Diurnal oscillations in hemispheric performance.

Folkard (1979, 1990) suggested that diurnal changes in performance may reflect a morning-evening decrease in the degree of left-hemisphere dominance. Forty-eight right-handed women performed verbal and spatial hemifield tachistoscopic tasks at four separate times of day. Supporting the above hypotheses, changes in accuracy over the day showed a left-hemisphere advantage at 12:00 and a right-hemisphere advantage at 19:45, whereas changes in speed were symmetrical both in verbal and spatial tasks. These changes occurred only when hemispheres received stimuli in the processing of which they were not specialized, that is, when verbal stimuli appeared at the left visual field and spatial stimuli at the right visual field. Evidence suggesting a separate oscillatory control of the circadian rhythms of left and right-hemisphere activity is discussed.