Interhemispheric interaction during the menstrual cycle.

Fluctuating levels of sex hormones and high levels of progesterone (P), in particular, have been suggested to reduce interhemispheric inhibition. The present study focuses on hormone-dependent modulation of interhemispheric integration. In two versions of the Banich-Belger task, participants were asked to match letters according to their physical (e.g., A vs. A) and semantic identity (e.g., A vs. a). Matches were presented either within or across visual half-fields. Moreover, a simple reaction-time task (Poffenberger task) that is assumed to estimate interhemispheric transfer time (IHTT) was used. Seventeen normally cycling women were tested during low P menses and high P midluteal phase. Saliva levels of P were analysed using chemiluminescence assays. Fifteen postmenopausal women performed the same tasks in corresponding time intervals. Additionally, 28 younger male controls were tested once. In agreement with previous results, the more demanding (semantic) interhemispheric-integration task revealed a typical across-field advantage (AFA) for all three groups. However, in normally cycling women, the AFA was significantly reduced during menses. IHTT did not change across the cycle phases. The results indicate that interhemispheric integration fluctuates across the menstrual cycle and is reduced during menses. During the luteal phase, however, the AFA is increased, suggesting that accompanying hormonal conditions favour an efficient interhemispheric integration. We conclude that transcallosal mechanisms involved in interhemispheric integration are profoundly altered when sex hormones are permanently reduced as in men and postmenopausal women. This difference enables an efficient interhemispheric integration without modulatory effects of P.