"Each woman watched three videos while having her brain imaged by positron emission tomography, better known as a PET scan. These scans detect minute changes in radioactivity in the brain that correspond to the amount of blood flowing to any given region. Regions with more blood flowing to them are considered more active.
One of the videos used in the study was a simple nature documentary about marine life in the Caribbean. The other two were selections from ;women-friendly' pornographic movies, one depicting only foreplay and manual stimulation and the other depicting oral sex and vaginal intercourse. Earlier studies had shown that the higher-intensity video showing intercourse produced stronger physical arousal in women than the foreplay-focused movie clip.
The scan results revealed that the high-intensity erotic video — and only the high-intensity erotic video — resulted in far less blood being sent to the primary visual cortex. The region is still active, just much less so. Usually, that effect is only seen when people are asked to conduct a nonvisual task, like remembering words, while also watching some sort of visual stimuli.
To [Gert] Holstege [a uroneurologist at the University of Groningen Medical Center in the Netherlands] those results suggest that the brain is focusing on sexual arousal as more important than visual processing during these erotic films."
— Stephanie Pappas, LiveScience