A hint towards the potential relevance of sex steroid differences to brain activity can be suggested by studies looking at brain activation across the different phases of the menstrual cycle. Brain activation levels themselves change as a function of menstrual phase (Goldstein, 2006). The relation to cognitive function also may change. For example, several studies have found that performance and brain activation fluctuate across the menstrual cycle on tasks including spatial ability (Hausmann & Gunturkun, 2000; Schöning et al., 2007) and semantic performance (Konrad et al., 2008). Tests of learning and memory also show fluctuations across the menstrual cycle, suggesting that temporary changes in sex steroid exposure can affect neuronal plasticity (Farage, Osborn, & Maclean, 2008; Sherwin, 2003). A recent study examining interhemispheric inhibition found that the influence of left hemispheric regions is much stronger during the menses, while lateralization decreases during the follicular phase as estradiol levels rise (Weis et al., 2008). Although brain imaging studies in general rarely explicitly account for influences such as menstrual phase, the opportunity to study the interaction of changing levels of steroid hormones
