Female sexual arousal and the law of initial value: assessment at several phases of the menstrual cycle.

Genital responses to erotic film and fantasy were recorded from 10 women at five phases of the menstrual cycle by means of vaginal photoplethysmography. Relationships between baseline (prestimulation) and difference (stimulation minus prestimulation) levels of response were examined. Neither vaginal blood volume nor average pulse amplitude conformed to the law of initial value ("the higher the baseline level, the less will be the difference score"). Baseline levels and difference scores generally were positively correlated at each phase of the menstrual cycle in the case of average pulse amplitude. For blood volume the correlations between baseline levels and difference scores were inconsistent in direction and small in magnitude, but trends differed between phases of the menstrual cycle. It is recommended that measures of female sexual arousal obtained through testing women who are at different phases of the cycle should not be aggregated unless it is established that the relationship between baseline levels and difference scores is uniform across phases.