Ultradian rhythms in cerebral laterality tested by a verbal dichotic listening paradigm

Dichotic listening (DL) performance of normal (healthy) adults was tested every 15 minutes continuously during 8-12 hours of daytime. The dichotic material consisted of 8 blocks, each containing 40 pairs of monosyllabic, concrete nouns in German. The subjects heard one block per each 15-min trial. DL performances determined by the number of correct free recalls gave no evidence for periodic variations, neither of right nor of left-ear performance. However, when determined by the number of first, but correctly recalled nouns DL performance showed distinct periodicities of about 3-4 hours at least in some subjects. Such periodicities were obvious in both right and left-ear DL performance and oscillated in counter-phase. This suggests that competitive processing of dichotic verbal stimuli mediated by the cerebral hemispheres could be modulated by an endogenous oscillatory system. Some subjects slept in the laboratory after they had performed in a long-term dichotic listening task. Since sleep stage was scored by means of electroencephalographic recordings phase relationships between ultradian cycles of DL performance and the sleep cycle could be analyzed. These findings suggest that sleep cycle of about 1 1/2-2 hours and the 3-4 hour periodicities of DL performance are organized in an approximately 1:2 period ratio of an intrinsic multi-oscillatory system.