There may have been multiple studies so I can't for sure say that the date and Person are wrong but the only study I could find that fits the description was in 1999 by W W Schults, P van Andel, I Sabelis, and E Mooyard.
Schultz, W. W., van Andel, P., Sabelis, I., & Mooyaart, E. (1999). Magnetic resonance imaging of male and female genitals during coitus and female sexual arousal. BMJ (Clinical research ed.), 319(7225), 1596–1600. https://doi.org/10.1136/bmj.319.7225.1596