frequencies for 24 males over a test period of eight hours. The verbal task of written word production cycled at 80 minutes; the spatial task of locating points in space cycled at 96 minutes. Multiple cycles were seen for a perceptual speed task that factor loads on both the verbal and spatial task. Replication of the results for the first 12 and second 12 subjects demonstrated their robustness. The verbal and spatial tasks were chosen to reflect specialized functions of the left and right cerebral hemispheres, respectively. Accordingly, the results are interpreted as evidence that specialized task performances are associated with independent neurochemical systems. In addition, blood samples were taken at task performance to assess cyclicity of hormone levels. Luteinizing hormone had a period of 120 minutes, testosterone and cortisol were inconsistent and none seemed to be related to the cognitive tasks. However when subjects were divided according to a winter or summer testing schedule, the spatial periodicity was absent for the summer group and the verbal periodicity was absent for the winter peak.
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