The cognitive aspects of sexual arousal in men are not completely known, but it does involve the appraisal and evaluation of the stimulus, categorization of the stimulus as sexual, and an affective response. Audition how the other's voice and movements sound ; Olfaction how the other smells, naturally or artificially; the wrong smell may be repellent. Androgen receptors occur in many different vertebrate body system tissues, and both males and females respond similarly to similar levels. The tendency for progesterone to have a regulatory effect, the presence of progesterone receptors in many types of body tissue, and the pattern of deterioration or tumor formation in many of those increasing in later years when progesterone levels have dropped, is prompting widespread research into the potential value of maintaining progesterone levels in both males and females. Furthermore, estrogens when administered shortly after natural or surgical menopause prevents decreases in verbal memory. Men with lower thresholds for sexual arousal have a greater likelihood to attend to sexual information and that testosterone may work by enhancing their attention to the relevant stimuli. When estrogen levels were raised through the increased activity of the enzyme aromatase in male lab mice, OCD rituals were dramatically decreased. However, there were some differences between homosexual and heterosexual women and men on these factors. Once erect, his penis may gain enough stimulation from contact with the inside of his clothing to maintain and encourage it for some time. The relationship between testosterone and female sexual motivation is somewhat ambiguous. In a study of cocaine users, it was reported that progesterone reduced craving and the feeling of being stimulated by cocaine. It has been observed in animal models that females have reduced susceptibility to traumatic brain injury and this protective effect has been hypothesized to be caused by increased circulating levels of estrogen and progesterone in females. The uterus typically contracts during orgasm and, with advancing age, those contractions may actually become painful. One of the serious effects of traumatic brain injury includes edema. Similarly, labial thermistor clips measure changes in temperature associated with genital engorgement; this method directly measures changes in temperature of the labia.