With the book "Fifty Shades of Grey" still flying off the shelves, women are discovering the power of sexual fantasy, some for the first time.According to neuroscientists Ogi Ogas and Sai Gaddam, authors of "A Billion Wicked Thoughts," while men tend to have visually driven fantasies, women prefer to fantasize about what a man might do to them.For example, the top five female fantasies involve having sex with a stranger, being worshipped in bed, being ravaged in bed, being watched by others and enjoying a threesome. In other words, women tend to fantasize about being the object of desire.Erotica is wonderful, but you don't need a book to feel sexy. In fact, indulging in a little bit of fantasy on your own may be even more effective at spurring both sexual desire and arousal, according to a recent study.Researchers at the University of Michigan have shown that different forms of sexual fantasy may have a real impact on arousal and desire, particularly for women.For their study, published in the Journal of Sexual Medicine, Katherine Goldey and Sari van Anders randomly assigned 128 women and 98 men to one of four groups, or "arousal conditions."One group completed an imagined social situation exercise in which they imagined a positive sexual encounter and then answered open-ended questions about it. The others engaged in typical unstructured fantasy (imagining sexual situations but not writing about them), while others read an erotic story of the researchers' choice or took part in a neutral exercise (writing about the room they were in).Then they rated their perceived genital arousal (how they felt physically), as well as their "state" desire (whether they wanted to have sex).The results: The first three groups all reported experiencing increased genital and psychological arousal, as well as higher "state" desire, compared with the neutral group. The group who engaged in unstructured fantasy had the greatest arousal.Interestingly, though, the researchers also measured "trait" desire -- sort of a person's general set point for sexual desire, once believed to be unchangeable -- and discovered that the imagined social situation exercise group, the ones imagining a positive sexual situation, reported significantly higher trait desire after that exercise.That's great news for all of us, but particularly for women, who often tend to experience sexual desire in response to the sex they're already having. In other words, you may not feel quite so "into" sex in the beginning, but your desire and arousal grow as the interlude progresses.Indeed, female sexual response is typically characterized by "responsive desire," while male sexual response is more likely characterized by "spontaneous desire," says sex educator Emily Nagoski, author of the "Good in Bed Guide to Female Orgasms.""'Responsive desire' is when motivation to have sex begins after sexual behavior has started: You're doing something else when your partner comes over and starts kissing you, and you think, 'Oh yeah! That's a good idea,'" she explains."'Spontaneous" desire, more typical of male sexuality, works more like this: You're walking down the street and for no immediately obvious reason you think, "Hm. I'd like to have sex!"As the new study suggests, actively fantasizing about sex isn't just a way to pass the time. It can create real changes in your body, moving you from thinking about sex in the abstract to fueling real desire and arousal.Fantasy is also a sign of a healthy sex life, and sharing those fantasies with your partner doesn't just have the potential to spice things up in the bedroom. It also means that you feel secure and confident in your relationship.It's often been said that the brain is our biggest sex organ, and this study only confirms that. So, give your brain a workout with a fantasy or two. You might be surprised where it leads you.